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AS Daily A Level Physics question

2025-08-24 OCR A A Level Physics 4.3.2(c)(i) 4.3.3(c)(i) 4.3.1(d)

A 6.0 V cell with internal resistance 3.0 Ω supplies a potential divider made from two identical 3.0 Ω resistors R1 and R2 in series. A high-resistance voltmeter measures V_out across R2. A third 3.0 Ω resistor is now connected in parallel with R2. Which statement must be true?

  1. A V_out decreases from 2.0 V to about 1.2 V, and if many more identical resistors are added in parallel with R2, V_out tends to 0 V. (correct)
  2. B V_out increases from 2.0 V to about 2.4 V because adding a parallel resistor increases the divider’s total resistance; with many such resistors, V_out tends to 6.0 V (the cell’s emf).
  3. C V_out remains 2.0 V because the divider proportion is still 1:1; with many such resistors, V_out tends to 3.0 V (half the emf).
  4. D V_out decreases only slightly from 2.0 V because the battery maintains 6.0 V across the divider; with many such resistors, V_out tends to 6.0 V as the lower branch draws more current.

Answer

The correct answer is A.

Correct: A — V_out decreases from 2.0 V to about 1.2 V, and if many more identical resistors are added in parallel with R2, V_out tends to 0 V. A This matches the calculation with internal resistance: initially V_out = 2.0 V; after making R2 → 1.5 Ω (3 || 3), V_out ≈ 1.2 V; as more parallels are added, R2,eq → 0 so V_out → 0. B Swaps series/parallel logic (parallel reduces total resistance, not increases) and also treats terminal pd as equal to the emf, hence the incorrect rise and 6.0 V limit. C Assumes the divider remains 1:1 (it becomes 3.0 Ω : 1.5 Ω) and again confuses terminal pd with emf when quoting a 3.0 V limit. D Treats the source as ideal (terminal pd fixed at 6.0 V) and infers the wrong limiting trend; in reality loading increases current, raises internal drop, reduces terminal pd, and V_out tends to 0.

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