A thermistor is connected in series with a fixed resistor and a battery. Explain what happens to the current through the circuit as the temperature of the thermistor increases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A thermistor and a fixed resistor of 200 Ω are connected in series with a 12 V battery. The thermistor is placed in water that is gradually heated from 10 °C to 60 °C.
Model answer (5 marks)
As the water temperature rises the thermistor’s resistance falls. The total resistance of the series circuit therefore decreases. With the battery voltage fixed at 12 V, Ohm’s law (V=IR) shows that a lower resistance gives a larger current. Consequently the current through the whole circuit increases. Because the current rises, the voltage drop across the fixed 200 Ω resistor also rises.
Examiner tips
- Use the command word ‘explain’ – give a clear cause–effect chain. Show the key equations (V=IR) and state that V is constant. Mention the fixed resistor’s role and its voltage drop. Keep the answer concise and use the exact terms from the mark scheme.
Common mistakes
- Saying the resistance of the thermistor increases instead of decreases. Forgetting to link the rise in current to the fixed resistor’s voltage drop. Using the wrong equation (e.g. V=IR for the whole circuit but not noting the fixed resistor’s drop).
Mark scheme (5 marks)
- As temperature increases, the resistance of the thermistor decreases
- The total resistance of the series circuit decreases
- Using V = IR / linking potential difference, current and resistance — since V is constant, a lower resistance means a larger current
- The current through the circuit increases
- The potential difference across the fixed resistor increases (as the current through it increases)
Key terms in this question
thermistor · current · series
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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