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A2 Daily A Level Physics question

2026-02-24 OCR A Internal resistance & terminal pd OCR Physics A (H556) Module 4.2.2: EMF and internal resistance; terminal pd under load OCR Physics A (H556) Module 4.3: DC circuits (qualitative application of energy conservation around loops)

In a lab test, a DC source with some internal resistance is connected to resistive loads. With a 6.0 Ω load, the voltmeter across the load reads 9.0 V. Replacing it with a 3.0 Ω load, the voltmeter reads 8.0 V. Assume the source parameters are constant. Which statement must be true?

  1. A The source’s emf is 9.0 V (the highest meter reading), its internal resistance is about 0.50 Ω, and a short would draw roughly 18 A.
  2. B The source’s emf is about 10.3 V and its internal resistance about 0.86 Ω; a short circuit would draw roughly 12 A. (correct)
  3. C The internal resistance is 3.0 Ω because halving the load roughly halves the terminal pd; the emf is about 11 V and the short‑circuit current about 3.7 A.
  4. D As the load tends to zero, the terminal pd would approach the emf, so the current cannot exceed about 3 A; the internal resistance must be below 0.40 Ω.

Answer

The correct answer is B.

Correct: B — The source’s emf is about 10.3 V and its internal resistance about 0.86 Ω; a short circuit would draw roughly 12 A. A treats the highest terminal pd as the emf and proposes an r that cannot reproduce both readings (with E = 9.0 V neither load gives 9.0 V at the terminals). B uses the change in terminal voltage (1.0 V) and change in current (8/3 − 1.5 A ≈ 1.167 A) to obtain r ≈ 1.0/1.167 ≈ 0.86 Ω, then E ≈ 9.0 + 1.5r ≈ 10.3 V, giving short‑circuit current E/r ≈ 12 A. C assumes terminal pd scales directly with external resistance; with internal resistance this is false and the quoted r and E contradict the two measurements. D reverses the limiting case: as load resistance → 0, terminal pd → 0 while current → E/r (here much larger than 3 A); the claimed r < 0.40 Ω is also incompatible with a 1.0 V drop when current increases by about 1.17 A.

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