# A student sets up a series circuit containing a battery, an ammeter, a filament lamp, and a thermistor. As the circuit is left switched on for several minutes, the student notices that the ammeter reading gradually decreases. Explain why the ammeter reading decreases over time, referring to both the filament lamp and the thermistor in your answer.

> OCR A-Level Physics B: Advancing Physics (H557) — 3.1 Motion · Explain · 5 marks

> A series circuit contains a battery, an ammeter, a filament lamp, and a thermistor connected end to end. The circuit is left switched on and the ammeter reading is observed to fall over several minutes.

## Mark scheme (5 marks)

1. As current flows, the filament lamp (and thermistor) heat up / temperature of both components increases
2. The resistance of the filament lamp increases as its temperature increases
3. The resistance of the thermistor also increases as its temperature increases
4. In a series circuit the total resistance is the sum of the individual resistances, so total resistance increases
5. Greater total resistance means less current flows, so the ammeter reading decreases (using voltage = current × resistance / Ohm's law reasoning)

## Key terms

- [filament lamp](https://www.gradenine.co.uk/glossary/filament-lamp)
- [thermistor](https://www.gradenine.co.uk/glossary/thermistor)
- [series](https://www.gradenine.co.uk/glossary/series)

## 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-student-sets-up-a-series-460a9474) · Published by Druglandscape Ltd.