A student connects a light dependent resistor (LDR) in series with a fixed resistor and a battery. The student then covers the LDR so that no light reaches it. Explain what happens to the potential difference across the fixed resistor when the LDR is covered.

AQA A-Level Physics (7408) — 3.5.1 Current Electricity · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Covering the LDR increases its resistance.
The total resistance of the series circuit therefore increases.
Because the battery voltage is unchanged, the current through the circuit decreases.
The fixed resistor has the same resistance as before, so the drop across it is V=IR.
With a lower current, the potential difference across the fixed resistor decreases.

Examiner tips

  • Use the V=IR relationship explicitly. Show the chain: LDR covered → R_LDR↑ → R_total↑ → I↓ → V_fixed↓. Mention that the fixed resistor’s resistance is unchanged.

Common mistakes

  • Assuming the fixed resistor’s voltage stays the same. Ignoring that the current through the whole series circuit must be the same. Using the wrong sign for the change in voltage (e.g., saying it increases).

Mark scheme (5 marks)

  1. Covering the LDR increases its resistance
  2. Total resistance of the circuit increases
  3. Current in the circuit decreases
  4. Potential difference across the fixed resistor decreases
  5. Because the potential difference across a component depends on both the current through it and its resistance (V = IR), and the current has decreased whilst the fixed resistance is unchanged

Key terms in this question

light dependent resistor (LDR) · potential difference · series

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