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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
A thermistor is a semiconductor whose resistance falls sharply as temperature rises, because more charge carriers are thermally excited.
1. Therefore, as the room warms, the thermistor’s resistance decreases.
2. The lower resistance allows a larger current to flow through the circuit for a given applied voltage.
3. The increased current (or the resulting change in voltage across the thermistor) is sensed by the control unit.
4. When the room reaches the set temperature, the sensed signal triggers the switch to cut off the heating element.
5. This automatic cut‑off keeps the room at a constant, desired temperature, acting as a thermostat.
1. Therefore, as the room warms, the thermistor’s resistance decreases.
2. The lower resistance allows a larger current to flow through the circuit for a given applied voltage.
3. The increased current (or the resulting change in voltage across the thermistor) is sensed by the control unit.
4. When the room reaches the set temperature, the sensed signal triggers the switch to cut off the heating element.
5. This automatic cut‑off keeps the room at a constant, desired temperature, acting as a thermostat.
Examiner tips
- State the temperature–resistance relationship first. Explain the effect on current. Show how the signal is used to switch the heater. Link to temperature maintenance.
Common mistakes
- Confusing the thermistor type (NTC vs PTC). Forgetting to mention the current change. Not linking the signal to the heater’s on/off action.
Mark scheme (5 marks)
- As temperature increases, the resistance of a thermistor decreases (stated or implied clearly)
- A lower resistance allows a greater current to flow through the circuit
- The change in current (or potential difference) can be detected / used as a signal in the circuit
- When the room reaches the target temperature, the signal from the thermistor can be used to switch the heating off
- This allows the heating system to maintain a steady / constant temperature (or act as a thermostat)
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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