A thermistor is used in a temperature-monitoring circuit. As the temperature of the thermistor increases, the current measured by the ammeter in the circuit also increases. Explain why the current increases as the temperature of the thermistor rises.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A thermistor is connected in series with an ammeter and a battery supplying a constant potential difference. When the thermistor is placed in warm water, the reading on the ammeter increases compared to when it is placed in cold water.
Model answer (5 marks)
As the temperature rises the resistance of the thermistor falls. The battery keeps the potential difference across the whole series circuit constant. From V=IR (or I=V/R) the current is inversely proportional to resistance. With a lower resistance the electrons encounter less opposition, so the rate of flow of charge increases. Consequently the ammeter reads a higher current.
Examiner tips
- State the temperature–resistance relationship first.
- Mention the constant voltage supplied by the battery.
- Use the equation I=V/R to link resistance to current.
- Explain that lower resistance means less opposition to charge flow.
Common mistakes
- Confusing a thermistor with a normal resistor (ignoring its temperature dependence).
- Forgetting to note that the battery voltage is constant.
- Using the wrong equation (e.g., V=IR but not solving for I).
Mark scheme (5 marks)
- As temperature increases, the resistance of the thermistor decreases
- The potential difference across the circuit remains constant (supplied by the battery)
- Using the relationship voltage = current × resistance (or current = voltage / resistance)
- Because resistance is lower, electrons can flow more easily / there is less opposition to the flow of charge
- Therefore the rate of flow of charge increases, so the current is greater
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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