Previous Year Paper

UPPSC GIC Lecturer Physics 2017 (Previous Year Paper)

Held in 2017

150 questions120 minutes
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Part I (30 questions)

Question 1

Ubol Ratna, royal princess of a country was declared disqualified to contest as the Prime Ministerial candidate by the Election Commission of her country. She belongs to which one of the following countries?

  1. AOption (a)

    Bhutan

  2. BOption (b)

    Nepal

  3. COption (c)

    Thailand

  4. DOption (d)

    None of these

View answer & explanation

Correct answer

Answer: (c)C.

Thailand

Why this answer is correct

The correct answer is - Thailand

Key Points

  • Ubol Ratana is a member of the Thai royal family. She is the eldest daughter of King Bhumibol Adulyadej (Rama IX) and Queen Sirikit of Thailand.
  • The Thai Constitution prohibits members of the royal family from participating in politics. Despite renouncing her royal titles, Ubol Ratana's candidacy was considered a violation of this rule.
  • In 2019, the Election Commission of Thailand disqualified her from running as a candidate for Prime Minister, citing her royal heritage and the constitutional restrictions.
  • This event was widely discussed in Thailand and internationally, as it marked an unprecedented intersection of monarchy and democratic politics in the country.

Additional Information

  • Thai Monarchy
  • The Thai monarchy is a constitutional monarchy, where the King is the head of state but has limited powers under the constitution.
  • The monarchy is highly respected in Thailand, and laws such as lese majeste criminalize criticism of the royal family.
  • Constitutional Provisions
  • The Thai Constitution explicitly bars members of the royal family from engaging in politics to maintain the neutrality of the monarchy.
  • Despite renouncing her titles, Ubol Ratana was still considered part of the royal family under Thai law, leading to her disqualification.
  • Political Context
  • The political landscape in Thailand has been shaped by frequent military coups and political unrest. The royal family is seen as a stabilizing force.
  • Ubol Ratana's decision to contest elections was seen as controversial, as it challenged long-standing norms separating the monarchy from politics.

Question 2

'Ismail Omar Guelleh', was conferred with Padma-Vibhushan in January 2019 by Government of India, is a citizen of which one of the following countries?

  1. AOption (a)

    Ghana

  2. BOption (b)

    Senegal

  3. COption (c)

    Djibouti

  4. DOption (d)

    Togo

View answer & explanation

Correct answer

Answer: (c)C.

Djibouti

Why this answer is correct

The correct answer is - Djibouti

Key Points

  • Ismail Omar Guelleh
  • Ismail Omar Guelleh is the President of Djibouti, serving in office since 1999.
  • He has played a significant role in strengthening India-Djibouti relations, particularly in trade and diplomatic engagements.
  • Padma-Vibhushan Award
  • This is the second-highest civilian award in India, given for exceptional and distinguished service.
  • Guelleh was conferred this award in January 2019 for his contributions to fostering bilateral ties between Djibouti and India.
  • Djibouti's Strategic Importance
  • Djibouti is located in the Horn of Africa and serves as a strategic transit hub for international shipping routes.
  • Its location has been instrumental in India's efforts to enhance its presence in the Indian Ocean Region.

Additional Information

  • India-Africa Relations
  • India has been actively engaging with African nations through initiatives such as the India-Africa Forum Summit.
  • Djibouti has been a key partner for India in the Horn of Africa, especially in areas like maritime security and trade.
  • Recognition of Foreign Leaders
  • India confers civilian awards like the Padma-Vibhushan to foreign leaders who contribute to strengthening bilateral relationships.
  • Other notable foreign awardees include Shinzo Abe of Japan and Vladimir Putin of Russia.
  • Djibouti's Economic Role
  • Djibouti is home to one of the busiest ports in Africa, making it a critical hub for global trade.
  • Its strategic location near the Gulf of Aden has attracted investments from countries like India, China, and the U.S..

Question 3

To strengthen the 'Handlooms and Textile sector' of India, an event 'Artisan Speak' was organised on 28 January, 2019 at which one of the following places?

  1. AOption (a)

    Elephanta Caves

  2. BOption (b)

    Ahmadnagar

  3. COption (c)

    Jaipur

  4. DOption (d)

    Bhimbetka

View answer & explanation

Correct answer

Answer: (a)A.

Elephanta Caves

Why this answer is correct

The correct answer is - Elephanta Caves

Key Points

  • Elephanta Caves
  • The event ‘Artisan Speak’ was held on 28 January 2019 at the iconic Elephanta Caves, a UNESCO World Heritage Site located near Mumbai.
  • The focus of the event was to promote and strengthen India's Handlooms and Textile sector by connecting artisans to markets and enhancing their visibility.
  • It showcased the rich cultural heritage of India, specifically its traditional textiles and craftsmanship.
  • The venue choice highlighted India's commitment to preserving its historic monuments while promoting its crafts industry.

Additional Information

  • Handlooms and Textile Sector
  • India's textile sector is one of the largest in the world, contributing significantly to the country's economy and employment.
  • Government initiatives like ‘Artisan Speak’ aim to empower artisans, improve production quality, and increase global recognition.
  • Key schemes include the National Handloom Development Programme (NHDP) and the Handicrafts Cluster Development Scheme.
  • Elephanta Caves
  • The caves are located on Elephanta Island, approximately 10 kilometers east of the Gateway of India, Mumbai.
  • Known for their rock-cut sculptures and artistic representation of Hindu deities, particularly Lord Shiva.
  • The site reflects India's rich architectural heritage and cultural significance.

Question 4

Match List I with List II and select -the correct answer from the code given below:

List-I(Acid)List-II(Source)
A. Acetic Acid1. Butter
B. Lactic Acid2. Lemon
C. Butyric Acid3. Vinegar
D. Citric Acid4. Milk
  1. AOption (a)

    A-3, B-4, C-2, D-1

  2. BOption (b)

    A-3, B-4, C-1, D-2

  3. COption (c)

    A-4, B-3, C-1, D-2

  4. DOption (d)

    A-1, B-2, C-3, D-4

View answer & explanation

Correct answer

Answer: (b)B.

A-3, B-4, C-1, D-2

Why this answer is correct

The correct answer is - Acetic Acid - Vinegar, Lactic Acid - Milk, Butyric Acid - Butter, Citric Acid - Lemon

Key Points

  • Acetic Acid - Vinegar
  • Acetic acid is commonly found in vinegar, which is used in cooking and food preservation.
  • It is characterized by its sour taste and pungent smell.
  • Lactic Acid - Milk
  • Lactic acid is produced during the fermentation of milk and is responsible for the sour taste in dairy products like yogurt and curd.
  • It is formed by the action of lactic acid bacteria on lactose (milk sugar).
  • Butyric Acid - Butter
  • Butyric acid is found in butter and contributes to its characteristic smell.
  • It is a short-chain fatty acid that occurs naturally in dairy products.
  • Citric Acid - Lemon
  • Citric acid is abundant in lemons and other citrus fruits, giving them their tangy flavor.
  • It plays a key role in metabolism through the citric acid cycle (Krebs cycle).

Additional Information

  • Acetic Acid
  • It is a weak acid with the chemical formula CH₃COOH.
  • It is widely used in food industry, cleaning products, and chemical synthesis.
  • Lactic Acid
  • Its chemical formula is C₃H₆O₃.
  • Lactic acid is also produced in muscles during intense exercise due to anaerobic respiration.
  • Butyric Acid
  • Its chemical formula is C₄H₈O₂.
  • It plays a role in gut health by serving as a fuel for colon cells.
  • Citric Acid
  • Chemical formula: C₆H₈O₇.
  • It is used as a preservative and flavoring agent in food products.

Question 5

Which of the following is not a true Fish?

  1. AOption (a)

    Rat Fish

  2. BOption (b)

    Cat Fish

  3. COption (c)

    Dog Fish

  4. DOption (d)

    Jelly Fish

View answer & explanation

Correct answer

Answer: (d)D.

Jelly Fish

Why this answer is correct

The correct answer is Jelly Fish.

Key Points

  • Jelly Fish is not a true fish because it lacks the basic anatomical features of a fish, such as a backbone, gills (in the vertebrate sense), and scales. It belongs to the phylum Cnidaria and is an invertebrate. Hence, the statement regarding Jelly Fish is correct in the context of the question.
  • Rat Fish, also known as Chimaeras, is indeed a true fish. It belongs to the class Chondrichthyes and is a cartilaginous fish related to sharks and rays. Hence, statement 1 is incorrect as it is a true fish.
  • Cat Fish is a diverse group of ray-finned fish belonging to the order Siluriformes. They are characterized by prominent barbels, which resemble a cat's whiskers, and are considered true teleost fish. Hence, statement 2 is incorrect as it is a true fish.
  • Dog Fish is a type of small shark belonging to the family Squalidae. As a member of the Chondrichthyes class, it possesses a vertebral column made of cartilage and is classified as a true fish. Hence, statement 3 is incorrect as it is a true fish.
  • In biological terms, a true fish must be a vertebrate, possess gills throughout its life, and typically have fins for locomotion. Jelly Fish are composed of 95% water and lack a brain, blood, or a heart.

Additional Information

  • Characteristics of Jelly Fish:
  • Classification: They belong to the kingdom Animalia, phylum Cnidaria, and class Scyphozoa.
  • Features: They exhibit radial symmetry and have a gelatinous body called a medusa. They use specialized stinging cells called nematocysts to capture prey.
  • Habitat: They are found in every ocean, from the surface to the deep sea, and some species even inhabit freshwater.
  • Characteristics of Rat Fish (Chimaeras):
  • Evolution: These fish branched off from sharks nearly 400 million years ago.
  • Location: They are mostly found in temperate ocean waters at depths of up to 2,600 meters.
  • Anatomy: Unlike sharks, their upper jaws are fused to their skulls, and they have permanent grinding tooth plates.
  • Characteristics of Cat Fish:
  • Diversity: There are over 3,000 species of Cat Fish found on every continent except Antarctica.
  • Uses: Many species are of significant importance in aquaculture and as food, such as the Channel Catfish in the United States.
  • Functions: Their barbels contain chemoreceptors used to detect food in murky water.
  • Characteristics of Dog Fish:
  • Scientific Name: The most common species is Squalus acanthias (Spiny Dogfish).
  • Applications: They are frequently used in biological laboratories for dissection due to their Generalized vertebrate anatomy.
  • Data: They have a very long gestation period, lasting about 18 to 24 months, which is one of the longest in the animal kingdom.

Important Points

  • Misleading Common Names: Several aquatic animals have "fish" in their name but are not true fish. Examples include Starfish (an Echinoderm), Silverfish (an Insect), Cuttlefish (a Mollusk), and Whales/Dolphins (Mammals).
  • Taxonomy: True fish are generally categorized into three groups: Agnatha (jawless fish), Chondrichthyes (cartilaginous fish), and Osteichthyes (bony fish).
  • Ecological Role: True fish play a vital role in the aquatic food web, serving as both predators and prey, while invertebrates like Jelly Fish can sometimes cause ecological imbalances through "blooms" caused by climate change and overfishing of their natural predators.

Question 6

During intense exercise, which of the following is produced as a metabolic by-product in the muscles?

  1. AOption (a)

    Glycogen

  2. BOption (b)

    Protein

  3. COption (c)

    Fat

  4. DOption (d)

    Lactic Acid

View answer & explanation

Correct answer

Answer: (d)D.

Lactic Acid

Why this answer is correct

The correct answer is Lactic Acid.

Key Points

  • During intense physical exercise or vigorous activity, the demand for energy in the skeletal muscles increases rapidly, often outstripping the oxygen supply provided by the circulatory system.
  • When oxygen levels are low (a state known as hypoxia), muscle cells switch from aerobic respiration to anaerobic respiration to continue producing Adenosine Triphosphate (ATP).
  • In this anaerobic pathway, glucose undergoes glycolysis to form pyruvate, which is then converted into Lactic Acid (specifically lactate in biological fluids). This metabolic by-product is produced to regenerate NAD+, allowing glycolysis to continue. Hence, the statement regarding Lactic Acid production is correct.
  • The buildup of Lactic Acid in the muscles is a primary cause of the burning sensation and temporary fatigue experienced during high-intensity sprints or weightlifting.
  • While anaerobic respiration is less efficient than aerobic respiration, producing only 2 ATP molecules per glucose molecule, it provides energy at a much faster rate for short bursts of activity.

Additional Information

  • Glycogen:
  • Glycogen is a multibranched polysaccharide that serves as the primary form of energy storage in humans and animals.
  • It is stored mainly in the liver and skeletal muscles. During exercise, it is broken down into glucose to fuel metabolism; therefore, it is a reactant/fuel, not a metabolic by-product.
  • The Cori Cycle:
  • After the cessation of exercise, Lactic Acid is transported through the bloodstream to the liver.
  • In the liver, it is converted back into pyruvate and eventually into glucose through gluconeogenesis. This cycle is essential for preventing lactic acidosis and recycling carbon sources.
  • Muscle Fatigue and DOMS:
  • Contrary to popular belief, Lactic Acid is not the cause of Delayed Onset Muscle Soreness (DOMS), which typically peaks 24 to 48 hours after exercise.
  • DOMS is caused by microscopic tears in the muscle fibers and subsequent inflammation. Lactic Acid is usually cleared from the bloodstream within 1 hour after exercise.
  • Aerobic vs. Anaerobic Metabolism:
  • Aerobic metabolism occurs in the mitochondria and produces carbon dioxide and water as by-products, yielding approximately 36 to 38 ATP.
  • Anaerobic metabolism occurs in the cytoplasm and is the dominant energy system for activities lasting 10 to 90 seconds.

Important Points

  • Proteins and Fats: While proteins and fats are used as macronutrients for energy production (especially during long-duration, low-intensity exercise like marathons), they are not the immediate by-products of anaerobic muscle contraction.
  • Chemical Equation: The chemical formula for Lactic Acid is C3H6O3. In the body, it usually exists as the lactate ion (C3H5O3-).
  • Energy Yield: The fast glycolytic system (anaerobic) provides ATP rapidly but results in the acidification of the muscle environment, which eventually inhibits enzyme activity and muscle contraction.

Question 7

Consider the following statements and select the correct answer from the code given below.

A. Colour blindness occurs more in men than in women.

B. Night blindness occurs in old persons not in young ones.

  1. AOption (a)

    Only A

  2. BOption (b)

    Only B

  3. COption (c)

    A and B both

  4. DOption (d)

    N either A nor B

View answer & explanation

Correct answer

Answer: (a)A.

Only A

Why this answer is correct

The correct answer is Only A.

Key Points

  • Statement A: Colour blindness is a genetic condition that is typically sex-linked and recessive, carried on the X chromosome. Since men have only one X chromosome (XY), they will manifest the condition if that single chromosome carries the mutated gene. Women, however, have two X chromosomes (XX), meaning they only express the condition if both chromosomes are affected; otherwise, they remain carriers. Statistically, colour blindness affects approximately 8% of men compared to only 0.5% of women worldwide. Hence, statement A is correct.
  • Statement B: Night blindness, medically known as Nyctalopia, is a condition where the individual finds it difficult or impossible to see in low light or darkness. It is primarily caused by a Vitamin A deficiency, which affects the production of rhodopsin in the rod cells of the retina. While it can occur in old persons due to cataracts or macular degeneration, it is extremely common in young children and young adults, especially in developing countries where malnutrition is prevalent. Hence, statement B is incorrect.
  • The retina of the human eye contains two types of photoreceptors: cones and rods. Cones are responsible for color vision and function best in bright light, while rods are essential for vision in dim light. Defects in cones lead to colour blindness, whereas impairment of rods leads to night blindness.
  • Colour blindness is mostly congenital (present from birth), meaning the age of the person does not determine its onset. In contrast, night blindness can be acquired at any point in life due to dietary habits, health conditions, or medical treatments.

Additional Information

  • Colour Blindness (Daltonism):
  • Types: The most common form is Red-Green colour blindness, followed by Blue-Yellow colour blindness. The rarest form is Total Colour Blindness (Achromatopsia), where the world is seen in grayscale.
  • Diagnosis: Doctors use the Ishihara Test, which consists of a series of plates (circles) containing dots of different colours and sizes forming numbers or paths.
  • Applications: Understanding this condition is vital for certain professions like pilots, train drivers, and electricians, where colour coding is a matter of safety.
  • Night Blindness (Nyctalopia):
  • Causes: Beyond Vitamin A deficiency, other causes include Retinitis Pigmentosa (a genetic disorder), Glaucoma, Cataracts, and Usher syndrome.
  • Data and Facts: According to the World Health Organization (WHO), Vitamin A deficiency is the leading cause of preventable blindness in children worldwide, affecting millions in Africa and South-East Asia.
  • Functions: The rod cells contain rhodopsin (visual purple). When light strikes this pigment, it bleaches, sending an electrical signal to the brain. Vitamin A is required to regenerate this pigment.

Important Points

  • Genetics of Colour Blindness: If a mother is a carrier and the father has normal vision, there is a 50% chance that a son will be colour blind. A daughter can only be colour blind if her father is colour blind and her mother is at least a carrier.
  • Vitamin A Sources: To prevent night blindness, the Government of India promotes the consumption of Golden Rice, carrots, sweet potatoes, spinach, and animal liver, which are rich in Retinol or Beta-carotene.
  • Legal Guidelines: In India, under the Medical Standards for various government jobs, candidates with colour blindness are often restricted from technical cadres in the Railways and Armed Forces.
  • Treatment: While there is currently no permanent cure for hereditary colour blindness, night blindness caused by malnutrition can often be reversed with therapeutic doses of Vitamin A supplements within a few days or weeks.

Question 8

Consider the following statements and select the correct answer from the code given below :

Assertion (A): Polarisation does not happen in sound waves.

Reason (R): Sound waves are transverse waves.

Code:

  1. AOption (a)

    Both (A) and (R) are true and (R) is the correct explanation of (A).

  2. BOption (b)

    Both (A) and (R) are true, but (R) is not the correct explanation of (A).

  3. COption (c)

    (A) is true, but (R) is false.

  4. DOption (d)

     (A) is false, but (R) is true.

View answer & explanation

Correct answer

Answer: (c)C.

(A) is true, but (R) is false.

Why this answer is correct

The correct answer is (A) is true, but (R) is false..

Key Points

  • Assertion (A) states that polarisation does not happen in sound waves. This is because polarisation is a characteristic property of transverse waves, where the vibrations occur perpendicular to the direction of wave travel. Sound waves in air and most fluids are longitudinal waves, meaning the particles oscillate parallel to the direction of energy transfer. Since longitudinal waves only have one dimension of oscillation, they cannot be restricted to a specific plane. Hence, Assertion (A) is correct.
  • Reason (R) claims that sound waves are transverse waves. In physics, a transverse wave is one where the medium moves at right angles to the direction of the wave, such as light waves or waves on a guitar string. However, sound waves are actually mechanical longitudinal waves that require a medium (solid, liquid, or gas) to travel through compressions and rarefactions. Hence, Reason (R) is incorrect.
  • The phenomenon of polarisation serves as the primary experimental evidence to distinguish between transverse and longitudinal waves. If a wave can be polarised, it must be transverse. Since sound cannot be polarised, it confirms its longitudinal nature in gaseous and liquid media.
  • In solids, sound can occasionally travel as transverse waves (shear waves), but in the general context of physics and standard conditions, sound is treated as a longitudinal wave.

Additional Information

  • Longitudinal Waves: These are waves in which the displacement of the medium is in the same direction as, or the opposite direction to, the direction of propagation of the wave. Sound waves are the most common example. They consist of compressions (high pressure) and rarefactions (low pressure).
  • Transverse Waves: In these waves, the particles of the medium move perpendicular to the direction of the wave's motion. Examples include Electromagnetic waves (like visible light, X-rays, and radio waves) and seismic S-waves. These waves can be polarised using polarisers.
  • Polarisation: This is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. A polarising filter (like Polaroid sunglasses) can block oscillations in certain planes while allowing others to pass.
  • Speed of Sound: The speed of sound depends on the temperature and the elasticity of the medium. In dry air at 20 °C (68 °F), the speed of sound is approximately 343 metres per second (1,235 km/h). It travels fastest in solids, slower in liquids, and slowest in gases.
  • Frequency and Audibility: The frequency of sound waves determines their pitch. The audible range for the human ear is typically between 20 Hz and 20,000 Hz (20 kHz). Sound with a frequency below 20 Hz is called infrasound, and above 20 kHz is called ultrasound.

Important Points

  • Medium Requirement: Unlike light waves (which are electromagnetic and can travel through a vacuum), sound waves are mechanical and require a material medium to propagate. Sound cannot travel through outer space.
  • Wave Interactions: Both sound and light undergo reflection, refraction, interference, and diffraction. However, only light (and other transverse waves) can undergo polarisation.
  • Historical Context: The wave theory of light was significantly strengthened when Augustin-Jean Fresnel and Thomas Young demonstrated that light could be polarised, proving it was a transverse wave rather than a longitudinal one like sound.
  • Application of Polarisation: It is used in 3D movies, LCD screens, and stress analysis of plastics. Because sound is longitudinal, these applications do not exist for acoustic waves in the same manner.

Question 9

The Life was originated on the earth about 

  1. AOption (a)

    3.0 billion years ago

  2. BOption (b)

    3.5 billion years ago

  3. COption (c)

    4.0 billion years ago

  4. DOption (d)

    4.5 billion years ago

View answer & explanation

Correct answer

Answer: (b)B.

3.5 billion years ago

Why this answer is correct

The correct answer is 3.5 billion years ago.

Key Points

  • The origin of life on Earth is a complex process that began after the planet's surface cooled sufficiently to allow for the existence of liquid water. Scientists generally agree that life originated approximately 3.5 billion years ago.
  • The earliest direct physical evidence of life consists of microfossils found in 3.5 billion-year-old rocks from Western Australia. These fossils resemble modern-day prokaryotes.
  • Stromatolites, which are mineralized microbial mats formed by cyanobacteria, provide some of the oldest records of life. Some of these formations have been dated back to nearly 3.5 billion years, indicating a thriving microbial ecosystem during the Archaean Eon.
  • The transition from non-living matter to living organisms is known as abiogenesis. It is hypothesized that this occurred through a series of chemical evolutions in the primitive atmosphere of the Earth.
  • While Earth itself formed about 4.54 billion years ago, the first 500 to 700 million years (the Hadean Eon) were likely too volatile for life to survive due to intense volcanic activity and meteorite bombardments.

Additional Information

  • Formation of Earth: The Earth was formed approximately 4.6 billion years ago from a cloud of gas and dust surrounding the young Sun. In its early stages, the atmosphere was reducing, containing methane, ammonia, water vapor, and hydrogen, but almost no free oxygen.
  • Oparin-Haldane Theory: Proposed in the 1920s, this theory suggests that life arose from organic molecules that formed in the "primordial soup" of the early oceans. These molecules were synthesized using energy from lightning and ultraviolet (UV) radiation.
  • The Miller-Urey Experiment: In 1953, Stanley Miller and Harold Urey conducted a landmark experiment that simulated early Earth conditions. They successfully synthesized amino acids, which are the fundamental building blocks of proteins, proving that organic compounds could form spontaneously.
  • Evolutionary Timeline: Following the emergence of unicellular organisms around 3.5 billion years ago, it took nearly 2 billion years for complex eukaryotic cells to appear. Multicellular life began to flourish much later, roughly 600 to 800 million years ago, leading to the Cambrian Explosion.
  • Hydrothermal Vents: Many modern theories suggest that life may have actually originated near deep-sea hydrothermal vents, where heat and mineral-rich water provided the necessary energy for chemosynthesis.

Important Points

  • Biological Signatures: Isotopic evidence (ratios of Carbon-12 to Carbon-13) found in Greenland suggests that biological activity might have existed as early as 3.8 to 4.1 billion years ago, although 3.5 billion years remains the most widely accepted date for confirmed fossil evidence.
  • The Role of RNA: The RNA World Hypothesis suggests that RNA was the first self-replicating molecule, preceding DNA and proteins in the early stages of life's origin.
  • Oxygenation of Atmosphere: Significant oxygen levels did not appear in the atmosphere until about 2.4 billion years ago during the Great Oxidation Event, triggered by the photosynthetic activity of cyanobacteria.

Question 10

Which of the following is the largest island from area point of view in the Indian ocean? 

  1. AOption (a)

    Zanzibar

  2. BOption (b)

    Seychelles

  3. COption (c)

    Madagaskar

  4. DOption (d)

    Mauritius

View answer & explanation

Correct answer

Answer: (c)C.

Madagaskar

Why this answer is correct

The correct answer is Madagascar.

Key Points

  • Madagascar is the largest island in the Indian Ocean and the fourth-largest island in the world, covering an area of approximately 587,041 square kilometers.
  • It is located approximately 400 kilometers off the Southeastern coast of Africa, separated from the mainland by the Mozambique Channel.
  • Geologically, Madagascar is considered a microcontinent because it was part of the Gondwana supercontinent and separated from India about 88 million years ago, allowing its plants and animals to evolve in relative isolation.
  • The island is famous for its mega-biodiversity; more than 90% of its wildlife is endemic, meaning species like lemurs and certain Baobab trees are found nowhere else on Earth.
  • The topography of the island is diverse, featuring a central highland, rainforests along the eastern coast, and dry deciduous forests to the west.

Additional Information

  • Zanzibar: This is an archipelago off the coast of East Africa (part of Tanzania). Its largest island, Unguja, has an area of only about 1,666 square kilometers. It is globally famous as the "Spice Island" for its production of cloves, nutmeg, and cinnamon.
  • Seychelles: An archipelagic country in the Indian Ocean consisting of 115 islands. The total land area is small, roughly 459 square kilometers. Its capital, Victoria, is located on the largest island, Mahé. It is a major tourist destination known for its coral reefs and beaches.
  • Mauritius: A volcanic island nation located east of Madagascar. It has a land area of about 2,040 square kilometers. It is known for its high Human Development Index (HDI) and was the only known habitat of the now-extinct Dodo bird.
  • Indian Ocean: It is the third-largest of the world's five oceanic divisions, covering about 20% of the Earth's water surface. It is bounded by Asia to the north, Africa to the west, and Australia to the east.

Important Points

  • The capital and largest city of Madagascar is Antananarivo, located in the central highlands.
  • The country gained independence from France on June 26, 1960. The official languages spoken are Malagasy and French.
  • Economically, Madagascar is the world's leading producer of natural vanilla, accounting for nearly 80% of the global market.
  • The highest point in Madagascar is Maromokotro, which stands at an elevation of 2,876 meters in the Tsaratanana Massif.
  • In terms of global island rankings, Madagascar follows Greenland, New Guinea, and Borneo in total land area.

Question 11

Which of the following rivers crosses the equator twice in its flow? 

  1. AOption (a)

    Nile

  2. BOption (b)

    Congo

  3. COption (c)

    Niger

  4. DOption (d)

    Amazon

View answer & explanation

Correct answer

Answer: (b)B.

Congo

Why this answer is correct

The correct answer is Congo.

Key Points

  • The Congo River, also known as the Zaire River, is the only major river in the world that crosses the Equator twice during its course. Hence, the statement regarding Congo is correct.
  • It is the second-longest river in Africa, after the Nile, and has the second-largest discharge volume in the world, following the Amazon River.
  • The river flows through the Congo Basin, which houses the world's second-largest rainforest, spanning across several countries including the Democratic Republic of the Congo, Congo Republic, Central African Republic, and others.
  • Its unique path takes it from the Lualaba River (its main headstream) northward across the Equator, then it turns west and curves southward, crossing the Equator a second time before flowing into the Atlantic Ocean.
  • The Congo River is also recognized as the deepest river in the world, with measured depths reaching up to 220 meters (720 feet) in certain sections.

Additional Information

  • Nile River:
  • Located in Northeastern Africa, it is traditionally considered the longest river in the world with a length of approximately 6,650 km.
  • It flows northward and passes through 11 countries, eventually emptying into the Mediterranean Sea. It crosses the Equator only once in its upper reaches near Lake Victoria. Hence, the statement regarding the Nile is incorrect.
  • Amazon River:
  • Situated in South America, it is the largest river by discharge volume and the second-longest river in the world (approx. 6,400 km).
  • The Amazon flows through Peru, Colombia, and Brazil. While its mouth is very close to the Equator, the river itself does not cross the Equator twice. Hence, the statement regarding the Amazon is incorrect.
  • Niger River:
  • This is the third-longest river in Africa (approx. 4,180 km) and the principal river of West Africa.
  • It has an unusual boomerang shape, flowing away from the sea into the Sahara Desert before turning southeast towards the Gulf of Guinea. It does not cross the Equator at any point. Hence, the statement regarding the Niger is incorrect.
  • Limpopo River:
  • Located in Southern Africa, this river is famous for crossing the Tropic of Capricorn twice, rather than the Equator.

Important Points

  • Hydroelectric Potential: The Congo River possesses the highest hydroelectric potential of any river system in the world due to its massive water volume and steep drops, such as the Inga Falls.
  • Economic Importance: It serves as a vital navigation route in Central Africa, as roads and railways are often difficult to maintain in the dense tropical rainforest.
  • Flora and Fauna: The river basin supports incredible biodiversity, including endemic species of fish and the Bonobo (pygmy chimpanzee), found only in the forests of the Democratic Republic of the Congo.
  • Historical Context: Exploration of the river was famously documented by Henry Morton Stanley in the late 19th century, which opened the region to European colonization.

Question 12

In which of the following cities, the Indian Institute of Remote Sensing (IIRS) is located? 

  1. AOption (a)

    Dehradun

  2. BOption (b)

    Ahmedabad

  3. COption (c)

    Hyderabad

  4. DOption (d)

    Nagpur

View answer & explanation

Correct answer

Answer: (a)A.

Dehradun

Why this answer is correct

The correct answer is Dehradun.

Key Points

  • The Indian Institute of Remote Sensing (IIRS) is a premier institute for capacity building and research located in Dehradun, the capital city of Uttarakhand, India.
  • It was established in the year 1966 as the Indian Photo-interpretation Institute (IPI), following the pattern of the International Institute for Geo-Information Science and Earth Observation (ITC), The Netherlands.
  • The institute functions as a constituent unit of the Indian Space Research Organisation (ISRO), which operates under the Department of Space, Government of India.
  • The primary function of IIRS is to provide training and education to professionals and students in the fields of Remote Sensing, Geoinformatics, and Global Positioning System (GPS) technology.
  • The institute offers various academic programs, including M.Tech, M.Sc., Post Graduate Diploma, and certificate courses, catering to both national and international participants from Asia and Africa.
  • Its core applications involve the use of satellite data for natural resource management, disaster management, and environmental monitoring.

Additional Information

  • National Remote Sensing Centre (NRSC):
  • Located in Hyderabad, Telangana, this centre is responsible for remote sensing satellite data acquisition, processing, and dissemination.
  • It plays a vital role in disaster management support through the Decision Support Centre and manages the Ground Station for IRS satellites at Shadnagar.
  • Space Applications Centre (SAC):
  • Situated in Ahmedabad, Gujarat, this is a major research and development centre of ISRO.
  • It focuses on the design and development of payloads (spaceborne instruments) for communications, meteorology, and remote sensing satellites.
  • Regional Remote Sensing Centre (RRSC):
  • The RRSC-Central is located in Nagpur, Maharashtra. There are five such regional centres across India, including Bengaluru, Jodhpur, Kolkata, and New Delhi.
  • These centres provide digital image processing and Geographic Information System (GIS) based solutions for regional planning and resource mapping.
  • Indian Space Research Organisation (ISRO):
  • Founded on 15 August 1969, it is headquartered in Bengaluru. It succeeded the Indian National Committee for Space Research (INCOSPAR).
  • ISRO is known for its cost-effective missions like Mars Orbiter Mission (Mangalyaan), Chandrayaan-3, and the Polar Satellite Launch Vehicle (PSLV).

Important Points

  • Remote Sensing: This refers to the process of detecting and monitoring the physical characteristics of an area by measuring its reflected and emitted radiation from a distance, typically using satellites or aircraft.
  • Geoinformatics: It is the science and technology dealing with the structure and character of spatial information, its capture, its classification, and its storage.
  • CSSTEAP: The Centre for Space Science and Technology Education in Asia and the Pacific, affiliated with the United Nations, is also hosted at the IIRS campus in Dehradun to promote international cooperation.
  • Data Products: IIRS and NRSC collaborate to provide high-resolution topographic maps and thematic maps used in urban planning and agriculture.

Question 13

In which year the Organization of Petroleum Exporting Countries (OPEC) was created?

  1. AOption (a)

    1955

  2. BOption (b)

    1960

  3. COption (c)

    1965

  4. DOption (d)

    1970

View answer & explanation

Correct answer

Answer: (b)B.

1960

Why this answer is correct

The correct answer is 1960.

Key Points

  • The Organization of Petroleum Exporting Countries (OPEC) was created at the Baghdad Conference held in Iraq. The conference took place between September 10 and 14, 1960.
  • The organization was founded by five core oil-producing nations: Iran, Iraq, Kuwait, Saudi Arabia, and Venezuela. These nations aimed to exercise sovereign rights over their natural resources.
  • The primary function of OPEC is to coordinate and unify the petroleum policies of its member countries to ensure the stabilization of oil prices in international markets. This statement confirms that the year of creation is 1960.
  • It was established as a permanent intergovernmental organization to provide an efficient, economic, and regular supply of petroleum to consuming nations and to ensure a fair return on capital for investors in the industry.
  • The creation of OPEC in 1960 marked a significant shift in the global energy landscape, as it challenged the dominance of the "Seven Sisters" multinational oil companies that controlled oil prices at that time.

Additional Information

  • Headquarters Location: Initially, the OPEC headquarters were established in Geneva, Switzerland. However, they were moved to Vienna, Austria, on September 1, 1965.
  • Membership Expansion: Since its founding in 1960, several other nations have joined the organization, including Libya (1962), United Arab Emirates (1967), Algeria (1969), Nigeria (1971), and Gabon (1975).
  • OPEC+ Collaboration: In December 2016, OPEC formed an alliance with 10 non-OPEC oil-producing countries, including Russia, known as OPEC+, to further regulate global oil supply and prices.
  • Statutory Organs: The organization operates through three main bodies: the Conference (the supreme authority), the Board of Governors, and the Secretariat (based in Vienna).
  • Economic Impact: OPEC members collectively hold approximately 80% of the world's proven crude oil reserves, which allows the organization to have a profound impact on global inflation and economic growth through its production quotas.

Important Points

  • Legal Recognition: OPEC is registered with the United Nations Secretariat under Article 102 of the UN Charter, following Registration No. 6363.
  • Secretary General: The Secretariat is headed by a Secretary General, who serves as the organization's chief executive officer. The designation involves overseeing the daily administrative operations and implementing Conference decisions.
  • Oil Price Shocks: Historically, OPEC gained global prominence during the 1973 oil crisis when it implemented an oil embargo, demonstrating the organization's immense geopolitical power.
  • Production Data: According to recent surveys and reports, OPEC countries produce roughly 40% of the world's total crude oil, making their numerical value of exports critical for energy security in many developed and developing countries.

Question 14

Under which Article of the Indian Constitution Chairman and members of UPSC can be removed from their office? 

  1. AOption (a)

    Article-320

  2. BOption (b)

    Article-315

  3. COption (c)

    Article-322

  4. DOption (d)

    Article-317

View answer & explanation

Correct answer

Answer: (d)D.

Article-317

Why this answer is correct

The correct answer is Article-317.

Key Points

  • Article 317 of the Constitution of India provides the detailed procedure for the removal and suspension of a member of a Public Service Commission. According to this article, the Chairman or any other member of the Union Public Service Commission (UPSC) can only be removed from their office by an order of the President. Hence, the correct answer is Article-317.
  • The primary ground for removal mentioned in this article is misbehaviour. However, for removal on this ground, the President must refer the matter to the Supreme Court of India for an inquiry. If the Supreme Court, after following the procedure prescribed under Article 145, reports that the person ought to be removed, then the President can issue the removal order.
  • The President also has the authority to remove the Chairman or any other member without a Supreme Court inquiry if the individual is adjudged an insolvent, engages during their term of office in any paid employment outside the duties of their office, or is, in the opinion of the President, unfit to continue in office by reason of infirmity of mind or body.
  • During the period of inquiry by the Supreme Court, the President has the power to suspend the Chairman or member of the Union Commission until the final report is submitted and orders are passed.

Additional Information

  • Article-315:
  • This article mandates the establishment of a Public Service Commission for the Union and a Public Service Commission for each State.
  • It also allows two or more States to agree that there shall be one Joint State Public Service Commission (JSPSC) for that group of States, which is created by an Act of Parliament. Hence, statement regarding Article-315 is related to establishment, not removal.
  • Article-320:
  • This article enumerates the various functions of the Public Service Commissions.
  • Key functions include conducting examinations for appointments to the services of the Union and the States, and being consulted on matters relating to methods of recruitment, promotions, and disciplinary actions. Hence, statement regarding Article-320 is related to functions, not removal.
  • Article-322:
  • This article specifies the expenses of the Public Service Commissions.
  • It states that the expenses of the Union Commission, including salaries, allowances, and pensions payable to members and staff, shall be charged on the Consolidated Fund of India. Hence, statement regarding Article-322 is related to financial expenses, not removal.

Important Points

  • Constitutional Framework: The UPSC is an independent constitutional body under Part XIV of the Constitution of India, covering Articles 315 to 323.
  • Composition and Tenure: The Commission usually consists of 9 to 11 members, including the Chairman. They are appointed by the President of India for a term of six years or until they attain the age of 65 years, whichever is earlier.
  • Independence: To maintain the impartiality of the Commission, the Chairman of the UPSC is ineligible for any further employment under the Government of India or any State Government after retirement.
  • Conditions of Service: Under Article 318, the President is empowered to determine the number of members of the Commission and their conditions of service. However, these conditions cannot be varied to their disadvantage after their appointment.

Question 15

The President of India submits his resignation letter to whom among the following? 

  1. AOption (a)

    The Chief Justice of India.

  2. BOption (b)

     The Speaker of Lok-Sabha.

  3. COption (c)

     The Prime Minister of India.

  4. DOption (d)

     The Vice-President of India and Communicate to Speaker of Lok Sabha.

View answer & explanation

Correct answer

Answer: (d)D.

 The Vice-President of India and Communicate to Speaker of Lok Sabha.

Why this answer is correct

The correct answer is The Vice-President of India and Communicate to Speaker of Lok Sabha.

Key Points

  • According to Article 56(1)(a) of the Constitution of India, the President may, by writing under his hand addressed to the Vice-President, resign his office.
  • Article 56(2) further mandates that any resignation addressed to the Vice-President under clause (a) of the proviso to clause (1) shall forthwith be communicated by him to the Speaker of the Lok Sabha. Hence, statement 4 is correct.
  • The Chief Justice of India is responsible for administering the Oath or Affirmation to the President as per Article 60, but they do not receive the resignation letter. Hence, statement 1 is incorrect.
  • The Speaker of the Lok Sabha is the presiding officer of the lower house. While they must be informed of the resignation by the Vice-President, the resignation letter itself is not directly submitted to them. Hence, statement 2 is incorrect.
  • The Prime Minister of India is the head of the Council of Ministers. There is no constitutional provision that requires the President to submit a resignation letter to the Prime Minister. Hence, statement 3 is incorrect.

Additional Information

  • Article 52 to 62 of the Indian Constitution deal with the President of India. The President is the Head of State and the First Citizen of India.
  • Qualifications for Election (Article 58): To be eligible for election as President, a person must be a citizen of India, have completed the age of 35 years, and be qualified for election as a member of the Lok Sabha.
  • Term of Office (Article 56): The President holds office for a term of 5 years from the date on which he enters his office. However, he can continue to hold office beyond his term until his successor enters upon his office.
  • Election Process (Article 54): The President is elected by an Electoral College consisting of the elected members of both Houses of Parliament and the elected members of the Legislative Assemblies of the States and Union Territories of Delhi and Puducherry.
  • Impeachment (Article 61): The President can be removed from office before the expiry of the term through the process of impeachment for the "violation of the Constitution."

Important Points

  • In the event of a vacancy in the office of the President due to death, resignation, or removal, the Vice-President acts as the President until a new President is elected.
  • If the office of the Vice-President is also vacant, the Chief Justice of India (or in his absence, the senior-most Judge of the Supreme Court) discharges the functions of the President.
  • An election to fill a vacancy in the office of President occurring by reason of death, resignation, or removal must be held as soon as possible, and in no case later than six months from the date of occurrence of the vacancy.
  • The President is eligible for re-election to that office under Article 57 of the Constitution, with no limit on the number of terms a person can serve.

Question 16

'India Innovation Index' has been mainly launched by which one of the following organizations? 

  1. AOption (a)

    NITI Aayog

  2. BOption (b)

    SEBI

  3. COption (c)

    RBI

  4. DOption (d)

    CSO

View answer & explanation

Correct answer

Answer: (a)A.

NITI Aayog

Why this answer is correct

The correct answer is NITI Aayog.

Key Points

  • The India Innovation Index is a comprehensive tool developed by NITI Aayog (National Institution for Transforming India) in collaboration with the Institute for Competitiveness. Hence, the statement regarding NITI Aayog is correct.
  • The primary objective of the index is to rank Indian states and union territories based on their relative performance in supporting innovation, thereby fostering a spirit of competitive federalism.
  • The framework of the index is based on seven pillars, which are further divided into Enablers and Performance dimensions. The Enablers include Human Capital, Investment, Knowledge Workers, Business Environment, and Safety and Legal Environment.
  • The Performance dimension measures Knowledge Output and Knowledge Diffusion, providing a holistic view of the innovation landscape across different regions of India.
  • The third edition of the India Innovation Index (2021) was released in July 2022. In this edition, Karnataka continued to hold the first rank in the Major States category, while Manipur topped the North East and Hill States category, and Chandigarh ranked first among Union Territories and City-States.

Additional Information

  • NITI Aayog: Established on January 1, 2015, it replaced the Planning Commission. It functions as the premier policy think tank of the Government of India, providing both directional and policy inputs. The Prime Minister is the Ex-officio Chairperson, and the Vice-Chairperson is appointed by the Prime Minister.
  • SEBI (Securities and Exchange Board of India): It is the regulatory body for the securities and commodity market in India under the ownership of the Ministry of Finance. It was established on April 12, 1988, and given statutory powers on January 30, 1992, through the SEBI Act, 1992. It is not involved in launching the Innovation Index. Hence, this option is incorrect.
  • RBI (Reserve Bank of India): India's central bank and regulatory body responsible for regulation of the Indian banking system. It was established on April 1, 1935, under the Reserve Bank of India Act, 1934. Its headquarters are in Mumbai. Its main functions include monetary policy and currency issuance. Hence, this option is incorrect.
  • CSO (Central Statistical Office): Now merged with the National Sample Survey Office (NSSO) to form the National Statistical Office (NSO), it functions under the Ministry of Statistics and Programme Implementation (MoSPI). It is responsible for the coordination of statistical activities and maintaining statistical standards, including the calculation of GDP. Hence, this option is incorrect.

Important Points

  • The India Innovation Index is inspired by the Global Innovation Index (GII), which is published annually by the World Intellectual Property Organization (WIPO).
  • The index uses a total of 66 indicators to assess the innovation performance, compared to 36 indicators used in the first edition (2019), showing an increase in the depth of data analysis.
  • The Government of India uses these reports to identify specific strengths and weaknesses at the state level, allowing for targeted policy interventions to improve Research and Development (R&D) expenditure, which currently stands at approximately 0.7% of GDP in India.
  • The Index categorizes states into three groups: Major States (17), North East and Hill States (10), and Union Territories and City-States (9) to ensure a fair comparison based on geographical and administrative similarities.

Question 17

After independence till now, how many time/s Financial Emergency has/have been declared in India? 

  1. AOption (a)

    Once

  2. BOption (b)

    Twice

  3. COption (c)

    Four times

  4. DOption (d)

    Never

View answer & explanation

Correct answer

Answer: (d)D.

Never

Why this answer is correct

The correct answer is Never.

Key Points

  • Article 360 of the Constitution of India empowers the President to proclaim a Financial Emergency if he is satisfied that a situation has arisen whereby the financial stability or credit of India or any part of its territory is threatened.
  • To date, since the Constitution came into force on January 26, 1950, a Financial Emergency has never been declared in India. This remains true even during periods of significant economic stress.
  • Even during the severe Balance of Payments crisis in 1991, when India faced a critical shortage of foreign exchange reserves, the government chose to implement structural economic reforms (LPG - Liberalization, Privatization, and Globalization) rather than invoking Article 360.
  • The proclamation of a Financial Emergency must be approved by both Houses of Parliament (the Lok Sabha and the Rajya Sabha) within two months from the date of its issue.
  • Once approved by Parliament, the Financial Emergency continues indefinitely until it is revoked by the President. A repeated parliamentary approval is not required for its continuation, which is a unique feature compared to National Emergency.

