# A student sets up a circuit to investigate how the resistance of a light dependent resistor (LDR) changes. The circuit uses a constant voltage supply and an ammeter connected in series with the LDR. The student gradually moves a lamp further away from the LDR, taking current readings at each distance. Explain what happens to the resistance of the LDR as the lamp is moved further away, and explain why the LDR behaves in this way.

> OCR A-Level Physics B: Advancing Physics (H557) — 3.2 Forces in action · Explain · 5 marks

> A student sets up a circuit using a constant voltage supply, an ammeter in series, and an LDR. The student moves a lamp progressively further from the LDR and records the current at each position.

## Mark scheme (5 marks)

1. As the lamp moves further away, the light intensity reaching the LDR decreases.
2. The resistance of the LDR increases as light intensity decreases.
3. Because the voltage is constant, the current in the circuit decreases as resistance increases.
4. The resistance of an LDR is greatest in darkness / when light intensity is at its lowest.
5. This property makes LDRs useful in automatic circuits that respond to light levels, such as automatic night lights.

## 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 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-sets-up-a-circuit-0359d4a0) · Published by Druglandscape Ltd.