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AS Daily A Level Physics question

2026-06-04 OCR A Applied contexts: sensor circuits (M4.3) M4.3.2 Electrical circuits: potential dividers and sensor circuits (LDR/thermistor) M4.3.1 Series circuits: voltage sharing proportional to resistance

An automatic night‑light uses a 5.0 V supply and a potential divider with an LDR to +5 V and a fixed 10 kΩ resistor to ground. The output voltage V_out (to a microcontroller) is taken across the 10 kΩ resistor. In bright light the LDR is 10 kΩ; when the room is shaded it rises to 30 kΩ. Which statement must be true when the room is shaded?

  1. A Increases from 2.5 V to 3.75 V (the LDR has a bigger share of 5.0 V).
  2. B Decreases from 2.5 V to 1.25 V (the 10 kΩ now has 10/(30+10) of the 5.0 V). (correct)
  3. C Stays at 2.5 V (the supply splits equally regardless of light level).
  4. D Decreases slightly to about 2.0 V (total resistance rises so V_out drops a little).

Answer

The correct answer is B.

Correct: B — Decreases from 2.5 V to 1.25 V (the 10 kΩ now has 10/(30+10) of the 5.0 V). A assumes the output is taken across the LDR; with V_out across the fixed 10 kΩ, increasing the LDR resistance reduces the share of 5.0 V. B is the only statement consistent with the divider ratio: total goes from 20 kΩ to 40 kΩ so the 10 kΩ share halves. C treats the split as always equal, which is only true when resistances are equal. D suggests only a slight drop, but the fraction falls from 10/20 to 10/40 — a 50% decrease.

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