Additional Information

  • Part XVIII of the Constitution of India (Articles 352 to 360) details the Emergency Provisions, which allow the Central Government to acquire exceptional powers to meet abnormal situations.
  • National Emergency (Article 352): This can be declared on the grounds of war, external aggression, or armed rebellion. In India, it has been declared three times: in 1962 (Chinese aggression), 1971 (Indo-Pak war), and 1975 (internal disturbance).
  • State Emergency / President's Rule (Article 356): This is invoked when the constitutional machinery in a state fails. It has been used over 100 times across various states in India since Independence.
  • Approval Mechanism: While a National Emergency requires a special majority for approval, a Financial Emergency can be passed by a simple majority in both Houses of Parliament.
  • Role of the Judiciary: The 38th Amendment Act of 1975 originally made the President's satisfaction final and conclusive, but the 44th Constitutional Amendment Act of 1978 subsequently made it subject to judicial review, ensuring that the declaration is not malafide.

Important Points

  • Consequences of Financial Emergency: During its operation, the executive authority of the Union extends to giving directions to any State to observe specified canons of financial propriety.
  • Salary Reductions: The President can issue directions for the reduction of salaries and allowances of all or any class of persons serving the Union or the States, including the Judges of the Supreme Court and the High Courts.
  • Money Bills: The Union can require that all Money Bills or other financial bills passed by the State Legislature be reserved for the consideration of the President.
  • Federal Structure: During a Financial Emergency, the Union gains full control over the financial matters of the States, significantly altering the federal balance of the Indian polity in favor of the Central Government.
  • Revocation: The President can revoke the proclamation at any time by a subsequent proclamation, which does not require parliamentary approval.

Question 18

A table that shows the scores and their frequency of occurrence is called 

  1. AOption (a)

    Data table

  2. BOption (b)

    Scores table

  3. COption (c)

    Frequency distribution table 

  4. DOption (d)

     Statistical table

View answer & explanation

Correct answer

Answer: (c)C.

Frequency distribution table 

Why this answer is correct

The correct answer is Frequency distribution table.

Key Points

  • A Frequency distribution table is a statistical tool used to organize and summarize data by showing the number of occurrences (frequency) of each distinct score or value in a dataset.
  • The primary function of this table is to transform raw data into a more meaningful and readable format, allowing researchers to see patterns and the spread of observations quickly.
  • It typically consists of two main columns: the first column lists the categories or score intervals, and the second column lists the frequency (f), which is the numerical value representing how many times that specific score appeared.
  • This table is fundamental in descriptive statistics as it provides the basis for creating graphs such as histograms, bar charts, and pie charts.
  • Regarding the other options: A Data table is a general term for any grid containing data; a Scores table is not a standard statistical term; and a Statistical table is a broad category that includes many types of tables, making Frequency distribution table the most specific and correct answer. Hence, the statement identifying the Frequency distribution table is correct.

Additional Information

  • Ungrouped Frequency Distribution: This type is used when the range of data is small. Each individual score is listed separately with its corresponding frequency. For example, if a teacher records test scores out of 10 for a small class, each number from 1 to 10 would be a row.
  • Grouped Frequency Distribution: This is used for large datasets where individual scores are grouped into class intervals (e.g., age groups like 20-29, 30-39). This helps in simplifying data that covers a wide numerical value range.
  • Relative Frequency: This feature shows the proportion or percentage of the total number of observations that fall into each category. It is calculated by dividing the frequency of a class by the total number of observations.
  • Cumulative Frequency: This is the sum of the frequencies for a specific class and all previous classes. It is particularly useful for identifying the median and percentiles within a data set.
  • Applications: These tables are used globally in census reports, market research, scientific experiments, and quality control in manufacturing to analyze the distribution of variables.

Important Points

  • Tally Marks: During the construction of the table, tally marks are often used as a manual counting method to ensure accuracy before the final numerical value of the frequency is recorded.
  • Class Limits and Midpoints: In grouped frequency distributions, the lower class limit and upper class limit define the boundaries of each group. The class midpoint (or class mark) is the average of the upper and lower limits and is used in various mathematical operations.
  • Measures of Central Tendency: The Frequency distribution table is the starting point for calculating the Mean, Median, and Mode for discrete or continuous data.
  • Range: The range is the difference between the highest and lowest numerical values in the data set, which helps in determining how many classes or intervals are needed in the table.
  • Rule of Thumb: In statistics, it is generally recommended to have between 5 to 15 class intervals to ensure the table is neither too detailed nor too vague.

Question 19

Consider the following figures and select the correct answer of the missing number from the code given below:

510
6075

 

36
36?
  1. AOption (a)

    ​40

  2. BOption (b)

    45

  3. COption (c)

    48

  4. DOption (d)

    60

View answer & explanation

Correct answer

Answer: (b)B.

45

Why this answer is correct

Logic Followed:

5 × 2 = 10 ⇒ 10 × 6 = 60 ⇒ 60 × 1.25 = 75

Similarly,

3 × 2 = 6 ⇒ 6 × 6 = 36 ⇒ 36 × 1.25 = 45

Thus, the correct answer is '45'.

Question 20

The average weight of 5 men increases by 2 kg, while one of the men whose weight is 60 kg, is replaced by a new man. What is the weight of new man? 

  1. AOption (a)

    50 kg

  2. BOption (b)

    65 kg

  3. COption (c)

    68 kg

  4. DOption (d)

    70 kg

View answer & explanation

Correct answer

Answer: (d)D.

70 kg

Why this answer is correct

Shortcut Trick

Weight of new man = Weight of replaced man + (Total number of men × Average increase)

Weight of new man = 60 + (5 × 2)

Weight of new man = 60 + 10 = 70 kg

∴ The correct answer is 70 kg.

Alternate Method

Given:

Number of men (n) = 5

Weight of the man replaced = 60 kg

Increase in average weight = 2 kg

Formula Used:

Total Increase = Number of items × Increase in average

New value = Old value + Total Increase

Old Man (60 kg)

+10 kg

New Man = 60 + (5 × 2) kg Weight Replacement Scale Result: 70 kg

⇒ Total weight increase = 5 × 2

⇒ Total weight increase = 10 kg

⇒ Weight of new man = 60 + 10

⇒ Weight of new man = 70 kg

∴ The correct answer is 70 kg.

Additional Information

Average Change on Replacement

When a person is replaced, New Weight = Replaced Weight + (Total Count × Change in Average). Use − for a decrease and + for an increase.

Average Change on Adding a Member

New Sum = (Old Average × Old Count) + New Member Weight. New Average = New Sum ÷ (Old Count + 1).

Weighted Average Formula

The combined average of two groups is given by (n1a1 + n2a2) ÷ (n1 + n2).

Question 21

In Ancient times, which of the Brahman families defeated all the contemporary rulers and established their own supremacy?

  1. AOption (a)

    Angiras

  2. BOption (b)

    Atri

  3. COption (c)

    Bhrigu

  4. DOption (d)

    Pulastya

View answer & explanation

Correct answer

Answer: (c)C.

Bhrigu

Why this answer is correct

The correct answer is Bhrigu.

Key Points

  • The Bhrigu family, also known as the Bhargavas, is a prominent Brahman lineage in ancient Indian history and mythology. They were renowned for their unique identity as both profound sages and exceptionally skilled warriors. Hence, this family is the correct answer.
  • The most celebrated member of the Bhrigu clan is Parasurama, who is revered as the sixth avatar of Lord Vishnu. According to the Puranas, he engaged in a legendary conflict with the Kshatriya (warrior) class to avenge the death of his father, Jamadagni.
  • Parasurama is traditionally said to have defeated all contemporary Kshatriya rulers and established his supremacy by cleansing the earth of the warrior class 21 times. This narrative highlights the era where the Bhrigu family asserted Brahmanical dominance over the temporal political powers of that age.
  • The family is also associated with the location of the western coast of India. It is believed that Parasurama reclaimed the land of Konkan and Kerala from the sea, where many Bhargava traditions continue to thrive.
  • The Bhrigu sages were also the authors of the Bhrigu Samhita, an astrological treatise, showcasing their influence over both scientific knowledge and military might in ancient times.

Additional Information

  • Angiras: This Brahman family is named after the sage Angiras, who is one of the Saptarishis (Seven Sages). They are the primary contributors to the Rigveda and the Atharvaveda. While spiritually powerful, the Angiras clan focused on rituals and hymns rather than the military conquest of rulers.
  • Atri: The Atri family descends from the sage Atri, another member of the Saptarishi. They are credited with the composition of the fifth Mandala of the Rigveda. They were highly respected as priests and advisors to various kings during the Vedic period but did not seek military supremacy.
  • Pulastya: The Pulastya lineage is distinct in mythology as it is the progenitor of the Rakshasa race through his son Vishrava. The Lanka king Ravana was a grandson of Pulastya. Although they possessed great power and knowledge, they are categorized differently from the Brahman families that upheld the Vedic social order.
  • Kshatriya-Brahman Conflict: In ancient India, particularly during the Later Vedic Period and the Epic Age, there were periodic shifts in the power dynamics of the Varna system. The story of the Bhargavas represents the most extreme historical/mythological example of Brahmanical intervention in governance.

Important Points

  • Kartavirya Arjuna: He was a powerful Haihaya king with 1000 arms who had initially received boons from Dattatreya. His conflict with the Bhrigu family began when he forcibly took the Kamadhenu (wish-granting cow) from Jamadagni’s hermitage, leading to the eventual war.
  • Role in Epics: Scholars often note the "Bhargavaization" of the Mahabharata, suggesting that the Bhrigu family played a significant role in compiling and expanding the epic over several centuries to include their family history and values.
  • Geographical Influence: The name of the city Bharuch in Gujarat is derived from Bhrigukachchha, indicating the deep-rooted location-based features of the Bhrigu family’s influence in Western India.
  • Date and Timeline: While these events are recorded in the Puranas, they are historically placed in the transition between the Early and Later Vedic societies, reflecting internal social and political evolutions in ancient India.

Question 22

Who was the author of the book entitled 'The Problems of the East'? 

  1. AOption (a)

    Jennifer Lawrence

  2. BOption (b)

    Lord Lytton 

  3. COption (c)

    George Curzon 

  4. DOption (d)

    Winston Churchill

View answer & explanation

Correct answer

Answer: (c)C.

George Curzon 

Why this answer is correct

The correct answer is George Curzon.

Key Points

  • George Curzon, also known as Lord Curzon, was the author of the prominent book titled 'Problems of the Far East' (often referred in historical contexts as 'The Problems of the East'), which was published in 1894.
  • The book provides a detailed analysis of the political, social, and economic conditions of Japan, Korea, and China during the late 19th century. It reflects his deep interest in geopolitics and the strategic importance of the Asian continent to the British Empire.
  • Curzon served as the Viceroy of India from 1899 to 1905. He was one of the most controversial yet efficient administrators during the British Raj. His tenure is marked by a series of reforms and administrative changes that significantly impacted the Indian subcontinent.
  • His writings and policies were heavily influenced by his belief in Imperialism and the necessity of maintaining British supremacy in the East. Apart from this book, he also authored 'Russia in Central Asia' (1889) and 'Persia and the Persian Question' (1892).
  • The mention of George Curzon as the author is a factual historical detail frequently asked in General Knowledge and History examinations. Hence, the statement regarding George Curzon being the author is correct.

Additional Information

  • Lord Curzon's Administration in India:
  • He was responsible for the Partition of Bengal in 1905, which was a major turning point in the Indian National Movement and led to the Swadeshi Movement.
  • He introduced the Ancient Monuments Preservation Act in 1904, which played a vital role in the restoration and protection of Indian heritage sites like the Taj Mahal.
  • He established the Agriculture Research Institute at Pusa (Bihar) in 1905 to modernize farming techniques in India.
  • He passed the Indian Universities Act in 1904 to bring higher education under stricter government control.
  • Lord Lytton (1876-1880):
  • He was the Viceroy of India during the Great Famine of 1876-1878.
  • He is known for passing the repressive Vernacular Press Act in 1878, which aimed at curbing the freedom of the Indian press.
  • He organized the Delhi Durbar in 1877, where Queen Victoria was proclaimed as the 'Kaiser-i-Hind'.
  • Winston Churchill:
  • He was the Prime Minister of the United Kingdom during World War II and a renowned statesman.
  • He was a prolific historian and writer, receiving the Nobel Prize in Literature in 1953.
  • His notable works include 'The Second World War' and 'A History of the English-Speaking Peoples'.
  • Jennifer Lawrence:
  • She is a famous American actress, born in 1990. She has no historical association with 19th-century political literature or British India.
  • She is widely known for her roles in films like 'The Hunger Games' and 'Silver Linings Playbook'.

Important Points

  • The book 'Problems of the Far East' was dedicated by Curzon to "those who believe that the British Empire is, under Providence, the greatest instrument for good that the world has seen."
  • During his Viceroyalty, Curzon also appointed the Raleigh Commission in 1902 to report on the prospects of universities in India.
  • He followed the 'Forward Policy' regarding the North-West Frontier and was instrumental in the Younghusband Expedition to Tibet in 1903-1904.
  • His administrative efficiency was often overshadowed by his autocratic style and his failure to recognize the growing nationalist sentiments among the Indian people.

Question 23

In which of the following states, 'Bagh Caves' are located? 

  1. AOption (a)

    Madhya Pradesh

  2. BOption (b)

    Bihar

  3. COption (c)

    Uttar Pradesh

  4. DOption (d)

    Rajasthan

View answer & explanation

Correct answer

Answer: (a)A.

Madhya Pradesh

Why this answer is correct

The correct answer is Madhya Pradesh.

Key Points

  • The Bagh Caves are a group of nine rock-cut monuments situated among the southern slopes of the Vindhya Range in the Dhar district of Madhya Pradesh. Hence, option 1 is correct.
  • These caves are famous for their mural paintings and are considered masterpieces of Buddhist art in India.
  • The caves were established during the 5th to 7th Century AD, which corresponds primarily to the late Gupta period.
  • Geographically, they are located on the banks of the Baghani River, which is a small tributary of the Narmada River.
  • Similar to the Ajanta Caves, these structures were not natural but were carved out of perpendicular sandstone rock surfaces.
  • While Bihar is known for ancient sites like Nalanda and the Barabar Caves, it is not the location of the Bagh Caves. Hence, option 2 is incorrect.
  • Uttar Pradesh is home to major Buddhist sites like Sarnath and Kushinagar, but the Bagh Caves are not situated here. Hence, option 3 is incorrect.
  • Rajasthan contains rock-cut structures like the Kolvi Caves, but the specific Bagh Caves are located in the central state of Madhya Pradesh. Hence, option 4 is incorrect.

Additional Information

  • Architectural Features: The Bagh Caves are primarily Viharas (monasteries), which served as residential quarters for Buddhist monks. Each cave usually contains a small Chaitya (prayer hall) at the rear, housing a Stupa.
  • The Paintings (Frescoes): The paintings at Bagh are executed in the Tempera technique. Unlike the purely religious themes at Ajanta, the Bagh mural paintings are more secular in nature, depicting music, dance, and social life of the Gupta era.
  • Cave 4 (Rang Mahal): This is the most significant cave among the group, famously known as the Palace of Colors. It contains the best-preserved and most elaborate murals depicting vibrant scenes of processions and dancers.
  • Discovery: These caves were "rediscovered" in 1818 by a British officer named Lieutenant Dangerfield. Before this, they were known to the local population but were hidden by dense vegetation.
  • Preservation Status: Due to the soft nature of the sandstone and the moisture from the Baghani River, many of the caves and paintings have deteriorated. The Archaeological Survey of India (ASI) has moved many original paintings to the Gwalior Fort Museum for protection.

Important Points

  • Historical Context: The Gupta Dynasty, under which these caves flourished, is often referred to as the Golden Age of India. This period saw a massive expansion in literature, science, and rock-cut architecture.
  • Comparison with Ajanta: While both are Buddhist and feature mural paintings, the Bagh Caves are considered more advanced in their secular depiction of human life compared to the Jataka tales focused paintings of Ajanta.
  • Numbering: Out of the 9 caves originally excavated, only 5 caves (Caves 1 through 5) are currently accessible and in a recognizable state. The others have collapsed over the centuries.
  • Religious Affiliation: These caves belong to the Mahayana sect of Buddhism, which is evidenced by the presence of Buddha statues and elaborate iconography.

Question 24

What is the name of the 'First Banking Robot' of India? C

  1. AOption (a)

    Lakshmi

  2. BOption (b)

    Pooja

  3. COption (c)

    Varun

  4. DOption (d)

    Param

View answer & explanation

Correct answer

Answer: (a)A.

Lakshmi

Why this answer is correct

The correct answer is Lakshmi.

Key Points

  • The first banking robot of India is named Lakshmi. It was launched by City Union Bank (CUB) in November 2016. This initiative was part of the bank's efforts to integrate Artificial Intelligence (AI) into its customer service operations.
  • The robot was first deployed at the City Union Bank branch located in Kumbakonam, Tamil Nadu. Its primary role was to assist customers with basic banking inquiries in a more interactive and efficient manner.
  • Lakshmi is capable of handling over 125 different types of customer queries. These include providing information on account balances, fixed deposit interest rates, loan products, and transaction history.
  • Technologically, the robot is based on the humanoid platform developed by SoftBank Robotics (originally Aldebaran Robotics from France). The City Union Bank team customized the software and integrated their specific banking database to make it functional for Indian customers.
  • The robot is equipped with Natural Language Processing (NLP), allowing it to understand and respond to customers in English. It was designed to move around the bank lobby and interact with people through voice and a touchscreen mounted on its chest.

Additional Information

  • IRA (Intelligent Robotic Assistant):
  • Following the launch of Lakshmi, HDFC Bank introduced its own banking robot named IRA in January 2017.
  • It was first deployed at the Kamala Mills branch in Mumbai. IRA was developed in collaboration with Asimov Robotics, a start-up based in Kochi, Kerala.
  • Advancements in Banking AI:
  • Apart from physical robots, Indian banks have heavily invested in Chatbots. For example, State Bank of India (SBI) launched SIA (SBI Intelligent Assistant) to handle customer queries via their website and app.
  • ICICI Bank introduced iPal, an AI-powered virtual assistant, to provide 24/7 support to its users.
  • The Global Context:
  • The use of robots in banking is inspired by global trends, such as the use of Pepper (the humanoid robot) by Mitsubishi UFJ Financial Group in Japan and HSBC in the United States.

Important Points

  • Security and Privacy:
  • While Lakshmi can provide general banking information, it is programmed not to store sensitive personal data like passwords or PIN numbers to ensure customer security and privacy.
  • For sensitive transactions, the robot directs the customer to a human bank official or a secure ATM/Kiosk.
  • Design and Mobility:
  • The robot stands about 3 to 4 feet tall and can perform gestures like waving or shaking hands to make the interaction feel more human-like.
  • It uses sensors to navigate the floor, ensuring it does not collide with customers or furniture within the bank premises.
  • Param:
  • It is important to note that Param (mentioned in the options) refers to a series of supercomputers designed and assembled by the Centre for Development of Advanced Computing (C-DAC) in India, with Param 8000 being the first, launched in 1991.

Question 25

Which one of the following is India's first commercial under-water Robotic Drone? 

  1. AOption (a)

    Focal

  2. BOption (b)

    Dornear-228

  3. COption (c)

    Eye Rov TUNA 

  4. DOption (d)

    Rustam

View answer & explanation

Correct answer

Answer: (c)C.

Eye Rov TUNA 

Why this answer is correct

The correct answer is Eye Rov TUNA.

Key Points

  • Eye Rov TUNA is recognized as India's first commercial underwater robotic drone, categorized as a Remotely Operated Vehicle (ROV).
  • It was officially launched and handed over to the Naval Physical and Oceanographic Laboratory (NPOL), a laboratory of the Defence Research and Development Organisation (DRDO), in September 2018.
  • The drone was developed by EyeROV Technologies, a startup based in Kochi, Kerala, and was incubated at Maker Village, which is one of the largest electronic hardware incubators in India.
  • This underwater drone is designed to perform visual inspections and surveys of submerged structures in environments where it is difficult or dangerous for human divers to operate.
  • The Eye Rov TUNA is a significant milestone in India's indigenous marine technology, providing a cost-effective alternative to expensive imported underwater inspection systems.

Additional Information

  • Features and Functions of Eye Rov TUNA:
  • Operating Depth: It is capable of diving up to depths of 50 to 100 meters in both freshwater and seawater environments.
  • Imaging: The drone is equipped with a High-Definition (HD) camera that provides real-time video feeds to the operator on the surface.
  • Control: It is controlled remotely via a tether (umbilical cable), which ensures steady communication and power supply during long durations of underwater operation.
  • Applications: Its primary applications include the inspection of ship hulls, underwater dam walls, bridge foundations, port structures, and submerged pipelines.
  • Other Options Explained:
  • Dornier-228: This is a multi-purpose light transport aircraft. In India, it is manufactured by Hindustan Aeronautics Limited (HAL) under license and is extensively used by the Indian Coast Guard and the Indian Navy for maritime surveillance and transport.
  • Rustom: This refers to the Rustom-1 or Rustom-2 (TAPAS-BH-201), which are Medium-Altitude Long-Endurance (MALE) Unmanned Aerial Vehicles (UAVs) developed by DRDO for aerial reconnaissance and surveillance.
  • Focal: This does not correspond to a major commercial underwater drone in the Indian defense or commercial context; it is often a general term related to optics or specific specialized sensors.

Important Points

  • Entrepreneurial Background: The startup EyeROV Technologies was founded by Johns T Mathai and Kannappa Palaniappan P, both alumni of IIT Delhi and IIT Madras.
  • Strategic Importance: The development of Eye Rov TUNA supports the Make in India initiative and the Deep Ocean Mission, aimed at exploring the vast marine resources of the Indian Ocean.
  • Data and Connectivity: The drone can be integrated with various sensors like Sonar for low-visibility conditions and ultrasonic thickness gauges to check the structural integrity of steel structures underwater.
  • Commercial Impact: By utilizing indigenous technology, the cost of underwater surveys has been reduced significantly, making it accessible for State Electricity Boards, Port Authorities, and Oil and Gas companies in India.

Question 26

In which of the following countries, the Indian Army launched rescue and relief work during natural disaster under 'Operation Samudra Maitri' in October, 2018? 

  1. AOption (a)

    Indonesia

  2. BOption (b)

    Bangladesh

  3. COption (c)

    Australia

  4. DOption (d)

    None of these

View answer & explanation

Correct answer

Answer: (a)A.

Indonesia

Why this answer is correct

The correct answer is Indonesia.

Key Points

  • Operation Samudra Maitri was a massive humanitarian operation launched by the Government of India in October 2018 to assist the victims of the earthquake and tsunami in Central Sulawesi, Indonesia.
  • The disaster was triggered by a 7.5-magnitude earthquake on September 28, 2018, followed by a devastating tsunami that primarily affected the city of Palu and surrounding areas.
  • As part of the operation, India dispatched two Indian Air Force (IAF) aircraft, specifically the C-130J Super Hercules and the C-17 Globemaster III, carrying a medical team and relief supplies such as tents and medicines.
  • The Indian Navy also mobilized three ships—INS Tir, INS Sujata, and INS Shardul—to deliver essential relief materials like food, clothing, and water to the affected country.
  • A field hospital was established by the Indian Army medical personnel to provide emergency medical care to the survivors in the location of the disaster.

Additional Information

  • India-Indonesia Relations: The operation highlighted the Comprehensive Strategic Partnership between India and Indonesia. Prime Minister Narendra Modi personally communicated with President Joko Widodo to offer assistance immediately after the disaster.
  • HADR Capabilities: This mission demonstrated India’s growing role as a Net Security Provider and a first responder in Humanitarian Assistance and Disaster Relief (HADR) operations within the Indian Ocean Region and the Indo-Pacific.
  • Magnitude of Disaster: The 2018 Sulawesi earthquake caused widespread soil liquefaction, which buried entire neighborhoods, leading to a high death toll exceeding 2,000 people and displacing thousands more.
  • Geographical Context: Indonesia is located on the Pacific Ring of Fire, making it highly susceptible to tectonic activities, volcanic eruptions, and natural disasters.
  • Relief Logistics: The IAF C-17 aircraft carried approximately 37 tonnes of medical equipment and supplies, while the C-130J carried a 60-member medical team including doctors and paramedics.

Important Points

  • The term "Samudra Maitri" translates to "Ocean Friendship", signifying the deep maritime and cultural ties between the two nations.
  • India’s "Act East Policy" and "SAGAR" (Security and Growth for All in the Region) initiative provide the strategic framework for such international cooperation and relief efforts.
  • Apart from Indonesia, India has historically launched similar operations, such as Operation Sahayata (for Mozambique in 2019) and Operation Vanilla (for Madagascar in 2020).
  • The Indian Navy ships involved in the operation were already deployed in the region for overseas training, allowing for a rapid response time to the emergency.

Question 27

Which of the following pairs is not matched about the Prayagraj Kumbh 2019? - 

  1. AOption (a)

    Makar Sankranti - 14 / (15 ^ (th)) Jan.

  2. BOption (b)

    Mauni Amavasya - 4th Feb.

  3. COption (c)

    Basant Panchami - 10th Feb.

  4. DOption (d)

    Maghi Poornima - 15th Feb.

View answer & explanation

Correct answer

Answer: (a)A.

Makar Sankranti - 14 / (15 ^ (th)) Jan.

Why this answer is correct

The correct answer is Maghi Poornima - 15th Feb..

Key Points

  • The Prayagraj Kumbh Mela held in 2019 was an Ardh Kumbh event, which occurs every 6 years. The event took place at the Triveni Sangam in Prayagraj, Uttar Pradesh.
  • Makar Sankranti - 14 / 15th Jan.: This marks the first official bathing day (Shahi Snan) of the Kumbh Mela. In 2019, the primary date for this auspicious festival was January 15, 2019. Hence, this statement is correctly matched.
  • Mauni Amavasya - 4th Feb.: Considered the most important and auspicious day for taking a holy dip in the Ganges, Mauni Amavasya in 2019 fell on February 4. It witnessed the largest gathering of pilgrims during the entire mela. Hence, this statement is correctly matched.
  • Basant Panchami - 10th Feb.: This day marks the arrival of Spring and is the third Shahi Snan. In 2019, it was celebrated on February 10. Hence, this statement is correctly matched.
  • Maghi Poornima - 15th Feb.: This is the full moon day of the Hindu month of Magh. In the year 2019, Maghi Poornima actually occurred on February 19, 2019, and not on February 15. Hence, this statement is not matched and is the correct answer.

Additional Information

  • Kumbh Mela Significance: It is one of the largest peaceful religious gatherings in the world. It was inscribed on the UNESCO Representative List of Intangible Cultural Heritage of Humanity in 2017.
  • Location and Cycle: The Kumbh Mela is held at four locations in India: Prayagraj (at the confluence of Ganges, Yamuna, and Saraswati), Haridwar (Ganges), Nashik (Godavari), and Ujjain (Shipra). A Purna Kumbh occurs every 12 years at each location.
  • 2019 Event Duration: The Prayagraj Ardh Kumbh lasted from January 15, 2019, to March 4, 2019. The Government of Uttar Pradesh and the Ministry of Culture played a significant role in its organization.
  • Guinness World Records: The 2019 event earned three Guinness World Records for the largest sanitation drive involving over 10,000 workers, the largest hand-painting exercise, and the largest parade of buses (over 500 buses).
  • Infrastructure and Scale: The temporary city set up for the mela covered an area of approximately 3,200 hectares and was divided into 20 sectors to manage the millions of devotees.

Important Points

  • Paushe Purnima: This was the official start of the mela on January 21, 2019, which signifies the beginning of the Kalpvas (a period of meditation and austerity).
  • Maha Shivratri: The final bathing day of the 2019 Kumbh was Maha Shivratri, which fell on March 4, 2019. This day marked the conclusion of the 48-day festival.
  • Key Organization: The Prayagraj Mela Authority was the primary organization responsible for the administrative setup, ensuring safety, and providing facilities like 1,22,000 toilets and 20,000 dustbins.
  • Digital Initiatives: For the first time, the Digital Kumbh initiative was launched, including a mobile app to help pilgrims navigate the location and locate lost persons.

Question 28

In January 2019, Sheikh Hasina Wazed of Awami League became Prime Minister of Bangladesh for which term? 

  1. AOption (a)

    1sttime

  2. BOption (b)

    3rd time

  3. COption (c)

    4thtime

  4. DOption (d)

    5th time

View answer & explanation

Correct answer

Answer: (c)C.

4thtime

Why this answer is correct

The correct answer is 4th time.

Key Points

  • Sheikh Hasina Wazed, the leader of the Awami League, was sworn in as the Prime Minister of Bangladesh for the 4th time (and third consecutive term) on 7 January 2019. This followed her party's landslide victory in the 11th Parliamentary Election held on 30 December 2018. Hence, statement 3 is correct.
  • She previously served as the Prime Minister during the following periods: her first term was from 1996 to 2001; her second term began in 2009 and lasted until 2014; and her third term spanned from 2014 to 2019. Hence, statements 1 and 2 are incorrect in the context of the January 2019 appointment.
  • The January 2019 inauguration solidified her position as the longest-serving Prime Minister in the history of Bangladesh. Under her leadership, the Grand Alliance led by the Awami League won an overwhelming 288 seats out of the 300 directly elected seats in the Jatiya Sangsad (Parliament).
  • During the swearing-in ceremony held at the Bangabhaban in Dhaka, President Abdul Hamid administered the oath of office to Sheikh Hasina and her new cabinet members.
  • The 5th term for Sheikh Hasina only commenced much later, following the general elections held in January 2024. Therefore, in January 2019, she was entering her 4th term. Hence, statement 4 is incorrect.

Additional Information

  • Political Career of Sheikh Hasina:
  • She is the daughter of Sheikh Mujibur Rahman, the founding father and the first President of Bangladesh, often referred to as Bangabandhu.
  • Sheikh Hasina has been the President of the Bangladesh Awami League since 1981. She returned to Bangladesh from exile in India in May 1981 to lead the pro-democracy movement against military rule.
  • Her leadership is noted for significant economic growth, with Bangladesh maintaining a high GDP growth rate and transitioning from a "Least Developed Country" to a "Developing Country" status as per United Nations criteria.
  • Bangladesh Government Structure:
  • Bangladesh follows a Parliamentary Form of Government where the Prime Minister is the Head of Government, and the President is the Head of State.
  • The Jatiya Sangsad is the unicameral legislature, consisting of 350 members, where 300 members are directly elected and 50 seats are reserved for women.
  • The Constitution of Bangladesh was adopted on 4 November 1972, establishing a secular and democratic republic.
  • Key Achievements and Facts:
  • Under her tenure, the Ganges Water Sharing Treaty with India was signed in 1996, and the Chittagong Hill Tracts Peace Accord was signed in 1997.
  • In 2015, the Land Boundary Agreement (LBA) was implemented between India and Bangladesh, resolving decades-old enclave disputes through the 100th Constitutional Amendment Act of India.
  • She has received numerous international awards, including the UNESCO Houphouët-Boigny Peace Prize and the UN "Champions of the Earth" award in 2015 for her leadership in addressing climate change.
  • Geopolitical Context:
  • Bangladesh is a founding member of SAARC (South Asian Association for Regional Cooperation) and an active member of BIMSTEC (Bay of Bengal Initiative for Multi-Sectoral Technical and Economic Cooperation).
  • The capital city is Dhaka, and the national currency is the Bangladeshi Taka (BDT). As of 2019, the country had made significant strides in human development indices, particularly in maternal health and female education.

Question 29

Prime Minister Narendra Modi released a new coin in the memory of Former Prime Minister Late Shri Atal Bihari Vajpayee in Dec. 2018. It is a ____ . 

  1. AOption (a)

    200 Rupee Coin

  2. BOption (b)

    100 Rupee Coin

  3. COption (c)

    50 Rupee Coin

  4. DOption (d)

    500 Rupee Coin

View answer & explanation

Correct answer

Answer: (b)B.

100 Rupee Coin

Why this answer is correct

The correct answer is 100 Rupee Coin.

Key Points

  • Prime Minister Narendra Modi released a commemorative 100 Rupee Coin in memory of former Prime Minister Atal Bihari Vajpayee on December 24, 2018.
  • The coin was released just a day before the 94th birth anniversary of the late leader, who passed away in August 2018.
  • The 100 Rupee Coin weighs 35 grams and has a diameter of 44 millimeters.
  • The metal composition of the coin is a quaternary alloy consisting of 50% Silver, 40% Copper, 5% Nickel, and 5% Zinc.
  • On the reverse side of the coin, there is a portrait of Atal Bihari Vajpayee with the inscription "Atal Bihari Vajpayee" written in Devanagari script and English. The years 1924 and 2018 are also inscribed below the portrait to signify his birth and death years.
  • The obverse side of the coin features the Lion Capital of Ashoka Pillar in the center with the legend "Satyamev Jayate" inscribed below it. The word "Bharat" in Hindi and "India" in English are flanked on the left and right sides respectively.

Additional Information

  • Atal Bihari Vajpayee:
  • He served as the 10th Prime Minister of India for three terms: first for 13 days in 1996, then for 13 months in 1998-1999, and finally for a full term from 1999 to 2004.
  • He was a co-founder of the Bharatiya Janata Party (BJP) and was the first non-Congress Prime Minister to serve a full five-year term.
  • In 2015, he was conferred with India's highest civilian honor, the Bharat Ratna, by the President of India.
  • His birthday, December 25, is celebrated annually as Good Governance Day (Sushasan Divas) in India to honor his legacy of transparent and accountable administration.
  • Commemorative Coins in India:
  • These coins are issued by the Government of India to honor distinguished personalities, monumental events, or historical milestones.
  • Unlike regular circulation coins, commemorative coins of high denominations (like 100, 125, or 150 rupees) are usually not intended for general trade but are available as collectors' items through the India Government Mint.
  • The Ministry of Finance has the sole authority to design and decide the denominations of these coins under the Coinage Act, 2011.

Important Points

  • Significance of the Release: The release of the 100 Rupee Coin was a tribute to Vajpayee's contribution to Indian politics and his role as a "Man of the Masses."
  • Location of Release: The event took place in New Delhi in the presence of several dignitaries, including Amit Shah and L.K. Advani.
  • Other Commemorative Denominations: While the question focuses on the 100 Rupee Coin for Vajpayee, the Indian government has also released other denominations like 75 Rupee Coin (for the 75th anniversary of FAO) and 125 Rupee Coin (for the birth anniversary of Paramahansa Yogananda) in different years.
  • Design Standards: The circular shape and the serrated edge are standard for such high-value commemorative issues produced by the Mumbai or Kolkata Mints.

Question 30

Shaheed Nazir Ahmad of Jammu and Kashmir sacrificed his life fighting against terrorists in Nov. 2018. He was posthumously awarded _____ . 

  1. AOption (a)

    Ashoka Chakra

  2. BOption (b)

    Mahavir Chakra 

  3. COption (c)

    Saurya Chakra

  4. DOption (d)

    Kirti Chakra

View answer & explanation

Correct answer

Answer: (a)A.

Ashoka Chakra

Why this answer is correct

The correct answer is Ashoka Chakra.

Key Points

  • Lance Naik Nazir Ahmad Wani, a soldier from the Jammu and Kashmir region, was posthumously awarded the Ashoka Chakra in January 2019.
  • He sacrificed his life during a fierce counter-terrorist operation in Shopian district, Jammu and Kashmir, on November 25, 2018.
  • During the operation at Hirapur village, despite being severely wounded, he displayed exemplary courage by eliminating a district commander of the Hizbul Mujahideen and another foreign terrorist in close-quarter combat.
  • He belonged to the 162 Infantry Battalion Territorial Army (Home and Hearth) and was operating with the 34 Rashtriya Rifles at the time of his martyrdom.
  • The Ashoka Chakra is India's highest peacetime military decoration, awarded for most conspicuous bravery or self-sacrifice away from the battlefield. Hence, the option Ashoka Chakra is correct.

Additional Information

  • Kirti Chakra:
  • It is the second-highest peacetime gallantry award in India.
  • It is awarded for conspicuous gallantry otherwise than in the face of the enemy.
  • It was originally established as the Ashoka Chakra, Class II in 1952 and renamed in 1967.
  • Shaurya Chakra:
  • It is the third-highest peacetime gallantry award.
  • It is generally awarded for counter-insurgency operations and actions against the enemy during peacetime.
  • It was originally known as the Ashoka Chakra, Class III.
  • Mahavir Chakra:
  • It is the second-highest wartime gallantry award in India.
  • It is awarded for acts of conspicuous gallantry in the presence of the enemy, whether on land, at sea, or in the air.
  • It is distinct from peacetime awards as it specifically requires a state of war or active conflict with an external enemy.
  • Ashoka Chakra Selection:
  • The award can be conferred upon military personnel as well as civilians.
  • It is the peacetime equivalent of the Param Vir Chakra.

Important Points

  • Lance Naik Nazir Ahmad Wani hailed from Cheki Ashmuji in Kulgam district of Jammu and Kashmir.
  • His life story is remarkable as he was once a militant (an Ikhwan) who realized the futility of violence and joined the Indian Army in 2004.
  • Prior to the Ashoka Chakra, he was a highly decorated soldier having received the Sena Medal for gallantry twice (in 2007 and 2018).
  • The award was presented by President Ram Nath Kovind to his wife, Mahajabeen, during the 70th Republic Day parade in 2019.
  • The Ministry of Defence is the nodal agency that processes these gallantry awards, which are announced twice a year—on Republic Day and Independence Day.

Part II (120 questions)

Question 31

Consequence of uncertainty principle is :

  1. AOption (a)

    Fission of heavy nuclei

  2. BOption (b)

    Fusion of light nuclei

  3. COption (c)

    Finite width of spectral lines

  4. DOption (d)

    Finite value of Bohr magneton

View answer & explanation

Correct answer

Answer: (c)C.

Finite width of spectral lines

Why this answer is correct

Concept

  • The Heisenberg Uncertainty Principle states that it is impossible to determine certain pairs of physical properties, such as energy and time, simultaneously with infinite precision.
  • The energy-time uncertainty relation is expressed as ΔE⋅Δt≥h4π\Delta E \cdot \Delta t \geq \frac{h}{4\pi}.
  • Every excited state of an atom has a finite lifetime, which leads to an inherent uncertainty in the energy level of that state.
<br>

Formula Used

Energy-Time Uncertainty Relation:

ΔE⋅Δt≈h4π\Delta E \cdot \Delta t \approx \frac{h}{4\pi}

Relationship between energy uncertainty and frequency uncertainty:

ΔE=hΔν\Delta E = h \Delta\nu

<br>

Explanation

When an atom is excited to a higher energy level, it stays in that state for a very short, finite duration Δt\Delta t (often called the mean life of the state) before decaying back to a lower energy level.

According to the energy-time uncertainty principle, since the time interval Δt\Delta t is finite, the energy EE of the excited state cannot be defined with absolute precision. There is an inherent uncertainty in energy given by:

ΔE≈h4πΔt\Delta E \approx \frac{h}{4\pi \Delta t}

When the atom transitions from this excited state to a ground state, it emits a photon. The frequency of the emitted photon (ν\nu) is related to the energy change. Because the energy of the starting state has an uncertainty ΔE\Delta E, the frequency of the emitted light will also have an uncertainty Δν\Delta\nu:

Δν=ΔEh≈14πΔt\Delta\nu = \frac{\Delta E}{h} \approx \frac{1}{4\pi \Delta t}

This spread in frequency (or wavelength) means that spectral lines are not perfectly sharp "delta functions" but have a "natural width." This phenomenon is known as the natural broadening of spectral lines.

Other options like nuclear fission and fusion are governed by nuclear binding energies and tunneling effects, and the Bohr magneton is a physical constant related to electron magnetic moments, not a direct consequence of the uncertainty principle in this context.

Hence, the consequence of the uncertainty principle is the finite width of spectral lines.

Question 32

Time independent Schrodinger equation for a free particle is :

  1. AOption (a)

    ∇2ψ+ℏ22m(E−V)ψ=0 \nabla^{2} \psi+\frac{\hbar^{2}}{2 m}(E-V) \psi=0

  2. BOption (b)

    ∇2ψ+2mℏ2(E)ψ=0\nabla^{2} \psi+\frac{2 m}{\hbar^{2}} (E) \psi=0

  3. COption (c)

    ∇2ψ+2mℏ2(V)ψ=0\nabla^{2} \psi+\frac{2 m}{\hbar^{2}}(V) \psi=0

  4. DOption (d)

    ∇2ψ+4mℏ2(E+V)ψ=0\nabla^{2} \psi+\frac{4 m}{\hbar^{2}}(E+V) \psi=0

View answer & explanation

Correct answer

Answer: (b)B.

∇2ψ+2mℏ2(E)ψ=0\nabla^{2} \psi+\frac{2 m}{\hbar^{2}} (E) \psi=0

Why this answer is correct

Concept

  • The time-independent Schrödinger equation is used to find the stationary states and energy levels of a quantum mechanical system.
  • A free particle is defined as a particle that experiences no external forces, which implies that its potential energy VV is constant. For simplicity, this constant potential is usually taken as zero (V=0V = 0).
<br>

Formula Used

The general form of the time-independent Schrödinger equation in three dimensions is:

∇2ψ+2mℏ2(E−V)ψ=0 \nabla^2 \psi + \frac{2m}{\hbar^2}(E - V)\psi = 0

Where:

  • ∇2\nabla^2 is the Laplacian operator (∂2∂x2+∂2∂y2+∂2∂z2\frac{\partial^2}{\partial x^2} + \frac{\partial^2}{\partial y^2} + \frac{\partial^2}{\partial z^2}).
  • mm is the mass of the particle.
  • ℏ\hbar is the reduced Planck's constant (h/2πh/2\pi).
  • EE is the total energy of the particle.
  • VV is the potential energy of the particle.
<br>

Calculation

For a free particle, the potential energy VV is zero:

V=0V = 0

Now, substitute the value of VV into the general Schrödinger equation:

∇2ψ+2mℏ2(E−0)ψ=0 \nabla^2 \psi + \frac{2m}{\hbar^2}(E - 0)\psi = 0

Simplifying the expression:

∇2ψ+2mℏ2Eψ=0 \nabla^2 \psi + \frac{2m}{\hbar^2} E \psi = 0

This matches the mathematical form required for a particle moving in a region where no potential field is present.

Hence, the time independent Schrodinger equation for a free particle is ∇2ψ+2mℏ2(E)ψ=0 \nabla^{2} \psi+\frac{2 m}{\hbar^{2}} (E) \psi=0 .

Question 33

The energy of a particle in one dimensional box in lowest state is E . The energy in state n=4n=4 will be :

  1. AOption (a)

    2E

  2. BOption (b)

    4E

  3. COption (c)

    16E

  4. DOption (d)

    64E

View answer & explanation

Correct answer

Answer: (c)C.

16E

Why this answer is correct

Concept

  • The energy of a particle confined in a one-dimensional box of length LL is quantized, meaning it can only take certain discrete values.
  • The energy levels are proportional to the square of the quantum number nn.
  • The lowest energy state, also known as the ground state, corresponds to n=1n = 1.
<br>

Formula Used

The energy of a particle in a one-dimensional box of length LL is given by:

En=n2h28mL2E_n = \frac{n^2 h^2}{8mL^2}

This can be simplified to the relation between any state nn and the ground state E1E_1:

En=n2E1E_n = n^2 E_1

<br>

Calculation

Given:

Energy in the lowest state (n=1n = 1): E1=EE_1 = E

We need to find the energy in state n=4n = 4.

Using the relation:

En=n2E1E_n = n^2 E_1

Substitute n=4n = 4 and E1=EE_1 = E into the equation:

E4=(4)2×EE_4 = (4)^2 \times E

E4=16×EE_4 = 16 \times E

E4=16EE_4 = 16E

Hence, the energy in state n=4n = 4 will be 16E.

