Thermistors (OCR A Level Physics)

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Thermistors

  • A thermistor is a non-ohmic conductor and sensory resistor whose resistance varies with temperature
  • Most thermistors are negative temperature coefficient (ntc) components
    • This means that if the temperature increases, the resistance of the thermistor decreases (and vice versa)

Thermistor diagram, downloadable AS & A Level Physics revision notes

The resistance through a thermistor is dependent on the temperature of it

  • The temperature-resistance graph for a thermistor is shown below

Thermistor graph, downloadable AS & A Level Physics revision notes

Graph of temperature against resistance for a thermistor

  • Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers
    • As the thermistor gets hotter, its resistance decreases
    • As the thermistor gets cooler, its resistance increases

Digital Thermometer, downloadable IGCSE & GCSE Physics revision notes

A digital thermometer uses a thermistor

Worked example

A thermistor is connected in series with a resistor R and a battery.

WE - thermistor in circuit question image, downloadable AS & A Level Physics revision notes

The resistance of the thermistor is equal to the resistance of R at room temperature.

When the temperature of the thermistor decreases, which statement is correct?

A.     The p.d across the thermistor increases

B.     The current in R increases

C.     The current through the thermistor decreases

D.     The p.d across R increases

     ANSWER: A

  • The resistance of the thermistor increases as the temperature decreases
  • Since the thermistor and resistor R are connected in series, the current I in both of them is the same
  • Ohm’s law states that V = IR
  • Since the resistance of the thermistor increases, and I is the same, the potential difference V across it increases
  • Therefore, statement A is correct

Examiner Tip

Make sure you remember the shape of the temperature-resistance graph for a thermistor, as it is a common exam question to draw and interpret this.The graph should not touch the x-axis, as this implies 0 resistance which is only possible in superconductors.

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Ashika

Author: Ashika

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.