# A thermistor is used in a central heating system to monitor room temperature. As the room warms up, the resistance of the thermistor decreases. Explain why the resistance of the thermistor decreases as temperature increases, and describe how this property makes the thermistor useful in a central heating control system.

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

> A central heating system uses a thermistor connected in a circuit to monitor and control room temperature. When the room is cold, the thermistor has a high resistance. As the room warms up, the resistance of the thermistor falls.

## Mark scheme (5 marks)

1. As temperature increases, the resistance of a thermistor decreases (stated or implied clearly)
2. A lower resistance allows a greater current to flow through the circuit
3. The change in current (or potential difference) can be detected / used as a signal in the circuit
4. When the room reaches the target temperature, the signal from the thermistor can be used to switch the heating off
5. This allows the heating system to maintain a steady / constant temperature (or act as a thermostat)

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-thermistor-is-used-in-a-93c94678) · Published by Druglandscape Ltd.