A light-dependent resistor (LDR) and a fixed resistor are connected in series with a battery. The circuit is left in a dark room overnight and then exposed to bright sunlight in the morning. Explain how the current in the circuit and the potential difference across the fixed resistor change when the circuit is moved from darkness into bright sunlight.

Edexcel A-Level Physics (9PH0) — 10.1 Stars, life cycles and cosmology · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A series circuit contains a battery, a fixed resistor of constant resistance, and a light-dependent resistor (LDR).

Model answer (5 marks)

The resistance of the LDR falls when it is illuminated, so the total resistance of the series circuit decreases. With a lower total resistance the current supplied by the battery increases. The fixed resistor is unchanged, so the larger current flowing through it gives a larger voltage drop across it (V=IR).

Examiner tips

  • Use the command word ‘Explain’ – give a clear causal chain.
  • Show the relationship between light, LDR resistance, total resistance, current and V=IR.
  • Use correct terminology: resistance, current, potential difference, series circuit.
  • Mention that the fixed resistor’s resistance is constant.

Common mistakes

  • Saying the voltage across the fixed resistor decreases instead of increases.
  • Forgetting that the LDR’s resistance changes with light.
  • Using the wrong equation (e.g. V=IR for the whole circuit instead of for the fixed resistor).

Mark scheme (5 marks)

  1. The resistance of the LDR decreases as light intensity increases
  2. The total resistance of the series circuit therefore decreases
  3. Because total resistance decreases, the current in the circuit increases
  4. The potential difference across the fixed resistor increases
  5. This is because the same (larger) current flows through the fixed resistor and its resistance is unchanged, so by V = IR the potential difference across it is greater

Key terms in this question

light-dependent resistor (LDR) · potential difference · current · series

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