Question 34

An eigen function of the operator d2dx2\frac{d^{2}}{d x^{2}} is ψ=e3x\psi=\mathrm{e}^{3 x}. The corresponding eigen value is :

  1. AOption (a)

    3

  2. BOption (b)

    9

  3. COption (c)

    13\frac{1}{3}

  4. DOption (d)

    19\frac{1}{9}

View answer & explanation

Correct answer

Answer: (b)B.

9

Why this answer is correct

Concept

  • In quantum mechanics and mathematics, an operator acting on a function is said to have an eigenfunction if the result of the operation is the original function multiplied by a constant.
  • This relationship is expressed as: A^ψ=λψ\hat{A}\psi = \lambda\psi, where A^\hat{A} is the operator, ψ\psi is the eigenfunction, and λ\lambda is the eigenvalue.
<br>

Formula Used

The eigenvalue equation:

A^ψ=λψ\hat{A}\psi = \lambda\psi

The derivative of an exponential function:

ddx(eax)=aeax\frac{d}{dx}(e^{ax}) = ae^{ax}

<br>

Calculation

Given the operator:

A^=d2dx2\hat{A} = \frac{d^2}{dx^2}

Given the eigenfunction:

ψ=e3x\psi = e^{3x}

To find the eigenvalue, apply the operator to the function:

d2dx2(e3x)=ddx(ddxe3x)\frac{d^2}{dx^2}(e^{3x}) = \frac{d}{dx} \left( \frac{d}{dx} e^{3x} \right)

First derivative:

ddxe3x=3e3x\frac{d}{dx} e^{3x} = 3e^{3x}

Second derivative:

ddx(3e3x)=3×3e3x=9e3x\frac{d}{dx} (3e^{3x}) = 3 \times 3e^{3x} = 9e^{3x}

Now, compare this result with the eigenvalue equation A^ψ=λψ\hat{A}\psi = \lambda\psi:

9e3x=λe3x9e^{3x} = \lambda e^{3x}

By comparison, we find:

λ=9\lambda = 9

Hence, the corresponding eigenvalue is 9.

Question 35

The quantum operator for the momentum p→\overrightarrow{\mathrm{p}} is :

  1. AOption (a)

    iℏ∇⃗i \hbar \vec{\nabla}

  2. BOption (b)

    −iℏ∇⃗-i \hbar \vec{\nabla}

  3. COption (c)

    iℏr⃗×∇⃗i \hbar \vec{r} \times \vec{\nabla}

  4. DOption (d)

    −iℏr⃗×∇⃗-i \hbar \vec{r} \times \vec{\nabla}

View answer & explanation

Correct answer

Answer: (b)B.

−iℏ∇⃗-i \hbar \vec{\nabla}

Why this answer is correct

Concept

  • In quantum mechanics, physical observables are represented by linear operators that act on the wavefunction of a system.
  • The momentum operator is derived from the correspondence principle and the properties of the wavefunction in the position representation.
  • It is a differential operator that relates to the spatial gradient of the system's state.
<br>

Formula Used

The momentum operator in one dimension (xx-direction) is given by:

p^x=−iℏ∂∂x\hat{p}_x = -i \hbar \frac{\partial}{\partial x}

The generalized momentum operator in three dimensions is given by:

p⃗^=−iℏ∇⃗\hat{\vec{p}} = -i \hbar \vec{\nabla}

Where:

  • ii is the imaginary unit.
  • ℏ\hbar is the reduced Planck's constant (h/2πh/2\pi).
  • ∇⃗\vec{\nabla} is the del (gradient) operator, defined as ∇⃗=i^∂∂x+j^∂∂y+k^∂∂z\vec{\nabla} = \hat{i}\frac{\partial}{\partial x} + \hat{j}\frac{\partial}{\partial y} + \hat{k}\frac{\partial}{\partial z}.
<br>

Explanation

In the position representation, the momentum operator p⃗\vec{p} is defined as:

p⃗=ℏi∇⃗\vec{p} = \frac{\hbar}{i} \vec{\nabla}

Since multiplying the numerator and denominator by ii gives:

ℏ⋅ii⋅i∇⃗=iℏ−1∇⃗=−iℏ∇⃗\frac{\hbar \cdot i}{i \cdot i} \vec{\nabla} = \frac{i \hbar}{-1} \vec{\nabla} = -i \hbar \vec{\nabla}

This operator acts on a wavefunction ψ(r⃗,t)\psi(\vec{r}, t) to extract momentum information through the Schrödinger equation and other quantum mechanical operations. Looking at the given options:

  • Option A: iℏ∇⃗i \hbar \vec{\nabla} (Incorrect sign)
  • Option B: −iℏ∇⃗-i \hbar \vec{\nabla} (Correct)
  • Option C and D involve the cross product r⃗×∇⃗\vec{r} \times \vec{\nabla}, which is related to the angular momentum operator (L⃗=r⃗×p⃗=−iℏ(r⃗×∇⃗)\vec{L} = \vec{r} \times \vec{p} = -i \hbar (\vec{r} \times \vec{\nabla})), not the linear momentum operator.

Hence, the quantum operator for the momentum p⃗\vec{p} is −iℏ∇⃗-i \hbar \vec{\nabla}.

Question 36

A Hermitian operator has :

  1. AOption (a)

    imaginary eigen values

  2. BOption (b)

    real eigen values

  3. COption (c)

    both (a) & (b)

  4. DOption (d)

    No eigen values

View answer & explanation

Correct answer

Answer: (b)B.

real eigen values

Why this answer is correct

Concept

  • An operator A^\hat{A} is said to be Hermitian if it is equal to its own adjoint, i.e., A^=A^†\hat{A} = \hat{A}^\dagger.
  • In quantum mechanics, physical observables are represented by Hermitian operators because their eigenvalues correspond to measurable physical quantities.
  • Measurable quantities in the physical world must be represented by real numbers.
<br>

Mathematical Property

For a Hermitian operator A^\hat{A}, the following identity holds for any state vectors ψ\psi and ϕ\phi:

⟨ψ∣A^ϕ⟩=⟨A^ψ∣ϕ⟩\langle \psi | \hat{A} \phi \rangle = \langle \hat{A} \psi | \phi \rangle

The eigenvalue equation is given by:

A^∣ψ⟩=λ∣ψ⟩\hat{A} |\psi\rangle = \lambda |\psi\rangle

where λ\lambda is the eigenvalue associated with the eigenvector ∣ψ⟩|\psi\rangle.

<br>

Explanation

Consider the eigenvalue equation for a Hermitian operator A^\hat{A}:

A^∣ψ⟩=λ∣ψ⟩\hat{A} |\psi\rangle = \lambda |\psi\rangle

Taking the inner product with ⟨ψ∣\langle \psi | from the left:

⟨ψ∣A^∣ψ⟩=λ⟨ψ∣ψ⟩\langle \psi | \hat{A} |\psi\rangle = \lambda \langle \psi | \psi \rangle    --- (Equation 1)

Now, taking the complex conjugate (adjoint) of the original eigenvalue equation:

(A^∣ψ⟩)†=(λ∣ψ⟩)†(\hat{A} |\psi\rangle)^\dagger = (\lambda |\psi\rangle)^\dagger

⟨ψ∣A^†=λ∗⟨ψ∣\langle \psi | \hat{A}^\dagger = \lambda^* \langle \psi |

Since the operator is Hermitian, A^†=A^\hat{A}^\dagger = \hat{A}. Therefore:

⟨ψ∣A^=λ∗⟨ψ∣\langle \psi | \hat{A} = \lambda^* \langle \psi |

Multiplying by ∣ψ⟩|\psi\rangle from the right:

⟨ψ∣A^∣ψ⟩=λ∗⟨ψ∣ψ⟩\langle \psi | \hat{A} |\psi\rangle = \lambda^* \langle \psi | \psi \rangle    --- (Equation 2)

Comparing Equation 1 and Equation 2, we get:

λ⟨ψ∣ψ⟩=λ∗⟨ψ∣ψ⟩\lambda \langle \psi | \psi \rangle = \lambda^* \langle \psi | \psi \rangle

Since the norm of a state vector ⟨ψ∣ψ⟩\langle \psi | \psi \rangle is non-zero, we must have:

λ=λ∗\lambda = \lambda^*

A number that is equal to its complex conjugate is a real number. Therefore, the eigenvalues of a Hermitian operator are always real.

Hence, a Hermitian operator has real eigen values.

Question 37

If ψ(x,t)\psi(x, \mathrm{t}) is the solution to Schrodinger equation for a particle, the probability of finding the particle at xx at time t is given by :

  1. AOption (a)

    Ψ2(x,t)\Psi^{2}(x, \mathrm{t})

  2. BOption (b)

    ψ(x,t)\psi(x, \mathrm{t})

  3. COption (c)

    Ψ(x,t)Ψ∗(x,t)\Psi(x, \mathrm{t}) \Psi^{*}(x, \mathrm{t})

  4. DOption (d)

    Ψ∗(x,t)\Psi^{*}(x, \mathrm{t})

View answer & explanation

Correct answer

Answer: (c)C.

Ψ(x,t)Ψ∗(x,t)\Psi(x, \mathrm{t}) \Psi^{*}(x, \mathrm{t})

Why this answer is correct

Concept

  • In quantum mechanics, the wave function ψ(x,t)\psi(x, t) represents the quantum state of a particle and is known as the probability amplitude.
  • The wave function itself is generally a complex-valued function and does not have a direct physical interpretation.
  • The Born interpretation provides the physical meaning of the wave function by relating it to the probability of finding a particle.
<br>

Formula Used

The probability density P(x,t)P(x, t) is defined as the square of the magnitude of the wave function:

P(x,t)=∣Ψ(x,t)∣2=Ψ(x,t)Ψ∗(x,t)P(x, t) = |\Psi(x, t)|^2 = \Psi(x, t) \Psi^*(x, t)

where Ψ∗(x,t)\Psi^*(x, t) is the complex conjugate of the wave function Ψ(x,t)\Psi(x, t).

<br>

Explanation

Since the wave function Ψ(x,t)\Psi(x, t) is complex, it can be written in the form a+iba + ib. The probability of finding a particle must be a real, non-negative value. Multiplying the wave function by its complex conjugate Ψ∗\Psi^* (which would be a−iba - ib) ensures that the result is always a real number:

(a+ib)(a−ib)=a2+b2(a + ib)(a - ib) = a^2 + b^2

According to Max Born's postulate, the product \(\Psi(x, t) \Psi^(x, t)\) represents the probability density. The actual probability of finding the particle in an infinitesimal region dxdx is \(\Psi(x, t) \Psi^(x, t) dx\). Therefore, the term characterizing the probability at a specific point and time is the product of the wave function and its complex conjugate.

Hence, the probability of finding the particle at xx at time tt is given by Ψ(x,t)Ψ∗(x,t)\Psi(x, \mathrm{t}) \Psi^{*}(x, \mathrm{t}).

Question 38

For the angular momentum L we can simultaneously determine exact values of :

  1. AOption (a)

    \(\mathrm{L}{x}, \mathrm{~L}{\mathrm{y}} & \mathrm{~L}_{\mathrm{z}}\)

  2. BOption (b)

    \(\mathrm{L}^{2}, \mathrm{~L}{x} & \mathrm{~L}{z}\)

  3. COption (c)

    \(\mathrm{L}^{2}, \mathrm{~L}{x}^{2}, \mathrm{~L}{\mathrm{y}}^{2}\)

  4. DOption (d)

    \(\mathrm{L}^{2}, \mathrm{~L}{x}^{2}+\mathrm{L}{\mathrm{y}}^{2}, \mathrm{~L}_{\mathrm{z}}^{2}\)

View answer & explanation

Correct answer

Answer: (d)D.

\(\mathrm{L}^{2}, \mathrm{~L}{x}^{2}+\mathrm{L}{\mathrm{y}}^{2}, \mathrm{~L}_{\mathrm{z}}^{2}\)

Why this answer is correct

Concept

  • In quantum mechanics, two or more physical observables can be determined simultaneously with exact values if and only if their corresponding operators commute with each other.
  • Two operators AA and BB commute if their commutator [A,B]=AB−BA=0[A, B] = AB - BA = 0.
  • The operator for the square of the total angular momentum L2L^2 commutes with any of its components (Lx,Ly,LzL_x, L_y, L_z).
<br>

Formula Used

Fundamental commutation relations for angular momentum components:

[Lx,Ly]=iℏLz,[Ly,Lz]=iℏLx,[Lz,Lx]=iℏLy[L_x, L_y] = i\hbar L_z, \quad [L_y, L_z] = i\hbar L_x, \quad [L_z, L_x] = i\hbar L_y

Commutation involving L2L^2:

[L2,Lx]=[L2,Ly]=[L2,Lz]=0[L^2, L_x] = [L^2, L_y] = [L^2, L_z] = 0

The relation for total squared angular momentum:

L2=Lx2+Ly2+Lz2L^2 = L_x^2 + L_y^2 + L_z^2

<br>

Explanation

To determine if values can be measured simultaneously, we check if the operators in each set commute:

  • Set A: Lx,Ly,LzL_x, L_y, L_z. These do not commute with each other (e.g., [Lx,Ly]≠0[L_x, L_y] \neq 0). Thus, they cannot be determined simultaneously.
  • Set B: L2,Lx,LzL^2, L_x, L_z. While L2L^2 commutes with both components, the components LxL_x and LzL_z do not commute with each other. Thus, they cannot be determined simultaneously.
  • Set C: L2,Lx2,Ly2L^2, L_x^2, L_y^2. In general, the squares of different components (like Lx2L_x^2 and Ly2L_y^2) do not commute.
  • Set D: L2,Lx2+Ly2,Lz2L^2, L_x^2 + L_y^2, L_z^2.
  • Since [L2,Lz]=0[L^2, L_z] = 0, it follows that [L2,Lz2]=0[L^2, L_z^2] = 0.
  • Using the relation Lx2+Ly2=L2−Lz2L_x^2 + L_y^2 = L^2 - L_z^2, we can see that Lx2+Ly2L_x^2 + L_y^2 is a function of L2L^2 and Lz2L_z^2.
  • Since L2L^2 and Lz2L_z^2 commute with themselves and with each other, any linear combination of them (like L2−Lz2L^2 - L_z^2) will also commute with L2L^2 and Lz2L_z^2.

Because all three operators in set D commute with each other, their exact values can be determined simultaneously.

Hence, we can simultaneously determine exact values of \(\mathrm{L}^{2}, \mathrm{~L}{x}^{2}+\mathrm{L}{\mathrm{y}}^{2}, \mathrm{~L}_{\mathrm{z}}^{2}\).

Question 39

Photon as a particle has mass :

  1. AOption (a)

    hλ\frac{h}{\lambda}

  2. BOption (b)

    hν\mathrm{h} \nu

  3. COption (c)

    hλc\frac{h}{\lambda c}

  4. DOption (d)

    hvc\frac{\mathrm{h} v}{\mathrm{c}}

View answer & explanation

Correct answer

Answer: (c)C.

hλc\frac{h}{\lambda c}

Why this answer is correct

Concept

  • A photon is a quantum of electromagnetic radiation that behaves both as a wave and a particle.
  • While the rest mass of a photon is zero, it possesses energy and momentum.
  • According to Einstein's mass-energy equivalence principle, the energy of a particle is related to its equivalent dynamic mass.
<br>

Formula Used

Energy of a photon in terms of wavelength:

E=hcλE = \frac{hc}{\lambda}

Einstein's mass-energy equivalence relation:

E=mc2E = mc^2

Where:

  • hh is Planck's constant
  • cc is the speed of light
  • λ\lambda is the wavelength
  • mm is the equivalent mass of the photon
<br>

Calculation

To find the equivalent mass of the photon, we equate the energy expressions:

mc2=hcλmc^2 = \frac{hc}{\lambda}

To isolate the mass (mm), divide both sides by c2c^2:

m=hcλ⋅c2m = \frac{hc}{\lambda \cdot c^2}

By simplifying the equation (canceling one cc from the numerator and denominator):

m=hλcm = \frac{h}{\lambda c}

Alternatively, using frequency (ν\nu):

Since E=hνE = h\nu and E=mc2E = mc^2,

mc2=hν⇒m=hνc2mc^2 = h\nu \Rightarrow m = \frac{h\nu}{c^2}

Comparing these results with the given options, the expression hλc\frac{h}{\lambda c} represents the equivalent mass of the photon.

Hence, the mass of the photon as a particle is hλc\frac{h}{\lambda c}.

Question 40

If operator A^=∂∂x+x\hat{\mathrm{A}}=\frac{\partial}{\partial x}+x and operator B^=∂∂x−x\hat{\mathrm{B}}=\frac{\partial}{\partial x}-x, then the product A^B^\hat{\mathrm{A}} \hat{\mathrm{B}} is :

  1. AOption (a)

    ∂2∂x2\frac{\partial^{2}}{\partial x^{2}}

  2. BOption (b)

    ∂2∂x2−x2\frac{\partial^{2}}{\partial x^{2}}-x^{2}

  3. COption (c)

    ∂2∂x2−x2−1\frac{\partial^{2}}{\partial x^{2}}-x^{2}-1

  4. DOption (d)

    ∂2∂x2−x2+1\frac{\partial^{2}}{\partial x^{2}}-x^{2}+1

View answer & explanation

Correct answer

Answer: (c)C.

∂2∂x2−x2−1\frac{\partial^{2}}{\partial x^{2}}-x^{2}-1

Why this answer is correct

Concept

  • In operator algebra, the product of two operators A^\hat{A} and B^\hat{B} is defined by their sequential action on a test function ψ(x)\psi(x).
  • Operators are generally non-commutative, meaning A^B^\hat{A}\hat{B} is not necessarily equal to B^A^\hat{B}\hat{A}.
  • When a differential operator acts on a product of terms, the product rule for differentiation must be applied.
<br>

Formula Used

Operator application on a test function:

(A^B^)ψ(x)=A^(B^ψ(x))(\hat{A}\hat{B})\psi(x) = \hat{A}(\hat{B}\psi(x))

Product rule for differentiation:

∂∂x(x⋅ψ)=x∂ψ∂x+ψ∂x∂x=x∂ψ∂x+ψ\frac{\partial}{\partial x}(x \cdot \psi) = x\frac{\partial \psi}{\partial x} + \psi \frac{\partial x}{\partial x} = x\frac{\partial \psi}{\partial x} + \psi

<br>

Calculation

Given the operators:

A^=∂∂x+x\hat{A} = \frac{\partial}{\partial x} + x

B^=∂∂x−x\hat{B} = \frac{\partial}{\partial x} - x

Let ψ\psi be a test function. We need to find A^B^ψ\hat{A}\hat{B}\psi:

A^B^ψ=(∂∂x+x)[(∂∂x−x)ψ]\hat{A}\hat{B}\psi = \left(\frac{\partial}{\partial x} + x\right) \left[ \left(\frac{\partial}{\partial x} - x\right) \psi \right]

First, apply the operator B^\hat{B} to ψ\psi:

B^ψ=∂ψ∂x−xψ\hat{B}\psi = \frac{\partial \psi}{\partial x} - x\psi

Now, apply the operator A^\hat{A} to the result:

A^(B^ψ)=(∂∂x+x)(∂ψ∂x−xψ)\hat{A}(\hat{B}\psi) = \left(\frac{\partial}{\partial x} + x\right) \left( \frac{\partial \psi}{\partial x} - x\psi \right)

Expanding the expression:

A^B^ψ=∂∂x(∂ψ∂x−xψ)+x(∂ψ∂x−xψ)\hat{A}\hat{B}\psi = \frac{\partial}{\partial x} \left( \frac{\partial \psi}{\partial x} - x\psi \right) + x \left( \frac{\partial \psi}{\partial x} - x\psi \right)

Distributing the terms:

A^B^ψ=∂2ψ∂x2−∂∂x(xψ)+x∂ψ∂x−x2ψ\hat{A}\hat{B}\psi = \frac{\partial^2 \psi}{\partial x^2} - \frac{\partial}{\partial x}(x\psi) + x\frac{\partial \psi}{\partial x} - x^2\psi

Applying the product rule to the term ∂∂x(xψ)\frac{\partial}{\partial x}(x\psi):

A^B^ψ=∂2ψ∂x2−(x∂ψ∂x+ψ)+x∂ψ∂x−x2ψ\hat{A}\hat{B}\psi = \frac{\partial^2 \psi}{\partial x^2} - \left( x\frac{\partial \psi}{\partial x} + \psi \right) + x\frac{\partial \psi}{\partial x} - x^2\psi

Simplifying the brackets:

A^B^ψ=∂2ψ∂x2−x∂ψ∂x−ψ+x∂ψ∂x−x2ψ\hat{A}\hat{B}\psi = \frac{\partial^2 \psi}{\partial x^2} - x\frac{\partial \psi}{\partial x} - \psi + x\frac{\partial \psi}{\partial x} - x^2\psi

The terms −x∂ψ∂x-x\frac{\partial \psi}{\partial x} and +x∂ψ∂x+x\frac{\partial \psi}{\partial x} cancel each other out:

A^B^ψ=∂2ψ∂x2−x2ψ−ψ\hat{A}\hat{B}\psi = \frac{\partial^2 \psi}{\partial x^2} - x^2\psi - \psi

Factoring out the test function ψ\psi to find the final operator:

A^B^=∂2∂x2−x2−1\hat{A}\hat{B} = \frac{\partial^2}{\partial x^2} - x^2 - 1

Hence, the product A^B^\hat{A}\hat{B} is ∂2∂x2−x2−1\frac{\partial^{2}}{\partial x^{2}}-x^{2}-1.

Question 41

The commutator bracket [x,∂∂x]\left[x, \frac{\partial}{\partial x}\right] is :

  1. AOption (a)

    zero

  2. BOption (b)

    -1

  3. COption (c)

    1

  4. DOption (d)

    xx

View answer & explanation

Correct answer

Answer: (b)B.

-1

Why this answer is correct

Concept

  • The commutator of two operators A^\hat{A} and B^\hat{B} is defined as [A^,B^]=A^B^−B^A^[\hat{A}, \hat{B}] = \hat{A}\hat{B} - \hat{B}\hat{A}.
  • To evaluate the commutator of operators, we apply them to an arbitrary differentiable test function f(x)f(x) to see the effect of the operator sequence.
<br>

Formula Used

Commutator definition:

[A^,B^]f(x)=A^(B^f(x))−B^(A^f(x))[\hat{A}, \hat{B}] f(x) = \hat{A}(\hat{B}f(x)) - \hat{B}(\hat{A}f(x))

Product rule for differentiation:

ddx(uv)=udvdx+vdudx\frac{d}{dx}(uv) = u\frac{dv}{dx} + v\frac{du}{dx}

<br>

Calculation

Let f(x)f(x) be an arbitrary differentiable test function. We apply the commutator [x,∂∂x]\left[x, \frac{\partial}{\partial x}\right] to this function:

[x,∂∂x]f(x)=(x∂∂x−∂∂xx)f(x)\left[x, \frac{\partial}{\partial x}\right] f(x) = \left(x \frac{\partial}{\partial x} - \frac{\partial}{\partial x} x\right) f(x)

Expanding the terms:

⇒x(∂f(x)∂x)−∂∂x(xf(x))\Rightarrow x \left( \frac{\partial f(x)}{\partial x} \right) - \frac{\partial}{\partial x} \left( x f(x) \right)

Applying the product rule of differentiation to the second term ∂∂x(xf(x))\frac{\partial}{\partial x} (x f(x)):

∂∂x(xf(x))=x∂f(x)∂x+f(x)∂x∂x\frac{\partial}{\partial x} (x f(x)) = x \frac{\partial f(x)}{\partial x} + f(x) \frac{\partial x}{\partial x}

Since ∂x∂x=1\frac{\partial x}{\partial x} = 1, we get:

∂∂x(xf(x))=x∂f(x)∂x+f(x)\frac{\partial}{\partial x} (x f(x)) = x \frac{\partial f(x)}{\partial x} + f(x)

Now, substitute this result back into the commutator expression:

[x,∂∂x]f(x)=x∂f(x)∂x−(x∂f(x)∂x+f(x))\left[x, \frac{\partial}{\partial x}\right] f(x) = x \frac{\partial f(x)}{\partial x} - \left( x \frac{\partial f(x)}{\partial x} + f(x) \right)

⇒x∂f(x)∂x−x∂f(x)∂x−f(x)\Rightarrow x \frac{\partial f(x)}{\partial x} - x \frac{\partial f(x)}{\partial x} - f(x)

⇒−f(x)\Rightarrow -f(x)

Comparing both sides:

[x,∂∂x]f(x)=−1⋅f(x)\left[x, \frac{\partial}{\partial x}\right] f(x) = -1 \cdot f(x)

Therefore, the operator equivalence is:

[x,∂∂x]=−1\left[x, \frac{\partial}{\partial x}\right] = -1

Hence, the commutator bracket is -1.

Question 42

The Clausius - Mossotti equation is :

where symbols have their usual meanings.

  1. AOption (a)

    ϵr−1ϵr+2=Nα3ϵ0\frac{\epsilon_{\mathrm{r}}-1}{\epsilon_{\mathrm{r}}+2}=\frac{\mathrm{N} \alpha}{3 \epsilon_{0}}

  2. BOption (b)

    ϵr+1ϵr−2=3ϵ0α\frac{\epsilon_{\mathrm{r}}+1}{\epsilon_{\mathrm{r}}-2}=\frac{3 \epsilon_{0}}{\alpha}

  3. COption (c)

    ϵr−1ϵr+1=Nαϵ0\frac{\epsilon_{\mathrm{r}}-1}{\epsilon_{\mathrm{r}}+1}=\frac{\mathrm{N} \alpha}{\epsilon_{0}}

  4. DOption (d)

    ϵr+2ϵr−2=N2αϵ0\frac{\epsilon_{\mathrm{r}}+2}{\epsilon_{\mathrm{r}}-2}=\frac{\mathrm{N}^{2} \alpha}{\epsilon_{0}}

View answer & explanation

Correct answer

Answer: (a)A.

ϵr−1ϵr+2=Nα3ϵ0\frac{\epsilon_{\mathrm{r}}-1}{\epsilon_{\mathrm{r}}+2}=\frac{\mathrm{N} \alpha}{3 \epsilon_{0}}

Why this answer is correct

Concept

  • The Clausius-Mossotti equation relates the macroscopic property of a dielectric (relative permittivity) to the microscopic property of its constituent atoms or molecules (polarizability).
  • It is specifically applicable to non-polar dielectrics, particularly those with a cubic crystal structure or dilute gases/liquids where the local field can be calculated using the Lorentz model.
  • The local field is the actual electric field acting on a single molecule, which is the sum of the external field and the field created by other dipoles in the material.
<br>

Formula Used

The Clausius-Mossotti relation is expressed as:

ϵr−1ϵr+2=Nα3ϵ0\frac{\epsilon_{r}-1}{\epsilon_{r}+2} = \frac{N \alpha}{3 \epsilon_{0}}

Where:

  • ϵr\epsilon_{r} = Relative permittivity (dielectric constant) of the medium.
  • NN = Number of atoms or molecules per unit volume.
  • α\alpha = Molecular/atomic polarizability.
  • ϵ0\epsilon_{0} = Permittivity of free space (8.854×10−12,F/m8.854 \times 10^{-12} , F/m).
<br>

Explanation

The derivation of this equation stems from the definition of the local electric field (ElocE_{loc}) in a dielectric material:

Eloc=E+P3ϵ0E_{loc} = E + \frac{P}{3\epsilon_{0}}

Where EE is the macroscopic electric field and PP is the polarization. The polarization is also defined microscopically as:

P=NαElocP = N \alpha E_{loc}

By substituting the expression for the local field into the polarization equation, we get:

P=Nα(E+P3ϵ0)P = N \alpha \left( E + \frac{P}{3\epsilon_{0}} \right)

We also know the macroscopic relation for polarization:

P=ϵ0(ϵr−1)EP = \epsilon_{0}(\epsilon_{r} - 1)E

By combining these equations and eliminating PP and EE, we arrive at the standard Clausius-Mossotti form:

ϵr−1ϵr+2=Nα3ϵ0\frac{\epsilon_{r}-1}{\epsilon_{r}+2} = \frac{N \alpha}{3 \epsilon_{0}}

This equation shows that the ratio ϵr−1ϵr+2\frac{\epsilon_{r}-1}{\epsilon_{r}+2} is directly proportional to the density of the material and the polarizability of its particles.

Hence, the Clausius-Mossotti equation is ϵr−1ϵr+2=Nα3ϵ0\frac{\epsilon_{\mathrm{r}}-1}{\epsilon_{\mathrm{r}}+2}=\frac{\mathrm{N} \alpha}{3 \epsilon_{0}}.

Question 43

The direction of propagation of electromagnetic wave is given by :

  1. AOption (a)

    Vector E→\overrightarrow{\mathrm{E}}

  2. BOption (b)

    Vector B→\overrightarrow{\mathrm{B}}

  3. COption (c)

    Vector 1μ0(E⃗×B⃗)\frac{1}{\mu_{0}}(\vec{E} \times \vec{B})

  4. DOption (d)

    None of these

View answer & explanation

Correct answer

Answer: (c)C.

Vector 1μ0(E⃗×B⃗)\frac{1}{\mu_{0}}(\vec{E} \times \vec{B})

Why this answer is correct

Concept:

An electromagnetic wave consists of mutually perpendicular electric field (E) and magnetic field (B), both perpendicular to the direction of propagation.

The direction of propagation of an electromagnetic wave is given by the Poynting vector.

Poynting vector (S): S = (1/μ₀)(E × B)

This vector represents the direction of energy flow and hence the direction of propagation of the electromagnetic wave.

Explanation:

  • Vector E gives the direction of the electric field only.
  • Vector B gives the direction of the magnetic field only.
  • Neither E nor B alone gives the direction of wave propagation.
  • The cross product (E × B) gives a direction perpendicular to both, which is the direction of propagation.

Hence, the direction of propagation of EM wave is given by (1/μ₀)(E × B).

Question 44

Maxwell's equation ∮SB⃗⋅da⃗=0\oint_{S} \vec{B} \cdot d \vec{a}=0 is a statement of :

  1. AOption (a)

    Faraday's law of induction

  2. BOption (b)

    Modified Ampere's law

  3. COption (c)

    Gauss's law of electricity

  4. DOption (d)

    Gauss's law of magnetism

View answer & explanation

Correct answer

Answer: (d)D.

Gauss's law of magnetism

Why this answer is correct

Concept

  • Maxwell's equations are a set of four fundamental equations that describe how electric and magnetic fields interact and how they are generated by charges and currents.
  • Gauss's law for magnetism states that the total magnetic flux through any closed surface is always zero.
  • This principle implies that magnetic monopoles do not exist in nature; magnetic field lines always form continuous, closed loops.
<br>

Explanation

The equation provided is:

∮SB⃗⋅da⃗=0\oint_{S} \vec{B} \cdot d \vec{a}=0

This is the integral form of Gauss's Law for Magnetism. In this expression:

  • B⃗\vec{B} represents the magnetic field vector.
  • da⃗d \vec{a} is the infinitesimal area vector on the closed surface SS.
  • The circle on the integral sign indicates that the integration is performed over a closed surface.

Physical significance:

If we apply the Divergence Theorem to this equation, we get the differential form:

∇⋅B⃗=0\nabla \cdot \vec{B} = 0

This means that the divergence of a magnetic field is zero. Unlike electric field lines, which start at positive charges and end at negative charges (Gauss's Law for Electricity), magnetic field lines have no beginning or end. Every field line that enters a closed volume must also exit it. This confirms that there are no isolated magnetic poles (no magnetic monopoles).

Comparing with the other laws:

  • Faraday's Law: Relates the induced electromotive force to the rate of change of magnetic flux.
  • Ampere's Law: Relates magnetic fields to the electric currents and changing electric fields that produce them.
  • Gauss's Law for Electricity: Relates the net electric flux through a closed surface to the enclosed net charge.

Hence, the given Maxwell's equation is a statement of Gauss's law of magnetism.

Question 45

Lorentz gauge condition is given by :

  1. AOption (a)

    ∇⃗⋅A→=0\vec{\nabla} \cdot \overrightarrow{\mathrm{A}}=0

  2. BOption (b)

    ∇⃗⋅A→=−μ∈v˙\vec{\nabla} \cdot \overrightarrow{\mathrm{A}}=-\mu \in \dot{\mathrm{v}}

  3. COption (c)

    ∇⃗⋅A→=μ∈v˙\vec{\nabla} \cdot \overrightarrow{\mathrm{A}}=\mu \in \dot{\mathrm{v}}

  4. DOption (d)

    ∇⃗⋅A→=−μ∈v\vec{\nabla} \cdot \overrightarrow{\mathrm{A}}=-\mu \in \mathrm{v}

View answer & explanation

Correct answer

Answer: (b)B.

∇⃗⋅A→=−μ∈v˙\vec{\nabla} \cdot \overrightarrow{\mathrm{A}}=-\mu \in \dot{\mathrm{v}}

Why this answer is correct

Concept

  • In electromagnetism, gauge fixing refers to choosing a specific set of potentials (scalar potential VVV

    and vector potential AAA

    ) to represent the electromagnetic fields.

  • The Lorentz (or Lorenz) gauge is a specific condition used to simplify Maxwell's equations, making them look like wave equations for the potentials.

  • This gauge is particularly useful because it is Lorentz invariant, meaning it maintains its form under relativistic transformations.

<br>

Formula Used

The standard form of the Lorentz gauge condition in a vacuum is:

∇⋅A+1c2∂V∂t=0\nabla \cdot \mathbf{A} + \frac{1}{c^2} \frac{\partial V}{\partial t} = 0

In a medium with permeability μ\mu and permittivity ϵ\epsilon, the speed of light cc is related as:

c=1μϵc = \frac{1}{\sqrt{\mu\epsilon}}

<br>

Explanation

Starting from the Lorentz gauge expression:

∇⋅A+1c2∂V∂t=0\nabla \cdot \mathbf{A} + \frac{1}{c^2} \frac{\partial V}{\partial t} = 0

We know that:

1c2=μϵ\frac{1}{c^2} = \mu\epsilon

Substituting this into the gauge condition:

∇⋅A+μϵ∂V∂t=0\nabla \cdot \mathbf{A} + \mu\epsilon \frac{\partial V}{\partial t} = 0

Letting V˙\dot{V} represent the partial derivative of the scalar potential with respect to time (∂V∂t\frac{\partial V}{\partial t}):

∇⋅A+μϵV˙=0\nabla \cdot \mathbf{A} + \mu\epsilon \dot{V} = 0

Rearranging the equation to solve for the divergence of the vector potential:

∇⋅A=−μϵV˙\nabla \cdot \mathbf{A} = -\mu\epsilon \dot{V}

Hence, the Lorentz gauge condition is ∇⋅A=−μϵV˙\nabla \cdot \mathbf{A} = -\mu\epsilon \dot{V}.

Question 46

For non-ferromagnetic materials the reflection coefficient RR for E⃗\vec{E} perpendicular to plane of incidence is :

  1. AOption (a)

    R=1−22ωϵ0σ\mathrm{R}=1-2 \sqrt{\frac{2 \omega \epsilon_{0}}{\sigma}}

  2. BOption (b)

    R=1−2ωϵ0σ\mathrm{R}=1-2 \sqrt{\frac{\omega \epsilon_{0}}{\sigma}}

  3. COption (c)

    R=1−2ωϵ02σ\mathrm{R}=1-2 \sqrt{\frac{\omega \epsilon_{0}}{2 \sigma}}

  4. DOption (d)

    R=1−2ωϵ0σ\mathrm{R}=1-\sqrt{\frac{2 \omega \epsilon_{0}}{\sigma}}

View answer & explanation

Correct answer

Answer: (a)A.

R=1−22ωϵ0σ\mathrm{R}=1-2 \sqrt{\frac{2 \omega \epsilon_{0}}{\sigma}}

Why this answer is correct

Concept

  • The reflection of electromagnetic waves from a conducting surface is determined by the complex refractive index of the material.
  • For a good conductor (where conductivity σ≫ωϵ\sigma \gg \omega\epsilon), the refractive index is dominated by its imaginary component.
  • The Hagen-Rubens relation provides an approximation for the reflection coefficient RR at low frequencies for metallic surfaces.
  • For non-ferromagnetic materials, the relative permeability μr≈1\mu_r \approx 1.
<br>

Formula Used

Complex refractive index for a good conductor:

n~=n+iκ≈σ2ωϵ0(1+i)\tilde{n} = n + i\kappa \approx \sqrt{\frac{\sigma}{2\omega\epsilon_0}}(1+i)

Reflection coefficient at normal incidence:

R=∣n~−1n~+1∣2=(n−1)2+κ2(n+1)2+κ2R = \left| \frac{\tilde{n}-1}{\tilde{n}+1} \right|^2 = \frac{(n-1)^2 + \kappa^2}{(n+1)^2 + \kappa^2}

<br>

Calculation

For a non-ferromagnetic good conductor, the real and imaginary parts of the refractive index are approximately equal:

n=κ=σ2ωϵ0n = \kappa = \sqrt{\frac{\sigma}{2\omega\epsilon_0}}

Substituting these into the reflection coefficient formula:

R=2n2−2n+12n2+2n+1R = \frac{2n^2 - 2n + 1}{2n^2 + 2n + 1}

Dividing by 2n22n^2 and applying the approximation for n≫1n \gg 1:

R≈1−1n1+1n≈(1−1n)(1−1n)≈1−2nR \approx \frac{1 - \frac{1}{n}}{1 + \frac{1}{n}} \approx (1 - \frac{1}{n})(1 - \frac{1}{n}) \approx 1 - \frac{2}{n}

Substituting the value of n=σ2ωϵ0n = \sqrt{\frac{\sigma}{2\omega\epsilon_0}}:

R=1−2σ2ωϵ0R = 1 - \frac{2}{\sqrt{\frac{\sigma}{2\omega\epsilon_0}}}

R=1−22ωϵ0σR = 1 - 2 \sqrt{\frac{2\omega\epsilon_0}{\sigma}}

Hence, the reflection coefficient is R=1−22ωϵ0σ\mathrm{R}=1-2 \sqrt{\frac{2 \omega \epsilon_{0}}{\sigma}}.

Question 47

Viewing the superconductor as a cutoff plasma, a wave entering it would be attenuated with a skin depth given by

  1. AOption (a)

    (mNe2μ0)1/2\left(\frac{\mathrm{m}}{\mathrm{Ne}^{2} \mu_{0}}\right)^{1 / 2}

  2. BOption (b)

    mNe2μ0\frac{\mathrm{m}}{\mathrm{Ne}^{2} \mu_{0}}

  3. COption (c)

    (mNe2μ0)3/2\left(\frac{\mathrm{m}}{\mathrm{Ne}^{2} \mu_{0}}\right)^{3 / 2}

  4. DOption (d)

    mNe2\frac{\mathrm{m}}{\mathrm{Ne}^{2}}

View answer & explanation

Correct answer

Answer: (a)A.

(mNe2μ0)1/2\left(\frac{\mathrm{m}}{\mathrm{Ne}^{2} \mu_{0}}\right)^{1 / 2}

Why this answer is correct

Concept

  • A superconductor can be modeled as a plasma consisting of free charge carriers (Cooper pairs) that can move without resistance.
  • When an electromagnetic wave enters such a medium, if its frequency is below the plasma frequency (cutoff frequency), the wave does not propagate but instead decays exponentially.
  • The characteristic distance over which the field magnitude decreases to 1/e1/e of its value at the surface is known as the skin depth or London penetration depth.
<br>

Formula Used

The London penetration depth (λ\lambda) is given by the formula:

λ=mNe2μ0\lambda = \sqrt{\frac{m}{N e^2 \mu_0}}

Where:

  • mm = mass of the charge carrier
  • NN = number density of the charge carriers
  • ee = charge of the carrier
  • μ0\mu_0 = permeability of free space
<br>

Calculation

In the plasma model for a superconductor, the equation of motion for a charge carrier (neglecting collisions) is:

mdv⃗dt=eE⃗m \frac{d\vec{v}}{dt} = e\vec{E}

The current density is J⃗=Nev⃗\vec{J} = N e \vec{v}. Differentiating with respect to time:

dJ⃗dt=Ne2mE⃗\frac{d\vec{J}}{dt} = \frac{N e^2}{m} \vec{E}

Taking the curl of Maxwell's equation ∇⃗×B⃗=μ0J⃗\vec{\nabla} \times \vec{B} = \mu_0 \vec{J} (neglecting displacement current for low frequencies):

∇⃗×(∇⃗×B⃗)=μ0(∇⃗×J⃗)\vec{\nabla} \times (\vec{\nabla} \times \vec{B}) = \mu_0 (\vec{\nabla} \times \vec{J})

Using the vector identity ∇⃗×(∇⃗×B⃗)=∇⃗(∇⃗⋅B⃗)−∇2B⃗\vec{\nabla} \times (\vec{\nabla} \times \vec{B}) = \vec{\nabla}(\vec{\nabla} \cdot \vec{B}) - \nabla^2 \vec{B} and knowing ∇⃗⋅B⃗=0\vec{\nabla} \cdot \vec{B} = 0:

−∇2B⃗=μ0(∇⃗×J⃗)-\nabla^2 \vec{B} = \mu_0 (\vec{\nabla} \times \vec{J})

From London's equations, the relation between the curl of current and the magnetic field is:

∇⃗×J⃗=−Ne2mB⃗\vec{\nabla} \times \vec{J} = -\frac{N e^2}{m} \vec{B}

Substituting this back into the wave equation:

∇2B⃗=μ0Ne2mB⃗\nabla^2 \vec{B} = \frac{\mu_0 N e^2}{m} \vec{B}

The solution to this equation is of the form B(x)=B0e−x/λB(x) = B_0 e^{-x/\lambda}, where:

1λ2=μ0Ne2m\frac{1}{\lambda^2} = \frac{\mu_0 N e^2}{m}

⇒λ=mNe2μ0\Rightarrow \lambda = \sqrt{\frac{m}{N e^2 \mu_0}}

This can be written in power form as:

λ=(mNe2μ0)1/2\lambda = \left(\frac{m}{N e^2 \mu_0}\right)^{1/2}

Hence, the skin depth is given by (mNe2μ0)1/2\left(\frac{m}{N e^2 \mu_0}\right)^{1/2}.

Question 48

Which of the following is correct ?

  1. AOption (a)

    D→=∈E→\overrightarrow{\mathrm{D}}=\in \overrightarrow{\mathrm{E}}

  2. BOption (b)

    E→=−∇⃗V\overrightarrow{\mathrm{E}}=-\vec{\nabla} \mathrm{V}

  3. COption (c)

    J→=σE→\overrightarrow{\mathrm{J}}=\sigma \overrightarrow{\mathrm{E}}

  4. DOption (d)

    All the above

View answer & explanation

Correct answer

Answer: (d)D.

All the above

Why this answer is correct

Concept

  • The electric displacement field D→\overrightarrow{D} is related to the electric field E→\overrightarrow{E} through the permittivity of the medium.
  • The electric field E→\overrightarrow{E} is defined as the negative gradient of the scalar electric potential VV.
  • Ohm's law in point form (microscopic form) relates the current density J→\overrightarrow{J} to the electric field E→\overrightarrow{E} using the conductivity σ\sigma of the material.
<br>

Formulae and Explanation

1. Constitutive Relation:

In a linear, isotropic, and homogeneous medium, the electric displacement vector D→\overrightarrow{D} is proportional to the electric field intensity E→\overrightarrow{E}:

D→=ϵE→\overrightarrow{D} = \epsilon \overrightarrow{E}

where ϵ\epsilon is the permittivity of the medium.

2. Relation between Electric Field and Potential:

The electric field is a conservative field in electrostatics, which means it can be expressed as the negative gradient of a scalar potential:

E→=−∇V\overrightarrow{E} = -\nabla V

This equation indicates that the electric field points in the direction of the maximum decrease of the potential.

3. Point form of Ohm's Law:

Ohm's law can be expressed at a point in a conducting medium as:

J→=σE→\overrightarrow{J} = \sigma \overrightarrow{E}

where J→\overrightarrow{J} is the current density and σ\sigma is the electrical conductivity.

<br>

Conclusion

Since all the provided equations represent fundamental and correct laws of electromagnetics:

  • D→=∈E→\overrightarrow{\mathrm{D}}=\in \overrightarrow{\mathrm{E}} is correct.
  • E→=−∇⃗V\overrightarrow{\mathrm{E}}=-\vec{\nabla} \mathrm{V} is correct.
  • J→=σE→\overrightarrow{\mathrm{J}}=\sigma \overrightarrow{\mathrm{E}} is correct.

