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AS Daily A Level Physics question

2026-02-08 OCR A Foundations of Physics (M2) Module 2.2.3 Work, energy and power (AS) Module 2: Foundations of Physics — Mathematical skills (algebra and proportional reasoning)

Two electric kettles heat the same mass of water from the same starting temperature to boiling. Ignore heat losses so the energy required is the same in both cases. Kettle A is rated 2.0 kW and Kettle B is rated 1.5 kW. Which statement must be true about their heating times?

  1. A Kettle B (1.5 kW) takes about 33% longer than Kettle A (2.0 kW). (correct)
  2. B Kettle B (1.5 kW) takes about 25% longer than Kettle A (2.0 kW).
  3. C Kettle B (1.5 kW) takes about 50% longer than Kettle A (2.0 kW).
  4. D Both kettles take the same time because the energy needed is the same.

Answer

The correct answer is A.

Correct: A — Kettle B (1.5 kW) takes about 33% longer than Kettle A (2.0 kW).. For the same energy, time is inversely proportional to power, so t_B/t_A = 2.0/1.5 = 4/3 ≈ 1.33, i.e. about 33% longer. A is correct by using the inverse proportion and the correct base for the percentage. B is wrong because 25% longer confuses bases: 2.0 is 33% greater than 1.5, but that makes A about 25% faster, not B 25% longer. C is wrong because 50% longer would require t_B/t_A = 1.5, which would only occur if powers were in a 2:1 ratio, not 2.0:1.5. D is wrong because although the energy is the same, different power means different time; lower power takes longer.

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