Resistors: The applications of thermistors in circuits eg a thermostat is required.

The idea

A thermistor is a resistor whose resistance changes with temperature. In the negative temperature coefficient (NTC) thermistor used in GCSE courses, resistance decreases as temperature increases, so it is a temperature-sensitive component. In a thermostat circuit, the thermistor forms part of a potential divider or control circuit. At low temperature its resistance is high, so the potential difference across it is large; this can switch on a heater. As the room warms, its resistance falls, the potential difference across it falls, and the heater switches off. The circuit therefore keeps temperature roughly constant. You should be able to describe this application and explain how the changing resistance controls a device, rather than simply naming a thermostat.

What earns the marks

• States that the resistance of an NTC thermistor decreases as temperature increases.
• Explains that the thermistor is used as a temperature sensor in a control circuit such as a thermostat.
• Describes how the changing resistance alters the potential difference across the thermistor in a potential divider.
• Links the change in potential difference or current to the switching on or off of a heater or cooling device.
• Applies the idea to a familiar context, for example a room thermostat, oven thermostat or fridge control.
• Uses correct circuit language, such as series, potential difference, current and resistance.

Common mistakes and how to put them right

• Saying resistance increases with temperature for an NTC thermistor.

  Put it right by stating that resistance decreases as temperature rises.

• Confusing a thermistor with an LDR.

  Put it right by noting that a thermistor responds to temperature while an LDR responds to light intensity.

• Describing only that a thermostat contains a thermistor without explaining how the resistance change controls the heater.

  Put it right by linking resistance change to potential difference or current and then to switching.

Exam tips

• Name the application clearly, then explain the sequence: temperature change, resistance change, potential difference or current change, device switches.
• Use the phrase 'negative temperature coefficient' only if you can also state what it means for resistance and temperature.
• If a circuit diagram is given, refer to the thermistor's position and how the output is taken across it or across another component.