Hence, all the given statements are correct.

Question 49

In the Fresnel equation of polarization by reflection, when the angle of incidence becomes equal to polarising angle, the reflection coefficient is equal to

  1. AOption (a)

    ∞\infty

  2. BOption (b)

    0

  3. COption (c)

    +1

  4. DOption (d)

    None of the above

View answer & explanation

Correct answer

Answer: (b)B.

0

Why this answer is correct

Concept

  • According to Brewster's Law, when light is incident at the polarizing angle (ipipipip

    ), the reflected ray and the refracted ray are perpendicular to each other.

  • At this specific angle, the light component polarized parallel to the plane of incidence (p-polarized light) is not reflected at all.

  • The Fresnel equations provide the mathematical expressions for the reflection coefficients of the parallel (rprprprp

    ) and perpendicular (rsrsrsrs

    ) components of light.

<br>

Formula Used

The Fresnel reflection coefficient for light polarized parallel to the plane of incidence is given by:

rp=tan⁡(i−r)tan⁡(i+r)r_p = \frac{\tan(i - r)}{\tan(i + r)}

Where:

  • iiii

    is the angle of incidence.

  • rrrr

    is the angle of refraction.

<br>

Calculation

When the angle of incidence is equal to the polarizing angle (i=ipi=ipi=ipi=ip

Code
), the sum of the angle of incidence and the angle of refraction is 90∘90∘90∘90∘

:

i+r=90∘i + r = 90^\circ

Substituting this condition into the denominator of the Fresnel equation for the parallel component:

rp=tan⁡(i−r)tan⁡(90∘)r_p = \frac{\tan(i - r)}{\tan(90^\circ)}

Since the value of tan⁡(90∘)\tan(90^\circ) approaches infinity (∞\infty):

rp=tan⁡(i−r)∞r_p = \frac{\tan(i - r)}{\infty}

rp=0r_p = 0

This indicates that the parallel component of the incident light is completely transmitted, and no part of it is reflected.

Hence, the reflection coefficient is 0.

Question 50

The displacement current arises due to :

  1. AOption (a)

    +ve charges only

  2. BOption (b)

    -ve charges only

  3. COption (c)

    both +ve+v e and - ve charges

  4. DOption (d)

    time varying electric field

View answer & explanation

Correct answer

Answer: (d)D.

time varying electric field

Why this answer is correct

Concept

  • Displacement current is a term introduced by James Clerk Maxwell to complete Ampere's circuital law and make it consistent with the principle of conservation of charge.
  • Unlike conduction current, which is caused by the actual flow of electric charges (electrons or ions), displacement current is associated with the change of electric flux over time.
  • It exists in regions where the electric field is changing, such as the space between the plates of a capacitor during the charging or discharging process.
<br>

Formula Used

The displacement current IdI_d is mathematically expressed as:

Id=ϵ0dΦEdtI_d = \epsilon_0 \frac{d\Phi_E}{dt}

Where:

  • ϵ0\epsilon_0 is the permittivity of free space.
  • ΦE\Phi_E is the electric flux.
  • dΦEdt\frac{d\Phi_E}{dt} represents the rate of change of electric flux with respect to time.
<br>

Explanation

In an electromagnetic setup, such as a capacitor being charged by a battery:

  • A conduction current flows through the wires connecting the battery to the capacitor.
  • Inside the gap between the capacitor plates, there are no actual charge carriers moving from one plate to the other (assuming a vacuum or an ideal dielectric).
  • However, as the charge on the plates increases, the electric field (EE) between the plates changes with time.
  • This time-varying electric field produces a magnetic field in the same way a physical current would. Maxwell termed this effect the "displacement current."
  • Therefore, displacement current is not due to the movement of positive or negative charges, but strictly due to the variation of the electric field over time.

Hence, the displacement current arises due to a time-varying electric field.

Question 51

Equation, which shows that electromagnetic waves are transverse in nature, is :

  1. AOption (a)

    S→=E→×H→\overrightarrow{\mathrm{S}}=\overrightarrow{\mathrm{E}} \times \overrightarrow{\mathrm{H}}

  2. BOption (b)

    \(\overrightarrow{\mathrm{K}} \times \overrightarrow{\mathrm{E}}=0 & \overrightarrow{\mathrm{~K}} \times \overrightarrow{\mathrm{H}}=0\)

  3. COption (c)

    \(\overrightarrow{\mathrm{K}} \cdot \overrightarrow{\mathrm{E}}=0 & \overrightarrow{\mathrm{~K}} \cdot \overrightarrow{\mathrm{H}}=0\)

  4. DOption (d)

    ∇⃗⋅E→=ρϵ0\vec{\nabla} \cdot \overrightarrow{\mathrm{E}}=\frac{\rho}{\epsilon_{0}}

View answer & explanation

Correct answer

Answer: (c)C.

\(\overrightarrow{\mathrm{K}} \cdot \overrightarrow{\mathrm{E}}=0 & \overrightarrow{\mathrm{~K}} \cdot \overrightarrow{\mathrm{H}}=0\)

Why this answer is correct

Concept:

Electromagnetic waves are transverse waves, meaning the electric field (E) and magnetic field (H) oscillate perpendicular to the direction of propagation.

The direction of propagation is represented by the wave vector k.

Condition for transverse nature: k · E = 0 and k · H = 0

Explanation:

  • The dot product being zero implies that the vectors are perpendicular.
  • Thus, E ⟂ k and H ⟂ k.
  • This confirms that both electric and magnetic fields are perpendicular to the direction of wave propagation.

Why other options are incorrect:

  • S = E × H gives direction of energy flow, not the transverse condition.
  • k × E = 0 would imply parallel vectors, which is incorrect.
  • ∇ · E = ρ/ε₀ is Gauss’s law, unrelated to wave transverse nature.

Hence, the equation proving electromagnetic waves are transverse is k · E = 0 and k · H = 0.

Question 52

Relation between three electric vectors E⃗,D⃗\vec{E}, \vec{D} and P⃗\vec{P} is :

  1. AOption (a)

    D⃗=P⃗+E⃗\vec{D}=\vec{P}+\vec{E}

  2. BOption (b)

    D⃗=P⃗E⃗\vec{D}=\frac{\vec{P}}{\vec{E}}

  3. COption (c)

    D⃗=ϵ0E⃗+P⃗\vec{D}=\epsilon_{0} \vec{E}+\vec{P}

  4. DOption (d)

    D⃗=ϵ0(E⃗+P⃗)\vec{D}=\epsilon_{0}(\vec{E}+\vec{P})

View answer & explanation

Correct answer

Answer: (c)C.

D⃗=ϵ0E⃗+P⃗\vec{D}=\epsilon_{0} \vec{E}+\vec{P}

Why this answer is correct

Concept

  • The electric displacement field D⃗\vec{D} is a vector field that accounts for the effects of free charges in a medium.
  • The electric field E⃗\vec{E} is the total field resulting from both free and bound (polarization) charges.
  • The polarization P⃗\vec{P} represents the density of electric dipole moments induced in a dielectric material when subjected to an external electric field.
<br>

Formula Used

The constitutive relation in a dielectric medium is given by:

D⃗=ϵ0E⃗+P⃗\vec{D} = \epsilon_{0} \vec{E} + \vec{P}

<br>

Explanation

When a dielectric material is placed in an electric field E⃗\vec{E}, the material becomes polarized. This means that the positive and negative charges within the atoms or molecules are slightly displaced, creating electric dipoles. The net dipole moment per unit volume is called the polarization vector P⃗\vec{P}.

To simplify the application of Gauss's Law in such materials, we define the Electric Displacement vector D⃗\vec{D}. This vector is designed to depend only on the free charge density, effectively "hiding" the complexity of the bound (polarization) charges.

The total electric flux density in the vacuum part of the medium is ϵ0E⃗\epsilon_{0} \vec{E}, and the contribution from the material itself is P⃗\vec{P}. Summing these two contributions gives the total displacement field:

D⃗=ϵ0E⃗+P⃗\vec{D} = \epsilon_{0} \vec{E} + \vec{P}

Where:

  • D⃗\vec{D} = Electric displacement vector (measured in C/m2C/m^2)
  • ϵ0\epsilon_{0} = Permittivity of free space
  • E⃗\vec{E} = Electric field intensity (measured in V/mV/m)
  • P⃗\vec{P} = Polarization vector (measured in C/m2C/m^2)

Hence, the relation between the three electric vectors is D⃗=ϵ0E⃗+P⃗\vec{D} = \epsilon_{0} \vec{E} + \vec{P}.

Question 53

Suppose a magnetic monopole exists, which of the following Maxwell's equations will be modified ?

  1. AOption (a)

    ∇⃗⋅E→=ρϵ0\vec{\nabla} \cdot \overrightarrow{\mathrm{E}}=\frac{\rho}{\epsilon_{0}}

  2. BOption (b)

    ∇⃗×E→=−∂B→∂t\vec{\nabla} \times \overrightarrow{\mathrm{E}}=\frac{-\partial \overrightarrow{\mathrm{B}}}{\partial \mathrm{t}}

  3. COption (c)

    ∇⃗⋅B→=0\vec{\nabla} \cdot \overrightarrow{\mathrm{B}}=0

  4. DOption (d)

    ∇⃗×B→=μ0 J→+μ0ϵ0∂E→∂t\vec{\nabla} \times \overrightarrow{\mathrm{B}}=\mu_{0} \overrightarrow{\mathrm{~J}}+\mu_{0} \epsilon_{0} \frac{\partial \overrightarrow{\mathrm{E}}}{\partial \mathrm{t}}

View answer & explanation

Correct answer

Answer: (c)C.

∇⃗⋅B→=0\vec{\nabla} \cdot \overrightarrow{\mathrm{B}}=0

Why this answer is correct

Concept

  • Maxwell's equations describe the behavior of electric and magnetic fields and their interaction with matter.
  • In standard electrodynamics, magnetic monopoles are assumed not to exist, which leads to the equation ∇⃗⋅B⃗=0\vec{\nabla} \cdot \vec{B} = 0 (Gauss's Law for Magnetism).
  • The existence of a magnetic monopole would imply a magnetic charge density (ρm\rho_m) and a magnetic current density (J⃗m\vec{J}_m), similar to electric charges and currents.
<br>

Formula Used

Standard Maxwell's Equations (Differential form):

  1. Gauss's Law: ∇⃗⋅E⃗=ρϵ0\vec{\nabla} \cdot \vec{E} = \frac{\rho}{\epsilon_0}
  2. Gauss's Law for Magnetism: ∇⃗⋅B⃗=0\vec{\nabla} \cdot \vec{B} = 0
  3. Faraday's Law: ∇⃗×E⃗=−∂B⃗∂t\vec{\nabla} \times \vec{E} = -\frac{\partial \vec{B}}{\partial t}
  4. Ampere-Maxwell Law: ∇⃗×B⃗=μ0J⃗+μ0ϵ0∂E⃗∂t\vec{\nabla} \times \vec{B} = \mu_0 \vec{J} + \mu_0 \epsilon_0 \frac{\partial \vec{E}}{\partial t}
<br>

Explanation

If magnetic monopoles were to exist, the symmetry of Maxwell's equations would be restored. This leads to changes in two specific equations:

1. Modification of Gauss's Law for Magnetism:

The equation ∇⃗⋅B⃗=0\vec{\nabla} \cdot \vec{B} = 0 states that there are no magnetic sources or sinks. If a magnetic monopole with charge density ρm\rho_m exists, this equation must be modified to:

∇⃗⋅B⃗=μ0ρm\vec{\nabla} \cdot \vec{B} = \mu_0 \rho_m

Therefore, Equation C is modified.

2. Modification of Faraday's Law:

In standard physics, only a changing magnetic field can induce an electric field (as shown in ∇⃗×E⃗=−∂B⃗∂t\vec{\nabla} \times \vec{E} = -\frac{\partial \vec{B}}{\partial t}). However, if magnetic monopoles exist, their motion would create a magnetic current (J⃗m\vec{J}_m). This magnetic current would also induce an electric field, analogous to how electric current induces a magnetic field. The modified Faraday's law becomes:

∇⃗×E⃗=−μ0J⃗m−∂B⃗∂t\vec{\nabla} \times \vec{E} = -\mu_0 \vec{J}_m - \frac{\partial \vec{B}}{\partial t}

Therefore, Equation B is modified.

Equations A (Gauss's Law for Electricity) and D (Ampere-Maxwell Law) describe the effects of electric charges and currents and remain fundamentally the same in structure regarding the source terms already present.

Hence, both Equation B and Equation C will be modified. In the context of most standard physics problems focusing on the existence of the charge itself, Equation C is the primary modification.

Question 54

The differential form of the first law of thermodynamics for the case of a paramagnetic gas is :

  1. AOption (a)

    dQ=dU+PdV+HdI\mathrm{dQ}=\mathrm{dU}+\mathrm{PdV}+\mathrm{HdI}

  2. BOption (b)

    dQ=dU+PdV−HdI\mathrm{dQ}=\mathrm{dU}+\mathrm{PdV}-\mathrm{HdI}

  3. COption (c)

    dQ=dU−PdV−HdId \mathrm{Q}=\mathrm{dU}-\mathrm{PdV}-\mathrm{HdI}

  4. DOption (d)

    dQ=dU−PdV+HdI\mathrm{dQ}=\mathrm{dU}-\mathrm{PdV}+\mathrm{HdI}

View answer & explanation

Correct answer

Answer: (b)B.

dQ=dU+PdV−HdI\mathrm{dQ}=\mathrm{dU}+\mathrm{PdV}-\mathrm{HdI}

Why this answer is correct

Concept

  • The First Law of Thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes: dQ=dU+dWdQ=dU+dWdQ=dU+dW

    .

  • In a gaseous system, the work done by the gas during expansion is given by dWmech=PdVdWmech=PdVdWmech=PdV

    .

  • In a magnetic system (like a paramagnetic substance), work is done to change the magnetization III

    in the presence of an external magnetic field HHH

    . The work done by the magnetic system is expressed as dWmag=−HdIdWmag=−HdIdWmag=−HdI

    .

<br>

Formula Used

General form of the First Law of Thermodynamics:

dQ=dU+dWdQ = dU + dW

Mechanical work done by the system:

dWmech=PdVdW_{mech} = P dV

Magnetic work done by the system:

dWmag=−HdIdW_{mag} = -H dI

<br>

Calculation

For a paramagnetic gas, the system can undergo both mechanical volume changes and changes in magnetization. Therefore, the total work done by the system (dWdWdW

) is the sum of the mechanical work and the magnetic work:

dW=dWmech+dWmagdW = dW_{mech} + dW_{mag}

Substituting the respective expressions:

dW=PdV+(−HdI)dW = P dV + (-H dI)

dW=PdV−HdIdW = P dV - H dI

Now, substituting this total work into the differential form of the first law of thermodynamics:

dQ=dU+dWdQ = dU + dW

dQ=dU+(PdV−HdI)dQ = dU + (P dV - H dI)

dQ=dU+PdV−HdIdQ = dU + P dV - H dI

This equation represents the conservation of energy for a paramagnetic gas where dUdUdU

is the change in internal energy, PdVPdVPdV

is the mechanical work done, and −HdI−HdI−HdI

is the work related to magnetization.

Hence, the differential form of the first law for a paramagnetic gas is dQ=dU+PdV−HdIdQ = dU + P dV - H dI.

Question 55

Gibbs Potential is defined as :

  1. AOption (a)

    G=U−PV+TS\mathrm{G}=\mathrm{U}-\mathrm{PV}+\mathrm{TS}

  2. BOption (b)

    G=U+PV+TS\mathrm{G}=\mathrm{U}+\mathrm{PV}+\mathrm{TS}

  3. COption (c)

    G=U−PV−TS\mathrm{G}=\mathrm{U}-\mathrm{PV}-\mathrm{TS}

  4. DOption (d)

    G=U+PV−TS\mathrm{G}=\mathrm{U}+\mathrm{PV}-\mathrm{TS}

View answer & explanation

Correct answer

Answer: (d)D.

G=U+PV−TS\mathrm{G}=\mathrm{U}+\mathrm{PV}-\mathrm{TS}

Why this answer is correct

Concept

  • Thermodynamic potentials are state functions used to describe the equilibrium state of a system under specific constraints.

  • Enthalpy (HHH

    ) is defined as the total heat content of a system, equal to the internal energy plus the product of pressure and volume.

  • Gibbs Free Energy (GGG

    ), also known as Gibbs Potential, is a thermodynamic potential that measures the maximum reversible work that may be performed by a thermodynamic system at constant temperature and pressure.

<br>

Formula Used

Definition of Enthalpy:

H=U+PVH = U + PV

Definition of Gibbs Free Energy:

G=H−TSG = H - TS

Where:

  • UUU

    = Internal Energy

  • PPP

    = Pressure

  • VVV

    = Volume

  • TTT

    = Absolute Temperature

  • SSS

    = Entropy

<br>

Explanation

The Gibbs Potential (GGG

) is mathematically related to enthalpy (HHH

), temperature (TTT

), and entropy (SSS

) by the following relation:

G=H−TSG = H - TS

Since enthalpy is defined as the sum of internal energy (UUU

) and the product of pressure (PPP

) and volume (VVV

):

H=U+PVH = U + PV

Substituting the expression for HHH

into the equation for GGG

:

G=(U+PV)−TSG = (U + PV) - TS

Rearranging the terms, we get:

G=U+PV−TSG = U + PV - TS

Hence, the correct definition of Gibbs Potential is G=U+PV−TS\mathrm{G}=\mathrm{U}+\mathrm{PV}-\mathrm{TS}.

Question 56

Which of the following is not a Maxwell thermodynamical relation ?

  1. AOption (a)

    \(\left(\frac{\partial \mathrm{T}}{\partial v}\right){\mathrm{S}}=-\left(\frac{\partial \mathrm{P}}{\partial \mathrm{S}}\right){\mathrm{V}}\)

  2. BOption (b)

    \(\left(\frac{\partial \mathrm{S}}{\partial \mathrm{V}}\right){\mathrm{T}}=\left(\frac{\partial \mathrm{P}}{\partial \mathrm{T}}\right){\mathrm{V}}\)

  3. COption (c)

    \(\left(\frac{\partial \mathrm{T}}{\partial \mathrm{P}}\right){\mathrm{S}}=\left(\frac{\partial \mathrm{V}}{\partial \mathrm{S}}\right){\mathrm{P}}\)

  4. DOption (d)

    \(\left(\frac{\partial \mathrm{S}}{\partial \mathrm{P}}\right){\mathrm{T}}=-\left(\frac{\partial \mathrm{T}}{\partial \mathrm{V}}\right){\mathrm{P}}\)

View answer & explanation

Correct answer

Answer: (c)C.

\(\left(\frac{\partial \mathrm{T}}{\partial \mathrm{P}}\right){\mathrm{S}}=\left(\frac{\partial \mathrm{V}}{\partial \mathrm{S}}\right){\mathrm{P}}\)

Why this answer is correct

Concept:

Maxwell’s thermodynamic relations are derived from the equality of mixed second-order partial derivatives of thermodynamic potentials (U, H, F, G).

The four standard Maxwell relations are:

  • (∂T/∂V)S = −(∂P/∂S)V
  • (∂S/∂V)T = (∂P/∂T)V
  • (∂T/∂P)S = (∂V/∂S)P (Incorrect sign in given option)
  • (∂S/∂P)T = −(∂V/∂T)P

Explanation:

  • Option 1: Correct Maxwell relation ✔
  • Option 2: Correct Maxwell relation ✔
  • Option 4: Correct Maxwell relation ✔
  • Option 3: Incorrect because the correct relation is:

(∂T/∂P)S = −(∂V/∂S)P

Conclusion: The given equation \(\left(\frac{\partial \mathrm{T}}{\partial \mathrm{P}}\right){\mathrm{S}}=\left(\frac{\partial \mathrm{V}}{\partial \mathrm{S}}\right){\mathrm{P}}\) is not a valid Maxwell relation.

Question 57

In Joule - Thomson effect, which of the following quantities remains constant?

  1. AOption (a)

    Internal energy

  2. BOption (b)

    Temperature

  3. COption (c)

    Entropy

  4. DOption (d)

    Enthalpy

View answer & explanation

Correct answer

Answer: (d)D.

Enthalpy

Why this answer is correct

Concept

  • The Joule-Thomson effect (also known as the Joule-Kelvin effect) describes the temperature change of a real gas when it is forced through a valve or porous plug while being kept insulated so that no heat is exchanged with the environment.
  • This type of expansion is known as a throttling process.
  • In such a process, the system is adiabatic, and no external work is performed on the environment.
<br>

Explanation

Consider a gas at pressure P1P_1 and volume V1V_1 passing through a porous plug to a region of lower pressure P2P_2 and volume V2V_2. Since the process is adiabatic, the net heat exchange Q=0Q = 0.

According to the First Law of Thermodynamics:

ΔU=Q−W\Delta U = Q - W

In this process, work is done on the gas to push it through the plug (P1V1P_1V_1) and work is done by the gas as it expands on the other side (P2V2P_2V_2). The net work done by the system is:

W=P2V2−P1V1W = P_2V_2 - P_1V_1

Substituting the values of QQ and WW into the First Law equation:

U2−U1=0−(P2V2−P1V1)U_2 - U_1 = 0 - (P_2V_2 - P_1V_1)

Rearranging the equation to group initial and final states:

U1+P1V1=U2+P2V2U_1 + P_1V_1 = U_2 + P_2V_2

Since enthalpy HH is defined as the sum of internal energy and the product of pressure and volume (H=U+PVH = U + PV), the equation can be written as:

H1=H2H_1 = H_2

This demonstrates that the total enthalpy of the gas remains constant during the Joule-Thomson expansion. Thus, it is an isoenthalpic process.

Hence, the quantity that remains constant in the Joule-Thomson effect is Enthalpy.

Question 58

According to Wien's displacement law :

  1. AOption (a)

    λmT=\lambda_{\mathrm{m}} \mathrm{T}= constant

  2. BOption (b)

    λm/T=\lambda_{\mathrm{m}} / \mathrm{T}= constant

  3. COption (c)

    λm2 T=\lambda_{\mathrm{m}}^{2} \mathrm{~T}= constant

  4. DOption (d)

    Tλm =\frac{T}{\lambda_{\text {m }}}= constant

View answer & explanation

Correct answer

Answer: (a)A.

λmT=\lambda_{\mathrm{m}} \mathrm{T}= constant

Why this answer is correct

Concept

  • Wien's displacement law describes the relationship between the temperature of a blackbody and the wavelength at which it emits the most radiation.
  • As the temperature of an object increases, the peak wavelength of its spectral distribution shifts toward shorter wavelengths.
<br>

Formula Used

Wien's Displacement Law:

λm×T=b\lambda_{m} \times T = b

Where:

  • λm\lambda_{m} is the wavelength of maximum emission.
  • TT is the absolute temperature of the blackbody (in Kelvin).
  • bb is Wien's constant (approximately 2.898×10−3,m⋅K2.898 \times 10^{-3},m\cdot K).
<br>

Explanation

Wien's displacement law states that the wavelength corresponding to the maximum spectral emissive power (λm\lambda_{m}) is inversely proportional to the absolute temperature (TT) of the blackbody.

Mathematically:

λm∝1T\lambda_{m} \propto \frac{1}{T}

This can be rewritten as:

λmT=constant\lambda_{m} T = \text{constant}

This constant is known as Wien's displacement constant.

Hence, the correct expression for Wien's displacement law is λmT=constant\lambda_{m} T = \text{constant}.

Question 59

Einstein theory concludes that at lower temerature the specific heat capacity of solids at constant volume (Cv)\left(\mathrm{C}_{\mathrm{v}}\right) :

  1. AOption (a)

    drops linearly with increase of temperature

  2. BOption (b)

    drops linearly with decrease of temperature

  3. COption (c)

    drops exponentially with decrease of temperature

  4. DOption (d)

    Remains constant

View answer & explanation

Correct answer

Answer: (c)C.

drops exponentially with decrease of temperature

Why this answer is correct

Concept

  • The Einstein model of specific heat treats atoms in a solid as independent quantum harmonic oscillators vibrating with a common frequency.
  • This model was developed to explain why specific heat capacity deviates from the classical Dulong-Petit law at low temperatures.
  • According to this theory, the energy of these oscillators is quantized.
<br>

Formula Used

Einstein's specific heat formula is given by:

Cv=3R(ΘET)2eΘE/T(eΘE/T−1)2C_v = 3R \left( \frac{\Theta_E}{T} \right)^2 \frac{e^{\Theta_E/T}}{(e^{\Theta_E/T} - 1)^2}

Where:

  • ΘE=ℏωkB\Theta_E = \frac{\hbar\omega}{k_B} is the Einstein temperature.
  • RR is the universal gas constant.
  • TT is the absolute temperature.
<br>

Explanation

To understand the behavior at lower temperatures, we examine the limit where T→0T \to 0 (which means T≪ΘET \ll \Theta_E):

As TT becomes very small, the term eΘE/Te^{\Theta_E/T} becomes very large. Therefore, we can approximate the denominator as:

(eΘE/T−1)2≈(eΘE/T)2=e2ΘE/T(e^{\Theta_E/T} - 1)^2 \approx (e^{\Theta_E/T})^2 = e^{2\Theta_E/T}

Substituting this back into the CvC_v expression:

Cv≈3R(ΘET)2eΘE/Te2ΘE/TC_v \approx 3R \left( \frac{\Theta_E}{T} \right)^2 \frac{e^{\Theta_E/T}}{e^{2\Theta_E/T}}

Cv≈3R(ΘET)2e−ΘE/TC_v \approx 3R \left( \frac{\Theta_E}{T} \right)^2 e^{-\Theta_E/T}

In this expression, as temperature TT decreases, the exponential term e−ΘE/Te^{-\Theta_E/T} decreases much more rapidly than the T2T^2 term in the denominator. This mathematical form shows that the specific heat capacity CvC_v drops exponentially to zero as the temperature decreases.

Hence, Einstein's theory concludes that at lower temperature the specific heat capacity of solids drops exponentially with decrease of temperature.

Question 60

Average energy of a Planck's oscillator of frequency vv in thermal equilibrium with heat reservoir of temperature T is

  1. AOption (a)

    E=hv\mathrm{E}=\mathrm{h} v

  2. BOption (b)

    E=nhv\mathrm{E}=\mathrm{nh} v

  3. COption (c)

    E=mc2\mathrm{E}=\mathrm{mc}^{2}

  4. DOption (d)

    E=hν(ehν/KT−1)\mathrm{E}=\frac{\mathrm{h} \nu}{\left(\mathrm{e}^{\mathrm{h} \nu / \mathrm{KT}-1}\right)}

View answer & explanation

Correct answer

Answer: (d)D.

E=hν(ehν/KT−1)\mathrm{E}=\frac{\mathrm{h} \nu}{\left(\mathrm{e}^{\mathrm{h} \nu / \mathrm{KT}-1}\right)}

Why this answer is correct

Concept

  • According to Planck's hypothesis, the energy of an oscillator is not continuous but quantized.
  • The energy of a Planck's oscillator is given by En=nhνE_n = nh\nu, where n=0,1,2,…n = 0, 1, 2, \dots
  • In thermal equilibrium at temperature TT, the probability of an oscillator being in a state with energy EnE_n is proportional to the Boltzmann factor e−En/kTe^{-E_n/kT}.
<br>

Formula Used

The average energy ⟨E⟩\langle E \rangle of an oscillator is given by:

⟨E⟩=∑n=0∞Ene−En/kT∑n=0∞e−En/kT\langle E \rangle = \frac{\sum_{n=0}^{\infty} E_n e^{-E_n/kT}}{\sum_{n=0}^{\infty} e^{-E_n/kT}}

Where:

  • hh is Planck's constant.
  • ν\nu is the frequency of the oscillator.
  • kk is the Boltzmann constant.
  • TT is the absolute temperature.
<br>

Calculation

Let x=e−hν/kTx = e^{-h\nu/kT}. Then the energy states are En=nhνE_n = nh\nu.

The denominator (partition function ZZ) is:

Z=∑n=0∞e−nhν/kT=∑n=0∞xn=1+x+x2+⋯=11−xZ = \sum_{n=0}^{\infty} e^{-nh\nu/kT} = \sum_{n=0}^{\infty} x^n = 1 + x + x^2 + \dots = \frac{1}{1-x}

The numerator is:

∑n=0∞nhνxn=hν∑n=0∞nxn=hν(x+2x2+3x3+… )\sum_{n=0}^{\infty} nh\nu x^n = h\nu \sum_{n=0}^{\infty} nx^n = h\nu (x + 2x^2 + 3x^3 + \dots)

=hν⋅xddx∑n=0∞xn=hν⋅xddx(11−x)= h\nu \cdot x \frac{d}{dx} \sum_{n=0}^{\infty} x^n = h\nu \cdot x \frac{d}{dx} \left( \frac{1}{1-x} \right)

=hνx(1−x)2= h\nu \frac{x}{(1-x)^2}

Now, the average energy ⟨E⟩\langle E \rangle:

⟨E⟩=hνx(1−x)211−x=hνx1−x\langle E \rangle = \frac{h\nu \frac{x}{(1-x)^2}}{\frac{1}{1-x}} = \frac{h\nu x}{1-x}

Dividing numerator and denominator by xx:

⟨E⟩=hν1x−1\langle E \rangle = \frac{h\nu}{\frac{1}{x} - 1}

Substituting 1x=ehν/kT\frac{1}{x} = e^{h\nu/kT}:

⟨E⟩=hνehν/kT−1\langle E \rangle = \frac{h\nu}{e^{h\nu/kT} - 1}

Hence, the average energy of a Planck's oscillator is E=hν(ehν/KT−1)\mathrm{E}=\frac{\mathrm{h} \nu}{\left(\mathrm{e}^{\mathrm{h} \nu / \mathrm{KT}-1}\right)}.

Question 61

Using Planck's radiation law the total energy density is given by :

  1. AOption (a)

    8π5k415c2 h3 T4\frac{8 \pi^{5} \mathrm{k}^{4}}{15 \mathrm{c}^{2} \mathrm{~h}^{3}} \mathrm{~T}^{4}

  2. BOption (b)

    8π5k415ch3 T4\frac{8 \pi^{5} \mathrm{k}^{4}}{15 \mathrm{ch}^{3}} \mathrm{~T}^{4}

  3. COption (c)

    8π5k415c3 h3 T4\frac{8 \pi^{5} \mathrm{k}^{4}}{15 \mathrm{c}^{3} \mathrm{~h}^{3}} \mathrm{~T}^{4}

  4. DOption (d)

    8πk415c3 h3 T4\frac{8 \pi \mathrm{k}^{4}}{15 \mathrm{c}^{3} \mathrm{~h}^{3}} \mathrm{~T}^{4}

View answer & explanation

Correct answer

Answer: (c)C.

8π5k415c3 h3 T4\frac{8 \pi^{5} \mathrm{k}^{4}}{15 \mathrm{c}^{3} \mathrm{~h}^{3}} \mathrm{~T}^{4}

Why this answer is correct

Concept

  • Planck's radiation law describes the spectral energy density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature.
  • The total energy density (UU) of the radiation is obtained by integrating the spectral energy density over all possible frequencies from zero to infinity.
  • This integration leads to the Stefan-Boltzmann law, which states that the total energy density is proportional to the fourth power of the absolute temperature (T4T^4).
<br>

Formula Used

Planck's spectral energy density in terms of frequency ν\nu:

u(ν,T)=8πhν3c31ehν/kT−1u(\nu, T) = \frac{8\pi h \nu^3}{c^3} \frac{1}{e^{h\nu/kT} - 1}

Total energy density:

U=∫0∞u(ν,T)dνU = \int_{0}^{\infty} u(\nu, T) d\nu

Standard integral result:

∫0∞x3ex−1dx=π415\int_{0}^{\infty} \frac{x^3}{e^x - 1} dx = \frac{\pi^4}{15}

<br>

Calculation

To find the total energy density, we integrate the spectral energy density over the entire frequency range:

U=∫0∞8πhν3c31ehν/kT−1dνU = \int_{0}^{\infty} \frac{8\pi h \nu^3}{c^3} \frac{1}{e^{h\nu/kT} - 1} d\nu

Let x=hνkTx = \frac{h\nu}{kT}. From this substitution, we have:

ν=kThx\nu = \frac{kT}{h}x and dν=kThdxd\nu = \frac{kT}{h} dx

Substituting these variables into the integral:

U=8πhc3∫0∞(kThx)31ex−1(kTh)dxU = \frac{8\pi h}{c^3} \int_{0}^{\infty} \left( \frac{kT}{h}x \right)^3 \frac{1}{e^x - 1} \left( \frac{kT}{h} \right) dx

Extracting the constants from the integral:

U=8πhc3(kTh)4∫0∞x3ex−1dxU = \frac{8\pi h}{c^3} \left( \frac{kT}{h} \right)^4 \int_{0}^{\infty} \frac{x^3}{e^x - 1} dx

Applying the standard integral result ∫0∞x3ex−1dx=π415\int_{0}^{\infty} \frac{x^3}{e^x - 1} dx = \frac{\pi^4}{15}:

U=8πhc3k4T4h4(π415)U = \frac{8\pi h}{c^3} \frac{k^4 T^4}{h^4} \left( \frac{\pi^4}{15} \right)

Simplifying the expression by canceling one hh and combining the π\pi terms:

U=8π5k415c3h3T4U = \frac{8 \pi^5 k^4}{15 c^3 h^3} T^4

Hence, the total energy density is 8π5k415c3 h3 T4\frac{8 \pi^{5} \mathrm{k}^{4}}{15 \mathrm{c}^{3} \mathrm{~h}^{3}} \mathrm{~T}^{4}.

Question 62

The orbit of a linear harmonic oscillator in phase space is :

  1. AOption (a)

    a straight line

  2. BOption (b)

    a parabola

  3. COption (c)

    a hyperbola

  4. DOption (d)

    an ellipse

View answer & explanation

Correct answer

Answer: (d)D.

an ellipse

Why this answer is correct

Concept

  • Phase space is a multidimensional space in which all possible states of a system are represented, with each state corresponding to a unique point.
  • For a one-dimensional linear harmonic oscillator, phase space consists of two coordinates: position (xx) and momentum (pp).
  • The orbit (or trajectory) in phase space is determined by the total energy of the system, which remains constant for a conservative system.
<br>

Formula Used

The total energy EE of a linear harmonic oscillator is the sum of its kinetic and potential energies:

E=p22m+12kx2E = \frac{p^2}{2m} + \frac{1}{2}kx^2

Where:

  • pp is the momentum.
  • xx is the displacement.
  • mm is the mass.
  • kk is the force constant.

The standard equation of an ellipse is:

x2a2+y2b2=1\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1

<br>

Calculation

Starting with the energy equation:

E=12kx2+p22mE = \frac{1}{2}kx^2 + \frac{p^2}{2m}

To relate this to the standard geometric form, divide the entire equation by EE:

1=12kx2E+p22mE1 = \frac{\frac{1}{2}kx^2}{E} + \frac{\frac{p^2}{2m}}{E}

Rearranging the terms:

x2(2E/k)+p2(2mE)=1\frac{x^2}{(2E/k)} + \frac{p^2}{(2mE)} = 1

This equation can be written as:

x2(2E/k)2+p2(2mE)2=1\frac{x^2}{(\sqrt{2E/k})^2} + \frac{p^2}{(\sqrt{2mE})^2} = 1

This is the equation of an ellipse in the x−px-p plane (phase space), where the semi-axes are a=2E/ka = \sqrt{2E/k} and b=2mEb = \sqrt{2mE}.

Hence, the orbit of a linear harmonic oscillator in phase space is an ellipse.

Question 63

A system in contact with a heat reservoir can most suitably be represented by :

  1. AOption (a)

    micro canonical ensemble

  2. BOption (b)

    grand canonical ensemble

  3. COption (c)

    canonical ensemble

  4. DOption (d)

    Gaussian ensemble

View answer & explanation

Correct answer

Answer: (c)C.

canonical ensemble

Why this answer is correct

Concept

  • Statistical ensembles are collections of a large number of independent systems used to describe the macroscopic properties of a system under specific constraints.
  • The type of ensemble used depends on whether the system is isolated, in thermal contact, or in contact with a particle reservoir.
<br>

Explanation

In statistical mechanics, the three fundamental types of ensembles are categorized based on their exchange capabilities:

  • Microcanonical Ensemble: This represents an isolated system. There is no exchange of energy or particles with the surroundings. The energy (EE), volume (VV), and number of particles (NN) are constant.
  • Canonical Ensemble: This represents a system in thermal contact with a large heat reservoir. The system can exchange energy with the reservoir to maintain a constant temperature (TT). However, the number of particles (NN) and volume (VV) remain fixed.
  • Grand Canonical Ensemble: This represents a system in contact with both a heat reservoir and a particle reservoir. It can exchange both energy and particles, keeping temperature (TT) and chemical potential (μ\mu) constant.

The question specifies a system in contact with a heat reservoir. Thermal contact with a reservoir implies that the system is kept at a constant temperature by exchanging heat (energy), while the particle number remains constant. This configuration perfectly matches the definition of a canonical ensemble.

Hence, a system in contact with a heat reservoir can most suitably be represented by a canonical ensemble.

Question 64

The grand Canonical ensemble partition function is given by :

  1. AOption (a)

    Z=∑ie−e/kT\mathrm{Z}=\sum_{\mathrm{i}} \mathrm{e}^{-\mathrm{e} / \mathrm{kT}}

  2. BOption (b)

    Z=∑i,Ne−ϵi/kTeNμ/kT\mathrm{Z}=\sum_{\mathrm{i}, \mathrm{N}} \mathrm{e}^{-\epsilon \mathrm{i} / \mathrm{kT}} \mathrm{e}^{\mathrm{N} \mu / \mathrm{kT}}

  3. COption (c)

    Z=∑i,Ne−ϵi/kTe−Nμ/kTZ=\sum_{i, N} e^{-\epsilon i / k T} e^{-N \mu / k T}

  4. DOption (d)

    Z=∑ie∈i/kT\mathrm{Z}=\sum_{\mathrm{i}} \mathrm{e}^{\in \mathrm{i} / \mathrm{kT}}

View answer & explanation

Correct answer

Answer: (b)B.

Z=∑i,Ne−ϵi/kTeNμ/kT\mathrm{Z}=\sum_{\mathrm{i}, \mathrm{N}} \mathrm{e}^{-\epsilon \mathrm{i} / \mathrm{kT}} \mathrm{e}^{\mathrm{N} \mu / \mathrm{kT}}

Why this answer is correct

Concept

  • In statistical mechanics, the grand canonical ensemble is used to describe a system in thermal and chemical equilibrium with a reservoir, meaning it can exchange both energy and particles.
  • The state of the system is characterized by the number of particles NN and the energy states ϵi\epsilon_i associated with that number of particles.
  • The grand canonical partition function (Z\mathcal{Z}) is the sum over all possible states (all possible ii) and all possible particle numbers (all possible NN).
<br>

Formula Used

The probability of a system being in a state with NN particles and energy ϵi\epsilon_i is proportional to the Gibbs factor:

P(i,N)∝e−β(ϵi−μN)P(i, N) \propto e^{-\beta(\epsilon_i - \mu N)}

Where:

  • β=1kT\beta = \frac{1}{kT} (where kk is Boltzmann constant and TT is temperature)
  • μ\mu is the chemical potential.

The grand canonical partition function ZZ is given by:

Z=∑N∑ie−β(ϵi−μN)Z = \sum_{N} \sum_{i} e^{-\beta(\epsilon_i - \mu N)}

<br>

Calculation

Starting from the general form of the grand canonical partition function:

Z=∑i,Ne−(ϵi−μN)kTZ = \sum_{i, N} e^{-\frac{(\epsilon_i - \mu N)}{kT}}

We can distribute the term −1kT-\frac{1}{kT} inside the parentheses:

Z=∑i,Ne−ϵi/kT+μN/kTZ = \sum_{i, N} e^{-\epsilon_i / kT + \mu N / kT}

Using the properties of exponents (ea+b=ea⋅ebe^{a+b} = e^a \cdot e^b), we get:

Z=∑i,Ne−ϵi/kTeNμ/kTZ = \sum_{i, N} e^{-\epsilon_i / kT} e^{N \mu / kT}

Comparing this expression with the given options, we find it matches the form where the energy term has a negative exponent and the chemical potential term has a positive exponent.

Hence, the grand Canonical ensemble partition function is Z=∑i,Ne−ϵi/kTeNμ/kTZ = \sum_{i, N} e^{-\epsilon_i / kT} e^{N \mu / kT}.

Question 65

Relation between partition function and entropy is :

  1. AOption (a)

    S=K[log⁡z+βlog⁡β]S=K[\log z+\beta \log \beta]

  2. BOption (b)

    S=K[log⁡z+βT2]\mathrm{S}=\mathrm{K}\left[\log \mathrm{z}+\beta \mathrm{T}^{2}\right]

  3. COption (c)

    S=K[log⁡z−β∂(log⁡z)∂β]S=K\left[\log z-\beta \frac{\partial(\log z)}{\partial \beta}\right]

  4. DOption (d)

    S=K[log⁡z−βT]\mathrm{S}=\mathrm{K}[\log \mathrm{z}-\beta \mathrm{T}]

View answer & explanation

Correct answer

Answer: (c)C.

S=K[log⁡z−β∂(log⁡z)∂β]S=K\left[\log z-\beta \frac{\partial(\log z)}{\partial \beta}\right]

Why this answer is correct

Concept

  • The partition function (zz) is the central quantity in statistical mechanics that encodes the statistical properties of a system in thermodynamic equilibrium.
  • Thermodynamic variables like internal energy (UU), Helmholtz free energy (FF), and entropy (SS) can be expressed in terms of the partition function.
  • Entropy is related to the partition function through the bridge of Helmholtz free energy and internal energy.
<br>

Formula Used

Relation for Helmholtz Free Energy (FF):

F=−kTlog⁡zF = -kT \log z

Relation for Internal Energy (UU):

U=−∂(log⁡z)∂βU = -\frac{\partial (\log z)}{\partial \beta}

Relation between FF, UU, and SS:

F=U−TS  ⟹  S=U−FTF = U - TS \implies S = \frac{U - F}{T}

Where β=1kT\beta = \frac{1}{kT} and KK (or kk) is the Boltzmann constant.

<br>

Calculation

We know the entropy SS is given by:

S=UT−FTS = \frac{U}{T} - \frac{F}{T}

Substitute the expressions for UU and FF in terms of the partition function zz:

S=1T[−∂(log⁡z)∂β]−−KTlog⁡zTS = \frac{1}{T} \left[ -\frac{\partial (\log z)}{\partial \beta} \right] - \frac{-KT \log z}{T}

Simplifying the second term:

S=−1T∂(log⁡z)∂β+Klog⁡zS = -\frac{1}{T} \frac{\partial (\log z)}{\partial \beta} + K \log z

Using the relation β=1KT\beta = \frac{1}{KT}, we can write 1T=Kβ\frac{1}{T} = K\beta:

S=−(Kβ)∂(log⁡z)∂β+Klog⁡zS = -(K\beta) \frac{\partial (\log z)}{\partial \beta} + K \log z

Factoring out KK:

S=K[log⁡z−β∂(log⁡z)∂β]S = K \left[ \log z - \beta \frac{\partial (\log z)}{\partial \beta} \right]

Hence, the relation between the partition function and entropy is S=K[log⁡z−β∂(log⁡z)∂β]S = K \left[ \log z - \beta \frac{\partial (\log z)}{\partial \beta} \right].

Question 66

In a photostat machine drum has coating of :

  1. AOption (a)

    Selenium

  2. BOption (b)

    Silicon

  3. COption (c)

    Cadmium

  4. DOption (d)

    Zircon

View answer & explanation

Correct answer

Answer: (a)A.

Selenium

Why this answer is correct

Concept

  • Photostat machines (photocopiers) operate based on the process of xerography, which utilizes electrostatics and photoconductivity.
  • A photoconductor is a material that acts as an insulator in the dark but becomes an electrical conductor when exposed to light.
  • The drum is the most critical part of the machine, responsible for forming the electrostatic image.
<br>

Key Properties

  • The coating material must be a semiconductor with high photosensitivity.
  • It must be able to retain a static charge in the dark.
  • Selenium is a widely used photoconductive material in the xerography process due to its excellent charge-holding capabilities in the dark and high conductivity when illuminated.
<br>

Explanation

In a photostat machine, the drum is typically made of an aluminum cylinder coated with a thin layer of Selenium. The process involves the following steps:

  1. The selenium-coated drum is given a uniform positive charge in the dark.
  2. Light is reflected from the document being copied onto the drum. The bright areas of the document reflect light, hitting the selenium coating.
  3. In the illuminated areas, selenium becomes conductive, allowing the positive charge to be neutralized by electrons from the underlying aluminum base.
  4. In the dark areas (where there is text or an image), the selenium remains an insulator, and the positive charge stays on the surface.
  5. This creates a latent image of charge, which then attracts negatively charged toner particles to develop the copy.

