# 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 what happens inside both the filament lamp and the thermistor.

> OCR A-Level Physics A (H556) — 3.1 Motion · Explain · 5 marks

> A student sets up a series circuit containing a battery, a filament lamp, and a thermistor. The ammeter reading is recorded immediately after switch-on and then again after several minutes of operation. The reading is noticeably lower after several minutes.

## Mark scheme (5 marks)

1. As current flows through the filament lamp, electrons collide with atoms in the filament / the filament heats up
2. The resistance of the filament lamp increases as its temperature increases
3. The thermistor also warms up / its temperature increases over time
4. The resistance of the thermistor decreases as its temperature increases
5. The overall / total resistance in the series circuit increases (because the increase in the filament lamp's resistance is greater than the decrease in the thermistor's resistance), so the current decreases

## Key terms

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

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-sets-up-a-series-49804db1) · Published by Druglandscape Ltd.