Explain why a light dependent resistor (LDR) is useful in an automatic street-lighting circuit that switches the lights on when it gets dark.

AQA GCSE Physics (8463) — 4.2.1 Current, potential difference and resistance · Explain · 4 marks · View as Markdown

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

An automatic street light uses an LDR in its circuit. The street light switches on when the surrounding light intensity falls below a certain level.

Model answer (4 marks)

As light intensity falls, the resistance of the LDR rises.
The higher resistance reduces the current through the circuit for a given potential difference.
The reduced current (or increased resistance) is detected by the control circuit and triggers the street light to turn on.
Thus the LDR automatically senses darkness and switches the light without manual intervention.

Examiner tips

  • Use the exact terms: resistance, current, potential difference, trigger, automatic

Common mistakes

  • Confusing the direction of change (resistance increases when light decreases)
  • Not linking the resistance change to the control mechanism
  • Using vague phrases like "it turns on" without explaining the electrical effect

Mark scheme (4 marks)

  1. As light intensity decreases / it gets dark, the resistance of the LDR increases
  2. Greater resistance means a smaller current flows (for the same potential difference)
  3. The change in current (or resistance) can be detected / used to trigger / switch on the street light
  4. The LDR responds automatically to the level of light / no manual switching is needed

Key terms in this question

light dependent resistor (LDR)

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