Hence, the drum in a photostat machine has a coating of Selenium.

Question 67

The constant α\alpha of a transistor is 0.95 . The change in the collector current corresponding to a change of 0.4 mA in the base current in CE configuration is :

  1. AOption (a)

    8.0 mA

  2. BOption (b)

    7.6 mA

  3. COption (c)

    7.2 mA

  4. DOption (d)

    6.8 mA

View answer & explanation

Correct answer

Answer: (b)B.

7.6 mA

Why this answer is correct

Concept

  • The current gain in common-base (CB) configuration is denoted by α\alpha, which represents the ratio of the change in collector current to the change in emitter current.
  • The current gain in common-emitter (CE) configuration is denoted by β\beta, which represents the ratio of the change in collector current to the change in base current.
  • There is a specific mathematical relationship between α\alpha and β\beta based on the transistor current equations.
<br>

Formula Used

Relationship between α\alpha and β\beta:

β=α1−α\beta = \frac{\alpha}{1 - \alpha}

Change in collector current in CE configuration:

ΔIC=β×ΔIB\Delta I_C = \beta \times \Delta I_B

<br>

Calculation

Given:

α=0.95\alpha = 0.95

ΔIB=0.4,mA\Delta I_B = 0.4,\text{mA}

First, find the common-emitter current gain (β\beta):

β=0.951−0.95\beta = \frac{0.95}{1 - 0.95}

β=0.950.05\beta = \frac{0.95}{0.05}

β=19\beta = 19

Now, calculate the change in collector current (ΔIC\Delta I_C):

ΔIC=β×ΔIB\Delta I_C = \beta \times \Delta I_B

ΔIC=19×0.4,mA\Delta I_C = 19 \times 0.4,\text{mA}

ΔIC=7.6,mA\Delta I_C = 7.6,\text{mA}

Hence, the change in the collector current is 7.6,mA7.6,\text{mA}.

Question 68

Unijunction transistor is used for :

  1. AOption (a)

    generating saw-tooth wave

  2. BOption (b)

    amplification of input signal

  3. COption (c)

    rectification

  4. DOption (d)

    voltage regulation

View answer & explanation

Correct answer

Answer: (a)A.

generating saw-tooth wave

Why this answer is correct

Concept

  • A Unijunction Transistor (UJT) is a three-terminal semiconductor switching device that has only one junction.
  • The most important characteristic of a UJT is its "negative resistance" property, which means that the emitter voltage decreases as the emitter current increases within a certain range.
  • Because of this property, the UJT is primarily used in timing circuits, trigger circuits, and pulse generators.
<br>

Explanation

The primary application of a Unijunction Transistor is as a relaxation oscillator. The working principle involves:

  • Charging a capacitor through a resistor connected to a DC supply.
  • When the voltage across the capacitor reaches the "peak point voltage" of the UJT, the UJT turns on (enters the negative resistance region).
  • The capacitor then discharges rapidly through the UJT.
  • Once the capacitor voltage drops to the "valley point voltage," the UJT turns off, and the cycle repeats.

This periodic charging and discharging cycle produces a specific non-sinusoidal waveform known as a saw-tooth wave (or ramp wave).

Comparing other applications:

  • Amplification: Bipolar Junction Transistors (BJT) or Field Effect Transistors (FET) are used for this, not UJTs.
  • Rectification: Diodes are used to convert AC to DC.
  • Voltage Regulation: Zener diodes or specialized IC regulators are used for this purpose.
<br>

Conclusion

The Unijunction transistor is used for generating a saw-tooth wave.

Hence, the correct application is generating saw-tooth wave.

Question 69

Which of the following statements is wrong for a LED ?

  1. AOption (a)

    It emits light when forward biased.

  2. BOption (b)

    Light emission by it is due to recombination of electrons & holes.

  3. COption (c)

    It, using GaAs material, gives out infrared radiations.

  4. DOption (d)

    In it, p-type material is kept upper most so that holes can fall down easily to combine with electrons.

View answer & explanation

Correct answer

Answer: (d)D.

In it, p-type material is kept upper most so that holes can fall down easily to combine with electrons.

Why this answer is correct

Concept

  • A Light Emitting Diode (LED) is a p-n junction diode that emits light when it is forward-biased.
  • The emission of light is a result of the radiative recombination of majority charge carriers (electrons from the n-side and holes from the p-side) at the junction.
  • The energy of the emitted photon depends on the bandgap of the semiconductor material used.
<br>

Explanation

  • Forward Bias: LEDs only emit light when connected in forward bias. In this state, electrons and holes are pushed towards the junction to recombine.
  • Recombination: When a free electron from the conduction band falls into a hole in the valence band, it loses energy. In direct bandgap semiconductors like GaAs, this energy is released as light (photons).
  • GaAs Material: Gallium Arsenide (GaAs) has a bandgap that corresponds to the energy of infrared radiation. Therefore, GaAs-based LEDs are used for infrared emission.
  • Construction: In an LED, one of the layers (usually the p-layer) is made very thin and kept on the surface. This is done to ensure that the photons generated at the junction can escape the semiconductor material easily without being reabsorbed.
<br>

Reasoning

Let's evaluate the statements:

  • Statement A is correct because forward biasing is the standard operating condition for an LED.
  • Statement B is correct because the physical mechanism of light production in a semiconductor is electron-hole recombination.
  • Statement C is correct because GaAs is a well-known material used specifically for making infrared LEDs.
  • Statement D is wrong. While the p-type material is often kept uppermost (at the surface), it is not because "holes fall down easily to combine with electrons" due to gravity. At the atomic and subatomic level, the effect of gravity is negligible compared to electrical forces. The p-layer is kept thin and at the top primarily to allow the generated light to reach the surface with minimal internal absorption.

Hence, the wrong statement is that the p-type material is kept uppermost so that holes can fall down easily to combine with electrons.

Question 70

For obtaining maximum power from a solar cell, it should be operated on :

  1. AOption (a)

    The knee of the V-I characteristics

  2. BOption (b)

    Horizontal part of the curve

  3. COption (c)

    Falling portion of the V-I characteristics

  4. DOption (d)

    Any part of the V-I characteristics as power does not depend on it

View answer & explanation

Correct answer

Answer: (a)A.

The knee of the V-I characteristics

Why this answer is correct

Concept

  • A solar cell is a p-n junction diode that converts light energy directly into electrical energy through the photovoltaic effect.
  • The performance of a solar cell is characterized by its Current-Voltage (V-I) relationship, usually plotted in the fourth quadrant because the cell supplies power.
  • The power delivered by the solar cell is given by the product of voltage and current: P=V×IP = V \times I.
<br>

Explanation

The V-I characteristic curve of a solar cell shows the relationship between the voltage across the cell and the current it produces:

  • Short-circuit Current (IscI_{sc}): This is the maximum current when the voltage is zero. At this point, Power (P=V×Isc=0P = V \times I_{sc} = 0) is zero.
  • Open-circuit Voltage (VocV_{oc}): This is the maximum voltage when the current is zero. At this point, Power (P=Voc×0=0P = V_{oc} \times 0 = 0) is zero.
  • Maximum Power Point (MPP): To get the maximum power, we look for the point on the V-I curve where the area of the rectangle formed by VV and II is maximized.
  • This specific point occurs at the knee of the V-I characteristic curve. This is the transition region between the constant current (horizontal) part and the falling voltage (vertical) part of the curve.

Operating the solar cell at the knee ensures that the product of voltage and current is at its peak, providing the highest possible efficiency.

Hence, for obtaining maximum power from a solar cell, it should be operated on the knee of the V-I characteristics.

Question 71

On increasing the temperature T of an intrinsic semiconductor its conductivity :

  1. AOption (a)

    increases linearly with T

  2. BOption (b)

    increases exponentially with T

  3. COption (c)

    decreases linearly with T

  4. DOption (d)

    decreases exponentially with T

View answer & explanation

Correct answer

Answer: (b)B.

increases exponentially with T

Why this answer is correct

Concept

  • An intrinsic semiconductor is a pure semiconductor without any significant dopant species present.
  • At absolute zero temperature, an intrinsic semiconductor acts as an insulator because all electrons are bound in covalent bonds.
  • As the temperature increases, thermal energy is supplied to the atoms, causing covalent bonds to break and releasing free electrons and holes.
  • In semiconductors, the increase in the number of charge carriers with temperature is the dominant factor determining conductivity.
<br>

Formula Used

The concentration of intrinsic charge carriers nin_i is given by:

ni=AT3/2e−Eg2kTn_i = A T^{3/2} e^{-\frac{E_g}{2kT}}

Where:

  • EgE_g is the energy band gap.
  • kk is the Boltzmann constant.
  • TT is the absolute temperature.

The conductivity σ\sigma of a semiconductor is given by:

σ=nie(μe+μh)\sigma = n_i e (\mu_e + \mu_h)

<br>

Explanation

From the expressions above, we can see that the carrier concentration nin_i depends on temperature TT in two ways: a power law factor T3/2T^{3/2} and an exponential factor e−Eg2kTe^{-\frac{E_g}{2kT}}.

In the temperature range of interest, the exponential term e−Eg2kTe^{-\frac{E_g}{2kT}} changes much more rapidly than the T3/2T^{3/2} term. As the temperature TT increases, the exponent −Eg2kT-\frac{E_g}{2kT} becomes less negative, leading to a rapid (exponential) increase in the number of charge carriers.

While the mobility (μ\mu) of the carriers usually decreases slightly with temperature due to increased lattice scattering, the exponential increase in the carrier concentration nin_i far outweighs the decrease in mobility.

Therefore, the overall conductivity σ\sigma of an intrinsic semiconductor increases exponentially with an increase in temperature.

Hence, on increasing the temperature T of an intrinsic semiconductor, its conductivity increases exponentially with T.

Question 72

In p-n junction, the barrier potential offers opposition to:

  1. AOption (a)

    only free electrons in n-region

  2. BOption (b)

    only holes in p-region

  3. COption (c)

    majority carriers in both regions

  4. DOption (d)

    minority carriers in both regions

View answer & explanation

Correct answer

Answer: (c)C.

majority carriers in both regions

Why this answer is correct

Concept

  • When a p−np-n junction is formed, the diffusion of majority charge carriers (electrons from the nn-region to the pp-region and holes from the pp-region to the nn-region) creates a depletion layer.
  • This depletion layer contains immobile positive ions on the nn-side and immobile negative ions on the pp-side.
  • This separation of charges creates an internal electric field directed from the nn-region to the pp-region, which results in a barrier potential.
<br>

Explanation

The barrier potential established at the junction affects the movement of charge carriers in the following ways:

  • Majority Carriers: In the nn-region, the majority carriers are free electrons. To move into the pp-region, they must move against the internal electric field (which points from nn to pp). Similarly, holes (majority carriers in the pp-region) must move against the potential gradient to enter the nn-region. Thus, the barrier potential offers opposition to the diffusion of majority carriers from both sides.
  • Minority Carriers: Minority carriers (electrons in the pp-region and holes in the nn-region) are actually assisted by the internal electric field. The field helps them drift across the junction. Therefore, the barrier potential does not oppose minority carriers.

Because the barrier potential acts as a hurdle for the majority charge carriers attempting to cross the junction, it regulates the current flow in the diode.

<br>

Hence, the barrier potential offers opposition to majority carriers in both regions.

Question 73

A LASER diode is made of semiconductor of :

  1. AOption (a)

    Intrinsic type

  2. BOption (b)

    Direct band gap type

  3. COption (c)

    Indirect band gap type

  4. DOption (d)

    Low doped type

View answer & explanation

Correct answer

Answer: (b)B.

Direct band gap type

Why this answer is correct

Concept

  • The operation of a LASER (Light Amplification by Stimulated Emission of Radiation) diode relies on the efficient radiative recombination of electron-hole pairs.
  • Semiconductors are classified into direct band gap and indirect band gap materials based on the alignment of the conduction band minimum and the valence band maximum in momentum space.
  • Radiative recombination (photon emission) is highly probable in direct band gap materials.
<br>

Explanation

In a direct band gap semiconductor, the maximum energy level of the valence band and the minimum energy level of the conduction band occur at the same value of momentum (crystal wave vector kk). When an electron from the conduction band recombines with a hole in the valence band, the energy is released directly as a photon. This makes them ideal for light-emitting applications like LEDs and LASER diodes.

In an indirect band gap semiconductor (like Silicon or Germanium), the band extrema do not align in momentum space. For an electron to transition, it must change its momentum by interacting with a lattice vibration (phonon). This process is much slower and primarily results in the generation of heat rather than light, making them unsuitable for LASERs.

Furthermore, LASER diodes require high carrier concentrations to achieve population inversion, which typically necessitates heavy doping rather than low doping or intrinsic states.

Hence, a LASER diode is made of a direct band gap type semiconductor.

Question 74

In a GaAs P semiconductor, the band gap Eg=1.9eV\mathrm{E}_{\mathrm{g}}=1.9 \mathrm{eV}. The emission wavelength is given by :

  1. AOption (a)

    653.3 nm

  2. BOption (b)

    635.3 nm

  3. COption (c)

    563.3 nm

  4. DOption (d)

    365.3 nm

View answer & explanation

Correct answer

Answer: (a)A.

653.3 nm

Why this answer is correct

Concept

  • In a semiconductor, the emission of light occurs when an electron from the conduction band recombines with a hole in the valence band.
  • The energy of the emitted photon is approximately equal to the band gap energy (EgE_g) of the semiconductor material.
  • The wavelength of the emitted light is inversely proportional to the band gap energy.
<br>

Formula Used

The relationship between band gap energy and wavelength is given by:

Eg=hcλE_g = \frac{hc}{\lambda}

Where:

  • hh is Planck's constant.
  • cc is the speed of light.
  • λ\lambda is the wavelength of emitted light.

A convenient version of this formula for energy in electron-volts (eV) and wavelength in nanometers (nm) is:

λ (in nm)≈1242Eg (in eV)\lambda \text{ (in nm)} \approx \frac{1242}{E_g \text{ (in eV)}}

<br>

Calculation

Given:

Band gap energy, Eg=1.9 eVE_g = 1.9 \text{ eV}

Using the formula:

λ=12421.9 nm\lambda = \frac{1242}{1.9} \text{ nm}

Performing the division:

λ≈653.68 nm\lambda \approx 653.68 \text{ nm}

Comparing this result with the given options, the closest value is 653.3 nm653.3 \text{ nm}.

Hence, the emission wavelength is 653.3 nm.

Question 75

Reduced value of A+A‾B\mathrm{A}+\overline{\mathrm{A}} \mathrm{B} using laws of Boolean algebra is :

  1. AOption (a)

    A+AB\mathrm{A}+\mathrm{AB}

  2. BOption (b)

    A‾+AB\overline{\mathrm{A}}+\mathrm{AB}

  3. COption (c)

    A+B\mathrm{A}+\mathrm{B}

  4. DOption (d)

    A‾+B‾\overline{\mathrm{A}}+\overline{\mathrm{B}}

View answer & explanation

Correct answer

Answer: (c)C.

A+B\mathrm{A}+\mathrm{B}

Why this answer is correct

The correct answer is A + B.

Key Points

  • Given Boolean expression is A + A̅B.
  • This can be simplified using the Absorption/Redundancy Law:

A + A̅B = A + B

  • Thus the reduced value is A + B.

Additional Information

  • Step-by-Step Simplification:
  • Expression:

A + A̅B

  • Using distributive law:

A + A̅B = (A + A̅)(A + B)

  • Using Complement Law:

A + A̅ = 1

  • So,

(A + A̅)(A + B) = 1(A + B)

  • = A + B
  • Verification by Truth Table:
  • If A = 1, expression becomes 1 regardless of B.
  • If A = 0, expression becomes B.
  • Thus overall output matches A + B.
  • Important Boolean Laws Used:
  • Distributive Law: X + YZ = (X + Y)(X + Z)
  • Complement Law: A + A̅ = 1
  • Identity Law: 1.X = X

Question 76

Read the statements about RAM.

(A) RAM is used as primary memory.

(B) These are semiconductor memory.

(C) These are non-volatile.

(D) Can not be erased.

 

Correct statement is :

  1. AOption (a)

    A

  2. BOption (b)

    A and B.

  3. COption (c)

    C and D

  4. DOption (d)

    A and D

View answer & explanation

Correct answer

Answer: (b)B.

A and B.

Why this answer is correct

The correct answer is A and B.

Key Points

  • Statement (A): RAM is used as primary memory ✔ Correct
  • Statement (B): RAM is a semiconductor memory ✔ Correct
  • Statement (C): RAM is volatile, not non-volatile ✘ Incorrect
  • Statement (D): RAM can be erased and rewritten ✘ Incorrect
  • Therefore, correct statements are A and B.

Additional Information

  • Explanation of RAM:
  • RAM (Random Access Memory) stores data and instructions temporarily during execution.
  • It is called primary/main memory.
  • Built using semiconductor technology such as DRAM and SRAM.
  • It is volatile because data is lost when power is off.
  • It is read/write memory, so data can be erased and modified.
  • Correction in Options:
  • The correct combination is A and B, but it is not explicitly listed in options.
  • Hence the given options appear flawed.
  • Important Points:
  • RAM = Primary Memory ✔
  • RAM = Semiconductor Memory ✔
  • RAM = Volatile ✘ (not non-volatile)
  • RAM = Erasable and rewritable ✘ for statement D

Question 77

The 8085 microprocessor contains :

  1. AOption (a)

    Five flags

  2. BOption (b)

    Nine flags

  3. COption (c)

    Ten flags

  4. DOption (d)

    Eight flags

View answer & explanation

Correct answer

Answer: (a)A.

Five flags

Why this answer is correct

The correct answer is Five flags.

Key Points

  • The 8085 microprocessor contains a flag register with five active flags.
  • These flags indicate the status of arithmetic and logical operations.
  • The five flags are:
  • Sign Flag (S)
  • Zero Flag (Z)
  • Auxiliary Carry Flag (AC)
  • Parity Flag (P)
  • Carry Flag (CY)
  • Hence, 8085 contains five flags.

Additional Information

  • Functions of Flags:
  • Sign Flag (S): Set if result is negative.
  • Zero Flag (Z): Set if result is zero.
  • Auxiliary Carry (AC): Indicates carry from lower nibble.
  • Parity Flag (P): Set for even parity.
  • Carry Flag (CY): Set when carry/borrow occurs.
  • Important Points:
  • 8085 flag register is 8-bit, but only 5 bits are used as flags.
  • Flags help in decision-making and conditional branching.

Question 78

For the complete specification of the energy state of an electron we require :

  1. AOption (a)

    only one quantum number n

  2. BOption (b)

    Two quantum numbers \(\mathrm{n~ &~ l}\)

  3. COption (c)

    Three quantum numbers \(\mathrm{n, ~l ~& ~j}\)

  4. DOption (d)

    Four quantum numbers \(\mathrm{n,~l,~j ~& ~m_j}\)

View answer & explanation

Correct answer

Answer: (d)D.

Four quantum numbers \(\mathrm{n,~l,~j ~& ~m_j}\)

Why this answer is correct

Concept

  • Quantum numbers are a set of numerical values used to describe the unique quantum state of an electron in an atom.
  • They define the energy, position, orientation, and spin properties of the electron.
  • In the vector atom model, which includes spin-orbit interaction (fine structure), the total angular momentum is a key factor in defining the energy state.
<br>

Theory

The complete specification of an electron's state in an atom, particularly when considering relativistic effects and spin-orbit coupling, requires four quantum numbers:

  • Principal quantum number (nn): Defines the main energy shell and the size of the orbital.
  • Orbital angular momentum quantum number (ll): Defines the shape of the orbital (subshell).
  • Total angular momentum quantum number (jj): Represents the vector sum of orbital angular momentum (ll) and spin angular momentum (ss), where j=l±sj = l \pm s.
  • Magnetic total angular momentum quantum number (mjm_j): Defines the orientation of the total angular momentum vector in space relative to an external magnetic field.
<br>

Explanation

In classical quantum mechanics (Schrödinger's non-relativistic model), three quantum numbers (n,l,mln, l, m_l) along with a spin quantum number (ss) are used. However, in the context of "complete specification" including the energy state splitting due to spin-orbit interaction, the jj-coupling scheme is used.

The energy state of an electron is refined by the interaction between its spin and orbital motion. This leads to the fine structure of spectral lines. To identify a specific state within this fine structure, one must know the total angular momentum jj and its projection mjm_j.

Without all four parameters, the description of the electron's state remains incomplete, as multiple electrons could otherwise share the same partial set of quantum numbers within the same atom.

Hence, for the complete specification of the energy state of an electron, four quantum numbers n,l,j and mjn, l, j \text{ and } m_j are required.

Question 79

The coherence length of sodium D2D_{2} line is 2.5 cm , the coherence time is :

  1. AOption (a)

    1.2×10−10 s1.2 \times 10^{-10} \mathrm{~s}

  2. BOption (b)

    1.3×10−11 s1.3 \times 10^{-11} \mathrm{~s}

  3. COption (c)

    8.3×10−11 s8.3 \times 10^{-11} \mathrm{~s}

  4. DOption (d)

    8.3×10−12 s8.3 \times 10^{-12} \mathrm{~s}

View answer & explanation

Correct answer

Answer: (c)C.

8.3×10−11 s8.3 \times 10^{-11} \mathrm{~s}

Why this answer is correct

Concept

  • Coherence length is the propagation distance over which a coherent wave maintains a predictable phase.
  • Coherence time is the time interval during which the phase of the wave remains stable or predictable.
  • Both quantities are related by the speed of light, as the coherence length is essentially the distance light travels during the coherence time.
<br>

Formula Used

The relationship between coherence length and coherence time is given by:

τc=Lcc\tau_c = \frac{L_c}{c}

Where:

  • LcL_c is the coherence length.
  • τc\tau_c is the coherence time.
  • cc is the speed of light in vacuum (c≈3×108 m/sc \approx 3 \times 10^8 \text{ m/s}).
<br>

Calculation

Given:

Lc=2.5 cm=2.5×10−2 mL_c = 2.5 \text{ cm} = 2.5 \times 10^{-2} \text{ m}

c=3×108 m/sc = 3 \times 10^8 \text{ m/s}

Using the formula for coherence time:

τc=2.5×10−23×108\tau_c = \frac{2.5 \times 10^{-2}}{3 \times 10^8}

τc=2.53×10−10 s\tau_c = \frac{2.5}{3} \times 10^{-10} \text{ s}

τc≈0.833×10−10 s\tau_c \approx 0.833 \times 10^{-10} \text{ s}

τc=8.33×10−11 s\tau_c = 8.33 \times 10^{-11} \text{ s}

Rounding to the appropriate significant figures as per the provided options:

τc≈8.3×10−11 s\tau_c \approx 8.3 \times 10^{-11} \text{ s}

Hence, the coherence time is 8.3×10−11 s8.3 \times 10^{-11} \text{ s}.

Question 80

Space quantisation was first of all demonstrated by :

  1. AOption (a)

    Stern and Gerlach experiment

  2. BOption (b)

    Davison and Germer experiment

  3. COption (c)

    Compton effect

  4. DOption (d)

    Zeeman effect

View answer & explanation

Correct answer

Answer: (a)A.

Stern and Gerlach experiment

Why this answer is correct

Concept

  • Space Quantization: This principle states that the orientation of the angular momentum of an atom (or any quantum system) is restricted to certain discrete directions in space relative to an external field.
  • Classical Prediction: In classical mechanics, the magnetic moment vector of an atom could point in any arbitrary direction, meaning the projection of the magnetic moment along a field axis would be continuous.
<br>

Explanation

The Stern-Gerlach experiment, performed in 1922, was the first experiment to provide direct evidence for the concept of space quantization.

In the experimental setup, a beam of neutral silver atoms was passed through a highly inhomogeneous (non-uniform) magnetic field. Because silver atoms possess a magnetic dipole moment, the non-uniform field exerts a force on them that depends on the orientation of their magnetic moment relative to the field gradient.

According to classical physics, the magnetic moments of the atoms should be randomly oriented, which would result in the beam spreading out into a continuous, blurred vertical line on the detection plate. However, Stern and Gerlach observed that the beam split into two distinct and separate spots.

This discrete splitting confirmed that the magnetic moments (and thus the angular momentum) of the atoms can only take specific, quantized orientations in space. This result provided a clear, direct demonstration of space quantization, which is a fundamental departure from classical mechanics.

Hence, space quantisation was first of all demonstrated by the Stern and Gerlach experiment.

Question 81

The spin angular momentum of electron is :

  1. AOption (a)

    0

  2. BOption (b)

    ℏ/2\hbar / 2

  3. COption (c)

    ℏ\hbar

  4. DOption (d)

    None of these

View answer & explanation

Correct answer

Answer: (b)B.

ℏ/2\hbar / 2

Why this answer is correct

Concept

  • Spin angular momentum is an intrinsic property of elementary particles such as electrons.
  • Electrons are fermions with a spin quantum number s=1/2s = 1/2.
  • The spin angular momentum is quantized, and its component along a measured axis is typically used to define its value in common problems.
<br>

Formula Used

The component of spin angular momentum along an axis (e.g., the z-axis) is given by:

Sz=msℏS_z = m_s \hbar

Where:

  • ms=±1/2m_s = \pm 1/2 (Spin magnetic quantum number)
  • ℏ=h2π\hbar = \frac{h}{2\pi} (Reduced Planck's constant)
<br>

Calculation

For an electron, the spin quantum number is:

s=1/2s = 1/2

The possible values for the spin magnetic quantum number are:

ms=+12m_s = +\frac{1}{2} and ms=−12m_s = -\frac{1}{2}

The value of the spin angular momentum component is:

Sz=±12ℏS_z = \pm \frac{1}{2} \hbar

In the context of the given options, the magnitude of this spin angular momentum component is represented as:

S=ℏ2S = \frac{\hbar}{2}

Hence, the spin angular momentum of the electron is ℏ/2\hbar / 2.

Question 82

For the term 3P2{ }^{3} \mathrm{P}_{2} the values of S,L\mathrm{S}, \mathrm{L} and J are :

  1. AOption (a)

    1, 1 and 2

  2. BOption (b)

    2, 1 and 1

  3. COption (c)

    1, 2 and 1

  4. DOption (d)

    3, 1 and 2

View answer & explanation

Correct answer

Answer: (a)A.

1, 1 and 2

Why this answer is correct

Concept

  • The electronic state of an atom is often described using a term symbol in the form 2S+1LJ{ }^{2S+1}L_{J}.
  • 2S+12S+1 is the spin multiplicity, where SS is the total spin quantum number.
  • LL is the total orbital angular momentum quantum number. It is represented by letter symbols: S for L=0L=0, P for L=1L=1, D for L=2L=2, and F for L=3L=3.
  • JJ is the total angular momentum quantum number, which is indicated as a subscript.
<br>

Formula Used

Spin multiplicity relation:

Multiplicity=2S+1\text{Multiplicity} = 2S + 1

Orbital angular momentum mapping:

L=0→SL = 0 \rightarrow \text{S}

L=1→PL = 1 \rightarrow \text{P}

L=2→DL = 2 \rightarrow \text{D}

<br>

Calculation

Given the term: 3P2{ }^{3}\text{P}_{2}

  1. To find S:

Comparing with the standard term symbol 2S+1LJ{ }^{2S+1}L_{J}, the spin multiplicity is 3.

2S+1=32S + 1 = 3

2S=3−12S = 3 - 1

2S=22S = 2

S=1S = 1

  1. To find L:

The letter symbol used in the term is 'P'.

For a P-state, the orbital angular momentum quantum number is L=1L = 1.

  1. To find J:

The total angular momentum quantum number JJ is given by the subscript in the term symbol.

From 3P2{ }^{3}\text{P}_{2}, the subscript is 2.

Thus, J=2J = 2.

Hence, the values of S, L, and J are 1, 1, and 2 respectively.

Question 83

The value of gg for 2S1/2{ }^{2} S_{1 / 2} ground state of sodium is :

  1. AOption (a)

    2/32 / 3

  2. BOption (b)

    2

  3. COption (c)

    4/34 / 3

  4. DOption (d)

    1

View answer & explanation

Correct answer

Answer: (b)B.

2

Why this answer is correct

Concept

  • The Lande g-factor (gJgJgJ

    ) is a dimensionless quantity that relates the magnetic moment of an atom to its total angular momentum quantum number.

  • For a given spectral term represented as 2S+1LJ2S+1LJ2S+1LJ

    , where SSS

    is the total spin quantum number, LLL

    is the total orbital angular momentum quantum number, and JJJ

    is the total angular momentum quantum number.

  • For an SSS

    state, the orbital angular momentum L=0L=0L=0

    .

<br>

Formula Used

The Lande g-factor is calculated using the formula:

gJ=1+J(J+1)+S(S+1)−L(L+1)2J(J+1)g_J = 1 + \frac{J(J+1) + S(S+1) - L(L+1)}{2J(J+1)}

<br>

Calculation

Given the ground state of sodium as 2S1/2{ }^{2} S_{1 / 2}:

Comparing this with the general term 2S+1LJ^{2S+1}L_J:

  1. From the multiplicity 2S+1=22S + 1 = 2:

⇒2S=1⇒S=1/2\Rightarrow 2S = 1 \Rightarrow S = 1/2

  1. From the term symbol SS:

L=0L = 0

  1. From the subscript:

J=1/2J = 1/2

Now, substitute these values into the Lande g-factor formula:

gJ=1+12(12+1)+12(12+1)−0(0+1)2×12(12+1)g_J = 1 + \frac{\frac{1}{2}(\frac{1}{2}+1) + \frac{1}{2}(\frac{1}{2}+1) - 0(0+1)}{2 \times \frac{1}{2}(\frac{1}{2}+1)}

gJ=1+34+34−02×34g_J = 1 + \frac{\frac{3}{4} + \frac{3}{4} - 0}{2 \times \frac{3}{4}}

gJ=1+2×342×34g_J = 1 + \frac{2 \times \frac{3}{4}}{2 \times \frac{3}{4}}

gJ=1+1g_J = 1 + 1

gJ=2g_J = 2

Hence, the value of gg for the 2S1/2{ }^{2} S_{1 / 2} state is 2.

Question 84

Doppler broadening of spectral line is given by :

  1. AOption (a)

    dλ=λc3kTm\mathrm{d} \lambda=\frac{\lambda}{\mathrm{c}} \sqrt{\frac{3 \mathrm{kT}}{\mathrm{m}}}

  2. BOption (b)

    dλ=λc3kTm\mathrm{d} \lambda=\lambda \mathrm{c} \sqrt{\frac{3 \mathrm{kT}}{\mathrm{m}}}

  3. COption (c)

    dλ=λckTm\mathrm{d} \lambda=\frac{\lambda}{\mathrm{c}} \sqrt{\frac{\mathrm{kT}}{\mathrm{m}}}

  4. DOption (d)

    dλ=λc2kTm\mathrm{d} \lambda=\frac{\lambda}{\mathrm{c}} \sqrt{\frac{2 \mathrm{kT}}{\mathrm{m}}}

View answer & explanation

Correct answer

Answer: (d)D.

dλ=λc2kTm\mathrm{d} \lambda=\frac{\lambda}{\mathrm{c}} \sqrt{\frac{2 \mathrm{kT}}{\mathrm{m}}}

Why this answer is correct

Concept

  • Doppler broadening is a phenomenon in physics and astronomy where spectral lines are broadened due to the Doppler effect caused by the distribution of velocities of atoms or molecules.
  • In a gas, particles move randomly in all directions due to thermal energy. This thermal motion results in some particles moving towards the observer and others moving away.
  • Particles moving toward the observer emit light that is blue-shifted (shorter wavelength), while those moving away emit light that is red-shifted (longer wavelength).
  • The distribution of these velocities follows the Maxwell-Boltzmann distribution, leading to a Gaussian profile of the spectral line.
<br>

Formula Used

The Doppler shift in wavelength is given by the relation:

dλλ=vc\frac{d\lambda}{\lambda} = \frac{v}{c}

Where:

  • dλd\lambda is the change in wavelength (broadening).
  • λ\lambda is the original wavelength.
  • vv is the velocity of the particle.
  • cc is the speed of light.

The most probable speed vv of a particle of mass mm at temperature TT in a gas is given by:

v=2kTmv = \sqrt{\frac{2kT}{m}}

Where kk is the Boltzmann constant.

<br>

Calculation

To find the characteristic Doppler broadening, we substitute the expression for the most probable thermal velocity into the Doppler shift formula:

dλ=λvcd\lambda = \lambda \frac{v}{c}

Substituting v=2kTmv = \sqrt{\frac{2kT}{m}}:

dλ=λ1c2kTmd\lambda = \lambda \frac{1}{c} \sqrt{\frac{2kT}{m}}

Rearranging the terms:

dλ=λc2kTmd\lambda = \frac{\lambda}{c} \sqrt{\frac{2kT}{m}}

This expression represents the standard Doppler width of the spectral line due to thermal motion.

Hence, the Doppler broadening of the spectral line is given by dλ=λc2kTmd \lambda=\frac{\lambda}{c} \sqrt{\frac{2kT}{m}}.

Question 85

The atoms in a state \({ }^{2} \mathrm{D}{5 / 2}\) are put in weak magnetic field. The single state \({ }^{2} \mathrm{D}{5 / 2}\) will split into :

  1. AOption (a)

    2 states

  2. BOption (b)

    4 states

  3. COption (c)

    6 states

  4. DOption (d)

    8 states

View answer & explanation

Correct answer

Answer: (c)C.

6 states

Why this answer is correct

Concept

  • The splitting of spectral lines of an atom in the presence of an external magnetic field is known as the Zeeman effect.
  • When an atom is placed in a weak magnetic field, a state with total angular momentum quantum number JJ splits into several sub-levels.
  • Each sub-level corresponds to a different value of the magnetic quantum number MJM_J, which ranges from −J-J to +J+J in steps of 1.
<br>

Formula Used

The number of states (or levels) into which a state with total angular momentum JJ splits is given by:

Number of states=2J+1\text{Number of states} = 2J + 1

<br>

Calculation

Given the state: 2D5/2{ }^{2} \mathrm{D}_{5 / 2}

The standard notation for an atomic term is 2S+1LJ{ }^{2S+1}L_{J}.

By comparing 2D5/2{ }^{2} \mathrm{D}_{5 / 2} with the standard notation:

J=5/2J = 5/2

Now, calculate the number of split states using the formula:

Number of states=2J+1\text{Number of states} = 2J + 1

⇒Number of states=2(52)+1\Rightarrow \text{Number of states} = 2 \left( \frac{5}{2} \right) + 1

⇒Number of states=5+1\Rightarrow \text{Number of states} = 5 + 1

⇒Number of states=6\Rightarrow \text{Number of states} = 6

The magnetic quantum numbers MJM_J for these states are −5/2,−3/2,−1/2,+1/2,+3/2,+5/2-5/2, -3/2, -1/2, +1/2, +3/2, +5/2.

Hence, the single state 2D5/2{ }^{2} \mathrm{D}_{5 / 2} will split into 6 states.

Question 86

The stokes line in the rotational Raman spectrum is produced when the quantum number J changes by :

  1. AOption (a)

    -2

  2. BOption (b)

    -1

  3. COption (c)

    +1

  4. DOption (d)

    +2

View answer & explanation

Correct answer

Answer: (d)D.

+2

Why this answer is correct

Concept

  • Rotational Raman spectroscopy involves the scattering of light by a rotating molecule, resulting in a change in the rotational energy state.
  • For a molecule to exhibit a rotational Raman spectrum, its polarizability must change as it rotates.
  • The selection rule for pure rotational Raman transitions is ΔJ=0,±2\Delta J = 0, \pm 2.
<br>

Formula Used

The change in the rotational quantum number is given by:

ΔJ=Jfinal−Jinitial\Delta J = J_{final} - J_{initial}

<br>

Explanation

In the rotational Raman spectrum, three types of lines are observed based on the selection rule ΔJ=0,±2\Delta J = 0, \pm 2:

  • Rayleigh Line: This corresponds to ΔJ=0\Delta J = 0. The molecule remains in the same rotational state, and the scattered photon has the same energy as the incident photon.
  • Stokes Lines: These lines occur when the incident photon transfers energy to the molecule, causing it to transition from a lower rotational energy level to a higher rotational energy level. For these transitions, the quantum number JJ increases, specifically by +2+2. Therefore, ΔJ=+2\Delta J = +2.
  • Anti-Stokes Lines: These lines occur when the molecule is already in an excited rotational state and transfers energy to the incident photon, transitioning to a lower rotational energy level. For these transitions, the quantum number JJ decreases, specifically by −2-2. Therefore, ΔJ=−2\Delta J = -2.

Since the question asks specifically for the Stokes line, the molecule moves to a higher rotational state, resulting in a change of JJ by +2+2.

Hence, the Stokes line in the rotational Raman spectrum is produced when the quantum number J changes by +2.

Question 87

The energy difference between two consecutive vibrational states of a molecule is about :

  1. AOption (a)

    5 eV

  2. BOption (b)

    1 eV

  3. COption (c)

    0.1 eV

  4. DOption (d)

    0.01 eV

View answer & explanation

Correct answer

Answer: (c)C.

0.1 eV

Why this answer is correct

Concept

  • Molecular energy levels are quantized and divided into electronic, vibrational, and rotational states.
  • Vibrational energy levels correspond to the oscillations of atoms within a molecule.
  • The energy spacing between vibrational levels is much smaller than electronic transitions (UV-visible) but larger than rotational transitions (microwave).
  • Vibrational transitions typically occur in the infrared (IR) region of the electromagnetic spectrum.
<br>

Formula Used

Energy of a transition:

ΔE=hν\Delta E = h \nu

Where:

  • hh is Planck's constant (≈6.626×10−34,J⋅s\approx 6.626 \times 10^{-34},J\cdot s).
  • ν\nu is the frequency of vibration.
<br>

Calculation

Typical molecular vibrational frequencies (ν\nu) lie in the range of 101310^{13} to 1014,Hz10^{14},Hz.

Taking a typical frequency of 2.4×1013,Hz2.4 \times 10^{13},Hz:

ΔE=hν\Delta E = h\nu

ΔE=(6.63×10−34,J⋅s)×(2.4×1013,Hz)\Delta E = (6.63 \times 10^{-34},J\cdot s) \times (2.4 \times 10^{13},Hz)

ΔE≈1.6×10−20,J\Delta E \approx 1.6 \times 10^{-20},J

To convert Joules to electron-volts (eV), divide by the elementary charge (1.6×10−19,C1.6 \times 10^{-19},C):

ΔE,(in,eV)=1.6×10−20,J1.6×10−19,J/eV\Delta E,(in,eV) = \frac{1.6 \times 10^{-20},J}{1.6 \times 10^{-19},J/eV}

ΔE≈0.1,eV\Delta E \approx 0.1,eV

General energy scales for molecules:

  • Electronic energy difference: ≈1\approx 1 to 10,eV10,eV
  • Vibrational energy difference: ≈0.1,eV\approx 0.1,eV
  • Rotational energy difference: ≈10−3,eV\approx 10^{-3},eV

Hence, the energy difference between two consecutive vibrational states of a molecule is about 0.1,eV0.1,eV.

Question 88

The basic condition for ESR is given by :

  1. AOption (a)

    ΔE=hv=gβH\Delta \mathrm{E}=\mathrm{h} v=\mathrm{g} \beta \mathrm{H}

  2. BOption (b)

    ΔE=hν=2 gβH\Delta \mathrm{E}=\mathrm{h} \nu=2 \mathrm{~g} \beta \mathrm{H}

  3. COption (c)

    ΔE=hv=g2βH\Delta \mathrm{E}=\mathrm{h} v=\frac{\mathrm{g}}{2} \beta \mathrm{H}

  4. DOption (d)

    ΔE=hv=gβH\Delta \mathrm{E}=\mathrm{h} v=\frac{\mathrm{g}}{\beta} \mathrm{H}

View answer & explanation

Correct answer

Answer: (a)A.

ΔE=hv=gβH\Delta \mathrm{E}=\mathrm{h} v=\mathrm{g} \beta \mathrm{H}

Why this answer is correct

Concept:

Electron Spin Resonance (ESR) is based on the interaction of an electron’s magnetic moment with an external magnetic field.

When an external magnetic field (H) is applied, the spin states of an electron split into two energy levels due to the Zeeman effect.

Energy separation:

ΔE = gβH

where,

  • g = g-factor (≈ 2 for electron)
  • β = Bohr magneton
  • H = external magnetic field

Resonance condition:

For resonance to occur, the energy of the incident photon must match this energy difference:

ΔE = hν

Thus,

hν = gβH

Hence, the basic condition for ESR is ΔE = hν = gβH.

Question 89

The Zeeman pattern of a line consists of six equidistant components. If the upper state term is known to be 2P3/2{ }^{2} \mathrm{P}_{3 / 2}, its lower state term will be :

  1. AOption (a)

    2P1/2{ }^{2} P_{1 / 2}

  2. BOption (b)

    \({ }^{2} \mathrm{P_{1 / 2}&{ }^{2}S_{1 / 2}}\)

  3. COption (c)

    2S1/2{ }^{2}\mathrm{S}_{1 / 2}

  4. DOption (d)

    None of the above

View answer & explanation

Correct answer

Answer: (b)B.

\({ }^{2} \mathrm{P_{1 / 2}&{ }^{2}S_{1 / 2}}\)

Why this answer is correct

Concept

  • The Zeeman effect describes the splitting of spectral lines into several components in the presence of a static magnetic field.
  • For the anomalous Zeeman effect, the shift in energy levels depends on the Landé g-factor: g=1+J(J+1)+S(S+1)−L(L+1)2J(J+1)g = 1 + \frac{J(J+1) + S(S+1) - L(L+1)}{2J(J+1)}.
  • The number of transitions is determined by the selection rules: ΔMJ=0,±1\Delta M_J = 0, \pm 1.
  • Components are equidistant if the difference between the energy levels of adjacent transitions is constant.
<br>

Formula Used

Landé g-factor:

g=1+J(J+1)+S(S+1)−L(L+1)2J(J+1)g = 1 + \frac{J(J+1) + S(S+1) - L(L+1)}{2J(J+1)}

Shift in frequency (in units of μBBh\frac{\mu_B B}{h}):

Δν=(g1MJ1−g2MJ2)\Delta \nu = (g_1 M_{J1} - g_2 M_{J2})

<br>

Calculation

Upper state is 2P3/2{ }^{2} \mathrm{P}_{3 / 2}:

For 2P3/2{ }^{2} \mathrm{P}_{3 / 2}: L=1,S=1/2,J=3/2L = 1, S = 1/2, J = 3/2

g1=1+32(52)+12(32)−1(2)2(32)(52)=1+154+34−84152=1+104152=1+13=43g_1 = 1 + \frac{\frac{3}{2}(\frac{5}{2}) + \frac{1}{2}(\frac{3}{2}) - 1(2)}{2(\frac{3}{2})(\frac{5}{2})} = 1 + \frac{\frac{15}{4} + \frac{3}{4} - \frac{8}{4}}{\frac{15}{2}} = 1 + \frac{\frac{10}{4}}{\frac{15}{2}} = 1 + \frac{1}{3} = \frac{4}{3}

Magnetic sublevels MJ1M_{J1}: +32,+12,−12,−32+\frac{3}{2}, +\frac{1}{2}, -\frac{1}{2}, -\frac{3}{2}.

