A student investigates how resistance changes with temperature for two components: a thermistor and a fixed resistor. As the temperature of both components is increased from 0 °C to 60 °C, describe and explain the differences in how the resistance of each component changes.

Edexcel A-Level Physics (9PH0) — 10.1 Stars, life cycles and cosmology · Describe and explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A thermistor and a fixed resistor are each connected separately to identical circuits, each with a constant voltage supply, an ammeter in series, and a voltmeter in parallel. The temperature of each component is gradually increased from 0 °C to 60 °C.

Model answer (5 marks)

The resistance of the thermistor falls as the temperature rises from 0 °C to 60 °C, whereas the resistance of the fixed resistor remains essentially constant.

In a thermistor (a semiconductor) the increase in temperature supplies energy to the lattice, allowing more charge carriers (electrons or holes) to be generated. This reduces the number of collisions that impede charge flow, so the resistance drops.

A fixed resistor is usually a normal conductor. As its temperature rises the metal ions vibrate more strongly, increasing the frequency of electron‑ion collisions. This would tend to raise the resistance, but for the small temperature range the change is negligible compared with the thermistor.

Thus the thermistor’s resistance varies strongly with temperature, making it useful as a temperature sensor or thermostat, while the fixed resistor’s resistance is largely temperature‑insensitive.

Examiner tips

  • Use the phrase ‘falls as temperature rises’ for the thermistor and ‘remains constant’ for the fixed resistor. Explain the semiconductor mechanism (more carriers) for the thermistor. Mention the normal conductor mechanism (more collisions) for the fixed resistor. Link the behaviour to the practical use of the thermistor.

Common mistakes

  • Saying the fixed resistor’s resistance increases with temperature; it actually stays constant over the given range. Confusing the thermistor’s resistance behaviour with that of a normal resistor. Using vague terms like ‘changes’ without specifying direction or magnitude.

Mark scheme (5 marks)

  1. The resistance of the thermistor decreases as temperature increases
  2. The resistance of the fixed resistor stays (approximately) constant / does not change significantly with temperature
  3. In the thermistor, higher temperature means charge can flow more easily / fewer collisions between electrons and ions (accept: more charge carriers available)
  4. For the fixed resistor (or a normal conductor), as temperature increases, ions vibrate more, causing more collisions with electrons, which would tend to increase resistance
  5. Therefore the thermistor is useful as a temperature sensor / thermostat because its resistance changes significantly with temperature, whereas the fixed resistor is not temperature-sensitive

Key terms in this question

thermistor · fixed resistor · resistance

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