OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) negative temperature coefficient (NTC) thermistor; variation of…
Full specification wording
Resistivity: (c) negative temperature coefficient (NTC) thermistor; variation of resistance with temperature.
An NTC thermistor is a semiconductor device whose resistance falls as temperature rises.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An NTC thermistor is a semiconductor device whose resistance falls as temperature rises. This is a negative temperature coefficient of resistance. The effect is strong: resistance may drop by a factor of several over a modest temperature range, and the R–T graph is a steep curve, not a straight line. The microscopic reason is that heating releases many more charge carriers, so the number density n increases and resistance falls. Because the change is large and reproducible, NTC thermistors are used as temperature sensors and in potential-divider circuits. In multiple-choice questions you may be asked to read an R–T graph, to choose a circuit, or to explain the shape using carrier number density.
What earns the marks
- An NTC thermistor has a resistance that decreases as temperature increases.
- The R–T graph is a steep, non-linear curve, not a straight line.
- The cause is an increase in the number density of charge carriers as temperature rises.
- NTC thermistors are used as temperature sensors and in potential dividers.
- The percentage change in resistance per kelvin is much larger than for a metal.
- The resistance change is reversible, so the device can be used repeatedly.
Common mistakes and how to put them right
Saying the resistance of an NTC thermistor increases with temperature.
Put it right: It decreases, hence 'negative' temperature coefficient.
Treating the R–T graph as linear.
Put it right: It is a curve, steep at low temperature and flattening at high temperature.
Confusing the thermistor with a metal wire.
Put it right: A metal wire has a positive temperature coefficient of resistance.
Explaining the fall in resistance by saying ions vibrate less.
Put it right: The dominant cause is the increase in the number of charge carriers.
Exam tips
- Read the graph at the temperature asked for, and quote the resistance with its unit.
- Use the phrase 'negative temperature coefficient' to justify the direction of the change.
- Link the circuit use to a potential divider, where the changing resistance changes the output voltage.
- Check whether the question asks about resistance or resistivity; for a fixed specimen they change together.
Try the lesson
The start of the Resistivity: (c) negative temperature coefficient (NTC) thermistor; variation of resistance with temperature. lesson, made from checked teaching notes for this specification point.
Listen
An NTC thermistor is a semiconductor device whose resistance falls as temperature rises. This is a negative temperature coefficient of resistance. The effect is strong: resistance may drop by a factor of several over a modest temperature range, and the R–T graph is a steep curve, not a straight line. The microscopic reason is that heating releases many more charge carriers, so the number density n increases and resistance falls. Because the change is large and reproducible, NTC thermistors are used as temperature sensors and in potential-divider circuits. In multiple-choice questions you may be asked to read an R–T graph, to choose a circuit, or to explain the shape using carrier number density. A strong answer earns marks for these points. An NTC thermistor has a resistance that decreases as temperature increases. The R–T graph is a steep, non-linear curve, not a straight line. The cause is an increase in the number density of charge carriers as temperature rises. NTC thermistors are used as temperature sensors and in potential dividers. A common slip: Saying the resistance of an NTC thermistor increases with temperature. Instead, it decreases, hence 'negative' temperature coefficient. Exam tip: Read the graph at the temperature asked for, and quote the resistance with its unit.
Key words
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NTC thermistor
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Negative temperature coefficient of resistance
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Semiconductor charge-carrier model
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Thermistors in sensing circuits
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See every definition
- NTC thermistor
- A semiconductor device whose resistance falls as temperature rises, giving a negative temperature coefficient of resistance.
- Negative temperature coefficient of resistance
- The effect in which resistance falls as temperature rises, as shown by an NTC thermistor.
- Semiconductor charge-carrier model
- The explanation that heating releases many more charge carriers, so the number density n increases and resistance falls.
- Thermistors in sensing circuits
- Because the change is large and reproducible, NTC thermistors are used as temperature sensors and in potential-divider circuits.
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