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A2 Daily A Level Physics question

2026-05-10 OCR A DC Circuits (M4) OCR-A Module 4: Electricity — 4.2.3 Electrical circuits (potential divider; sensor circuits with LDR/thermistor)

A data logger reads V_out from the junction of a potential divider to ground. The divider uses a fixed 10 kΩ resistor and a light-dependent resistor (LDR). The LDR’s resistance decreases as light increases. Which arrangement will make V_out increase as the room gets brighter?

  1. A Put the LDR to ground (lower resistor); its resistance falling in bright light makes V_out larger.
  2. B Put the LDR to ground (lower resistor); the current rises in bright light so V_out rises.
  3. C Put the LDR to +5 V (upper resistor); its resistance falling in bright light means a larger share of the supply is across the lower fixed resistor, so V_out rises. (correct)
  4. D It does not matter which way round; V_out increases with brightness in both cases.

Answer

The correct answer is C.

Correct: C — Put the LDR to +5 V (upper resistor); its resistance falling in bright light means a larger share of the supply is across the lower fixed resistor, so V_out rises. A With the LDR at the bottom, its resistance falls in bright light so the fraction of the supply across it (and thus V_out) falls, not rises. B Focusing on current is misleading; at the bottom a lower resistance gives a smaller voltage across the LDR, so V_out decreases despite current increasing. C Placing the variable resistance on top means a fall in the upper resistance increases the fraction of the supply across the fixed lower resistor, raising V_out. D Orientation matters: swapping the LDR from top to bottom reverses the trend.

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