AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
The applications of thermistors in circuits eg a thermostat is required.
Full specification wording
Resistors: The applications of thermistors in circuits eg a thermostat is required.
Thermistors earn their place in circuits that have to respond to temperature.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 8 of 13
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Thermistors earn their place in circuits that have to respond to temperature. In a thermostat the thermistor is connected in series with a fixed resistor across the supply, forming a potential divider. As the room warms, the thermistor resistance falls, so the share of the supply potential difference across it falls, and when that reading drops past a set value the control circuit switches the heater off; as the room cools the resistance rises again and the heater comes back on. The same arrangement appears in an oven controller, a fridge, a car engine temperature gauge and a fire alarm, where a sudden rise in temperature produces a sudden fall in resistance that triggers the alarm.
What earns the marks
- Naming a suitable application such as a thermostat, oven, fridge or fire alarm.
- Stating the change in thermistor resistance when the temperature changes.
- Linking that to the change in potential difference across the thermistor in a potential divider.
- Describing the switching action that results, such as turning the heater off.
Common mistakes and how to put them right
Naming a thermostat without describing how the resistance change causes the switching.
Claiming the thermistor itself switches the heater, rather than triggering a control circuit.
Describing the potential difference across the thermistor as rising when its resistance falls.
Using a light dependent resistor in a temperature application by mistake.
Exam tips
- Give the full chain: temperature changes, resistance changes, potential difference changes, circuit switches.
- Name the application the question describes rather than listing every use you know.
- Say which way the temperature is moving at the start, because the whole answer depends on it.
Try the lesson
The start of the Resistors: The applications of thermistors in circuits eg a thermostat is required. lesson, made from checked teaching notes for this specification point.
Have a go first
In which of these devices is a thermistor used to respond to temperature changes?
Listen
Thermistors earn their place in circuits that have to respond to temperature. In a thermostat the thermistor is connected in series with a fixed resistor across the supply, forming a potential divider. As the room warms, the thermistor resistance falls, so the share of the supply potential difference across it falls, and when that reading drops past a set value the control circuit switches the heater off; as the room cools the resistance rises again and the heater comes back on. The same arrangement appears in an oven controller, a fridge, a car engine temperature gauge and a fire alarm, where a sudden rise in temperature produces a sudden fall in resistance that triggers the alarm. A strong answer earns marks for these points. naming a suitable application such as a thermostat, oven, fridge or fire alarm. stating the change in thermistor resistance when the temperature changes. linking that to the change in potential difference across the thermistor in a potential divider. describing the switching action that results, such as turning the heater off. Exam tip: Give the full chain: temperature changes, resistance changes, potential difference changes, circuit switches.
See it
Key words
Say what each word means before you turn the card.
potential dividers
Try answering aloud or in your head before turning the card.
temperature control
Try answering aloud or in your head before turning the card.
fire alarms
Try answering aloud or in your head before turning the card.
See every definition
- potential dividers
- A thermistor connected in series with a fixed resistor across the supply forms a potential divider, so the share of the supply potential difference across the thermistor falls as its resistance falls.
- temperature control
- As the room warms the thermistor resistance falls and the potential difference across it falls, and when that reading drops past a set value the control circuit switches the heater off.
- fire alarms
- A sudden rise in temperature produces a sudden fall in resistance that triggers the alarm.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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