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A2 Daily A Level Physics question

2026-06-04 OCR A DC circuits: potential dividers and sensors (Revision weeks 1–6) OCR A (H556) Module 4.2.3 Electrical circuits — Potential dividers; sensors (LDR, thermistor) OCR A (H556) Module 4.2.2 Energy, power and resistance — Component behaviour (qualitative I–V; resistance changes)

A light sensor uses a 9 V supply with a fixed 10 kΩ resistor in series with a light-dependent resistor (LDR). The output voltage is measured across the LDR. When the light level on the LDR increases, which statement best describes what happens to the output voltage, and why?

  1. A Increases, because the LDR’s resistance increases so it takes a larger share of the supply voltage.
  2. B No change, because the supply is fixed at 9 V regardless of component values.
  3. C Increases, because the current increases so the p.d. across every series component increases.
  4. D Decreases, because the LDR’s resistance falls with light so it gets a smaller fraction of the supply voltage. (correct)

Answer

The correct answer is D.

Correct: D — Decreases, because the LDR’s resistance falls with light so it gets a smaller fraction of the supply voltage. A is wrong because an LDR’s resistance decreases with increasing light, so it would not take a larger share. B is wrong because while the supply is fixed, the division of voltage between series components depends on their resistances. C is wrong because in a series circuit the increased current does not make the p.d. across every component increase; each p.d. depends on that component’s resistance relative to the total. D is correct because a lower LDR resistance means a smaller proportion of the total resistance, so a smaller p.d. appears across it.

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