An electric circuit consumes energy at a rate of 540 W per minute when heated. If the voltage across the circuit is 270 V, what is the resistance of the circuit?
- AOption (a)
135 ohm
- BOption (b)
270 ohm
- COption (c)
540 ohm
- DOption (d)
65 ohm
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Correct answer
135 ohm
Why this answer is correct
The correct answer is 135 ohm.
Key Points
- The problem states 540 W per minute. This is a tricky way of wording it. It doesn't mean the power is 540 Watts. It means the circuit consumes 540 Watt-minutes of energy. A Watt-minute is a unit of energy, not power.
- Here's how to get to the correct answer of 135 ohms:
- Energy Calculation: 540 Watt-minutes means the circuit uses 540 Watts of power for 1 minute (60 seconds).
- Power is already given: The wording "540 W per minute" is designed to be confusing. The instantaneous power is already given as 540 W. The "per minute" is extra information that isn't needed if you interpret it correctly. It's there to make you think about energy.
- Use the Power Formula: P = V²/R
- Solve for R: R = V²/P
- Plug in the values: R = (270 V)² / 540 W
- Calculate: R = 72900 / 540 = 135 ohms

