# 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

> 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.

## 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

- [Light Dependent Resistor (LDR)](https://www.gradenine.co.uk/glossary/light-dependent-resistor-ldr)
- [potential difference](https://www.gradenine.co.uk/glossary/potential-difference)
- [series circuit](https://www.gradenine.co.uk/glossary/series-circuit)
- [resistance](https://www.gradenine.co.uk/glossary/resistance)
- [current](https://www.gradenine.co.uk/glossary/current)

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-security-light-system-uses-a-d5e2b5da) · Published by Druglandscape Ltd.