A student investigates how the resistance of a thermistor changes with temperature. The student places the thermistor in water and gradually increases the water temperature from 10 °C to 60 °C, measuring the current at each temperature whilst keeping the potential difference constant. Explain what happens to the current through the thermistor as the temperature of the water increases, and why this behaviour makes thermistors useful in temperature-sensing devices.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A thermistor is a temperature-dependent component used in many electronic sensing circuits. Unlike ordinary resistors, its resistance changes significantly with temperature.
Model answer (5 marks)
As the water temperature rises from 10 °C to 60 °C the resistance of the thermistor falls.
Because the potential difference across the thermistor is held constant, a lower resistance means a higher current flows through it (Ohm’s law, I = V/R).
The resistance (or the resulting current) changes in a predictable way with temperature, giving a measurable output that can be calibrated to a specific temperature.
Thus the thermistor can be used in a circuit where the current or the voltage across it acts as an indicator of temperature – for example in a thermostat or a temperature‑alarm system.
In contrast, an ordinary ohmic conductor has a resistance that is essentially constant at a given temperature, so its current is simply proportional to the applied voltage and does not provide a temperature‑dependent signal.
Because the potential difference across the thermistor is held constant, a lower resistance means a higher current flows through it (Ohm’s law, I = V/R).
The resistance (or the resulting current) changes in a predictable way with temperature, giving a measurable output that can be calibrated to a specific temperature.
Thus the thermistor can be used in a circuit where the current or the voltage across it acts as an indicator of temperature – for example in a thermostat or a temperature‑alarm system.
In contrast, an ordinary ohmic conductor has a resistance that is essentially constant at a given temperature, so its current is simply proportional to the applied voltage and does not provide a temperature‑dependent signal.
Examiner tips
- Show the sequence: temperature ↑ → resistance ↓ → current ↑. Use the phrase "predictable, measurable output". Mention a practical application such as a thermostat. Contrast with an ordinary conductor to demonstrate understanding.
Common mistakes
- Saying the resistance increases instead of decreases. Confusing current with voltage – e.g. claiming current decreases. Failing to explain why the change is useful (missing the calibration point).
Mark scheme (5 marks)
- As temperature increases, the resistance of the thermistor decreases
- Because resistance decreases, the current through the thermistor increases (at constant potential difference)
- The changing resistance (or current) gives a measurable, predictable output that corresponds to a specific temperature
- This allows the thermistor to be used in a circuit where the current or voltage across it acts as an indicator of temperature (e.g. in a thermostat or temperature alarm)
- Contrast with an ordinary (ohmic) conductor: for an ohmic conductor at constant temperature, current is directly proportional to potential difference and resistance does not change in this way, whereas a thermistor's resistance changes significantly with temperature
Key terms in this question
thermistor · resistance · current · potential difference
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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