# A student sets up a circuit to investigate how a thermistor behaves at different temperatures. The student places the thermistor in water and gradually increases the water temperature, keeping the potential difference across the thermistor constant throughout. Explain what happens to the resistance of the thermistor and the current through it as the temperature of the water increases.

> OCR A-Level Physics A (H556) — 3.2 Forces in action · Explain · 5 marks

> A thermistor is a temperature-dependent component used in temperature detectors and thermostats. Its resistance changes with temperature in the opposite way to that of a normal wire.

## Mark scheme (5 marks)

1. The resistance of the thermistor decreases as temperature increases
2. This is the opposite behaviour to a normal wire / filament lamp (where resistance increases with temperature)
3. Because potential difference is kept constant, as resistance decreases the current through the thermistor increases
4. Current and resistance are linked by voltage = current × resistance (Ohm's law / V = IR), so a lower resistance at constant voltage gives a higher current
5. This property makes thermistors useful in temperature sensing / thermostats, because a change in temperature produces a measurable change in current

## Key terms

- [thermistor](https://www.gradenine.co.uk/glossary/thermistor)
- [resistance](https://www.gradenine.co.uk/glossary/resistance)
- [potential difference](https://www.gradenine.co.uk/glossary/potential-difference)
- [current](https://www.gradenine.co.uk/glossary/current)
- [temperature](https://www.gradenine.co.uk/glossary/temperature)

## 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-circuit-57569e77) · Published by Druglandscape Ltd.