AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Applications of thermistors to include temperature sensors and…
Full specification wording
Resistivity: Applications of thermistors to include temperature sensors and resistance–temperature graphs.
Thermistors are widely used as temperature sensors because their resistance changes predictably with temperature.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Thermistors are widely used as temperature sensors because their resistance changes predictably with temperature. For an ntc thermistor, resistance falls as temperature rises, so it can be placed in a potential divider to produce a potential difference that varies with temperature. A resistance–temperature graph shows this relationship: the curve is steep at low temperatures and flattens at higher temperatures. You should be able to read values from such a graph, describe the shape, and explain how a thermistor circuit can switch on a heater when it is cold or trigger a cooling fan when it is hot. You should also recognise that the graph is non-linear, so the sensitivity of the sensor changes with temperature.
What earns the marks
- Describes how a thermistor in a potential divider produces an output potential difference that depends on temperature.
- Explains that as temperature rises, the resistance of an ntc thermistor falls, changing the share of the supply potential difference across each component.
- Interprets a resistance–temperature graph by reading resistance values at given temperatures and describing the curve as non-linear.
- Explains a practical use, such as a thermostat or temperature sensor, where the thermistor controls a heater, fan or alarm.
- Compares the sensitivity of the thermistor at different temperatures using the gradient of the resistance–temperature graph.
Common mistakes and how to put them right
Assuming the output potential difference of a potential divider changes linearly with temperature.
Put it right by referring to the non-linear resistance–temperature graph.
Placing the thermistor in the wrong part of a potential divider and predicting the opposite change.
Put it right by tracing how the resistance change alters the potential difference across each component.
Reading a resistance–temperature graph as a straight line.
Put it right by describing the curve as steep at low temperature and flattening at higher temperature.
Exam tips
- When describing a temperature sensor, state the variable that is measured and how the thermistor changes it, for example output potential difference.
- Use the resistance–temperature graph to justify your answer rather than quoting a memorised value.
- In potential divider questions, calculate the output potential difference before and after a temperature change to show the direction of change clearly.
Try the lesson
The start of the Resistivity: Applications of thermistors to include temperature sensors and resistance–temperature graphs. lesson, made from checked teaching notes for this specification point.
Have a go first
Which graph shape shows how the resistance of an ntc thermistor changes as temperature increases?
Listen
Thermistors are widely used as temperature sensors because their resistance changes predictably with temperature. For an ntc thermistor, resistance falls as temperature rises, so it can be placed in a potential divider to produce a potential difference that varies with temperature. A resistance–temperature graph shows this relationship: the curve is steep at low temperatures and flattens at higher temperatures. You should be able to read values from such a graph, describe the shape, and explain how a thermistor circuit can switch on a heater when it is cold or trigger a cooling fan when it is hot. You should also recognise that the graph is non-linear, so the sensitivity of the sensor changes with temperature. A strong answer earns marks for these points. Describes how a thermistor in a potential divider produces an output potential difference that depends on temperature. Explains that as temperature rises, the resistance of an ntc thermistor falls, changing the share of the supply potential difference across each component. Interprets a resistance–temperature graph by reading resistance values at given temperatures and describing the curve as non-linear. Explains a practical use, such as a thermostat or temperature sensor, where the thermistor controls a heater, fan or alarm. A common slip: Assuming the output potential difference of a potential divider changes linearly with temperature. Instead, referring to the non-linear resistance–temperature graph. Exam tip: When describing a temperature sensor, state the variable that is measured and how the thermistor changes it, for example output potential difference.
Key words
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Temperature sensing
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Potential divider circuits
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Non-linear resistance–temperature graphs
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Practical control systems
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See every definition
- Temperature sensing
- Using a thermistor whose resistance changes predictably with temperature so that it can act as a sensor in a circuit.
- Potential divider circuits
- Arrangements in which a thermistor produces a potential difference that varies with temperature, changing the share of the supply potential difference across each component.
- Non-linear resistance–temperature graphs
- Curves that are steep at low temperatures and flatten at higher temperatures, so the sensitivity of the sensor changes with temperature.
- Practical control systems
- Circuits in which a thermistor can switch on a heater when it is cold or trigger a cooling fan when it is hot.
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