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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
1. As the greenhouse temperature rises, the resistance of the thermistor falls.
2. The thermistor is in series with a fixed resistor, so the total resistance of the circuit is the sum of the two resistances and therefore decreases.
3. With a lower total resistance, the current supplied by the 12 V source increases (I=V/R).
4. The higher current is sensed by the control circuit, which triggers the fan to turn on.
5. A thermistor is ideal here because its resistance changes predictably with temperature, enabling automatic, reliable control of the fan.
2. The thermistor is in series with a fixed resistor, so the total resistance of the circuit is the sum of the two resistances and therefore decreases.
3. With a lower total resistance, the current supplied by the 12 V source increases (I=V/R).
4. The higher current is sensed by the control circuit, which triggers the fan to turn on.
5. A thermistor is ideal here because its resistance changes predictably with temperature, enabling automatic, reliable control of the fan.
Examiner tips
- Use the command word ‘Explain’ – give a clear cause–effect chain. Show the relationship R_total = R_fixed + R_thermistor and link it to the current. Mention the role of the thermistor’s predictable temperature dependence.
Common mistakes
- Confusing the thermistor’s resistance change direction (it decreases, not increases). Forgetting that the thermistor is in series, so total resistance is additive. Not linking the increased current to the fan‑triggering mechanism.
Mark scheme (5 marks)
- As temperature increases, the resistance of the thermistor decreases
- Because the thermistor is in series, the total resistance of the circuit decreases
- Because total resistance decreases, the current in the circuit increases
- The increased current is detected and used to trigger / switch on the fan
- The thermistor is useful in this application because its resistance changes predictably with temperature, allowing automatic control
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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