# Thermistors

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A series of free GCSE/IGCSE Physics Notes and Lessons.

Thermistors

In this lesson, we will
• Recognise the symbol for a thermistor.
• Explain that the resistance of a thermistor changes with temperature.
• Describe applications of thermistors eg. in thermostats.

How thermistors can be used as thermostats?
The resistance of a thermistor decreases if the temperature increases.
Thermistors can be used as thermostats, for example in computers.

Under cool conditions, the resistance of the thermistor is high. This means that it takes a lot of energy for the current to pass through the thermistor. The potential difference across the thermistor is high. Since the potential difference is shared between components in series, the potential difference across the fan is small. Therefore, the fan operates at very low speed.

If the computer gets hot, the resistance of the thermistor falls. Now, it takes less energy for the current to pass through the thermistor. The potential difference across the thermistor is now very low so more electrical energy is available for the fan. This makes the potential difference across the fan very high so the fan powers up to high speed, cooling the computer back down.

Thermistor for measuring/controlling temperature
The difference between Positive Temperature Coefficient (PTC) thermistors and Negative Temperature Coefficient (NTC)
The effect of the thermistor's changing resistance on the electrical current running through a PC fan, to turn the fan on and off.

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