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A2 Daily A Level Physics question

2026-06-17 OCR A Energy & power in DC circuits OCR A (H556) Module 4.1 Charge and current OCR A (H556) Module 4.2 Energy, power and resistance OCR A (H556) Module 4.3 d.c. circuits (I–V basics)

An electric lab heater is powered by an ideal 12 V dc supply. When new, the heater's resistance is 6.0 Ω. After use, oxidation increases its resistance by 50% but the supply voltage remains 12 V. Which statement about the power dissipated in the heater is correct?

  1. A It decreases to 16 W, i.e. two-thirds of the original 24 W. (correct)
  2. B It stays at 24 W because the supply voltage is unchanged.
  3. C It increases to 36 W, i.e. 1.5 times the original power.
  4. D It halves to 12 W because power is inversely proportional to resistance.

Answer

The correct answer is A.

Correct: A — It decreases to 16 W, i.e. two-thirds of the original 24 W. A uses fixed supply voltage so power varies inversely with resistance; increasing R by 50% (6.0 Ω to 9.0 Ω) scales power by 1/1.5 = 2/3: 24 W to 16 W. B is wrong because power would stay the same only if both voltage and resistance stayed the same; here higher resistance reduces current and thus power. C is wrong because it treats current as unchanged and uses P ∝ R, but with a fixed-voltage source the current falls so power does not increase. D is wrong because halving to 12 W would happen if resistance doubled; here it increased by 1.5×, giving 2/3, not 1/2.

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