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AS Daily A Level Physics question

2026-03-03 OCR A Electricity (M4): energy & power in DC circuits Module 4.2.2 Energy, power and resistance (OCR A) Module 4.2.3 Ohmic conductors and I–V characteristics (OCR A)

In a lab test of a 6.0 V bench-powered heat pad that behaves as an ohmic resistor, the initial resistance is 4.0 Ω. You replace it with a longer strip of the same material so the resistance increases by 50%. The supply remains a regulated 6.0 V source. Which statement must be true about the electrical power dissipated in the pad after the change?

  1. A The power increases by 50% (e.g., from 9.0 W to 13.5 W).
  2. B The power decreases by 50% (e.g., from 9.0 W to 4.5 W).
  3. C The power decreases to two-thirds of its original value (e.g., from 9.0 W to 6.0 W). (correct)
  4. D The power is unchanged because the voltage is unchanged (still 9.0 W).

Answer

The correct answer is C.

Correct: C — The power decreases to two-thirds of its original value (e.g., from 9.0 W to 6.0 W). With constant supply voltage, power is inversely proportional to resistance, so increasing R by 50% (×1.5) reduces power by a factor of 1/1.5 = 2/3; numerically 36/4.0 = 9.0 W changes to 36/6.0 = 6.0 W. A confuses the relationship by assuming power rises with resistance at fixed V. B assumes a 50% drop, which would only follow if another quantity halved; here the correct ratio is 2/3, not 1/2. C is correct as shown by the 1/R reasoning and the numbers. D is wrong because constant voltage does not imply constant power; changing resistance changes current and hence power.

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