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A2 Daily A Level Physics question

2026-02-22 OCR A DC Circuits — internal resistance and combining cells (Module 4.2.3) OCR Physics A (H556) 4.2.3 DC circuits — Kirchhoff’s laws; internal resistance and terminal pd OCR Physics A (H556) 4.2.3 — Cells in series and parallel; practical power delivery to loads

A camping lamp has a 4.0 Ω LED driver. It can be powered by two identical 1.5 V cells, each with internal resistance 1.0 Ω, connected either in series or in parallel. Assume the driver’s resistance stays 4.0 Ω. Which statement must be true about the terminal potential difference across the 4.0 Ω load?

  1. A With the cells in parallel, the load pd is larger — about 1.5 times the series value.
  2. B The load pd is the same for series and parallel because doubling the cell count also doubles the internal resistance.
  3. C With the cells in parallel, the load pd is only slightly lower than series — by around 10%.
  4. D With the cells in series, the load pd is larger — roughly 50% higher than the parallel case. (correct)

Answer

The correct answer is D.

Correct: D — With the cells in series, the load pd is larger — roughly 50% higher than the parallel case. Series: emf = 3.0 V, internal = 2.0 Ω ⇒ I = 3.0/(4.0+2.0) = 0.50 A, so V_load ≈ 0.50×4.0 = 2.0 V. Parallel: emf = 1.5 V, internal = 0.50 Ω ⇒ I = 1.5/(4.0+0.50) ≈ 0.33 A, so V_load ≈ 0.33×4.0 ≈ 1.33 V; 2.0/1.33 ≈ 1.5 (about 50% higher). A reverses the conclusion: parallel gives ≈1.33 V, not larger than series. B assumes the effects cancel regardless of load; cancellation only happens if load equals total internal resistance, which is not the case here (4.0 Ω vs 2.0 Ω or 0.50 Ω). C greatly underestimates the difference; it is about 50%, not around 10%. D correctly compares the two, with a ratio close to 1.5 in this scenario.

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