A thermistor is used in a temperature-sensing circuit. Explain how and why the current through the thermistor changes as the temperature of its surroundings increases.

Edexcel GCSE Physics (1PH0) — 10.1 Electricity and circuits · Explain · 4 marks · View as Markdown

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. The circuit is placed in an environment where the temperature gradually rises.

Model answer (4 marks)

The resistance of the thermistor falls as the temperature of its surroundings rises. Because the thermistor is in series with the ammeter and the battery, the same potential difference from the battery is applied across the whole circuit. With a lower resistance, Ohm’s law (I = V/R) gives a larger current. Therefore, as the temperature increases, the resistance of the thermistor decreases and the current through it increases.

Examiner tips

  • Mention the thermistor’s negative temperature coefficient first; link resistance change to current via Ohm’s law; state that the battery voltage is constant; give the final conclusion in one sentence.

Common mistakes

  • Saying the resistance increases instead of decreases; forgetting that the battery voltage remains constant; not using the correct term ‘negative temperature coefficient’ or ‘NTC’ for a thermistor.

Mark scheme (4 marks)

  1. The resistance of the thermistor decreases as temperature increases
  2. Current is greater when resistance is lower (linking resistance decrease to current increase)
  3. Reference to potential difference (voltage) remaining the same / constant from the battery
  4. Correct overall conclusion: as temperature increases, resistance decreases, so current through the thermistor increases

Key terms in this question

thermistor · current

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