2.1.7Electricity
Thermistors — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 8 of 23
Part 1 of 4What a thermistor is
Temperature-dependent resistor
- Temperature-dependent resistor A thermistor is a resistor whose resistance changes strongly with its temperature.
- It is a non-ohmic component, so its I–V graph is a curve, not a straight line.
Negative temperature coefficient
- Negative temperature coefficient In the type met at GCSE (an NTC thermistor) resistance falls as the temperature rises.
- At low temperatures the resistance is high and little current flows.
Circuit symbol
- Circuit symbol The symbol is a resistor rectangle with a line through it, bent at the end like a hockey stick, showing it responds to temperature.
Part 2 of 4Why resistance falls when it warms
1Temperature rises
- Temperature rises The semiconductor material gains thermal energy.
2More charge carriers released
- More charge carriers released Extra electrons break free from their atoms and can carry current.
3Current can flow more easily
- Current can flow more easily With more charge carriers available, more charge passes each second for the same p.d.
4Resistance decreases
- Resistance decreases Since R = V ÷ I, a larger current for the same p.d. means a smaller resistance.
Part 3 of 4Using a thermistor in a circuit
1
Connect the thermistor in series with a fixed resistor and a power supply.
2
Connect a voltmeter across the fixed resistor or across the thermistor to act as the output.
3
When the surroundings warm, the thermistor's resistance drops and it takes a smaller share of the supply p.d.
4
A switching circuit or display responds to the changing p.d., turning something on or showing a temperature.
Part 4 of 4Everyday applications
| Device | Job of the thermistor |
|---|---|
| Digital thermometer | Its changing resistance is converted to a temperature reading |
| Oven or boiler thermostat | Signals the heater to switch off when the set temperature is reached |
| Refrigerator | Keeps the interior cold by switching the cooling system on and off |
| Fire alarm | Triggers the alarm when the air temperature rises sharply |
| Car engine | Monitors coolant temperature to warn of overheating |
Key terms
| Term | Meaning |
|---|---|
| Thermistor | A resistor whose resistance depends on its temperature. |
| Thermostat | A device that keeps a temperature steady by switching heating or cooling on and off. |
| Non-ohmic | Describes a component whose current is not directly proportional to p.d. |
| Charge carriers | Particles, such as electrons, that move to make an electric current. |
| Sensor | A component that detects a change in surroundings and produces an electrical response. |
Avoid these mistakes
1
Stating that resistance increases as a thermistor gets hotter — that is true for a metal wire, but a thermistor's resistance decreases.
2
Writing 'the thermistor gets hot so it has more resistance because particles vibrate' — the key idea is more charge carriers are released.
3
Using the symbol of an LDR or ordinary resistor when asked to draw a thermistor in a circuit diagram.
4
Explaining a thermistor's use without saying that the change in resistance changes the current or p.d. that controls the device.
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