# A filament lamp is connected to a battery in a simple series circuit. As the lamp is switched on and left running, the current through the lamp decreases over time, even though the potential difference of the battery stays the same. Explain why this happens.

> Edexcel GCSE Physics (1PH0) — 10.1 Electricity and circuits · Explain · 4 marks

> A filament lamp is connected to a battery in a simple series circuit. When the circuit is first switched on, the current is relatively large. After a short time, the current decreases and settles at a lower steady value.

## Mark scheme (4 marks)

1. As current flows, electrical energy is transferred to thermal energy in the filament, causing it to heat up / the filament's temperature increases
2. Electrons collide with ions in the filament, and the ions vibrate more as temperature rises, making it harder for electrons to flow
3. The resistance of the filament lamp increases as its temperature increases
4. Because resistance increases (and potential difference stays the same), the current through the lamp decreases

## Key terms

- [filament lamp](https://www.gradenine.co.uk/glossary/filament-lamp)
- [current](https://www.gradenine.co.uk/glossary/current)
- [potential difference](https://www.gradenine.co.uk/glossary/potential-difference)

## Related

- [Revision notes for Edexcel GCSE Physics (1PH0)](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-filament-lamp-is-connected-to-72abde1b) · Published by Druglandscape Ltd.