A student investigates how temperature affects the resistance of a thermistor. At room temperature the thermistor has a high resistance. When the thermistor is placed in warm water, its resistance decreases. Explain, in terms of electrons and their movement through the thermistor, why the resistance decreases as temperature increases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A thermistor is a component whose resistance changes with temperature. At 20 °C its resistance is approximately 10 000 Ω. When heated to 80 °C its resistance falls to around 200 Ω.
Model answer (5 marks)
Current in a thermistor is a flow of free electrons.
At low temperature only a small number of electrons are free; most are bound to atoms.
When the temperature rises, thermal energy frees more electrons from the atoms.
With more free electrons available, the same applied voltage drives a larger current.
Because resistance is defined by R = V/I, a larger I for the same V gives a lower resistance.
At low temperature only a small number of electrons are free; most are bound to atoms.
When the temperature rises, thermal energy frees more electrons from the atoms.
With more free electrons available, the same applied voltage drives a larger current.
Because resistance is defined by R = V/I, a larger I for the same V gives a lower resistance.
Examiner tips
- Use the term ‘free electrons’ and ‘charge carriers’. Explain the link between temperature, electron liberation and current. Show the final step that R = V/I gives lower resistance when I increases.
Common mistakes
- Confusing resistance with voltage or current. Failing to mention that electrons are freed from atoms. Using vague phrases like ‘more electrons’ without linking to current and resistance.
Mark scheme (5 marks)
- Current is a flow of free electrons (charge carriers) through the thermistor
- At low temperature there are fewer free electrons available to carry charge
- As temperature increases, more electrons are released / freed from atoms
- More free electrons means a greater flow of charge for the same voltage
- Greater current for the same voltage means resistance is lower (linking to R = V/I or resistance definition)
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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