# A heating engineer installs a thermistor-based sensor in a greenhouse to monitor temperature and control a fan. Explain how the thermistor and the rest of the circuit respond as the greenhouse temperature rises, and describe why the total resistance of the circuit changes in the way it does.

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

> A greenhouse uses a thermistor connected in series with a fixed resistor and a 12 V power supply. The thermistor is used to detect rising temperatures so that a fan can be triggered when it gets too hot.

## Mark scheme (5 marks)

1. As temperature increases, the resistance of the thermistor decreases
2. Because the thermistor is in series, the total resistance of the circuit decreases
3. Because total resistance decreases, the current in the circuit increases
4. The increased current is detected and used to trigger / switch on the fan
5. The thermistor is useful in this application because its resistance changes predictably with temperature, allowing automatic control

## Key terms

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

## 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-heating-engineer-installs-a-thermistor-based-a89b22a9) · Published by Druglandscape Ltd.