AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Resistors: The resistance of a thermistor decreases as the temperature increases.
A thermistor is a temperature-dependent resistor, usually made from a semiconductor oxide.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A thermistor is a temperature-dependent resistor, usually made from a semiconductor oxide. At low temperatures, few charge carriers are available, so its resistance is high. As temperature increases, more charge carriers are released, so the resistance decreases. This means the current through a thermistor increases for a given potential difference as it warms up. Thermistors are used in temperature sensors, thermostats and protection circuits. On a resistance–temperature graph the curve slopes downwards; on a current–potential difference graph the line curves upwards because resistance falls as temperature rises. The thermistor is therefore a non-ohmic component.
What earns the marks
- The resistance of a thermistor decreases as its temperature increases.
- At low temperatures the thermistor has a high resistance because there are few charge carriers.
- At higher temperatures more charge carriers are released, so the resistance falls.
- For a fixed potential difference, the current through a thermistor increases as temperature increases.
- A thermistor is a non-ohmic component because its resistance changes with temperature rather than remaining constant.
Common mistakes and how to put them right
Saying that resistance increases with temperature for a thermistor.
Put it right: Resistance decreases as temperature increases.
Confusing a thermistor with a filament lamp.
Put it right: A filament lamp's resistance increases as it heats up, whereas a thermistor's resistance decreases.
Thinking that temperature directly pushes current through the thermistor.
Put it right: Temperature changes the number of charge carriers and therefore the resistance.
Exam tips
- Use the phrase 'as temperature increases, resistance decreases' to make the inverse relationship explicit.
- Refer to charge carriers being released or made available when explaining the change in resistance.
- When describing a graph, state the direction of the slope and link it to the change in resistance.
Try the lesson
The start of the Resistors: The resistance of a thermistor decreases as the temperature increases. lesson, made from checked teaching notes for this specification point.
Have a go first
A thermistor is placed in a beaker of water and connected to a circuit to measure its resistance. As the water is heated from 20 °C to 80 °C, how does the resistance of the thermistor change?
Key words
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Temperature-dependent resistance
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Non-ohmic behaviour
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Charge carriers in semiconductors
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Temperature sensing and control
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See every definition
- Temperature-dependent resistance
- The resistance of a thermistor decreases as the temperature increases.
- Non-ohmic behaviour
- A thermistor is a non-ohmic component because its resistance changes with temperature rather than remaining constant.
- Charge carriers in semiconductors
- At low temperatures few charge carriers are available, so resistance is high; at higher temperatures more charge carriers are released, so resistance falls.
- Temperature sensing and control
- Thermistors are used in temperature sensors, thermostats and protection circuits.
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