# 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.

> OCR A-Level Physics A (H556) — 3.2 Forces in action · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. At midday the light intensity is high, so the resistance of the LDR is low
2. At midnight the light intensity is low (dark), so the resistance of the LDR is high
3. Because potential difference remains constant, when resistance increases (at midnight) the current in the circuit decreases
4. 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
5. At midday (high light intensity / low LDR resistance) the current is high, and the security light remains off

## Key terms

- [light dependent resistor (LDR)](https://www.gradenine.co.uk/glossary/light-dependent-resistor-ldr)
- [resistance](https://www.gradenine.co.uk/glossary/resistance)
- [current](https://www.gradenine.co.uk/glossary/current)

## Related

- [Revision notes for OCR A-Level Physics A (H556)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-security-light-outside-a-house-7c3af076) · Published by Druglandscape Ltd.