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A2 Daily A Level Physics question

2026-07-03 OCR A DC Circuits (M4): Sensors with potential dividers OCR-A 4.3.3 Electrical circuits: potential divider and sensor applications OCR-A 4.3.4 Electrical characteristics: I–V behaviour of components (LDR/thermistor qualitative)

A student builds a light sensor using a potential divider: an LDR is connected to the +5 V supply and in series with a fixed resistor R to 0 V. The output voltage V_out is taken across R. The LDR’s resistance decreases as light level increases. Which statement must be true when the light level increases?

  1. A V_out decreases because the LDR’s resistance increases with light.
  2. B V_out increases because a smaller fraction of the supply is dropped across the LDR. (correct)
  3. C V_out stays the same because the supply voltage is fixed.
  4. D V_out increases only if R is larger than the LDR’s resistance in the dark.

Answer

The correct answer is B.

Correct: B — V_out increases because a smaller fraction of the supply is dropped across the LDR. A The LDR’s resistance falls in brighter light, so its voltage drop falls rather than rises. B This is correct: with less resistance in the LDR, more of the supply appears across R, so V_out rises. C The supply is fixed, but the share of the voltage across each series component changes with resistance, so V_out does not stay the same. D The direction of change (V_out increases) does not depend on the absolute values of R and the LDR, only on the LDR decreasing; the absolute values affect only how much it increases.

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