A street lighting system uses light dependent resistors (LDRs) to automatically switch lights on at night and off during the day. Explain how the resistance of an LDR changes between daytime and night-time, and describe 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).
Street lighting engineers design circuits that respond automatically to changing light levels throughout the day and night, without any manual switching.
Model answer (5 marks)
During the day the LDR is exposed to high light intensity, so its resistance is low. At night the light intensity falls and the resistance rises.
Because current I = V/R, a low resistance gives a larger current for a given supply voltage, whereas a high resistance gives a smaller current.
Thus, when it is dark the LDR’s high resistance limits the current, keeping the lights off. When it is bright the low resistance allows a larger current to flow, turning the lights off.
During the day the low resistance allows a larger current to flow in the circuit, keeping the lights off. At night the high resistance reduces the current, allowing the lights to turn on.
Because current I = V/R, a low resistance gives a larger current for a given supply voltage, whereas a high resistance gives a smaller current.
Thus, when it is dark the LDR’s high resistance limits the current, keeping the lights off. When it is bright the low resistance allows a larger current to flow, turning the lights off.
During the day the low resistance allows a larger current to flow in the circuit, keeping the lights off. At night the high resistance reduces the current, allowing the lights to turn on.
Examiner tips
- Use the inverse relationship between I and R explicitly
- Mention the effect on the circuit (lights on/off)
- Keep answer concise and use exact terms (high/low resistance, current)
Common mistakes
- Confusing high resistance with high current
- Forgetting to state the effect on the lights
- Using vague terms like "more" or "less" instead of "high" or "low"
Mark scheme (5 marks)
- The resistance of the LDR is low during the day / when light intensity is high
- The resistance of the LDR is high at night / when light intensity is low
- Current and resistance are inversely related / as resistance increases, current decreases (and vice versa)
- At night, the LDR's high resistance causes a low current to flow in the circuit
- During the day, the LDR's low resistance allows a larger current to flow in the circuit
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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