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AS Daily A Level Physics question

2026-01-21 OCR A Electricity (M4) OCR-A Module 4.1.2 Energy, power and resistance OCR-A Module 4.1.3 Electrical circuits (series and parallel)

A portable heater has two identical heating elements and a switch that connects them either in series (LOW) or in parallel (HIGH) across the same 230 V supply. Treat each element as a constant resistor. When switching from LOW to HIGH, how does the total electrical power drawn from the supply change?

  1. A It becomes four times the LOW setting. (correct)
  2. B It becomes about twice the LOW setting.
  3. C It stays the same as the LOW setting.
  4. D It becomes half the LOW setting.

Answer

The correct answer is A.

Correct: A — It becomes four times the LOW setting. A The equivalent resistance falls from 2R in series to R/2 in parallel (a factor of 4 decrease), so with the same supply voltage the total power increases by a factor of 4. B Doubling is a common mistake that ignores both the lower total resistance and the fact that each element receives the full supply voltage in parallel, making the increase greater than 2×. C The total resistance changes markedly between series and parallel, so the power cannot remain the same. D Total resistance decreases, not increases, when moving to parallel, so power rises rather than halves.

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