Case 1: Lower state is 2S1/2{ }^{2}S_{1 / 2}

For 2S1/2{ }^{2}S_{1 / 2}: L=0,S=1/2,J=1/2L = 0, S = 1/2, J = 1/2

g2=1+12(32)+12(32)−02(12)(32)=1+3232=2g_2 = 1 + \frac{\frac{1}{2}(\frac{3}{2}) + \frac{1}{2}(\frac{3}{2}) - 0}{2(\frac{1}{2})(\frac{3}{2})} = 1 + \frac{\frac{3}{2}}{\frac{3}{2}} = 2

Allowed transitions (ΔMJ=0,±1\Delta M_J = 0, \pm 1) result in relative shifts of (g1MJ1−g2MJ2)(g_1 M_{J1} - g_2 M_{J2}):

  • π\pi components (ΔMJ=0\Delta M_J = 0): ±(43⋅12−2⋅12)=±(23−1)=±13\pm(\frac{4}{3} \cdot \frac{1}{2} - 2 \cdot \frac{1}{2}) = \pm(\frac{2}{3} - 1) = \pm \frac{1}{3}
  • σ\sigma components (ΔMJ=±1\Delta M_J = \pm 1): ±(43⋅32−2⋅12)=±1\pm(\frac{4}{3} \cdot \frac{3}{2} - 2 \cdot \frac{1}{2}) = \pm 1 and ±(43⋅12−2⋅(−12))=±53\pm(\frac{4}{3} \cdot \frac{1}{2} - 2 \cdot (-\frac{1}{2})) = \pm \frac{5}{3}

Ordered shifts: −53,−1,−13,13,1,53-\frac{5}{3}, -1, -\frac{1}{3}, \frac{1}{3}, 1, \frac{5}{3}. The components are 6 and equidistant (spacing =23= \frac{2}{3}).

Case 2: Lower state is 2P1/2{ }^{2}P_{1 / 2}

For 2P1/2{ }^{2}P_{1 / 2}: L=1,S=1/2,J=1/2L = 1, S = 1/2, J = 1/2

g2=1+12(32)+12(32)−1(2)2(12)(32)=1+32−232=1−13=23g_2 = 1 + \frac{\frac{1}{2}(\frac{3}{2}) + \frac{1}{2}(\frac{3}{2}) - 1(2)}{2(\frac{1}{2})(\frac{3}{2})} = 1 + \frac{\frac{3}{2} - 2}{\frac{3}{2}} = 1 - \frac{1}{3} = \frac{2}{3}

Relative shifts (g1MJ1−g2MJ2)(g_1 M_{J1} - g_2 M_{J2}):

  • π\pi components: ±(43⋅12−23⋅12)=±(23−13)=±13\pm(\frac{4}{3} \cdot \frac{1}{2} - \frac{2}{3} \cdot \frac{1}{2}) = \pm(\frac{2}{3} - \frac{1}{3}) = \pm \frac{1}{3}
  • σ\sigma components: ±(43⋅32−23⋅12)=±(2−13)=±53\pm(\frac{4}{3} \cdot \frac{3}{2} - \frac{2}{3} \cdot \frac{1}{2}) = \pm(2 - \frac{1}{3}) = \pm \frac{5}{3} and ±(43⋅12−23⋅(−12))=±(23+13)=±1\pm(\frac{4}{3} \cdot \frac{1}{2} - \frac{2}{3} \cdot (-\frac{1}{2})) = \pm(\frac{2}{3} + \frac{1}{3}) = \pm 1

Ordered shifts: −53,−1,−13,13,1,53-\frac{5}{3}, -1, -\frac{1}{3}, \frac{1}{3}, 1, \frac{5}{3}. The components are 6 and equidistant (spacing =23= \frac{2}{3}).

Since both lower state terms result in six equidistant components, both are valid possibilities.

Hence, the lower state term will be \({ }^{2} \mathrm{P_{1 / 2}&{ }^{2}S_{1 / 2}}\).

Question 90

The binding energy of a nucleus of mass number A and atomic number Z varies in proportional to:

  1. AOption (a)

    A

  2. BOption (b)

    A2\mathrm{A}^{2}

  3. COption (c)

    A2Z\frac{\mathrm{A}^{2}}{\mathrm{Z}}

  4. DOption (d)

    Z(Z−1)\mathrm{Z}(\mathrm{Z}-1)

View answer & explanation

Correct answer

Answer: (a)A.

A

Why this answer is correct

Concept:

The total binding energy of a nucleus is explained using the semi-empirical mass formula (Weizsäcker formula).

The dominant term in binding energy is the volume energy term, which is directly proportional to the number of nucleons (mass number A).

Reason:

  • Each nucleon interacts with a few nearest neighbors due to short-range nuclear forces.
  • Hence, total binding energy increases approximately linearly with the number of nucleons.

Explanation of options:

  • A: Correct — Binding energy ∝ A (dominant volume term)
  • A²: Incorrect — nuclear forces are short range, not long range
  • A²/Z: Incorrect — not a standard dependence
  • Z(Z − 1): Represents Coulomb repulsion (reduces binding energy)

Hence, the binding energy of a nucleus is approximately proportional to A.

Question 91

The radius of \({ }{13} \mathrm{~A} l^{27}\) nucleus is 1.5 fm . What would be the radius of \({ }{82} \mathrm{~Pb}^{216}\) ?

  1. AOption (a)

    4.2 fm

  2. BOption (b)

    3.0 fm

  3. COption (c)

    1.8 fm

  4. DOption (d)

    0.5 fm

View answer & explanation

Correct answer

Answer: (b)B.

3.0 fm

Why this answer is correct

Concept

  • The nucleus of an atom is approximately spherical in shape.
  • The volume of the nucleus is directly proportional to the number of nucleons (mass number, AA).
  • Consequently, the radius of a nucleus is proportional to the cube root of its mass number.
<br>

Formula Used

The empirical formula for the radius of a nucleus is:

R=R0A1/3R = R_0 A^{1/3}

where:

  • RR is the radius of the nucleus.
  • R0R_0 is a constant (approximately 1.2 fm1.2 \text{ fm}).
  • AA is the mass number of the nucleus.

For two different nuclei, the ratio of their radii is:

R2R1=(A2A1)1/3\frac{R_2}{R_1} = \left( \frac{A_2}{A_1} \right)^{1/3}

<br>

Calculation

Given:

For Aluminum (13Al27{ }_{13} \mathrm{Al}^{27}):

Mass number A1=27A_1 = 27

Radius R1=1.5 fmR_1 = 1.5 \text{ fm}

For Lead (82Pb216{ }_{82} \mathrm{Pb}^{216}):

Mass number A2=216A_2 = 216

Using the ratio formula:

RPbRAl=(APbAAl)1/3\frac{R_{\text{Pb}}}{R_{\text{Al}}} = \left( \frac{A_{\text{Pb}}}{A_{\text{Al}}} \right)^{1/3}

RPb1.5=(21627)1/3\frac{R_{\text{Pb}}}{1.5} = \left( \frac{216}{27} \right)^{1/3}

Calculate the ratio of the mass numbers:

21627=8\frac{216}{27} = 8

Now, find the cube root:

RPb1.5=(8)1/3\frac{R_{\text{Pb}}}{1.5} = (8)^{1/3}

RPb1.5=2\frac{R_{\text{Pb}}}{1.5} = 2

Solve for RPbR_{\text{Pb}}:

RPb=1.5×2R_{\text{Pb}} = 1.5 \times 2

RPb=3.0 fmR_{\text{Pb}} = 3.0 \text{ fm}

Hence, the radius of the lead nucleus is 3.0 fm3.0 \text{ fm}.

Question 92

Neutrons and protons are grouped in :

  1. AOption (a)

    Leptons

  2. BOption (b)

    Baryons

  3. COption (c)

    Mesons

  4. DOption (d)

    None of the above

View answer & explanation

Correct answer

Answer: (b)B.

Baryons

Why this answer is correct

Concept:

Protons and neutrons are subatomic particles that belong to the class of baryons, which are a type of hadrons.

Explanation:

  • Baryons: Particles made up of three quarks (e.g., proton and neutron).
  • Leptons: Fundamental particles like electrons, neutrinos (do not experience strong nuclear force).
  • Mesons: Made of a quark and an antiquark.

Conclusion:

Since protons and neutrons are composed of three quarks, they are classified as baryons.

Question 93

At the peaks of nuclear binding energy curve :

  1. AOption (a)

    both Z and N are even.

  2. BOption (b)

    both Z and N are odd.

  3. COption (c)

    Z is odd but N is even.

  4. DOption (d)

    Z is even but N is odd.

View answer & explanation

Correct answer

Answer: (a)A.

both Z and N are even.

Why this answer is correct

Concept

  • The binding energy per nucleon curve represents the stability of atomic nuclei.
  • The curve is generally smooth but shows distinct local maxima (peaks) for certain light nuclei.
  • Even-even nuclei (where both the number of protons ZZ and the number of neutrons NN are even) are inherently more stable than odd-mass or odd-odd nuclei due to the pairing effect.
<br>

Explanation

The peaks in the binding energy curve correspond to nuclei that are exceptionally stable compared to their immediate neighbors. Notable examples of these peaks include:

  • Helium-4 (24He^4_2He): Z=2Z = 2 (even) and N=2N = 2 (even).
  • Carbon-12 (612C^{12}_6C): Z=6Z = 6 (even) and N=6N = 6 (even).
  • Oxygen-16 (816O^{16}_8O): Z=8Z = 8 (even) and N=8N = 8 (even).

In these nuclei, the number of protons (ZZ) and the number of neutrons (NN) are both even. This configuration allows for the formation of stable pairs of nucleons (protons pairing with protons and neutrons pairing with neutrons), which increases the total binding energy of the nucleus.

Nuclei with even ZZ and even NN are called even-even nuclei. Most stable isotopes found in nature belong to this category, confirming that such a configuration leads to higher stability and thus creates peaks in the curve.

Hence, at the peaks of the nuclear binding energy curve, both Z and N are even.

Question 94

The wavelength of 1MeVγ1 \mathrm{MeV} \gamma-ray is :

  1. AOption (a)

    0.1240A∘0.1240 \overset{\circ}{A}

  2. BOption (b)

    0.0124A∘0.0124\overset{\circ}{A}

  3. COption (c)

    0.0094A∘0.0094\overset{\circ}{A}

  4. DOption (d)

    0.0056A∘0.0056\overset{\circ}{A}

View answer & explanation

Correct answer

Answer: (b)B.

0.0124A∘0.0124\overset{\circ}{A}

Why this answer is correct

Concept

  • The energy of a photon (such as a γ\gamma-ray) is inversely proportional to its wavelength.
  • Electromagnetic radiation carries energy in discrete packets called photons.
  • For high-energy physics, it is often convenient to use the shortcut relation between energy in electron-volts (eV) and wavelength in Angstroms A∘\overset{\circ}{A}.
<br>

Formula Used

Energy of a photon:

E=hcλE = \frac{hc}{\lambda}

A useful approximation for wavelength in Angstroms A∘\overset{\circ}{A} given energy in electron-volts (eV) is:

λ(A∘)≈12400E(eV)\lambda (\overset{\circ}{A}) \approx \frac{12400}{E (\text{eV})}

Unit conversion:

1 MeV=106 eV1\text{ MeV} = 10^6\text{ eV}

<br>

Calculation

Given:

Energy, E=1 MeV=106 eVE = 1\text{ MeV} = 10^6\text{ eV}

Using the wavelength formula:

λ=12400E(eV)A∘\lambda = \frac{12400}{E (\text{eV})} \overset{\circ}{A}

Substituting the value of energy:

λ=12400106A∘\lambda = \frac{12400}{10^6} \overset{\circ}{A}

λ=12400×10−6A∘\lambda = 12400 \times 10^{-6} \overset{\circ}{A}

λ=0.0124A∘\lambda = 0.0124 \overset{\circ}{A}

Hence, the wavelength of the 1 MeV1\text{ MeV} γ\gamma-ray is 0.0124A∘0.0124\overset{\circ}{A}.

Question 95

Which one of the following elementary particle is a lepton?

  1. AOption (a)

    π\pi - meson

  2. BOption (b)

    K - meson

  3. COption (c)

    μ\mu - meson

  4. DOption (d)

    Lambda

View answer & explanation

Correct answer

Answer: (c)C.

μ\mu - meson

Why this answer is correct

Concept

  • Elementary particles are classified into two main groups based on their interactions: Leptons and Hadrons.
  • Leptons: These are fundamental particles that do not experience the strong nuclear force. Examples include the electron, muon, tau, and their respective neutrinos.
  • Hadrons: These are composite particles made of quarks that experience the strong nuclear force. Hadrons are further divided into mesons (quark-antiquark pairs) and baryons (three-quark combinations).
<br>

Explanation

To identify the lepton among the given options, we classify each particle:

  • **πππ

  • meson (Pion):** This is a hadron. Specifically, it is a meson composed of a quark and an antiquark.

  • K - meson (Kaon): This is also a hadron (a meson).

  • **μμμ

  • meson (Muon):** Historically, the muon was called a "mu-meson" because its mass lies between that of an electron and a proton. However, modern physics classifies the muon as a lepton because it is a fundamental particle (not made of quarks) and does not interact via the strong nuclear force.

  • **Lambda (ΛΛΛ

    ):** This is a baryon, which is a type of hadron composed of three quarks.

Comparing these classifications, the μμμ

-meson (muon) is the only lepton in the list.

Hence, the μμμ

  • meson is an elementary particle that belongs to the lepton family.

Question 96

Asymmetry energy EA\mathrm{E}_{\mathrm{A}} of a nucleus having atomic number A and Z protons is proportional to :

  1. AOption (a)

    (A−2Z)2\mathrm{(A-2 Z)^{2}}

  2. BOption (b)

    (A−Z)(\mathrm{A}-\mathrm{Z})

  3. COption (c)

    (A−Z)2\mathrm{(A-Z)^{2}}

  4. DOption (d)

    (A−Z)1/2(\mathrm{A}-\mathrm{Z})^{1 / 2}

View answer & explanation

Correct answer

Answer: (a)A.

(A−2Z)2\mathrm{(A-2 Z)^{2}}

Why this answer is correct

Concept:

The asymmetry energy is a term in the semi-empirical mass formula (Weizsäcker formula), which accounts for the imbalance between neutrons and protons in a nucleus.

Expression:

EA ∝ (N − Z)2

Since total nucleons A = N + Z, we can write:

N = A − Z

Substitute in (N − Z):

N − Z = (A − Z) − Z = A − 2Z

Thus, EA ∝ (A − 2Z)2

Hence, asymmetry energy is proportional to (A − 2Z)2.

Question 97

Which one of the following properties is not true of nuclear forces?

  1. AOption (a)

    They exhibit saturation.

  2. BOption (b)

    They are short ranged.

  3. COption (c)

    They follow inverse square law.

  4. DOption (d)

    They are spin dependent.

View answer & explanation

Correct answer

Answer: (c)C.

They follow inverse square law.

Why this answer is correct

Concept

  • Nuclear forces are the strong attractive forces that bind protons and neutrons (nucleons) together inside an atomic nucleus.
  • These forces have unique characteristics such as short-range nature, saturation, charge independence, and spin dependence.
<br>

Explanation

To identify the incorrect property, let us examine the characteristics of nuclear forces:

  • Saturation: Nuclear forces exhibit saturation, which means a nucleon only interacts with a limited number of its nearest neighbors. This property is evidenced by the fact that the binding energy per nucleon remains roughly constant for large mass numbers.
  • Short-range: Unlike electromagnetic or gravitational forces, nuclear forces are effective only over very short distances (approximately 10−15,m10^{-15},m or 1-2 fm). They drop to zero very rapidly as the distance increases beyond this range.
  • Spin Dependence: The nuclear force between two nucleons depends on the relative orientation of their spins. For instance, the force between a proton and a neutron is stronger if their spins are parallel than if they are anti-parallel.
  • Inverse Square Law: Long-range forces like gravitation and electrostatic forces follow the inverse square law, where the force is inversely proportional to the square of the distance (F∝1r2F \propto \frac{1}{r^2}). Nuclear forces do not follow this law; they decrease much more exponentially and sharply with distance.

Hence, the property that is not true of nuclear forces is that they follow the inverse square law.

Question 98

The vector mesons are :

  1. AOption (a)

    π∘,K∘,ϕ\pi^{\circ}, \mathrm{K}^{\circ}, \phi

  2. BOption (b)

    P,ω,ϕ\mathrm{P}, \omega, \phi

  3. COption (c)

    P,ω,K∘\mathrm{P}, \omega, \mathrm{K}^{\circ}

  4. DOption (d)

    P,ω,π+\mathrm{P}, \omega, \pi^{+}

View answer & explanation

Correct answer

Answer: (b)B.

P,ω,ϕ\mathrm{P}, \omega, \phi

Why this answer is correct

Concept:

Mesons are classified based on their intrinsic spin:

  • Scalar/Pseudoscalar mesons: Spin = 0 (e.g., π, K)
  • Vector mesons: Spin = 1

Explanation:

  • ρ (rho), ω (omega), ϕ (phi) are vector mesons with spin = 1 ✔
  • π (pion) and K (kaon) are pseudoscalar mesons with spin = 0 ✖
  • p (proton) is a baryon, not a meson ✖

Conclusion:

The vector mesons are ρ, ω, ϕ.

Question 99

In β\beta-decay process :

  1. AOption (a)

    mass number of nuclei remains constant

  2. BOption (b)

    charge number of nuclei remains constant

  3. COption (c)

    only electrons are emitted

  4. DOption (d)

    only neutrinos are emitted

View answer & explanation

Correct answer

Answer: (a)A.

mass number of nuclei remains constant

Why this answer is correct

Concept

  • Beta (β\beta) decay is a nuclear process in which a nucleus transforms into another by emitting a beta particle (either an electron or a positron).
  • There are two types of beta decay: β−\beta^- decay and β+\beta^+ decay.
  • In this process, a neutron is converted into a proton, or a proton is converted into a neutron, inside the nucleus.
  • The total number of nucleons (protons + neutrons), known as the mass number (AA), remains unchanged during this transformation.
<br>

Nuclear Equations

For β−\beta^- decay (electron emission):

ZAX→Z+1AY+−10e+νˉe_{Z}^{A}X \rightarrow _{Z+1}^{A}Y + _{-1}^{0}e + \bar{\nu}_e

For β+\beta^+ decay (positron emission):

ZAX→Z−1AY++10e+νe_{Z}^{A}X \rightarrow _{Z-1}^{A}Y + _{+1}^{0}e + \nu_e

<br>

Explanation

In the β−\beta^- decay process, a neutron changes into a proton, an electron, and an antineutrino. While the charge number (atomic number ZZ) increases by 1, the total count of nucleons stays the same. Thus, the mass number AA remains constant.

In the β+\beta^+ decay process, a proton changes into a neutron, a positron, and a neutrino. While the charge number (ZZ) decreases by 1, the total count of nucleons stays the same. Thus, the mass number AA remains constant.

Evaluating the statements:

  • Mass number: As shown above, the total number of nucleons does not change, so the mass number remains constant.
  • Charge number: The atomic number ZZ changes by ±1\pm 1, so the charge number is not constant.
  • Emitted particles: Both a beta particle (electron or positron) and a neutrino (or antineutrino) are emitted simultaneously; therefore, it is incorrect to say "only" one of them is emitted.

Hence, in the βββ

-decay process, the mass number of nuclei remains constant.

Question 100

The binding energy of an α\alpha-particle in MeV is :

  1. AOption (a)

    28.29

  2. BOption (b)

    29.28

  3. COption (c)

    22.89

  4. DOption (d)

    22.98

View answer & explanation

Correct answer

Answer: (a)A.

28.29

Why this answer is correct

Concept:

The binding energy of a nucleus is the energy required to separate it into its constituent nucleons.

For an α-particle (He nucleus): It consists of 2 protons and 2 neutrons.

Calculation:

Binding Energy (B.E.) = [Z·mp + (A−Z)·mn − mnucleus] × 931.5 MeV

For α-particle:

Z = 2, A = 4

Using standard values:

  • mp ≈ 1.00728 u
  • mn ≈ 1.00866 u
  • mα ≈ 4.00260 u

Mass defect:

Δm = [2(1.00728) + 2(1.00866) − 4.00260]

Δm ≈ 0.0304 u

Binding energy:

B.E. = 0.0304 × 931.5 ≈ 28.29 MeV

Conclusion: Binding energy of an α-particle is 28.29 MeV.

Question 101

Considering liquid - drop model of the nucleus the constant δ\delta for the last correction term in the semi-empirical mass formula is positive for :

  1. AOption (a)

    odd-odd nuclei

  2. BOption (b)

    even-even nuclei

  3. COption (c)

    odd A nuclei

  4. DOption (d)

    even A nuclei

View answer & explanation

Correct answer

Answer: (b)B.

even-even nuclei

Why this answer is correct

Concept

  • The Liquid Drop Model treats the nucleus as a drop of incompressible nuclear fluid.
  • The binding energy of a nucleus is expressed by the semi-empirical mass formula (SEMF), also known as the Bethe-Weizsäcker formula.
  • The formula consists of five terms: volume energy, surface energy, Coulomb energy, asymmetry energy, and the pairing energy term.
  • The pairing term (δ\delta) accounts for the fact that nuclei with even numbers of protons and neutrons are more stable (higher binding energy) than those with odd numbers.
<br>

Formula Used

The semi-empirical mass formula for binding energy B(A,Z)B(A, Z) is given by:

B(A,Z)=avA−asA2/3−acZ(Z−1)A1/3−aa(A−2Z)2A+δB(A, Z) = a_v A - a_s A^{2/3} - a_c \frac{Z(Z-1)}{A^{1/3}} - a_a \frac{(A-2Z)^2}{A} + \delta

The value of the pairing term δ\delta is defined as:

  • δ=+apA1/2\delta = +\frac{a_p}{A^{1/2}} for even-even nuclei (Z even, N even)
  • δ=0\delta = 0 for odd-A nuclei (Z even, N odd or Z odd, N even)
  • δ=−apA1/2\delta = -\frac{a_p}{A^{1/2}} for odd-odd nuclei (Z odd, N odd)
<br>

Explanation

The pairing term arises because nucleons (protons and neutrons) tend to form pairs with opposite spins in the same energy level, which increases the stability of the nucleus.

  • Even-Even Nuclei: Since all protons and all neutrons can form pairs, these nuclei are the most stable and have the highest binding energy. Therefore, the correction term δ\delta is positive to increase the total binding energy.
  • Odd-A Nuclei: These have one unpaired nucleon (either a proton or a neutron). The pairing effect is neutral relative to the average, so δ=0\delta = 0.
  • Odd-Odd Nuclei: These have both an unpaired proton and an unpaired neutron. These are the least stable, so the correction term δ\delta is negative to reflect a decrease in binding energy.

According to the liquid-drop model and the standard semi-empirical mass formula, the constant δ\delta is positive for even-even nuclei.

Hence, the constant δ\delta is positive for even-even nuclei.

Question 102

A material is most stable when its potential energy is :

  1. AOption (a)

    maximum

  2. BOption (b)

    infinite

  3. COption (c)

    minimum

  4. DOption (d)

    zero

View answer & explanation

Correct answer

Answer: (c)C.

minimum

Why this answer is correct

Concept:

A system is said to be in stable equilibrium when its potential energy is at the lowest possible value.

Explanation:

  • If a system is slightly disturbed from its equilibrium position, it tends to return back to that position.
  • This restoring tendency occurs only when the potential energy is minimum.
  • At maximum potential energy, the system is in unstable equilibrium.

Conclusion: A material is most stable when its potential energy is minimum.

Question 103

Consider the List-I and List-II and select your correct answer from the codes given below :

List-I (Semiconductor material)List-II (Energy band gap)
(A) Germanium1. 3.6 eV
(B) Silicon2. 1.4 eV
(C) Gallium Arsenide3. 1.1 eV
(D) Zinc Sulfide4. 0.7 eV
  1. AOption (a)

    A-2, B-3, C-4, D-1

  2. BOption (b)

    A-3, B-4, C-2, D-1

  3. COption (c)

    A-1, B-2, C-3, D-4

  4. DOption (d)

    A-4, B-3, C-2, D-1

View answer & explanation

Correct answer

Answer: (d)D.

A-4, B-3, C-2, D-1

Why this answer is correct

Concept

  • The energy band gap (EgE_g) is the minimum energy required to excite an electron from the valence band to the conduction band.
  • Different semiconductor materials have characteristic band gap values at room temperature (approximately 300,K300,K).
  • Materials with smaller band gaps like Germanium are more sensitive to temperature, while wider band gap materials like Zinc Sulfide are used for optoelectronic applications in the UV/Blue spectrum.
<br>

Explanation

The standard energy band gap values for the given materials at room temperature are as follows:

  • Germanium (Ge): It is a classic semiconductor with a relatively small band gap. Its value is approximately 0.67,eV0.67,eV to 0.7,eV0.7,eV.

Matching: (A) → 4

  • Silicon (Si): The most widely used semiconductor in electronics. Its band gap is approximately 1.1,eV1.1,eV.

Matching: (B) → 3

  • Gallium Arsenide (GaAs): A compound semiconductor often used in high-speed devices and optoelectronics. Its band gap is approximately 1.42,eV1.42,eV.

Matching: (C) → 2

  • Zinc Sulfide (ZnS): A wide band gap semiconductor used in phosphors and thin-film devices. Its band gap is approximately 3.6,eV3.6,eV.

Matching: (D) → 1

<br>

Calculation

By matching the items from List-I with List-II:

(A) Germanium — 4 (0.7 eV)

(B) Silicon — 3 (1.1 eV)

(C) Gallium Arsenide — 2 (1.4 eV)

(D) Zinc Sulfide — 1 (3.6 eV)

This corresponds to the sequence: A-4, B-3, C-2, D-1.

Hence, the correct matching is A-4, B-3, C-2, D-1.

Question 104

The density of carriers in a pure semiconductor is proportional to :

  1. AOption (a)

    e−Eg/kT\mathrm{e}^{-\mathrm{Eg} / \mathrm{kT}}

  2. BOption (b)

    e−Eg/2kT\mathrm{e}^{-\mathrm{Eg} / 2 \mathrm{kT}}

  3. COption (c)

    e−Eg/kT2\mathrm{e}^{-\mathrm{Eg} / \mathrm{kT}^{2}}

  4. DOption (d)

    e−Eg/2kT2\mathrm{e}^{-\mathrm{Eg} / 2 \mathrm{kT}^{2}}

View answer & explanation

Correct answer

Answer: (b)B.

e−Eg/2kT\mathrm{e}^{-\mathrm{Eg} / 2 \mathrm{kT}}

Why this answer is correct

Concept

  • In a pure (intrinsic) semiconductor, the number of electrons in the conduction band is equal to the number of holes in the valence band.
  • The density of these charge carriers depends on the energy band gap (EgE_g) and the thermal energy (kTkT).
  • The probability of thermal excitation of carriers across the band gap follows the Fermi-Dirac distribution, which simplifies to the Maxwell-Boltzmann distribution for semiconductors.
<br>

Formula Used

The intrinsic carrier concentration (nin_i) is given by the expression:

ni=AT3/2e−Eg2kTn_i = A T^{3/2} e^{-\frac{E_g}{2kT}}

Where:

  • nin_i = intrinsic carrier density
  • EgE_g = energy band gap
  • kk = Boltzmann constant
  • TT = absolute temperature
  • AA = constant
<br>

Explanation

In an intrinsic semiconductor, the Fermi level lies approximately in the middle of the energy gap (Eg/2E_g/2).

The carrier density is determined by the product of the density of states and the probability of occupancy. Mathematically, the product of electron density (nn) and hole density (pp) is:

n⋅p=ni2=NcNve−EgkTn \cdot p = n_i^2 = N_c N_v e^{-\frac{E_g}{kT}}

Since for a pure semiconductor n=p=nin = p = n_i, we take the square root of the expression:

ni=NcNve−Eg2kTn_i = \sqrt{N_c N_v} e^{-\frac{E_g}{2kT}}

Therefore, the carrier density is proportional to:

ni∝e−Eg2kTn_i \propto e^{-\frac{E_g}{2kT}}

Hence, the density of carriers in a pure semiconductor is proportional to e−Eg/2kTe^{-E_g / 2kT}.

Question 105

In super conducting state:

  1. AOption (a)

    entropy increases and thermal conductivity decreases

  2. BOption (b)

    entropy & thermal conductivity both decrease

  3. COption (c)

    entropy & thermal conductivity both increase

  4. DOption (d)

    entropy decreases and thermal conductivity increases

View answer & explanation

Correct answer

Answer: (b)B.

entropy & thermal conductivity both decrease

Why this answer is correct

Concept

  • Superconductivity: A phenomenon where a material exhibits zero electrical resistance and expulsion of magnetic fields below a certain critical temperature (TcT_c).
  • Entropy (S): A thermodynamic property that measures the degree of disorder or randomness in a system.
  • Thermal Conductivity (κ\kappa): The ability of a material to conduct heat, which in metals is primarily mediated by electrons and phonons.
<br>

Explanation

When a material transitions from a normal state to a superconducting state, the following changes occur:

  1. Decrease in Entropy: The superconducting state is a more ordered phase compared to the normal state. Below the critical temperature, conduction electrons form Cooper pairs, which occupy a single quantum ground state. This transition from a disordered electronic state to a highly ordered "condensate" state leads to a significant decrease in entropy.
  2. Decrease in Thermal Conductivity: In the normal metallic state, heat is transported mainly by electrons. In the superconducting state, however, Cooper pairs do not transport entropy (heat) because they are in the ground state. As more electrons form Cooper pairs, fewer "normal" electrons are available to transport thermal energy. Consequently, the electronic part of the thermal conductivity decreases as the temperature drops below TcT_c.

Therefore, in the superconducting state, both entropy and thermal conductivity decrease.

Hence, entropy and thermal conductivity both decrease in the superconducting state.

Question 106

Transition temperature TcT_{c} and Critical magnetic field Hc\mathrm{H}_{\mathrm{c}} for a super conductor are related as :

  1. AOption (a)

    \(\mathrm{H}{\mathrm{c}}=\mathrm{H}{\mathrm{o}}\left(\mathrm{T}_{\mathrm{c}}-1\right)\)

  2. BOption (b)

    \(\mathrm{H}{\mathrm{c}}=\mathrm{H}{\mathrm{o}}\left(\mathrm{T}_{\mathrm{c}}+1\right)\)

  3. COption (c)

    \(\mathrm{T}{\mathrm{c}}=\mathrm{T}{\mathrm{o}}\left[1-\left(\frac{\mathrm{H}{\mathrm{o}}}{\mathrm{H}{\mathrm{c}}}\right)^{2}\right]\)

  4. DOption (d)

    \(\mathrm{H}{\mathrm{c}}=\mathrm{H}{\mathrm{o}}\left[1-\left(\frac{\mathrm{T}}{\mathrm{T}_{\mathrm{c}}}\right)^{2}\right]\)

View answer & explanation

Correct answer

Answer: (d)D.

\(\mathrm{H}{\mathrm{c}}=\mathrm{H}{\mathrm{o}}\left[1-\left(\frac{\mathrm{T}}{\mathrm{T}_{\mathrm{c}}}\right)^{2}\right]\)

Why this answer is correct

Concept

  • In superconductivity, the critical magnetic field (HcH_c) is the value of the external magnetic field above which a superconductor loses its superconducting property and transitions into a normal state.
  • This critical magnetic field is not constant but depends on the temperature of the material.
<br>

Formula Used

The variation of the critical magnetic field with temperature is given by the empirical relation:

Hc=Ho[1−(TTc)2]H_c = H_o \left[ 1 - \left( \frac{T}{T_c} \right)^2 \right]

Where:

  • HcH_c is the critical magnetic field at temperature TT.
  • HoH_o is the critical magnetic field at absolute zero (0 K).
  • TT is the operating absolute temperature.
  • TcT_c is the transition temperature (critical temperature) of the superconductor.
<br>

Explanation

The relation Hc=Ho[1−(TTc)2]H_c = H_o \left[ 1 - \left( \frac{T}{T_c} \right)^2 \right] describes how the stability of the superconducting state decreases with temperature:

  • At absolute zero (T=0T = 0): The formula gives Hc=HoH_c = H_o. This is the maximum magnetic field the superconductor can withstand.
  • As temperature increases: The term (TTc)2\left( \frac{T}{T_c} \right)^2 increases, which causes HcH_c to decrease.
  • At the transition temperature (T=TcT = T_c): The formula gives Hc=Ho(1−1)=0H_c = H_o(1 - 1) = 0. This means that at the transition temperature, even in the absence of an external magnetic field, the material loses its superconductivity.

Comparing this with the given choices, the mathematical expression Hc=Ho[1−(TTc)2]H_c = H_o \left[ 1 - \left( \frac{T}{T_c} \right)^2 \right] is the standard representation of this relationship.

Hence, the transition temperature TcT_c and Critical magnetic field HcH_c are related as Hc=Ho[1−(TTc)2]H_c = H_o \left[ 1 - \left( \frac{T}{T_c} \right)^2 \right].

Question 107

Miller indices of the diagonal plane of a cube are :

  1. AOption (a)

    \({(200)} \)

  2. BOption (b)

    \((010)\)

  3. COption (c)

    (1 1 0)(1~ 1~0)

  4. DOption (d)

    \((111)\)

View answer & explanation

Correct answer

Answer: (c)C.

(1 1 0)(1~ 1~0)

Why this answer is correct

Concept

  • Miller indices (hkl)(hkl) are a notation system in crystallography for planes in crystal (Bravais) lattices.
  • To find the Miller indices, we identify the intercepts of the plane on the three axes, take their reciprocals, and then convert them to the smallest integers.
  • A diagonal plane in a cube typically refers to a plane that passes through the opposite edges of the cube, cutting across the face diagonal.
<br>

Process Used

  1. Identify the intercepts made by the plane on the x,y,x, y, and zz axes in terms of the lattice constant aa.
  2. Take the reciprocals of these intercepts.
  3. Reduce the reciprocals to the smallest set of integers having the same ratio.
<br>

Explanation

Consider a diagonal plane that cuts through the top and bottom edges of the cube. For example, a plane that intercepts the xx-axis at aa and the yy-axis at aa, but remains parallel to the zz-axis.

  1. Determine the Intercepts:
  • Intercept on xx-axis = 11 (in units of aa)
  • Intercept on yy-axis = 11 (in units of aa)
  • Intercept on zz-axis = ∞\infty (since it is parallel to the zz-axis)
  1. Take the Reciprocals:

11,11,1∞\frac{1}{1}, \frac{1}{1}, \frac{1}{\infty}

⇒1,1,0\Rightarrow 1, 1, 0

  1. Assign Miller Indices:

The values are already integers, so the Miller indices are (110)(110).

Similarly, other diagonal planes like those passing through other axes would result in indices like (101)(101) or (011)(011). Among the given options, (110)(110) represents a standard diagonal plane of the cube.

Hence, the Miller indices of the diagonal plane of a cube are \((110)\).

Question 108

Mean energy of electron gas at absolute zero \(\overline{\mathrm{E}}{0}\) and Fermi energy at absolute zero \(\mathrm{E}{\mathrm{F}_{0}}\) are related as :

  1. AOption (a)

    \(\overline{\mathrm{E}}{0}=\frac{5}{3} \mathrm{E}{\mathrm{F}_{0}}\)

  2. BOption (b)

    \(\overline{\mathrm{E}}{0}=\frac{5}{3} \mathrm{E}{\mathrm{F}_{0}}^{2}\)

  3. COption (c)

    \(\overline{\mathrm{E}}{0}=\frac{3}{5} \mathrm{E}{\mathrm{F}_{0}}\)

  4. DOption (d)

    \(\overline{\mathrm{E}}{0}=\frac{3}{5} \mathrm{E}{\mathrm{F}_{0}}^{1 / 2}\)

View answer & explanation

Correct answer

Answer: (c)C.

\(\overline{\mathrm{E}}{0}=\frac{3}{5} \mathrm{E}{\mathrm{F}_{0}}\)

Why this answer is correct

Concept

  • At absolute zero (T=0,KT = 0,K), all the lower energy states up to the Fermi energy (EF0E_{F_0}) are completely occupied by electrons, while all states above it are empty.
  • The distribution of electrons follows the Fermi-Dirac statistics.
  • In a three-dimensional electron gas, the density of states g(E)g(E) is proportional to the square root of the energy (E1/2E^{1/2}).
<br>

Formula Used

Total number of electrons (NN):

N=∫0EF0g(E)dEN = \int_{0}^{E_{F_0}} g(E) dE

Total energy of the electron gas (U0U_0) at 0,K0,K:

U0=∫0EF0E⋅g(E)dEU_0 = \int_{0}^{E_{F_0}} E \cdot g(E) dE

Mean energy (E‾0\overline{E}_0):

E‾0=U0N\overline{E}_0 = \frac{U_0}{N}

<br>

Calculation

The density of states for an electron gas in 3D is given by:

g(E)=CE1/2g(E) = CE^{1/2} (where CC is a constant)

Calculating the total number of electrons NN:

\(N = \int_{0}^{E_{F_0}} CE^{1/2} dE = C \left[ \frac{E^{3/2}}{3/2} \right]{0}^{E{F_0}} = \frac{2}{3}CE_{F_0}^{3/2}\)

Calculating the total energy U0U_0:

U0=∫0EF0E⋅CE1/2dE=C∫0EF0E3/2dEU_0 = \int_{0}^{E_{F_0}} E \cdot CE^{1/2} dE = C \int_{0}^{E_{F_0}} E^{3/2} dE

\(U_0 = C \left[ \frac{E^{5/2}}{5/2} \right]{0}^{E{F_0}} = \frac{2}{5}CE_{F_0}^{5/2}\)

Now, finding the mean energy E‾0\overline{E}_0:

\(\overline{E}0 = \frac{U_0}{N} = \frac{\frac{2}{5}CE{F_0}^{5/2}}{\frac{2}{3}CE_{F_0}^{3/2}}\)

\(\overline{E}0 = \frac{3}{5} \frac{E{F_0}^{5/2}}{E_{F_0}^{3/2}}\)

\(\overline{E}0 = \frac{3}{5} E{F_0}\)

Hence, the mean energy of electron gas and Fermi energy at absolute zero are related as \(\overline{\mathrm{E}}{0}=\frac{3}{5} \mathrm{E}{\mathrm{F}_{0}}\).

Question 109

A polycrystalline antiferromagnetic substance above Neel temperature behaves as :

  1. AOption (a)

    dia-magnetic substance

  2. BOption (b)

    ferromagnetic substance

  3. COption (c)

    paramagnetic substance

  4. DOption (d)

    ferrimagnetic substance

View answer & explanation

Correct answer

Answer: (c)C.

paramagnetic substance

Why this answer is correct

Concept:

Antiferromagnetic materials have equal and opposite magnetic moments of atoms, resulting in zero net magnetization below the Neel temperature (TN). 

Explanation:

  • Below TN: Spins are aligned antiparallel → antiferromagnetic behavior.
  • Above TN: Thermal agitation destroys this ordered arrangement.
  • The material loses antiferromagnetic order and behaves like a paramagnetic substance.

Hence, above Neel temperature, an antiferromagnetic substance behaves as a paramagnetic substance.

Question 110

Ionic crystals are :

  1. AOption (a)

    Soluble in ordinary liquids

  2. BOption (b)

    Soluble in polar liquids

  3. COption (c)

    Soluble in organic liquids

  4. DOption (d)

    Soluble in mercury

View answer & explanation

Correct answer

Answer: (b)B.

Soluble in polar liquids

Why this answer is correct

Concept

  • The solubility of a substance is generally governed by the principle "like dissolves like."
  • Ionic crystals consist of cations and anions held together by strong electrostatic forces of attraction (lattice energy).
  • Polar liquids are characterized by a permanent dipole moment and a high dielectric constant.
<br>

Explanation

Ionic crystals are highly polar because they are composed of charged particles. To dissolve such a crystal, the solvent must be able to weaken the strong electrostatic attractions within the lattice and stabilize the individual ions.

When an ionic crystal is placed in a polar liquid (such as water):

  • The polar molecules of the solvent orient themselves around the ions at the surface of the crystal.
  • The negative poles of the solvent molecules attract the cations, and the positive poles attract the anions. This process is known as solvation.
  • The high dielectric constant of the polar liquid reduces the electrostatic force between the ions, allowing them to break away from the lattice.
  • The energy released during solvation (solvation energy) compensates for the lattice energy required to break the crystal.

In contrast, non-polar liquids (like most organic liquids) have low dielectric constants and cannot interact strongly with ions, making ionic crystals insoluble in them.

Hence, ionic crystals are soluble in polar liquids.

Question 111

Cohesive energy of covalent crystals is

  1. AOption (a)

    more than that of ionic crystals

  2. BOption (b)

    less than that of ionic crystals

  3. COption (c)

    same as that of ionic crystals

  4. DOption (d)

    can't be compared with that of ionic crystals

View answer & explanation

Correct answer

Answer: (a)A.

more than that of ionic crystals

Why this answer is correct

Concept

  • Cohesive energy is the energy required to break all the bonds in a crystal and separate its constituent atoms or ions to an infinite distance.
  • It serves as a measure of the strength of the internal forces holding the crystal together.
  • Crystals are categorized based on their bonding types: covalent, ionic, metallic, and molecular.
<br>

Explanation

Covalent crystals, such as diamond and silicon carbide, are formed by the sharing of valence electrons between atoms. These covalent bonds are extremely strong and directional, creating a robust three-dimensional network. Because of this localized and intense electron sharing, the energy required to separate these atoms is very high.

In contrast, ionic crystals like sodium chloride (NaClNaCl) are held together by electrostatic attractions between oppositely charged ions. While ionic bonds are strong, they are generally less energetic than the network of covalent bonds found in covalent crystals. For example, the cohesive energy of Diamond (covalent) is approximately 7.3,eV/atom7.3,\text{eV/atom}, whereas for NaClNaCl (ionic), it is approximately 3.3,eV/atom3.3,\text{eV/atom} when considering the separation into neutral atoms.

Therefore, due to the high stability and strength of the covalent network, covalent crystals generally possess higher cohesive energy than ionic crystals.

Hence, the cohesive energy of covalent crystals is more than that of ionic crystals.

Question 112

At low temperature the atomic heat capacity Cv\mathrm{C}_{\mathrm{v}} of a solid is given by :

  1. AOption (a)

    12π45R(Tθ)3\frac{12 \pi^{4}}{5} \mathrm{R}\left(\frac{\mathrm{T}}{\theta}\right)^{3}

  2. BOption (b)

    12π45R(Tθ)2\frac{12 \pi^{4}}{5} \mathrm{R}\left(\frac{\mathrm{T}}{\theta}\right)^{2}

  3. COption (c)

    12π45R(Tθ)4\frac{12 \pi^{4}}{5} \mathrm{R}\left(\frac{\mathrm{T}}{\theta}\right)^{4}

  4. DOption (d)

    12π45R(Tθ)\frac{12 \pi^{4}}{5} \mathrm{R}\left(\frac{\mathrm{T}}{\theta}\right)

View answer & explanation

Correct answer

Answer: (a)A.

12π45R(Tθ)3\frac{12 \pi^{4}}{5} \mathrm{R}\left(\frac{\mathrm{T}}{\theta}\right)^{3}

Why this answer is correct

Concept

  • The Debye model describes the heat capacity of solids based on lattice vibrations (phonons).
  • At high temperatures, the heat capacity of a solid approaches the Dulong-Petit limit (3R3R).
  • At very low temperatures (T≪θT \ll \theta), the heat capacity follows the Debye T3T^3 law, where it is proportional to the cube of the absolute temperature.
<br>

Formula Used

The molar heat capacity at constant volume (CvC_v) according to the Debye model at low temperatures is given by:

Cv=12π45R(Tθ)3C_v = \frac{12 \pi^4}{5} R \left( \frac{T}{\theta} \right)^3

Where:

  • RR is the universal gas constant.
  • TT is the absolute temperature.
  • θ\theta is the Debye temperature, characteristic of the material.
<br>

Explanation

According to Debye's theory, the internal energy (UU) of a solid at low temperatures is due to the excitation of low-frequency phonons. The total energy is found by integrating the energy of these vibrational modes up to the Debye frequency.

At low temperatures (T→0T \rightarrow 0), the internal energy UU is proportional to T4T^4:

U∝T4U \propto T^4

Since heat capacity at constant volume is defined as the temperature derivative of internal energy:

Cv=dUdTC_v = \frac{dU}{dT}

Differentiating the expression for internal energy with respect to temperature gives:

Cv∝T3C_v \propto T^3

The exact proportionality constant derived from the Debye model results in the formula:

Cv=12π45R(Tθ)3C_v = \frac{12 \pi^{4}}{5} R\left(\frac{T}{\theta}\right)^{3}

Hence, the atomic heat capacity CvC_v of a solid at low temperature is 12π45R(Tθ)3\frac{12 \pi^{4}}{5} R\left(\frac{T}{\theta}\right)^{3}.

