A security light system uses a Light Dependent Resistor (LDR) connected in series with a fixed resistor and a 12 V power supply. The LDR controls whether the light switches on. Explain how the resistance of the LDR changes between daytime and night-time, and describe how this change in resistance affects the current in the series circuit and the potential difference shared between the LDR and the fixed resistor.

OCR A-Level Physics B: Advancing Physics (H557) — 3.4 Materials · Explain · 5 marks · View as Markdown

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

Security lights are designed to switch on automatically when it gets dark. Engineers use LDRs in the circuit because their resistance changes depending on the intensity of light falling on them.

Model answer (5 marks)

During the day the LDR is exposed to high light intensity, so its resistance is low (≈ a few hundred ohms). At night the light intensity falls to almost zero, and the LDR’s resistance rises sharply (several kilo‑ohms). In the series circuit the total resistance is the sum of the LDR resistance and the fixed resistor. When the LDR’s resistance increases at night, the total resistance increases, so by Ohm’s law I=V/R the current through the whole circuit decreases. Because the LDR now has a much larger resistance than the fixed resistor, a larger fraction of the 12 V supply voltage falls across the LDR. Consequently the voltage drop across the fixed resistor becomes smaller, while the two voltage drops still add up to 12 V.

Examiner tips

  • Use the command word ‘explain’ – describe the change and its effect, not just state it.
  • Show the relationship I=V/R and V_total=V_LDR+V_fixed to justify the voltage changes.
  • Mention the LDR’s low resistance in daylight and high resistance at night.
  • Keep the answer concise – 5 marks, so one sentence per point is enough.

Common mistakes

  • Confusing the LDR’s resistance behaviour (writing high resistance in daylight).
  • Forgetting that the voltage drops must sum to the supply voltage.
  • Not linking the increased resistance to a decreased current in the series circuit.

Mark scheme (5 marks)

  1. The resistance of the LDR is lower during the day (when light intensity is high) / greater at night (when light intensity is low)
  2. At night, the higher resistance of the LDR means the total resistance in the series circuit is greater
  3. The current in the series circuit decreases at night (because resistance increases and voltage is constant)
  4. At night, a greater share of the potential difference is across the LDR (because it has a greater resistance compared to the fixed resistor)
  5. Therefore the potential difference across the fixed resistor decreases at night (the two p.d.s must sum to 12 V / the supply voltage)

Key terms in this question

Light Dependent Resistor (LDR) · potential difference · series circuit · resistance · current

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