Back to Daily Question archive

A2 Daily A Level Physics question

2026-03-21 OCR A High level GCSE electricity: series/parallel rules; energy & power in DC circuits OCR-A 4.2.2 Energy, power and resistance OCR-A 4.2.3 Electrical circuits: series and parallel combinations

A lab power supply is fixed at 12 V. Two identical 10 Ω resistors are first connected in series across it, then reconnected in parallel across the same supply. Which statement must be true about the total electrical power drawn from the supply when changing from series to parallel?

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

Answer

The correct answer is A.

Correct: A — It becomes four times larger. A At fixed 12 V, total resistance changes from 20 Ω (series) to 5 Ω (parallel), so with P ∝ 1/R_total the power scales by 20/5 = 4. B This underestimates the change: R_total falls by a factor of 4, so power increases by 4×, not 2×. C This reverses the effect: power would halve only if R_total doubled, but here R_total decreases. D This ignores that changing R_total at fixed voltage must change both current and power, so it cannot stay the same.

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