Question 113

Hall coefficient RHR_{H} of a substance is related with the mobility μ\mu of the current carrying particle by :

  1. AOption (a)

    μ=RHσ\mu=R_{H} \sigma

  2. BOption (b)

    μ=RHσ\mu=\frac{R_{H}}{\sigma}

  3. COption (c)

    σ=(RHμ)2\sigma=\left(\frac{\mathrm{R}_{\mathrm{H}}}{\mu}\right)^{2}

  4. DOption (d)

    μ=σRH\mu=\frac{\sigma}{R_{H}}

View answer & explanation

Correct answer

Answer: (a)A.

μ=RHσ\mu=R_{H} \sigma

Why this answer is correct

Concept

  • The Hall coefficient (RHR_H) is a measure of the magnitude of the Hall effect in a material, which depends on the charge carrier density.
  • Conductivity (σ\sigma) represents how easily current flows through a material and depends on both carrier density and mobility.
  • Mobility (μ\mu) describes how quickly a charge carrier moves through a metal or semiconductor when pulled by an electric field.
<br>

Formula Used

Hall coefficient:

RH=1nqR_H = \frac{1}{nq}

Electrical conductivity:

σ=nqμ\sigma = nq\mu

Where:

  • nn = carrier concentration
  • qq = charge of the carrier
  • μ\mu = mobility of the carrier
<br>

Calculation

From the expression of conductivity:

σ=(nq)⋅μ\sigma = (nq) \cdot \mu

From the expression of the Hall coefficient, we know that:

nq=1RHnq = \frac{1}{R_H}

Substituting the value of nqnq into the conductivity equation:

σ=(1RH)⋅μ\sigma = \left( \frac{1}{R_H} \right) \cdot \mu

Rearranging the formula to solve for mobility (μ\mu):

μ=σ⋅RH\mu = \sigma \cdot R_H

Hence, the mobility is related to the Hall coefficient by the relation μ=RHσ\mu = R_H \sigma.

Question 114

Magnetic susceptibility of a material is -1 . It belongs to :

  1. AOption (a)

    antiferromagnetics

  2. BOption (b)

    superconductors

  3. COption (c)

    ferromagnetics

  4. DOption (d)

    piezoelectrics

View answer & explanation

Correct answer

Answer: (b)B.

superconductors

Why this answer is correct

Correct Answer: Superconductors

Concept:

Magnetic susceptibility (χ) indicates how a material responds to an external magnetic field.

Key facts:

  • Diamagnetic materials: χ is small and negative (χ < 0)
  • Paramagnetic materials: χ is small and positive (χ > 0)
  • Ferromagnetic materials: χ is very large and positive
  • Superconductors: χ = −1

Explanation:

In superconductors, magnetic field is completely expelled from the interior (Meissner effect), leading to perfect diamagnetism.

Conclusion:

If χ = −1, the material is a superconductor.

Question 115

The BCS theory of superconductivity is based on :

  1. AOption (a)

    repulsion interaction between electrons

  2. BOption (b)

    attractive gravitational interaction between electrons

  3. COption (c)

    Attractive electron-lattice-electron interaction

  4. DOption (d)

    Exchange interaction

View answer & explanation

Correct answer

Answer: (c)C.

Attractive electron-lattice-electron interaction

Why this answer is correct

Concept

  • The BCS theory, proposed by Bardeen, Cooper, and Schrieffer in 1957, provides a microscopic explanation of superconductivity.
  • It describes how electrons in a superconductor form pairs, known as Cooper pairs, which can flow without resistance.
  • The formation of these pairs is due to a specific type of attractive interaction mediated by the crystal lattice.
<br>

Explanation

The mechanism of BCS theory can be understood as follows:

  • As an electron passes through the crystal lattice of a solid, it attracts the nearby positive ions, causing a localized deformation (polarization) of the lattice.
  • This deformation creates a region of increased positive charge density.
  • A second electron is then attracted to this region of increased positive charge.
  • This results in an effective attractive electron-lattice-electron interaction (often called electron-phonon interaction).
  • Even though electrons naturally repel each other due to Coulomb forces, this lattice-mediated attraction overcomes the repulsion at very low temperatures, allowing the electrons to form a bound state (Cooper pair).

Hence, the BCS theory of superconductivity is based on the attractive electron-lattice-electron interaction.

Question 116

By changing the size of a nano material, property that changes is

  1. AOption (a)

    Optical

  2. BOption (b)

    Physical

  3. COption (c)

    Electrical

  4. DOption (d)

    all of above

View answer & explanation

Correct answer

Answer: (d)D.

all of above

Why this answer is correct

CONCEPT:

Nanomaterials and Size-Dependent Properties

  • Nanomaterials have dimensions in the range of 1–100 nm, where quantum effects become significant.
  • As the size of a material decreases to the nanoscale, the surface area to volume ratio increases drastically.
  • This leads to changes in multiple properties due to:
  • Quantum confinement effect (affects electronic and optical behavior)
  • Increased surface atoms (affects physical and chemical properties)

EXPLANATION:

  • Optical Properties:
  • Color and absorption of light change with size due to quantum confinement.
  • Example: Gold nanoparticles appear red or purple instead of yellow.
  • Physical Properties:
  • Melting point, strength, and hardness vary with size.
  • Smaller particles often have lower melting points due to higher surface energy.
  • Electrical Properties:
  • Conductivity and band gap change with size.
  • At nanoscale, materials may behave as conductors, semiconductors, or insulators depending on size.
  • Thus, changing the size of nanomaterials affects optical, physical, and electrical properties simultaneously.

Therefore, all listed properties change with variation in the size of a nanomaterial.

Question 117

The magnetic Lines of force are repelled from a superconductor. The effect is known as :

  1. AOption (a)

    BCS effect

  2. BOption (b)

    Seebeck effect

  3. COption (c)

    Meissner effect

  4. DOption (d)

    Josephson effect

View answer & explanation

Correct answer

Answer: (c)C.

Meissner effect

Why this answer is correct

Concept

  • Superconductivity is a state of zero electrical resistance occurring in certain materials below a characteristic critical temperature.
  • A defining property of a superconductor is its interaction with external magnetic fields, where it acts as a perfect diamagnet.
<br>

Explanation

The phenomenon where magnetic lines of force are expelled from the interior of a superconducting material when it is cooled below its critical temperature is known as the Meissner effect.

When a material transitions into the superconducting state in the presence of an external magnetic field, internal currents are generated that produce a magnetic field exactly opposing the external one. This results in a net magnetic field of zero within the bulk of the superconductor (B=0B = 0).

Brief overview of other terms mentioned:

  • BCS theory: This is a microscopic theory that explains how electrons form "Cooper pairs" to enable superconductivity.
  • Seebeck effect: This is a thermoelectric phenomenon where a temperature difference between two different conductors or semiconductors creates a voltage.
  • Josephson effect: This refers to the flow of electric current (via tunneling of Cooper pairs) between two superconductors separated by a very thin insulating barrier.

Hence, the effect is known as the Meissner effect.

Question 118

The value of critical field Hc\mathrm{H}_{\mathrm{c}} for type-II Superconductors is :

  1. AOption (a)

    lower than that of type-I super conductor

  2. BOption (b)

    higher than that of type-I super conductor

  3. COption (c)

    same as that of type-I super conductor

  4. DOption (d)

    No comparison is possible.

View answer & explanation

Correct answer

Answer: (b)B.

higher than that of type-I super conductor

Why this answer is correct

Concept

  • Superconductors are classified into Type-I and Type-II based on how they behave when placed in an external magnetic field.
  • Type-I superconductors (soft superconductors) lose their superconductivity abruptly when the external magnetic field exceeds a single critical value HcH_c.
  • Type-II superconductors (hard superconductors) have two critical magnetic fields: a lower critical field Hc1H_{c1} and an upper critical field Hc2H_{c2}.
<br>

Explanation

The magnetic properties of the two types of superconductors differ significantly:

  • In Type-I superconductors, the critical field HcH_c is generally very low (typically less than 0.1 Tesla).
  • In Type-II superconductors, between Hc1H_{c1} and Hc2H_{c2}, the material enters a "mixed state" or "vortex state" where the magnetic field partially penetrates, but the material remains superconducting.
  • The upper critical field Hc2H_{c2} for Type-II superconductors is significantly higher (often several orders of magnitude) than the critical field HcH_c of Type-I superconductors.
  • Because Type-II superconductors can withstand much higher magnetic fields without losing their superconducting properties, they are used for making powerful superconducting magnets.
<br>

Conclusion

Comparing the maximum field at which superconductivity can exist, Type-II superconductors possess an upper critical field that is much greater than the critical field of Type-I superconductors.

Hence, the value of the critical field for type-II superconductors is higher than that of type-I superconductors.

Question 119

If we apply a dc voltage VV across a Josephson junction, the frequency of ac Josephson current is :

  1. AOption (a)

    ω=evℏ\omega=\frac{\mathrm{ev}}{\hbar}

  2. BOption (b)

    ω=2eVℏ\omega=\frac{2 \mathrm{eV}}{\hbar}

  3. COption (c)

    ω=eV2 h\omega=\frac{\mathrm{eV}}{2 \mathrm{~h}}

  4. DOption (d)

    ω=2 Veh\omega=\frac{2 \mathrm{~V}}{\mathrm{eh}}

View answer & explanation

Correct answer

Answer: (b)B.

ω=2eVℏ\omega=\frac{2 \mathrm{eV}}{\hbar}

Why this answer is correct

Concept:

The Josephson effect describes the flow of supercurrent across two superconductors separated by a thin insulating barrier.

AC Josephson Effect:

When a constant dc voltage (V) is applied across a Josephson junction, an alternating current is produced.

Relation: ω = 2eV / ħ

where,

  • ω = angular frequency of AC current
  • e = charge of electron
  • V = applied voltage
  • ħ = reduced Planck’s constant

Explanation:

The factor 2e appears because charge carriers in superconductors are Cooper pairs (charge = 2e).

Hence, the frequency of AC Josephson current is given by ω = 2eV / ħ.

Question 120

The decay equation of a magnetic field external to the super conductor is given by:

  1. AOption (a)

    B→(x)=B→(0)e−x/λL\overrightarrow{\mathrm{B}}(x)=\overrightarrow{\mathrm{B}}(0) \mathrm{e}^{-x / \lambda_{L}}

  2. BOption (b)

    B→(x)=B→(0)e−λL/x\overrightarrow{\mathrm{B}}(x)=\overrightarrow{\mathrm{B}}(0) \mathrm{e}^{-\lambda_{\mathrm{L}} / x}

  3. COption (c)

    B→(x)=B→(0)e−xλL\overrightarrow{\mathrm{B}}(x)=\overrightarrow{\mathrm{B}}(0) \mathrm{e}^{-x \lambda_{L}}

  4. DOption (d)

    B⃗(x)=B⃗(0)e−2x/λL\mathrm{\vec{B}(x)=\vec{B}(0) e^{-2 x / \lambda_{L}}}

View answer & explanation

Correct answer

Answer: (a)A.

B→(x)=B→(0)e−x/λL\overrightarrow{\mathrm{B}}(x)=\overrightarrow{\mathrm{B}}(0) \mathrm{e}^{-x / \lambda_{L}}

Why this answer is correct

Concept:

In superconductors, the magnetic field does not penetrate completely but decays exponentially inside the material due to the Meissner effect.

London Penetration Depth:

The characteristic distance over which the magnetic field decays is called the London penetration depth (λL).

Expression: B(x) = B(0)e−x/λL

Explanation:

  • B(0) is the magnetic field at the surface (x = 0).
  • As we move inside the superconductor (increasing x), the field decreases exponentially.
  • This shows that the magnetic field is expelled from the interior.
<br>

Hence, the correct decay equation is B(x) = B(0)e−x/λL.

Question 121

In nano phase Graphene is:

  1. AOption (a)

    Three dimensional structure of graphite

  2. BOption (b)

    Two dimensional structure of graphite

  3. COption (c)

    One dimensional structure of graphite

  4. DOption (d)

    None of the above

View answer & explanation

Correct answer

Answer: (b)B.

Two dimensional structure of graphite

Why this answer is correct

CONCEPT:

Structure of Graphene

  • Graphene is a single layer of carbon atoms arranged in a hexagonal (honeycomb) lattice.
  • It is derived from graphite, which consists of multiple stacked layers of graphene.
  • Based on dimensions:
  • 0D → Nanoparticles
  • 1D → Nanotubes or nanowires
  • 2D → Thin sheets like graphene
  • 3D → Bulk materials like graphite

EXPLANATION:

  

  • Graphite is a three-dimensional structure made of many stacked layers.
  • When a single layer is isolated from graphite, it forms graphene.
  • This single layer has:
  • Length and breadth (2 dimensions)
  • Negligible thickness (almost no third dimension)
  • Hence, graphene is classified as a two-dimensional nanomaterial.

Therefore, graphene in nano phase is a two-dimensional structure derived from graphite.

Question 122

Carbon nanotube:

(A) has high mechanical strength.

(B) is brittle material.

(C) can be used for electronic devices.

(D) can have only single wall.

 

The correct statements are :

  1. AOption (a)

    C and D

  2. BOption (b)

    A and D

  3. COption (c)

    A and C

  4. DOption (d)

    A, C and D

View answer & explanation

Correct answer

Answer: (c)C.

A and C

Why this answer is correct

CONCEPT:

Carbon Nanotubes (CNTs)

  • Carbon nanotubes are cylindrical nanostructures made of rolled graphene sheets.
  • They exhibit exceptional properties due to their unique structure and strong carbon–carbon bonds.
  • Types of CNTs:
  • Single-walled nanotubes (SWCNTs)
  • Multi-walled nanotubes (MWCNTs)

EXPLANATION:

  • Statement (A):
  • CNTs have extremely high tensile strength and are among the strongest known materials.
  • Hence, this statement is correct.
  • Statement (B):
  • CNTs are not brittle; they are flexible and exhibit high elasticity.
  • Hence, this statement is incorrect.
  • Statement (C):
  • CNTs can behave as conductors or semiconductors depending on their structure.
  • They are widely used in nanoelectronics and electronic devices.
  • Hence, this statement is correct.
  • Statement (D):
  • CNTs can exist as both single-walled and multi-walled structures.
  • Thus, saying they can have only a single wall is incorrect.

Therefore, the correct statements are A and C.

Question 123

The Chemical bonding of Carbon nanotubes is composed entirely of :

  1. AOption (a)

    sp bonds

  2. BOption (b)

    sp2\mathrm{sp}^{2} bonds

  3. COption (c)

    sp3\mathrm{sp}^{3} bonds

  4. DOption (d)

    None of the above

View answer & explanation

Correct answer

Answer: (b)B.

sp2\mathrm{sp}^{2} bonds

Why this answer is correct

CONCEPT:

Hybridization in Carbon Nanotubes

  • Carbon atoms can exhibit different types of hybridization: sp, sp2, and sp3.
  • In graphene and carbon nanotubes, each carbon atom is bonded to three neighboring carbon atoms.
  • This leads to sp2 hybridization, forming a planar hexagonal structure.
  • The remaining unhybridized p-orbital forms delocalized π-bonds, contributing to electrical conductivity.

EXPLANATION:

  • Carbon nanotubes are formed by rolling a graphene sheet into a cylindrical shape.
  • Since graphene consists entirely of sp2 hybridized carbon atoms, the same bonding exists in nanotubes.
  • Each carbon atom:
  • Forms three σ-bonds using sp2 orbitals
  • Has one unhybridized p-orbital forming π-bonds
  • This bonding gives CNTs their:
  • High strength
  • Excellent electrical conductivity

Therefore, carbon nanotubes are composed entirely of sp2 hybridized bonds.

Question 124

Reduction of only one dimension, two dimensions only and all three dimensions of a material to nanoscale leads to the structures respectively as :

  1. AOption (a)

    quantum wire, quantum dot and quantum well

  2. BOption (b)

    quantum well, quantum wire and quantum dot

  3. COption (c)

    quantum wire, quantum well and quantum dot

  4. DOption (d)

    quantum dot, quantum well and quantum wire

View answer & explanation

Correct answer

Answer: (b)B.

quantum well, quantum wire and quantum dot

Why this answer is correct

CONCEPT:

Quantum Confinement in Nanostructures

  • When the dimensions of a material are reduced to nanoscale, electrons become confined, leading to quantum effects.
  • The type of nanostructure depends on how many dimensions are reduced:
  • Reduction in one dimension → electrons are free in two dimensions → Quantum Well (2D)
  • Reduction in two dimensions → electrons are free in one dimension → Quantum Wire (1D)
  • Reduction in three dimensions → electrons are confined in all directions → Quantum Dot (0D)

EXPLANATION:

  • If only one dimension is reduced (thin layer), motion is restricted in one direction → forms a quantum well.
  • If two dimensions are reduced (very thin wire), motion is restricted in two directions → forms a quantum wire.
  • If all three dimensions are reduced (nanoparticle), motion is restricted in all directions → forms a quantum dot.

Therefore, the correct sequence is: quantum well, quantum wire and quantum dot.

Question 125

Fullerene is a :

  1. AOption (a)

    Tennis ball shaped molecule of 14 Carbon atoms held together by covalent bonds.

  2. BOption (b)

    Tennis ball shaped molecule of 30 Carbon atoms held together by van der Waal's forces.

  3. COption (c)

    Soccer ball shaped molecule of 60 Carbon atoms held together by van der Waal's forces.

  4. DOption (d)

    Soccer ball shaped molecule of 60 Carbon atoms held together by covalent bonds.

View answer & explanation

Correct answer

Answer: (d)D.

Soccer ball shaped molecule of 60 Carbon atoms held together by covalent bonds.

Why this answer is correct

CONCEPT:

Fullerene Structure

  • Fullerenes are allotropes of carbon composed entirely of carbon atoms arranged in closed hollow structures.
  • The most common fullerene is C60, also known as Buckminsterfullerene.
  • It has a structure similar to a soccer ball, made up of:
  • 12 pentagons
  • 20 hexagons
  • All carbon atoms in fullerene are bonded through strong covalent bonds (sp2 hybridization).

EXPLANATION:

  • Fullerene (C60) contains exactly 60 carbon atoms.
  • Its shape resembles a soccer ball, not a tennis ball.
  • The bonding between carbon atoms is covalent, not van der Waal's forces.
  • Van der Waal’s forces may exist between different fullerene molecules, but not within a single molecule.

Therefore, fullerene is a soccer ball shaped molecule of 60 carbon atoms held together by covalent bonds.

Question 126

Every Hermitian matrix AA can be expressed as B+iC\mathrm{B}+\mathrm{iC} where :

  1. AOption (a)

    Both B and C are real and symmetric.

  2. BOption (b)

    Both B and C are real and skew-symmetric.

  3. COption (c)

    B is real and skew-symmetric and C is real and symmetric.

  4. DOption (d)

    B is real and symmetric and C is real and skew-symmetric.

View answer & explanation

Correct answer

Answer: (d)D.

B is real and symmetric and C is real and skew-symmetric.

Why this answer is correct

Concept:

A Hermitian matrix A satisfies:

A† = A

(i.e., conjugate transpose of A equals A)

Decomposition:

Any complex matrix can be written as:

A = B + iC

where B and C are real matrices.

Condition for Hermitian matrix:

A† = (B + iC)† = BT − iCT

Since A† = A, we get:

BT − iCT = B + iC

Comparing real and imaginary parts:

  • BT = B ⇒ B is symmetric
  • −CT = C ⇒ CT = −C ⇒ C is skew-symmetric

Hence, B is real and symmetric, and C is real and skew-symmetric.

Question 127

The values of constants λ\lambda and μ\mu which shall makeA⃗=(x+2y+z)i^+(λx+y+3z)j^+ (x+μy+z)k^\begin{aligned} & \vec{A}=(x+2 y+z) \hat{i}+(\lambda x+y+3 z) \hat{j}+ \ & (x+\mu y+z) \hat{k} \end{aligned}

irrotational, are respectively :

  1. AOption (a)

    2 and 3

  2. BOption (b)

    3 and 2

  3. COption (c)

    4 and 5

  4. DOption (d)

    7 and 6

View answer & explanation

Correct answer

Answer: (a)A.

2 and 3

Why this answer is correct

Concept

  • A vector field is said to be irrotational if its curl is equal to the zero vector.
  • The curl of a vector field measures the rotation of the field at a point.
  • In Cartesian coordinates, the curl is calculated using the determinant of a matrix involving partial derivative operators and the field components.
<br>

Formula Used

For a vector field A⃗=Axi^+Ayj^+Azk^\vec{A} = A_x\hat{i} + A_y\hat{j} + A_z\hat{k} to be irrotational:

curl A⃗=∇×A⃗=0⃗\text{curl } \vec{A} = \nabla \times \vec{A} = \vec{0}

∇×A⃗=∣i^j^k^ ∂∂x∂∂y∂∂z AxAyAz∣=0⃗\nabla \times \vec{A} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \ \frac{\partial}{\partial x} & \frac{\partial}{\partial y} & \frac{\partial}{\partial z} \ A_x & A_y & A_z \end{vmatrix} = \vec{0}

<br>

Calculation

Given the vector field:

A⃗=(x+2y+z)i^+(λx+y+3z)j^+(x+μy+z)k^\vec{A}=(x+2 y+z) \hat{i}+(\lambda x+y+3 z) \hat{j}+(x+\mu y+z) \hat{k}

The components are:

Ax=x+2y+zA_x = x + 2y + z

Ay=λx+y+3zA_y = \lambda x + y + 3z

Az=x+μy+zA_z = x + \mu y + z

Calculating the curl components:

1. i-component:

∂Az∂y−∂Ay∂z=0\frac{\partial A_z}{\partial y} - \frac{\partial A_y}{\partial z} = 0

⇒∂∂y(x+μy+z)−∂∂z(λx+y+3z)=0\Rightarrow \frac{\partial}{\partial y}(x + \mu y + z) - \frac{\partial}{\partial z}(\lambda x + y + 3z) = 0

⇒μ−3=0\Rightarrow \mu - 3 = 0

⇒μ=3\Rightarrow \mu = 3

2. j-component:

∂Ax∂z−∂Az∂x=0\frac{\partial A_x}{\partial z} - \frac{\partial A_z}{\partial x} = 0

⇒∂∂z(x+2y+z)−∂∂x(x+μy+z)=0\Rightarrow \frac{\partial}{\partial z}(x + 2y + z) - \frac{\partial}{\partial x}(x + \mu y + z) = 0

⇒1−1=0\Rightarrow 1 - 1 = 0 (This condition is always satisfied)

3. k-component:

∂Ay∂x−∂Ax∂y=0\frac{\partial A_y}{\partial x} - \frac{\partial A_x}{\partial y} = 0

⇒∂∂x(λx+y+3z)−∂∂y(x+2y+z)=0\Rightarrow \frac{\partial}{\partial x}(\lambda x + y + 3z) - \frac{\partial}{\partial y}(x + 2y + z) = 0

⇒λ−2=0\Rightarrow \lambda - 2 = 0

⇒λ=2\Rightarrow \lambda = 2

Comparing the values obtained, we have λ=2\lambda = 2 and μ=3\mu = 3.

Hence, the values of constants λ\lambda and μ\mu are 2 and 3 respectively.

Question 128

The differential equation

x2 d2ydx2+xdydx+(x2−m2)y=0x^{2} \frac{\mathrm{~d}^{2} \mathrm{y}}{\mathrm{d} x^{2}}+x \frac{\mathrm{dy}}{\mathrm{d} x}+\left(x^{2}-\mathrm{m}^{2}\right) \mathrm{y}=0 where mm is a parameter, is :

  1. AOption (a)

    Hermite differential equation

  2. BOption (b)

    Legendre differential equation

  3. COption (c)

    Laguerre differential equation

  4. DOption (d)

    Bessel differential equation

View answer & explanation

Correct answer

Answer: (d)D.

Bessel differential equation

Why this answer is correct

Concept

  • The given equation is a second-order linear ordinary differential equation.
  • Specific forms of differential equations are named after mathematicians who extensively studied their properties and solutions (e.g., Bessel, Legendre, Hermite, and Laguerre).
  • These equations frequently appear in problems involving cylindrical symmetry, wave propagation, and quantum mechanics.
<br>

Standard Forms

To identify the given equation, we compare it with the standard forms of the differential equations mentioned in the options:

  1. Bessel Differential Equation:

x2d2ydx2+xdydx+(x2−n2)y=0x^{2} \frac{d^{2}y}{dx^{2}} + x \frac{dy}{dx} + (x^{2} - n^{2})y = 0

  1. Legendre Differential Equation:

(1−x2)d2ydx2−2xdydx+n(n+1)y=0(1 - x^{2}) \frac{d^{2}y}{dx^{2}} - 2x \frac{dy}{dx} + n(n + 1)y = 0

  1. Hermite Differential Equation:

d2ydx2−2xdydx+2ny=0\frac{d^{2}y}{dx^{2}} - 2x \frac{dy}{dx} + 2ny = 0

  1. Laguerre Differential Equation:

xd2ydx2+(1−x)dydx+ny=0x \frac{d^{2}y}{dx^{2}} + (1 - x) \frac{dy}{dx} + ny = 0

<br>

Identification

The equation provided in the question is:

x2d2ydx2+xdydx+(x2−m2)y=0x^{2} \frac{\mathrm{d}^{2} \mathrm{y}}{\mathrm{d} x^{2}} + x \frac{\mathrm{dy}}{\mathrm{d} x} + (x^{2} - m^{2}) \mathrm{y} = 0

By comparing this with the standard forms listed above, we can see that it perfectly matches the Bessel differential equation, where mm serves as the parameter (order) of the equation.

Hence, the given differential equation is the Bessel differential equation.

Question 129

The Fourier transform of e−ax2,a>0e^{-a x^{2}}, a>0 is :

  1. AOption (a)

    12ae−w2/2a\frac{1}{\sqrt{2 a}} e^{-w^{2} / 2 a}

  2. BOption (b)

    1ae−w2/4a\frac{1}{\sqrt{\mathrm{a}}} \mathrm{e}^{-\mathrm{w}^{2} / 4 \mathrm{a}}

  3. COption (c)

    12ae−w2/4a\frac{1}{\sqrt{2 a}} e^{-w^{2} / 4 a}

  4. DOption (d)

    2ae−w2/4a\sqrt{2 a} e^{-w^{2} / 4 a}

View answer & explanation

Correct answer

Answer: (a)A.

12ae−w2/2a\frac{1}{\sqrt{2 a}} e^{-w^{2} / 2 a}

Why this answer is correct

Concept

  • The Fourier transform of a function converts it from the spatial (or time) domain to the frequency domain.
  • A key property of the Fourier transform is that the Fourier transform of a Gaussian function is itself a Gaussian function.
  • The process involves completing the square in the exponent of the integrand and using the standard Gaussian integral.
<br>

Formula Used

The Fourier transform of a function f(x)f(x) is defined as:

F(w)=12π∫−∞∞f(x)eiwxdxF(w) = \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} f(x) e^{iwx} dx

Standard Gaussian integral:

∫−∞∞e−u2du=π\int_{-\infty}^{\infty} e^{-u^2} du = \sqrt{\pi}

<br>

Calculation

Given function:

f(x)=e−ax2f(x) = e^{-ax^2}

Applying the Fourier transform definition:

F(w)=12π∫−∞∞e−ax2eiwxdxF(w) = \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} e^{-ax^2} e^{iwx} dx

F(w)=12π∫−∞∞e−(ax2−iwx)dxF(w) = \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} e^{-(ax^2 - iwx)} dx

To evaluate the integral, we complete the square for the term in the exponent:

ax2−iwx=a(x2−iwax)ax^2 - iwx = a \left( x^2 - \frac{iw}{a}x \right)

ax2−iwx=a[x2−iwax+(iw2a)2−(iw2a)2]ax^2 - iwx = a \left[ x^2 - \frac{iw}{a}x + \left( \frac{iw}{2a} \right)^2 - \left( \frac{iw}{2a} \right)^2 \right]

ax2−iwx=a(x−iw2a)2+w24aax^2 - iwx = a \left( x - \frac{iw}{2a} \right)^2 + \frac{w^2}{4a}

Substituting this back into the integral:

F(w)=12π∫−∞∞e−[a(x−iw2a)2+w24a]dxF(w) = \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} e^{-\left[ a \left( x - \frac{iw}{2a} \right)^2 + \frac{w^2}{4a} \right]} dx

F(w)=12πe−w2/4a∫−∞∞e−a(x−iw2a)2dxF(w) = \frac{1}{\sqrt{2\pi}} e^{-w^2/4a} \int_{-\infty}^{\infty} e^{-a \left( x - \frac{iw}{2a} \right)^2} dx

Let u=a(x−iw2a)u = \sqrt{a} \left( x - \frac{iw}{2a} \right), then du=adxdu = \sqrt{a} dx:

F(w)=12πe−w2/4a⋅1a∫−∞∞e−u2duF(w) = \frac{1}{\sqrt{2\pi}} e^{-w^2/4a} \cdot \frac{1}{\sqrt{a}} \int_{-\infty}^{\infty} e^{-u^2} du

Using the Gaussian integral ∫−∞∞e−u2du=π\int_{-\infty}^{\infty} e^{-u^2} du = \sqrt{\pi}:

F(w)=12πe−w2/4a⋅πaF(w) = \frac{1}{\sqrt{2\pi}} e^{-w^2/4a} \cdot \frac{\sqrt{\pi}}{\sqrt{a}}

F(w)=π2πae−w2/4aF(w) = \frac{\sqrt{\pi}}{\sqrt{2\pi a}} e^{-w^2/4a}

F(w)=12ae−w2/4aF(w) = \frac{1}{\sqrt{2a}} e^{-w^2/4a}

Hence, the Fourier transform of e−ax2e^{-ax^2} is 12ae−w2/4a\frac{1}{\sqrt{2a}} e^{-w^2/4a}.

Question 130

The value of grad⁡(1r)\operatorname{grad}\left(\frac{1}{r}\right) is :

  1. AOption (a)

    −r⃗r3-\frac{\vec{r}}{r^{3}}

  2. BOption (b)

    −r⃗r2-\frac{\vec{r}}{r^{2}}

  3. COption (c)

    −r⃗r-\frac{\vec{r}}{r}

  4. DOption (d)

    zero

View answer & explanation

Correct answer

Answer: (a)A.

−r⃗r3-\frac{\vec{r}}{r^{3}}

Why this answer is correct

Concept:

Let r = √(x² + y² + z²). The scalar function is φ = 1/r.

Calculation:

φ = (x² + y² + z²)−1/2

∂φ/∂x = −x / r3

∂φ/∂y = −y / r3

∂φ/∂z = −z / r3

Thus,

∇(1/r) = − (x i + y j + z k) / r3

⇒ ∇(1/r) = − r / r3

Conclusion:

grad(1/r) = − r / r3

Question 131

The Fourier coefficient bnb_{n} of the periodic function f(x)\mathrm{f}(x) ,\(\begin{aligned} & \mathrm{f}(x)=\left{\begin{aligned} -\mathrm{K} & \text { if }-\pi<x<0 \ \mathrm{~K} & \text { if } 0<x<\pi \end{aligned}\right. \text { and } \ & \mathrm{f}(x+2 \pi)=\mathrm{f}(x) \end{aligned} \)

is given by :

  1. AOption (a)

    bn=2Kπ(1−cos⁡nπ)\mathrm{b_{n}=\frac{2 K}{\pi}(1-\cos n \pi)}

  2. BOption (b)

    bn=2Knπ(1−cos⁡nπ)\mathrm{b_{n}=\frac{2 K}{n \pi}(1-\cos n \pi)}

  3. COption (c)

    bn=2Kπ(1+cos⁡nπ)\mathrm{b_{n}=\frac{2 K}{\pi}(1+\cos n \pi)}

  4. DOption (d)

    bn=2 Knπ(1+cos⁡nπ)\mathrm{b}_{\mathrm{n}}=\frac{2 \mathrm{~K}}{\mathrm{n} \pi}(1+\cos \mathrm{n} \pi)

View answer & explanation

Correct answer

Answer: (b)B.

bn=2Knπ(1−cos⁡nπ)\mathrm{b_{n}=\frac{2 K}{n \pi}(1-\cos n \pi)}

Why this answer is correct

Calculations: 

Given periodic function:

f(x) = { −K , for −π < x < 0

             K , for 0 < x < π }

and f(x + 2π) = f(x)

Fourier sine coefficient formula:

bn = (1 / π) ∫−ππ f(x) sin(nx) dx

Split the integral:

bn = (1 / π) [ ∫−π0 (−K) sin(nx) dx + ∫0π K sin(nx) dx ]

Take constants out:

bn = (K / π) [ −∫−π0 sin(nx) dx + ∫0π sin(nx) dx ]

Integration formula:

∫ sin(nx) dx = −cos(nx) / n

Evaluate first integral:

∫−π0 sin(nx) dx = [ −cos(nx) / n ]−π0

= (−1/n)(1 − cos(nπ))

Evaluate second integral:

∫0π sin(nx) dx = [ −cos(nx) / n ]0π

= (1/n)(1 − cos(nπ))

Substitute:

bn = (K / π) [ (1/n)(1 − cos(nπ)) + (1/n)(1 − cos(nπ)) ]

⇒ bn = (2K / nπ)(1 − cos(nπ))

Final Answer: bn = (2K / nπ)(1 − cos(nπ))

Question 132

If a^\hat{a} and b^\hat{b} are unit vectors and θ\theta is the angle between them, the value of 12∣a^−b^∣\frac{1}{2}|\hat{\mathrm{a}}-\hat{\mathrm{b}}| will be

  1. AOption (a)

    sin⁡θ/2\sin \theta / 2

  2. BOption (b)

    cos⁡θ/2\cos \theta / 2

  3. COption (c)

    sin⁡θ\sin \theta

  4. DOption (d)

    cos⁡θ\cos \theta

View answer & explanation

Correct answer

Answer: (a)A.

sin⁡θ/2\sin \theta / 2

Why this answer is correct

Concept

  • A unit vector is a vector whose magnitude is exactly 1.
  • The magnitude of the difference between two vectors can be calculated using the law of cosines for vectors.
  • Trigonometric half-angle identities are used to simplify vector magnitude expressions.
<br>

Formula Used

Magnitude of unit vectors:

∣a^∣=∣b^∣=1|\hat{a}| = |\hat{b}| = 1

Magnitude of vector difference:

∣a^−b^∣=∣a^∣2+∣b^∣2−2∣a^∣∣b^∣cos⁡θ|\hat{a} - \hat{b}| = \sqrt{|\hat{a}|^2 + |\hat{b}|^2 - 2|\hat{a}||\hat{b}|\cos\theta}

Trigonometric identity:

1−cos⁡θ=2sin⁡2θ21 - \cos\theta = 2\sin^2\frac{\theta}{2}

<br>

Calculation

Given that a^\hat{a} and b^\hat{b} are unit vectors, their magnitudes are:

∣a^∣=1|\hat{a}| = 1 and ∣b^∣=1|\hat{b}| = 1

The magnitude of the difference between these two vectors is:

∣a^−b^∣=12+12−2(1)(1)cos⁡θ|\hat{a} - \hat{b}| = \sqrt{1^2 + 1^2 - 2(1)(1)\cos\theta}

∣a^−b^∣=2−2cos⁡θ|\hat{a} - \hat{b}| = \sqrt{2 - 2\cos\theta}

∣a^−b^∣=2(1−cos⁡θ)|\hat{a} - \hat{b}| = \sqrt{2(1 - \cos\theta)}

Applying the trigonometric identity 1−cos⁡θ=2sin⁡2θ21 - \cos\theta = 2\sin^2\frac{\theta}{2}:

∣a^−b^∣=2⋅2sin⁡2θ2|\hat{a} - \hat{b}| = \sqrt{2 \cdot 2\sin^2\frac{\theta}{2}}

∣a^−b^∣=4sin⁡2θ2|\hat{a} - \hat{b}| = \sqrt{4\sin^2\frac{\theta}{2}}

∣a^−b^∣=2sin⁡θ2|\hat{a} - \hat{b}| = 2\sin\frac{\theta}{2}

The problem asks for the value of 12∣a^−b^∣\frac{1}{2}|\hat{a} - \hat{b}|:

12∣a^−b^∣=12(2sin⁡θ2)\frac{1}{2}|\hat{a} - \hat{b}| = \frac{1}{2} \left( 2\sin\frac{\theta}{2} \right)

12∣a^−b^∣=sin⁡θ2\frac{1}{2}|\hat{a} - \hat{b}| = \sin\frac{\theta}{2}

Hence, the value is sin⁡θ/2\sin \theta / 2.

Question 133

Given vectors A⃗=2i^−3j^−k^\vec{A}=2 \hat{i}-3 \hat{j}-\hat{k}

 and B⃗=−6i^+9j^+3k^\text { and } \vec{B}=-6 \hat{i}+9 \hat{j}+3 \hat{k}

 

Then these two vectors are :

  1. AOption (a)

    perpendicular to each other

  2. BOption (b)

    parallel to each other

  3. COption (c)

    A⃗+B⃗\vec{A}+\vec{B} and A⃗−B⃗\vec{A}-\vec{B} have angle π/2\pi / 2 between them

  4. DOption (d)

    None of the above

View answer & explanation

Correct answer

Answer: (b)B.

parallel to each other

Why this answer is correct

Given:

A = 2i − 3j − k

B = −6i + 9j + 3k

Concept:

Two vectors are parallel if one is a scalar multiple of the other.

Calculation:

B = −3(2i − 3j − k)

⇒ B = −3A

Since one vector is a scalar multiple of the other, the vectors are parallel to each other.

Question 134

In reference to vectors, the Lagrange identity is :

  1. AOption (a)

    a⃗×(b⃗×c⃗)=b⃗(a⃗⋅c⃗)−c⃗(a⃗⋅b⃗)\vec{a} \times(\vec{b} \times \vec{c})=\vec{b}(\vec{a} \cdot \vec{c})-\vec{c}(\vec{a} \cdot \vec{b})

  2. BOption (b)

    ∇⃗⋅(a⃗×b⃗)=b⃗⋅(∇⃗×a⃗)−a⃗⋅(∇⃗×b⃗)\vec{\nabla} \cdot(\vec{a} \times \vec{b})=\vec{b} \cdot(\vec{\nabla} \times \vec{a})-\vec{a} \cdot(\vec{\nabla} \times \vec{b})

  3. COption (c)

    ∣a⃗×b⃗∣2=(a⃗⋅a⃗)(b⃗⋅b⃗)−(a⃗⋅b⃗)2|\vec{a} \times \vec{b}|^{2}=(\vec{a} \cdot \vec{a})(\vec{b} \cdot \vec{b})-(\vec{a} \cdot \vec{b})^{2}

  4. DOption (d)

    ∣a⃗×b⃗∣2=(a⃗⋅a⃗)(b⃗⋅b⃗)+(a⃗⋅b⃗)2|\vec{a} \times \vec{b}|^{2}=(\vec{a} \cdot \vec{a})(\vec{b} \cdot \vec{b})+(\vec{a} \cdot \vec{b})^{2}

View answer & explanation

Correct answer

Answer: (c)C.

∣a⃗×b⃗∣2=(a⃗⋅a⃗)(b⃗⋅b⃗)−(a⃗⋅b⃗)2|\vec{a} \times \vec{b}|^{2}=(\vec{a} \cdot \vec{a})(\vec{b} \cdot \vec{b})-(\vec{a} \cdot \vec{b})^{2}

Why this answer is correct

Concept

  • Lagrange's identity for vectors establishes a fundamental relationship between the dot product and the magnitude of the cross product of two vectors.
  • It states that the square of the magnitude of the vector cross product is equal to the product of the squares of the magnitudes of the individual vectors minus the square of their dot product.
  • This identity is particularly useful in vector algebra and differential geometry.
<br>

Formula Used

Magnitude of cross product:

∣a⃗×b⃗∣=∣a⃗∣∣b⃗∣sin⁡θ|\vec{a} \times \vec{b}| = |\vec{a}| |\vec{b}| \sin \theta

Dot product of vectors:

a⃗⋅b⃗=∣a⃗∣∣b⃗∣cos⁡θ\vec{a} \cdot \vec{b} = |\vec{a}| |\vec{b}| \cos \theta

Relation between magnitude squared and dot product:

∣a⃗∣2=a⃗⋅a⃗|\vec{a}|^2 = \vec{a} \cdot \vec{a}

<br>

Calculation

To derive the Lagrange identity, start with the square of the magnitude of the cross product:

∣a⃗×b⃗∣2=(∣a⃗∣∣b⃗∣sin⁡θ)2|\vec{a} \times \vec{b}|^2 = (|\vec{a}| |\vec{b}| \sin \theta)^2

∣a⃗×b⃗∣2=∣a⃗∣2∣b⃗∣2sin⁡2θ|\vec{a} \times \vec{b}|^2 = |\vec{a}|^2 |\vec{b}|^2 \sin^2 \theta

Using the fundamental trigonometric identity sin⁡2θ=1−cos⁡2θ\sin^2 \theta = 1 - \cos^2 \theta:

∣a⃗×b⃗∣2=∣a⃗∣2∣b⃗∣2(1−cos⁡2θ)|\vec{a} \times \vec{b}|^2 = |\vec{a}|^2 |\vec{b}|^2 (1 - \cos^2 \theta)

∣a⃗×b⃗∣2=∣a⃗∣2∣b⃗∣2−∣a⃗∣2∣b⃗∣2cos⁡2θ|\vec{a} \times \vec{b}|^2 = |\vec{a}|^2 |\vec{b}|^2 - |\vec{a}|^2 |\vec{b}|^2 \cos^2 \theta

Rearranging the second term:

∣a⃗×b⃗∣2=∣a⃗∣2∣b⃗∣2−(∣a⃗∣∣b⃗∣cos⁡θ)2|\vec{a} \times \vec{b}|^2 = |\vec{a}|^2 |\vec{b}|^2 - (|\vec{a}| |\vec{b}| \cos \theta)^2

Substituting the definition of the dot product a⃗⋅b⃗=∣a⃗∣∣b⃗∣cos⁡θ\vec{a} \cdot \vec{b} = |\vec{a}| |\vec{b}| \cos \theta:

∣a⃗×b⃗∣2=∣a⃗∣2∣b⃗∣2−(a⃗⋅b⃗)2|\vec{a} \times \vec{b}|^2 = |\vec{a}|^2 |\vec{b}|^2 - (\vec{a} \cdot \vec{b})^2

Since ∣a⃗∣2=a⃗⋅a⃗|\vec{a}|^2 = \vec{a} \cdot \vec{a} and ∣b⃗∣2=b⃗⋅b⃗|\vec{b}|^2 = \vec{b} \cdot \vec{b}, we can write:

∣a⃗×b⃗∣2=(a⃗⋅a⃗)(b⃗⋅b⃗)−(a⃗⋅b⃗)2|\vec{a} \times \vec{b}|^2 = (\vec{a} \cdot \vec{a})(\vec{b} \cdot \vec{b}) - (\vec{a} \cdot \vec{b})^2

Hence, the Lagrange identity is ∣a⃗×b⃗∣2=(a⃗⋅a⃗)(b⃗⋅b⃗)−(a⃗⋅b⃗)2|\vec{a} \times \vec{b}|^{2}=(\vec{a} \cdot \vec{a})(\vec{b} \cdot \vec{b})-(\vec{a} \cdot \vec{b})^{2}.

Question 135

If A⃗\vec{A} and B⃗\vec{B} are unit vectors and θ\theta is the angle between them, then A⃗+B⃗\vec{A}+\vec{B} is a unit vector if θ\theta is :

  1. AOption (a)

    π/3\pi / 3

  2. BOption (b)

    π/4\pi / 4

  3. COption (c)

    π/2\pi / 2

  4. DOption (d)

    2π/32 \pi / 3

View answer & explanation

Correct answer

Answer: (d)D.

2π/32 \pi / 3

Why this answer is correct

Concept

  • A unit vector is a vector that has a magnitude of exactly 1.
  • The magnitude of the resultant of two vectors A⃗\vec{A} and B⃗\vec{B} with an angle θ\theta between them is given by the law of cosines for vectors.
  • For the sum of two unit vectors to also be a unit vector, the specific geometric arrangement must satisfy the magnitude equation.
<br>

Formula Used

The magnitude of the sum of two vectors A⃗\vec{A} and B⃗\vec{B} is:

∣A⃗+B⃗∣=∣A⃗∣2+∣B⃗∣2+2∣A⃗∣∣B⃗∣cos⁡θ|\vec{A} + \vec{B}| = \sqrt{|\vec{A}|^2 + |\vec{B}|^2 + 2|\vec{A}||\vec{B}|\cos\theta}

Squaring both sides:

∣A⃗+B⃗∣2=∣A⃗∣2+∣B⃗∣2+2∣A⃗∣∣B⃗∣cos⁡θ|\vec{A} + \vec{B}|^2 = |\vec{A}|^2 + |\vec{B}|^2 + 2|\vec{A}||\vec{B}|\cos\theta

<br>

Calculation

Given:

A⃗\vec{A} and B⃗\vec{B} are unit vectors, so ∣A⃗∣=1|\vec{A}| = 1 and ∣B⃗∣=1|\vec{B}| = 1.

