A security light outside a house uses a light dependent resistor (LDR) connected in a circuit to automatically switch the light on when it gets dark. Explain how the resistance of the LDR changes from midday to midnight, and explain how this change in resistance affects the current in the circuit.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An LDR is connected in series with a fixed resistor and a 12 V battery. The total potential difference across the circuit remains constant throughout the day.
Model answer (5 marks)
At midday the light intensity is high, so the resistance of the LDR is low. At midnight the light intensity is low (dark), so the resistance of the LDR is high.
Because the potential difference across the circuit is constant, when the resistance of the LDR increases (at midnight) the current in the circuit decreases (I=V/R). From midday to midnight the resistance of the LDR increases and the current in the circuit decreases – this change in current is used to trigger the light switching on.
At midday, when the light intensity is high and the LDR resistance is low, the current is high and the security light remains off.
Because the potential difference across the circuit is constant, when the resistance of the LDR increases (at midnight) the current in the circuit decreases (I=V/R). From midday to midnight the resistance of the LDR increases and the current in the circuit decreases – this change in current is used to trigger the light switching on.
At midday, when the light intensity is high and the LDR resistance is low, the current is high and the security light remains off.
Examiner tips
- Use the command word ‘explain’ – describe the change in resistance and the resulting change in current. Include the key terms ‘high light intensity’, ‘low resistance’, ‘low light intensity’, ‘high resistance’, ‘constant V’, ‘I=V/R’, and ‘current decreases’.
- Show the causal chain: light intensity → LDR resistance → current → light switch.
- Mention that the light turns on when the current falls below the threshold set by the switch.
Common mistakes
- Confusing low resistance with high current without linking to the switch.
- Forgetting that the potential difference is constant and using Ohm’s law incorrectly.
Mark scheme (5 marks)
- At midday the light intensity is high, so the resistance of the LDR is low
- At midnight the light intensity is low (dark), so the resistance of the LDR is high
- Because potential difference remains constant, when resistance increases (at midnight) the current in the circuit decreases
- From midday to midnight, the resistance of the LDR increases and the current in the circuit decreases — this change in current is used to trigger the light switching on
- At midday (high light intensity / low LDR resistance) the current is high, and the security light remains off
Key terms in this question
light dependent resistor (LDR) · resistance · current
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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