Back to Daily Question archive

AS Daily A Level Physics question

2026-05-28 OCR A Foundations of Physics (M2) 4.2.2 Energy, power and resistance (AS)

A camping heater is marked 60 W at 12 V. For a short test it is powered from a 6.0 V DC supply. Assuming its resistance does not change significantly at the lower temperature, which is the best estimate of the power it now dissipates?

  1. A About 30 W, because halving the voltage halves the power for a fixed resistance.
  2. B About 60 W, because the 60 W rating is independent of the supply voltage.
  3. C About 15 W, because with resistance unchanged, halving the voltage makes the power one quarter. (correct)
  4. D About 7.5 W, because both the voltage and the current are halved, so the power halves twice.

Answer

The correct answer is C.

Correct: C — About 15 W, because with resistance unchanged, halving the voltage makes the power one quarter. A assumes power is directly proportional to voltage for a fixed resistor, but for fixed resistance power scales with the square of voltage, giving a quarter not a half. B misinterprets the rating as a fixed power independent of supply; the rating applies at 12 V only. C matches the correct scaling: (6/12)^2 = 1/4, so 60 W becomes about 15 W. D incorrectly compounds reductions, treating power as halving twice; with fixed resistance, halving voltage already accounts for the current change, giving one quarter, not one eighth.

Try today's guest Daily MCQ A Level Physics MCQ practice Problem-solving previews