A student sets up a circuit to investigate how a thermistor behaves at different temperatures. The student places the thermistor in water and gradually increases the water temperature, keeping the potential difference across the thermistor constant throughout. Explain what happens to the resistance of the thermistor and the current through it as the temperature of the water increases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A thermistor is a temperature-dependent component used in temperature detectors and thermostats. Its resistance changes with temperature in the opposite way to that of a normal wire.
Model answer (5 marks)
The resistance of the thermistor falls as the water temperature rises.
This is the opposite of a normal conductor, where resistance rises with temperature.
Because the applied voltage is held constant, a lower resistance means a higher current.
Ohm’s law (V=IR) shows that at constant V, I∝1/R, so I increases as R decreases.
This temperature‑dependent change in current is what makes thermistors useful for temperature sensing and in thermostats.
This is the opposite of a normal conductor, where resistance rises with temperature.
Because the applied voltage is held constant, a lower resistance means a higher current.
Ohm’s law (V=IR) shows that at constant V, I∝1/R, so I increases as R decreases.
This temperature‑dependent change in current is what makes thermistors useful for temperature sensing and in thermostats.
Examiner tips
- State the change in resistance and contrast with a normal wire. Explain the effect on current using V=IR. Mention the practical application of the property.
Common mistakes
- Confusing the direction of resistance change (saying it increases instead of decreases). Forgetting to link the constant voltage condition to the current change. Using vague terms like "more current" without showing the reasoning.
Mark scheme (5 marks)
- The resistance of the thermistor decreases as temperature increases
- This is the opposite behaviour to a normal wire / filament lamp (where resistance increases with temperature)
- Because potential difference is kept constant, as resistance decreases the current through the thermistor increases
- Current and resistance are linked by voltage = current × resistance (Ohm's law / V = IR), so a lower resistance at constant voltage gives a higher current
- This property makes thermistors useful in temperature sensing / thermostats, because a change in temperature produces a measurable change in current
Key terms in this question
thermistor · resistance · potential difference · current · temperature
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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