AQA A Level Physics

Revision Notes

5.2.2 Resistance in a Thermistor

Temperature & Resistance

  • All solids are made up of vibrating atoms
    • The higher the temperature, the faster these atoms vibrate
  • Electric current is the flow of free electrons in a material
    • The electrons collide with the vibrating atoms which impede their flow, hence the current decreases

Resistance & Temperature in a Metal, downloadable AS & A Level Physics revision notes

Metal atoms and free electrons at low and high temperatures

  • So, if the current decreases, then the resistance will increase (from V = IR)
  • Therefore, for a metallic conductor which obeys Ohm’s law:
    • An increase in temperature causes an increase in resistance
    • A decrease in temperature causes a decrease in resistance
  • This is not the case for components such as a thermistor
    • For a thermistor, an increase in temperature causes a decrease in resistance

Application of a Thermistor

  • 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)
  • 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

Thermistor diagram, downloadable AS & A Level Physics revision notes

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

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

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