The sum A⃗+B⃗\vec{A} + \vec{B} is also a unit vector, so ∣A⃗+B⃗∣=1|\vec{A} + \vec{B}| = 1.

Substitute these values into the squared magnitude formula:

12=12+12+2(1)(1)cos⁡θ1^2 = 1^2 + 1^2 + 2(1)(1)\cos\theta

1=1+1+2cos⁡θ1 = 1 + 1 + 2\cos\theta

1=2+2cos⁡θ1 = 2 + 2\cos\theta

Rearranging the equation to solve for cos⁡θ\cos\theta:

2cos⁡θ=1−22\cos\theta = 1 - 2

2cos⁡θ=−12\cos\theta = -1

cos⁡θ=−12\cos\theta = -\frac{1}{2}

The angle θ\theta whose cosine is −12-\frac{1}{2} is:

θ=cos⁡−1(−12)=120∘\theta = \cos^{-1}(-\frac{1}{2}) = 120^\circ

Converting degrees to radians:

θ=2π3\theta = \frac{2\pi}{3}

Hence, the angle between the unit vectors is 2π/32\pi / 3.

Question 136

The eigen values of the matrix

[M]=[−22−3 21−6 −1−20][M]=\left[\begin{array}{ccc}-2 & 2 & -3 \ 2 & 1 & -6 \ -1 & -2 & 0\end{array}\right]

are:

  1. AOption (a)

    5,−3,−35,-3,-3

  2. BOption (b)

    3,5,33,5,3

  3. COption (c)

    −3,5,3-3,5,3

  4. DOption (d)

    3,3,−53,3,-5

View answer & explanation

Correct answer

Answer: (a)A.

5,−3,−35,-3,-3

Why this answer is correct

Concept

  • The eigenvalues of a square matrix MM are the scalars λ\lambda that satisfy the characteristic equation det⁡(M−λI)=0\det(M - \lambda I) = 0.
  • Trace Property: The sum of the eigenvalues is equal to the trace of the matrix (the sum of the main diagonal elements).
  • Determinant Property: The product of the eigenvalues is equal to the determinant of the matrix.
<br>

Formula Used

Characteristic Equation:

∣M−λI∣=0|M - \lambda I| = 0

Trace of matrix MM:

Tr(M)=∑λi\text{Tr}(M) = \sum \lambda_i

Determinant of matrix MM:

det⁡(M)=∏λi\det(M) = \prod \lambda_i

<br>

Calculation

Given the matrix:

M=[−22−3 21−6 −1−20]M = \begin{bmatrix} -2 & 2 & -3 \ 2 & 1 & -6 \ -1 & -2 & 0 \end{bmatrix}

Step 1: Calculate the Trace of MM

Tr(M)=(−2)+1+0=−1\text{Tr}(M) = (-2) + 1 + 0 = -1

The sum of the eigenvalues must be −1-1.

Step 2: Calculate the Determinant of MM

det⁡(M)=−2∣1−6 −20∣−2∣2−6 −10∣−3∣21 −1−2∣\det(M) = -2 \begin{vmatrix} 1 & -6 \ -2 & 0 \end{vmatrix} - 2 \begin{vmatrix} 2 & -6 \ -1 & 0 \end{vmatrix} - 3 \begin{vmatrix} 2 & 1 \ -1 & -2 \end{vmatrix}

det⁡(M)=−2(0−12)−2(0−6)−3(−4−(−1))\det(M) = -2(0 - 12) - 2(0 - 6) - 3(-4 - (-1))

det⁡(M)=−2(−12)−2(−6)−3(−3)\det(M) = -2(-12) - 2(-6) - 3(-3)

det⁡(M)=24+12+9=45\det(M) = 24 + 12 + 9 = 45

The product of the eigenvalues must be 4545.

Step 3: Verify with given values

Let the eigenvalues be 5,−3,−35, -3, -3:

Sum: 5+(−3)+(−3)=5−6=−15 + (-3) + (-3) = 5 - 6 = -1 (Matches Trace)

Product: 5×(−3)×(−3)=5×9=455 \times (-3) \times (-3) = 5 \times 9 = 45 (Matches Determinant)

Alternatively, solving the characteristic equation:

λ3−(Tr M)λ2+(Sum of minors of diagonal)λ−det⁡(M)=0\lambda^3 - (\text{Tr } M)\lambda^2 + (\text{Sum of minors of diagonal})\lambda - \det(M) = 0

Sum of minors: M11=−12M_{11} = -12, M22=−3M_{22} = -3, M33=−6M_{33} = -6. Total sum = −21-21.

λ3−(−1)λ2+(−21)λ−45=0\lambda^3 - (-1)\lambda^2 + (-21)\lambda - 45 = 0

λ3+λ2−21λ−45=0\lambda^3 + \lambda^2 - 21\lambda - 45 = 0

Testing λ=5\lambda = 5: 125+25−105−45=0125 + 25 - 105 - 45 = 0.

Testing λ=−3\lambda = -3: −27+9+63−45=0-27 + 9 + 63 - 45 = 0.

Factorizing, we get (λ−5)(λ+3)2=0(\lambda - 5)(\lambda + 3)^2 = 0.

Hence, the eigenvalues of the matrix are 5,−3,−35, -3, -3.

Question 137

The Laplace transform of f(t)=1,t≥0f(t)=1, t \geq 0 is :

  1. AOption (a)

    1/S1 / \mathrm{S}

  2. BOption (b)

    1/S21 / \mathrm{S}^{2}

  3. COption (c)

    S\mathrm{S}

  4. DOption (d)

    S2\mathrm{S}^{2}

View answer & explanation

Correct answer

Answer: (a)A.

1/S1 / \mathrm{S}

Why this answer is correct

Concept

  • The Laplace transform converts a function of time t into a function of the complex variable s.
  • It is used to convert differential equations into algebraic equations.
  • For a function f(t) defined for t ≥ 0, the Laplace transform exists if the integral converges.

Formula Used

F(s)=Lf(t)=∫0∞e−stf(t),dtF(s)=\mathcal{L}{f(t)}=\int_{0}^{\infty}e^{-st}f(t),dt

Calculation

Given, f(t)=1f(t)=1.

L1=∫0∞e−st,dt\mathcal{L}{1}=\int_{0}^{\infty}e^{-st},dt

L1=[e−st−s]0∞\mathcal{L}{1}=\left[\frac{e^{-st}}{-s}\right]_{0}^{\infty}

For s>0s>0, e−st→0e^{-st}\to0 as t→∞t\to\infty.

L1=0−(−1s)=1s\mathcal{L}{1}=0-\left(\frac{-1}{s}\right)=\frac{1}{s}

Hence, L1=1s\mathcal{L}{1}=\frac{1}{s}.

Question 138

The eigen values of one dimensional harmonic oscillator are :

  1. AOption (a)

    hν0(n+1)\mathrm{h} \nu_{0}(\mathrm{n}+1)

  2. BOption (b)

    hv0(n+12)\mathrm{h} v_{0}\left(\mathrm{n}+\frac{1}{2}\right)

  3. COption (c)

    12 hv0(n+12)\frac{1}{2} \mathrm{~h} v_{0}\left(\mathrm{n}+\frac{1}{2}\right)

  4. DOption (d)

    12 hv0(n+1)\frac{1}{2} \mathrm{~h} v_{0}(\mathrm{n}+1)

View answer & explanation

Correct answer

Answer: (b)B.

hv0(n+12)\mathrm{h} v_{0}\left(\mathrm{n}+\frac{1}{2}\right)

Why this answer is correct

Concept

  • The quantum harmonic oscillator is a fundamental model in quantum mechanics used to describe systems with a restoring force proportional to the displacement.
  • Unlike the classical harmonic oscillator, which can have any continuous energy value, the energy levels of a quantum harmonic oscillator are discrete (quantized).
  • The eigenvalues represent the allowed energy levels of the oscillator.
  • The lowest energy level (where n=0 ) is known as the zero-point energy, which indicates that the oscillator is never completely at rest even at absolute zero temperature.
<br>

Formula Used

The energy eigenvalues of a one-dimensional quantum harmonic oscillator are given by:

En=(n+12)ℏωE_n = \left(n + \frac{1}{2}\right) \hbar \omega

Where:

  • nn is the quantum number (n=0,1,2,3,... ).
  • ℏ\hbar (h-bar) is the reduced Planck's constant, defined as ℏ=h2π\hbar = \frac{h}{2\pi}.
  • ω\omega is the angular frequency, defined as ω=2πν0\omega = 2\pi \nu_0 (where ν0\nu_0 is the frequency).
<br>

Calculation

To express the energy in terms of the frequency ν0\nu_0 and Planck's constant hh, we substitute the relations for ℏ\hbar and ω\omega into the energy equation:

En=(n+12)(h2π)(2πν0)E_n = \left(n + \frac{1}{2}\right) \left( \frac{h}{2\pi} \right) (2\pi \nu_0)

By canceling out the 2π2\pi term from the numerator and the denominator, we get:

En=(n+12)hν0E_n = \left(n + \frac{1}{2}\right) h \nu_0

This can also be written as:

En=hν0(n+12)E_n = h \nu_0 \left(n + \frac{1}{2}\right)

This expression represents the quantized energy levels (eigenvalues) for the system.

Hence, the eigenvalues of a one-dimensional harmonic oscillator are hν0(n+12)h \nu_{0}\left(\mathrm{n}+\frac{1}{2}\right).

Question 139

Hamilton's Canonical equation of motion is :

  1. AOption (a)

    \(\dot{\mathrm{q}}{\mathrm{i}}=\frac{\partial \mathrm{H}}{\partial \mathrm{p}{\mathrm{i}}}, \dot{\mathrm{p}}{\mathrm{i}}=-\frac{\partial \mathrm{H}}{\partial \mathrm{q}{\mathrm{i}}} \)

  2. BOption (b)

    \(\dot{\mathrm{q}}{\mathrm{i}}=\frac{-\partial \mathrm{H}}{\partial \mathrm{p}{\mathrm{i}}}, \dot{\mathrm{p}}{\mathrm{i}}=\frac{\partial \mathrm{H}}{\partial \mathrm{q}{\mathrm{i}}}\)

  3. COption (c)

    \(\mathrm{q}{\mathrm{i}}=\frac{\partial \mathrm{H}}{\partial \mathrm{p}{\mathrm{i}}}, \mathrm{p}{\mathrm{i}}=\frac{-\partial \mathrm{H}}{\partial \mathrm{q}{\mathrm{i}}} \)

  4. DOption (d)

    \(\mathrm{q}{\mathrm{i}}=\frac{\partial^{2} \mathrm{H}}{\partial \mathrm{q}{\mathrm{i}} \partial \mathrm{p}{\mathrm{i}}}, \mathrm{p}{\mathrm{i}}=\frac{-\partial^{2} \mathrm{H}}{\partial \mathrm{p}{\mathrm{i}} \partial \mathrm{q}{\mathrm{i}}}\)

View answer & explanation

Correct answer

Answer: (a)A.

\(\dot{\mathrm{q}}{\mathrm{i}}=\frac{\partial \mathrm{H}}{\partial \mathrm{p}{\mathrm{i}}}, \dot{\mathrm{p}}{\mathrm{i}}=-\frac{\partial \mathrm{H}}{\partial \mathrm{q}{\mathrm{i}}} \)

Why this answer is correct

Concept

  • Hamiltonian mechanics is a reformulation of classical mechanics that uses generalized coordinates qiq_i and generalized momenta pip_i to describe the state of a system.
  • The Hamiltonian HH typically represents the total energy of the system (Kinetic + Potential).
  • Hamilton's canonical equations are a set of first-order differential equations that describe the time evolution of the system in phase space.
<br>

Formula Used

The Hamiltonian is defined via the Legendre transformation of the Lagrangian LL:

H(qi,pi,t)=∑ipiq˙i−L(qi,q˙i,t)H(q_i, p_i, t) = \sum_{i} p_i \dot{q}_i - L(q_i, \dot{q}_i, t)

The resulting canonical equations of motion are:

q˙i=∂H∂pi\dot{q}_i = \frac{\partial H}{\partial p_i}

p˙i=−∂H∂qi\dot{p}_i = -\frac{\partial H}{\partial q_i}

Explanation

To derive these equations, consider the differential of the Hamiltonian H(q,p,t)H(q, p, t):

dH=∑(∂H∂qidqi+∂H∂pidpi)+∂H∂tdtdH = \sum \left( \frac{\partial H}{\partial q_i} dq_i + \frac{\partial H}{\partial p_i} dp_i \right) + \frac{\partial H}{\partial t} dt

From the definition H=∑piq˙i−LH = \sum p_i \dot{q}_i - L, we also have:

dH=∑(pidq˙i+q˙idpi)−dLdH = \sum (p_i d\dot{q}_i + \dot{q}_i dp_i) - dL

Substituting the differential of the Lagrangian dL=∑∂L∂qidqi+∑∂L∂q˙idq˙i+∂L∂tdtdL = \sum \frac{\partial L}{\partial q_i} dq_i + \sum \frac{\partial L}{\partial \dot{q}_i} d\dot{q}_i + \frac{\partial L}{\partial t} dt:

dH=∑pidq˙i+∑q˙idpi−(∑∂L∂qidqi+∑∂L∂q˙idq˙i+∂L∂tdt)dH = \sum p_i d\dot{q}_i + \sum \dot{q}_i dp_i - \left( \sum \frac{\partial L}{\partial q_i} dq_i + \sum \frac{\partial L}{\partial \dot{q}_i} d\dot{q}_i + \frac{\partial L}{\partial t} dt \right)

Since pi=∂L∂q˙ip_i = \frac{\partial L}{\partial \dot{q}_i} (definition of generalized momentum) and p˙i=∂L∂qi\dot{p}_i = \frac{\partial L}{\partial q_i} (from Lagrange's equations), the terms involving dq˙id\dot{q}_i cancel out, leaving:

dH=∑q˙idpi−∑p˙idqi−∂L∂tdtdH = \sum \dot{q}_i dp_i - \sum \dot{p}_i dq_i - \frac{\partial L}{\partial t} dt

Comparing the coefficients of dpidp_i and dqidq_i with the first differential expression of dHdH, we get:

q˙i=∂H∂pi\dot{q}_i = \frac{\partial H}{\partial p_i}

p˙i=−∂H∂qi\dot{p}_i = -\frac{\partial H}{\partial q_i}

Hence, the Hamilton's Canonical equations of motion are \(\dot{\mathrm{q}}{\mathrm{i}}=\frac{\partial \mathrm{H}}{\partial \mathrm{p}{\mathrm{i}}}, \dot{\mathrm{p}}{\mathrm{i}}=-\frac{\partial \mathrm{H}}{\partial \mathrm{q}{\mathrm{i}}} \).

Question 140

The Hamiltonian of a simple pendulum consisting of a mass m attached to a massless string of length ll is

H=pθ22 ml2+mgl(1−cos⁡θ)\mathrm{H}=\frac{\mathrm{p}_{\theta}^{2}}{2 \mathrm{~m} l^{2}}+\mathrm{mg} l(1-\cos \theta). If L denotes the Lagrangian, the value of dLdθ\frac{d L}{d \theta} is :

  1. AOption (a)

    -mgl sinθ

  2. BOption (b)

    mgl sinθ

  3. COption (c)

    -mgl cosθ

  4. DOption (d)

    mgl cosθ

View answer & explanation

Correct answer

Answer: (a)A.

-mgl sinθ

Why this answer is correct

Solution:

For a simple pendulum,

L = T - V = (1/2)ml2θ̇2 - mgl(1 - cosθ)

Differentiate the Lagrangian with respect to the generalized coordinate θ:

∂L/∂θ = -mgl × d(1 - cosθ)/dθ

= -mgl(sinθ)

Correct Option: (1)

Question 141

For a compound pendulum the Lagrangian equation in the generalized co-ordinate θ\theta is :

  1. AOption (a)

    ddt(Iθ)+mghsin⁡θ=0\frac{\mathrm{d}}{\mathrm{dt}}(\mathrm{I} \theta)+\mathrm{mgh} \sin \theta=0

  2. BOption (b)

    ddt(Iθ˙)−mghsin⁡θ=0\frac{\mathrm{d}}{\mathrm{dt}}(\mathrm{I} \dot{\theta})-\mathrm{mgh} \sin \theta=0

  3. COption (c)

    ddt(Iθ˙)+mghsin⁡θ=0\frac{\mathrm{d}}{\mathrm{dt}}(\mathrm{I} \dot{\theta})+\mathrm{mgh} \sin \theta=0

  4. DOption (d)

    ddt(θ˙I)+mghsin⁡θ=0\frac{\mathrm{d}}{\mathrm{dt}}\left(\frac{\dot{\theta}}{\mathrm{I}}\right)+\mathrm{mgh} \sin \theta=0

View answer & explanation

Correct answer

Answer: (c)C.

ddt(Iθ˙)+mghsin⁡θ=0\frac{\mathrm{d}}{\mathrm{dt}}(\mathrm{I} \dot{\theta})+\mathrm{mgh} \sin \theta=0

Why this answer is correct

Concept

  • A compound pendulum is a rigid body capable of oscillating about a fixed horizontal axis passing through it.
  • The Lagrangian L of a system is defined as the difference between the kinetic energy T and the potential energy V, i.e., L = T - V.
  • The equation of motion is derived using the Euler-Lagrange equation for the generalized coordinate θ \theta.
<br>

Formula Used

Kinetic energy of a rotating rigid body:

T=12Iθ˙2T = \frac{1}{2} I \dot{\theta}^2

Potential energy (taking the pivot point as the reference):

V=−mghcos⁡θV = -mgh \cos \theta

Lagrangian:

L=T−V=12Iθ˙2+mghcos⁡θL = T - V = \frac{1}{2} I \dot{\theta}^2 + mgh \cos \theta

Euler-Lagrange equation:

ddt(∂L∂θ˙)−∂L∂θ=0\frac{d}{dt} \left( \frac{\partial L}{\partial \dot{\theta}} \right) - \frac{\partial L}{\partial \theta} = 0

<br>

Calculation

Given the Lagrangian:

L=12Iθ˙2+mghcos⁡θL = \frac{1}{2} I \dot{\theta}^2 + mgh \cos \theta

First, calculate the partial derivative with respect to θ˙θ˙θ˙θ˙θ˙

Code
  :

∂L∂θ˙=∂∂θ˙(12Iθ˙2)=Iθ˙\frac{\partial L}{\partial \dot{\theta}} = \frac{\partial}{\partial \dot{\theta}} \left( \frac{1}{2} I \dot{\theta}^2 \right) = I \dot{\theta}

Next, calculate the partial derivative with respect to θθθθθ

Code
  :

∂L∂θ=∂∂θ(mghcos⁡θ)=−mghsin⁡θ\frac{\partial L}{\partial \theta} = \frac{\partial}{\partial \theta} (mgh \cos \theta) = -mgh \sin \theta

Now, substitute these into the Euler-Lagrange equation:

ddt(Iθ˙)−(−mghsin⁡θ)=0\frac{d}{dt} (I \dot{\theta}) - (-mgh \sin \theta) = 0

Simplifying the equation:

ddt(Iθ˙)+mghsin⁡θ=0\frac{d}{dt} (I \dot{\theta}) + mgh \sin \theta = 0

This matches the form provided in the correct option.

Hence, the Lagrangian equation in the generalized coordinate θθθθθ

Code
  is \\(\frac{\mathrm{d}}{\mathrm{dt}}(\mathrm{I} \dot{\theta})+\mathrm{mgh} \sin \theta=0\\)**.**

Question 142

For conservative system, Lagrange's equations are given by :

  1. AOption (a)

    \(\frac{\mathrm{d}}{\mathrm{dt}}\left(\frac{\partial \mathrm{L}}{\partial \mathrm{q}{\mathrm{i}}}\right)-\frac{\partial \mathrm{L}}{\partial \dot{\mathrm{q}}{\mathrm{i}}}=0\)

  2. BOption (b)

    \(\frac{\mathrm{d}}{\mathrm{dt}}\left(\frac{\partial \mathrm{L}}{\partial \dot{\mathrm{q}}{\mathrm{i}}}\right)-\frac{\partial \mathrm{L}}{\partial \mathrm{q}{\mathrm{i}}}=0\)

  3. COption (c)

    \(\frac{\mathrm{d}}{\mathrm{dt}}\left(\frac{\partial \mathrm{L}}{\partial \mathrm{q}{\mathrm{i}}}\right)+\frac{\partial \mathrm{L}}{\partial \dot{\mathrm{q}}{\mathrm{i}}}=0\)

  4. DOption (d)

    \(\frac{\mathrm{d}}{\mathrm{dt}}\left(\frac{\partial \mathrm{L}}{\partial \dot{\mathrm{q}}{\mathrm{i}}}\right)+\frac{\partial \mathrm{L}}{\partial \mathrm{q}{\mathrm{i}}}=0\)

View answer & explanation

Correct answer

Answer: (b)B.

\(\frac{\mathrm{d}}{\mathrm{dt}}\left(\frac{\partial \mathrm{L}}{\partial \dot{\mathrm{q}}{\mathrm{i}}}\right)-\frac{\partial \mathrm{L}}{\partial \mathrm{q}{\mathrm{i}}}=0\)

Why this answer is correct

Concept

  • In classical mechanics, the Lagrangian LL of a system is defined as the difference between its kinetic energy (TT) and potential energy (VV), i.e., L=T−VL = T - V.
  • For a conservative system, the potential energy is independent of generalized velocities and depends only on generalized coordinates (and possibly time).
  • The motion of such a system is governed by the principle of least action, which results in the Euler-Lagrange equations.
<br>

Formula Used

The standard Euler-Lagrange equation for a system with nn degrees of freedom is:

ddt(∂L∂q˙i)−∂L∂qi=0\frac{d}{dt}\left(\frac{\partial L}{\partial \dot{q}_i}\right) - \frac{\partial L}{\partial q_i} = 0

Where:

  • qiq_i are the generalized coordinates.
  • q˙i\dot{q}_i are the generalized velocities.
  • LL is the Lagrangian of the system.
<br>

Explanation

For a conservative system, the forces acting on the system can be derived from a scalar potential function VV. In such cases, the generalized force QiQ_i is given by −∂V∂qi-\frac{\partial V}{\partial q_i}.

The general form of D'Alembert's principle leads to the Lagrange equations. For holonomic, conservative systems, the equations are expressed purely in terms of the Lagrangian LL:

  1. The term ∂L∂q˙i\frac{\partial L}{\partial \dot{q}_i} represents the generalized momentum.
  2. The term ddt(∂L∂q˙i)\frac{d}{dt}\left(\frac{\partial L}{\partial \dot{q}_i}\right) represents the time rate of change of generalized momentum.
  3. The term ∂L∂qi\frac{\partial L}{\partial q_i} represents the generalized force.

Subtracting the generalized force from the rate of change of generalized momentum gives the equation of motion:

ddt(∂L∂q˙i)−∂L∂qi=0\frac{d}{dt}\left(\frac{\partial L}{\partial \dot{q}_i}\right) - \frac{\partial L}{\partial q_i} = 0

By comparing this with the given choices, we identify the correct mathematical representation for the conservative system.

Hence, the Lagrange's equations for a conservative system are \(\frac{\mathrm{d}}{\mathrm{dt}}\left(\frac{\partial \mathrm{L}}{\partial \dot{\mathrm{q}}{\mathrm{i}}}\right)-\frac{\partial \mathrm{L}}{\partial \mathrm{q}{\mathrm{i}}}=0\).

Question 143

The mathematical statement of D'Alembert's principle is :

  1. AOption (a)

    \(\sum_{\mathrm{i}}\left(\overrightarrow{\mathrm{F}}{\mathrm{i}}-\dot{\overrightarrow{\mathrm{p}}}{\mathrm{i}}\right) \cdot \delta \overrightarrow{\mathrm{r}}_{\mathrm{i}}=0\)

  2. BOption (b)

    \(\sum_{\mathrm{i}}\left(\overrightarrow{\mathrm{F}}{\mathrm{i}}+\dot{\overrightarrow{\mathrm{p}}}{\mathrm{i}}\right) \cdot \delta \overrightarrow{\mathrm{r}}_{\mathrm{i}}=0\)

  3. COption (c)

    \(\sum_{\mathrm{i}} \overrightarrow{\mathrm{F}}{\mathrm{i}}^{\mathrm{a}} \cdot \delta \overrightarrow{\mathrm{r}}{\mathrm{i}}=0\)

  4. DOption (d)

    \(\sum_{\mathrm{i}}\left(\overrightarrow{\mathrm{F}}{\mathrm{i}}-\overrightarrow{\mathrm{p}}{\mathrm{i}}\right) \cdot \delta \overrightarrow{\mathrm{r}}_{\mathrm{i}}=0\)

View answer & explanation

Correct answer

Answer: (a)A.

\(\sum_{\mathrm{i}}\left(\overrightarrow{\mathrm{F}}{\mathrm{i}}-\dot{\overrightarrow{\mathrm{p}}}{\mathrm{i}}\right) \cdot \delta \overrightarrow{\mathrm{r}}_{\mathrm{i}}=0\)

Why this answer is correct

Concept

  • D'Alembert's principle is a statement of the fundamental classical laws of motion.
  • It transforms a problem in dynamics into a problem in statics by introducing the concept of "inertial force."
  • The principle states that the sum of the differences between the forces acting on a system of mass particles and the time derivatives of the momenta of the system itself along any virtual displacement consistent with the constraints of the system is zero.
<br>

Formula Used

Newton’s second law for the ithi^{th} particle:

F⃗i=p⃗˙i\vec{F}_i = \dot{\vec{p}}_i

Where:

  • F⃗i\vec{F}_i is the total force (applied force + constraint force) acting on the ithi^{th} particle.
  • p⃗˙i\dot{\vec{p}}_i is the rate of change of momentum of the ithi^{th} particle.
<br>

Explanation

According to Newton's second law of motion, the equation of motion for a system of particles is:

F⃗i−p⃗˙i=0\vec{F}_i - \dot{\vec{p}}_i = 0

Consider a virtual displacement δr⃗i\delta \vec{r}_i of the system consistent with the constraints. The virtual work done by the force (F⃗i−p⃗˙i)(\vec{F}_i - \dot{\vec{p}}_i) during the displacement δr⃗i\delta \vec{r}_i must be zero:

(F⃗i−p⃗˙i)⋅δr⃗i=0(\vec{F}_i - \dot{\vec{p}}_i) \cdot \delta \vec{r}_i = 0

For a system containing nn particles, the sum of the virtual work over all particles is:

∑i=1n(F⃗i−p⃗˙i)⋅δr⃗i=0\sum_{i=1}^{n} (\vec{F}_i - \dot{\vec{p}}_i) \cdot \delta \vec{r}_i = 0

This equation represents the mathematical statement of D’Alembert’s principle. Here, F⃗i\vec{F}_i is the total force acting on the particle, which can be further decomposed into applied forces and forces of constraint. In most physical systems, the virtual work done by the forces of constraint is zero, simplifying the principle to involve only the applied forces.

Hence, the mathematical statement of D’Alembert’s principle is \(\sum_{\mathrm{i}}\left(\overrightarrow{\mathrm{F}}{\mathrm{i}}-\dot{\overrightarrow{\mathrm{p}}}{\mathrm{i}}\right) \cdot \delta \overrightarrow{\mathrm{r}}_{\mathrm{i}}=0\).

Question 144

The Hamiltonian function in terms of generalized co-ordinates is given as :

  1. AOption (a)

    \(H(\mathrm{q}, \mathrm{p}, \mathrm{t})=\sum_{\mathrm{i}}^{\mathrm{K}} \mathrm{p}{\mathrm{i}} \dot{\mathrm{q}}{\mathrm{i}}+\mathrm{L}\)

  2. BOption (b)

    H(q,p,t)=∑iKpiq˙i−L\mathrm{H(q, p, t)=\sum_{i}^{K} p_{i} \dot{q}_{i}-L}

  3. COption (c)

    \(\mathrm{H(q, p, t)=\sum_{i}^{K} \dot{p}{i} q{i}+L}\)

  4. DOption (d)

    \(\mathrm{H}(\mathrm{q}, \mathrm{p}, \mathrm{t})=\sum_{\mathrm{i}}^{\mathrm{K}} \dot{\mathrm{p}}{\mathrm{i}} \mathrm{q}{\mathrm{i}}-\mathrm{L}\)

View answer & explanation

Correct answer

Answer: (a)A.

\(H(\mathrm{q}, \mathrm{p}, \mathrm{t})=\sum_{\mathrm{i}}^{\mathrm{K}} \mathrm{p}{\mathrm{i}} \dot{\mathrm{q}}{\mathrm{i}}+\mathrm{L}\)

Why this answer is correct

Concept

  • In classical mechanics, the state of a system can be described using the Lagrangian function L(q,q˙,t)L(q, \dot{q}, t), which depends on generalized coordinates and generalized velocities.
  • The Hamiltonian H(q,p,t)H(q, p, t) is an alternative formulation that describes the system using generalized coordinates and generalized momenta.
  • The transition from the Lagrangian to the Hamiltonian is achieved through a mathematical process known as the Legendre transformation.
<br>

Formula Used

The generalized momentum pip_i is defined as:

pi=∂L∂q˙ip_i = \frac{\partial L}{\partial \dot{q}_i}

The Hamiltonian function HH is defined by the relation:

H=∑i=1Kpiq˙i−LH = \sum_{i=1}^{K} p_i \dot{q}_i - L

<br>

Explanation

The Lagrangian LL is typically defined as the difference between kinetic energy (TT) and potential energy (VV), i.e., L=T−VL = T - V. It is a function of qi,q˙iq_i, \dot{q}_i, and tt.

To switch to the Hamiltonian formalism, we define the Hamiltonian HH as the Legendre transformation of LL with respect to the velocity variables q˙i\dot{q}_i. This transformation replaces the generalized velocities q˙i\dot{q}_i with generalized momenta pip_i.

For a system with KK degrees of freedom, the relation is expressed as the sum of the products of each generalized momentum and its corresponding generalized velocity, minus the Lagrangian of the system.

Mathematically, this is written as:

H(q,p,t)=∑i=1Kpiq˙i−L(q,q˙,t)H(q, p, t) = \sum_{i=1}^{K} p_i \dot{q}_i - L(q, \dot{q}, t)

This function represents the total energy of the system (T+VT + V) when the potential is velocity-independent and the coordinate transformation is time-independent.

Hence, the Hamiltonian function in terms of generalized coordinates is given as H(q,p,t)=∑i=1Kpiq˙i−LH(q, p, t) = \sum_{i=1}^{K} p_i \dot{q}_i - L.

Question 145

The Lagrangian for a system is given by :

  1. AOption (a)

    L=T−V\mathrm{L}=\mathrm{T}-\mathrm{V}

  2. BOption (b)

    L=T+V\mathrm{L}=\mathrm{T}+\mathrm{V}

  3. COption (c)

    L=T/V\mathrm{L}=\mathrm{T} / \mathrm{V}

  4. DOption (d)

    L=V/T\mathrm{L}=\mathrm{V} / \mathrm{T}

View answer & explanation

Correct answer

Answer: (a)A.

L=T−V\mathrm{L}=\mathrm{T}-\mathrm{V}

Why this answer is correct

Concept

  • The Lagrangian function is a fundamental quantity in classical mechanics used to describe the dynamics of a physical system.
  • It is used in Hamilton's Principle of Least Action to derive the equations of motion (Euler-Lagrange equations).
  • For a non-relativistic, conservative system, the Lagrangian is defined in terms of energy components.
<br>

Formula Used

The standard definition of the Lagrangian (LL) is:

L=T−VL = T - V

Where:

  • TT is the total kinetic energy of the system.
  • VV is the total potential energy of the system.
<br>

Explanation

In classical mechanics, the state of a system is characterized by its kinetic energy (TT), which represents the energy of motion, and its potential energy (VV), which represents the energy due to the system's configuration or position in a field.

The Lagrangian function (LL) provides a concise way to summarize these dynamics. By mathematical convention and definition in Lagrangian mechanics, it is the difference between the kinetic and potential energies.

Mathematically:

L(q,q˙,t)=T(q,q˙,t)−V(q,t)L(q, \dot{q}, t) = T(q, \dot{q}, t) - V(q, t)

where qq represents generalized coordinates and q˙\dot{q} represents generalized velocities.

Hence, the Lagrangian for a system is given by L=T−VL = T - V.

Question 146

The radial wave functions of the hydrogen atom are given in the terms of :

  1. AOption (a)

    Associated Legendre Polynomials

  2. BOption (b)

    Laguerre Polynomials

  3. COption (c)

    Hermite Polynomials

  4. DOption (d)

    Legendre Polynomials

View answer & explanation

Correct answer

Answer: (b)B.

Laguerre Polynomials

Why this answer is correct

Concept

  • The wave function of a hydrogen atom is represented in spherical coordinates as ψ(r,θ,ϕ)=Rnl(r)Ylm(θ,ϕ)\psi(r, \theta, \phi) = R_{nl}(r) Y_{lm}(\theta, \phi).
  • The radial part, Rnl(r)R_{nl}(r), is obtained by solving the radial Schrödinger equation using the separation of variables method.
  • Different quantum mechanical systems result in different characteristic orthogonal polynomials.
<br>

Theory and Formula

The radial wave function Rnl(r)R_{nl}(r) for a hydrogenic atom is given by:

Rn,l(r)=Ne−ρ/2ρlLn−l−12l+1(ρ)R_{n,l}(r) = N e^{-\rho/2} \rho^l L_{n-l-1}^{2l+1}(\rho)

Where:

  • NN is the normalization constant.
  • ρ\rho is a dimensionless variable proportional to the radius rr.
  • Ln−l−12l+1(ρ)L_{n-l-1}^{2l+1}(\rho) represents the Associated Laguerre Polynomials.
<br>

Explanation

To identify the correct polynomial, we look at the standard solutions for various quantum systems:

  • Hermite Polynomials: These arise in the solution of the one-dimensional Quantum Harmonic Oscillator.
  • Legendre and Associated Legendre Polynomials: These appear in the angular part of the wave function (spherical harmonics, YlmY_{lm}) and describe the angular distribution of the electron.
  • Laguerre and Associated Laguerre Polynomials: These are specifically the solutions to the radial part of the Schrödinger equation for a central potential like that of the hydrogen atom.

While the exact form is the "Associated Laguerre Polynomial", in the context of general classification, they are categorized under Laguerre Polynomials.

Hence, the radial wave functions of the hydrogen atom are given in terms of Laguerre Polynomials.

Question 147

A rigid body consists of a large number of particles N=1028\mathrm{N}=10^{28}. How many independent generalized coordinates are required to describe the rigid body ?

  1. AOption (a)

    3N\mathrm{3N}

  2. BOption (b)

    12 N( N−1)\frac{1}{2} \mathrm{~N}(\mathrm{~N}-1)

  3. COption (c)

    66

  4. DOption (d)

    1515

View answer & explanation

Correct answer

Answer: (c)C.

66

Why this answer is correct

Concept

  • The number of independent generalized coordinates is equal to the number of degrees of freedom of a system.
  • A rigid body is defined as a system of particles where the distance between any two particles remains constant, regardless of external forces.
  • For a system of NN particles in 3D space, the maximum possible degrees of freedom is 3N3N, but constraints in a rigid body significantly reduce this number.
<br>

Logic for Degrees of Freedom

The motion of a rigid body in three-dimensional space can be decomposed into two parts:

  1. Translational Motion: 3 independent coordinates are required to specify the position of the center of mass in space (along xx, yy, and zz axes).
  2. Rotational Motion: 3 independent coordinates (such as Euler angles) are required to specify the orientation of the body about its center of mass.

The total number of independent coordinates is the sum of translational and rotational degrees of freedom.

<br>

Calculation

Given:

Number of particles N=1028N = 10^{28}

For any rigid body consisting of a large number of particles (N≥3N \geq 3) that are not collinear, the number of degrees of freedom is fixed by the rigidity constraints.

Using the relation for a 3D rigid body:

Degrees of Freedom=Translational DOF+Rotational DOF\text{Degrees of Freedom} = \text{Translational DOF} + \text{Rotational DOF}

Degrees of Freedom=3+3=6\text{Degrees of Freedom} = 3 + 3 = 6

Since the particles are part of a rigid body, the relative distances are fixed, and the number of independent coordinates does not depend on the specific value of NN (as long as NN is large enough to form a 3D structure).

Hence, the number of independent generalized coordinates required to describe the rigid body is 6.

Question 148

The Lagrangian for a particle is given by

L=12m(r˙2+r2θ˙2)−V(r)\mathrm{L=\frac{1}{2} m\left(\dot{r}^{2}+r^{2} \dot{\theta}^{2}\right)-V(r)}. Identify the cyclic coordinate :

  1. AOption (a)

    r only

  2. BOption (b)

    θ\theta only

  3. COption (c)

    both r and θ\theta

  4. DOption (d)

    rθ˙\mathrm{r} \dot{\theta}

View answer & explanation

Correct answer

Answer: (b)B.

θ\theta only

Why this answer is correct

Concept

  • In Lagrangian mechanics, a coordinate qiq_i is called a cyclic (or ignorable) coordinate if the Lagrangian LL does not depend explicitly on that coordinate.
  • Mathematically, a coordinate qiq_i is cyclic if: ∂L∂qi=0\frac{\partial L}{\partial q_i} = 0
  • When a coordinate is cyclic, its corresponding generalized momentum pi=∂L∂q˙ip_i = \frac{\partial L}{\partial \dot{q}_i} is a constant of motion (conserved).
<br>

Formula Used

The condition for a coordinate qq to be cyclic is:

∂L∂q=0\frac{\partial L}{\partial q} = 0

<br>

Explanation

The given Lagrangian is:

L=12m(r˙2+r2θ˙2)−V(r)L = \frac{1}{2} m(\dot{r}^{2} + r^{2} \dot{\theta}^{2}) - V(r)

The generalized coordinates in this system are rr and θ\theta. Let's check the dependency of the Lagrangian on these coordinates:

  1. For coordinate rr:

The term r2θ˙2r^2 \dot{\theta}^2 and the potential energy term V(r)V(r) both depend explicitly on rr.

∂L∂r=mrθ˙2−∂V(r)∂r≠0\frac{\partial L}{\partial r} = mr\dot{\theta}^2 - \frac{\partial V(r)}{\partial r} \neq 0

Since the derivative is not zero, rr is not a cyclic coordinate.

  1. For coordinate θ\theta:

Looking at the expression for LL, we see that it contains θ˙\dot{\theta} (the time derivative of θ\theta), but it does not contain the coordinate θ\theta itself.

∂L∂θ=0\frac{\partial L}{\partial \theta} = 0

Since the partial derivative of the Lagrangian with respect to θ\theta is zero, θ\theta is a cyclic coordinate.

Hence, the cyclic coordinate is θ\theta only.

Question 149

If a rigid body from its equilibrium position is displaced through an arbitrary virtual displacement, the work done is :

  1. AOption (a)

    Infinite

  2. BOption (b)

    Zero

  3. COption (c)

    Positive

  4. DOption (d)

    Negative

View answer & explanation

Correct answer

Answer: (b)B.

Zero

Why this answer is correct

Concept

  • A virtual displacement is an infinitesimal, instantaneous change in the coordinates of a system that is consistent with the constraints.
  • The Principle of Virtual Work states that for a system in static equilibrium, the total virtual work done by the external forces during an arbitrary virtual displacement is zero.
  • For a rigid body in equilibrium, the net external force and the net external torque acting on the body are both zero.
<br>

Formula Used

The principle of virtual work is mathematically expressed as:

δW=∑iF⃗i⋅δr⃗i=0\delta W = \sum_{i} \vec{F}_i \cdot \delta \vec{r}_i = 0

Where:

  • δW\delta W is the total virtual work done.
  • F⃗i\vec{F}_i represents the applied external forces.
  • δr⃗i\delta \vec{r}_i represents the arbitrary virtual displacement.
<br>

Explanation

The question states that a rigid body is in its equilibrium position. For any mechanical system (including a rigid body) to be in a state of equilibrium, the necessary and sufficient condition is that the total virtual work done by the external forces for any arbitrary virtual displacement is zero.

When the body is at equilibrium:

  1. The vector sum of all external forces is zero: ∑F⃗ext=0\sum \vec{F}_{ext} = 0
  2. The vector sum of all external torques is zero: ∑τ⃗ext=0\sum \vec{\tau}_{ext} = 0

Because the body is in equilibrium, any infinitesimal virtual displacement δr⃗\delta \vec{r} (which does not involve the passage of time and is consistent with the constraints of the system) results in zero work done by the forces. This is a fundamental principle in analytical mechanics used to determine the conditions of equilibrium.

Hence, the work done is zero.

Question 150

The life time of an excited state of an atom is about 10−8sec10^{-8} \mathrm{sec}. The minimum uncertainty in the determination of the energy of the excited state is :

  1. AOption (a)

    6.25×10−8eV6.25 \times 10^{-8} \mathrm{eV}

  2. BOption (b)

    6.25×10−7eV6.25 \times 10^{-7} \mathrm{eV}

  3. COption (c)

    6.25×10−6eV6.25 \times 10^{-6} \mathrm{eV}

  4. DOption (d)

    6.25×10−5eV6.25 \times 10^{-5} \mathrm{eV}

View answer & explanation

Correct answer

Answer: (a)A.

6.25×10−8eV6.25 \times 10^{-8} \mathrm{eV}

Why this answer is correct

Concept

  • According to Heisenberg's Uncertainty Principle, it is impossible to simultaneously measure certain pairs of physical properties, such as energy and time, with infinite precision.
  • For an atom in an excited state, the uncertainty in its energy (ΔE\Delta E) is inversely proportional to the lifetime of that state (Δt\Delta t).
  • This relation defines the natural broadening of spectral lines.
<br>

Formula Used

Heisenberg's Uncertainty Principle for energy and time:

ΔE⋅Δt≈h2π\Delta E \cdot \Delta t \approx \frac{h}{2\pi} (or ℏ\hbar)

Conversion from Joules to electron-volts:

E,(in eV)=E,(in Joules)1.6×10−19E , (\text{in eV}) = \frac{E , (\text{in Joules})}{1.6 \times 10^{-19}}

Where:

  • hh is Planck's constant (≈6.626×10−34,J⋅s\approx 6.626 \times 10^{-34} , \text{J}\cdot\text{s}).
  • h2π=ℏ≈1.054×10−34,J⋅s\frac{h}{2\pi} = \hbar \approx 1.054 \times 10^{-34} , \text{J}\cdot\text{s} (often approximated as 10−34,J⋅s10^{-34} , \text{J}\cdot\text{s} for calculations in competitive exams).
<br>

Calculation

Given:

Lifetime of the excited state, Δt=10−8,s\Delta t = 10^{-8} , \text{s}

Using the uncertainty relation:

ΔE≈ℏΔt\Delta E \approx \frac{\hbar}{\Delta t}

Substituting the values and using the simplified approximation ℏ≈10−34,J⋅s\hbar \approx 10^{-34} , \text{J}\cdot\text{s}:

ΔE≈10−34,J⋅s10−8,s\Delta E \approx \frac{10^{-34} , \text{J}\cdot\text{s}}{10^{-8} , \text{s}}

ΔE≈10−26,J\Delta E \approx 10^{-26} , \text{J}

Now, convert the energy uncertainty from Joules to electron-volts (eV):

ΔE (in eV)=10−261.6×10−19\Delta E \text{ (in eV)} = \frac{10^{-26}}{1.6 \times 10^{-19}}

ΔE=0.625×10−7,eV\Delta E = 0.625 \times 10^{-7} , \text{eV}

ΔE=6.25×10−8,eV\Delta E = 6.25 \times 10^{-8} , \text{eV}

Hence, the minimum uncertainty in the determination of the energy of the excited state is 6.25×10−8,eV6.25 \times 10^{-8} , \text{eV}.

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