# A light-dependent resistor (LDR) and a fixed resistor are connected in series with a battery. The circuit is left in a dark room overnight and then exposed to bright sunlight in the morning. Explain how the current in the circuit and the potential difference across the fixed resistor change when the circuit is moved from darkness into bright sunlight.

> Edexcel A-Level Physics (9PH0) — 10.1 Stars, life cycles and cosmology · Explain · 5 marks

> A series circuit contains a battery, a fixed resistor of constant resistance, and a light-dependent resistor (LDR).

## Mark scheme (5 marks)

1. The resistance of the LDR decreases as light intensity increases
2. The total resistance of the series circuit therefore decreases
3. Because total resistance decreases, the current in the circuit increases
4. The potential difference across the fixed resistor increases
5. This is because the same (larger) current flows through the fixed resistor and its resistance is unchanged, so by V = IR the potential difference across it is greater

## 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)
- [current](https://www.gradenine.co.uk/glossary/current)
- [series](https://www.gradenine.co.uk/glossary/series)

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](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-light-dependent-resistor-ldr-and-a-930b3ec5) · Published by Druglandscape Ltd.