AQA GCSE Physics

Revision Notes

2.1.9 Investigating Resistance in Thermistors & LDRs

Investigating Resistance in Thermistors & LDRs

  • To investigate the relationship between the resistance of a thermistor and temperature, or the resistance of an LDR and light intensity, the following circuits must be set up:

LDR & Thermistor Circuits, downloadable IGCSE & GCSE Physics revision notes

  • For the LDR circuit:
    • Begin with the lamp turned off in a dark room
    • Record the reading on the voltmeter and ammeter
    • Slowly increase the light intensity of the lamp using the dimmer switch
    • Record the reading on the voltmeter and ammeter for each increase in light intensity
  • For the thermistor circuit:
    • Begin with the heater turned off
    • Record the reading on the voltmeter and ammeter
    • Slowly increase the heat of the heater using the dimmer switch
    • Record the reading on the voltmeter and ammeter for each increase in temperature of the heater
  • In both situations, make sure the lamp and heater are close, but not touching, the LDR and thermistor respectively
  • Wait a few seconds before taking the voltmeter and ammeter readings to allow the LDR and thermistor to react to the change in the environment
  • Calculate the resistance of the LDR or thermistor for each change in light intensity or temperature using the equation:

Resistance Equation

  • Therefore, to measure the resistance of any component in a circuit, make sure the following are included in the circuit diagrams:
    • The ammeter is connected in series to the component
    • The voltmeter is connected in parallel to the component
    • The component with the appropriate circuit symbol
    • The component is connected to a power supply with a low voltage (below 15 V) otherwise too high a current in the circuit will start to affect the resistance of the component

IV-circuit, IGCSE & GCSE Physics revision notes

Circuit diagram for investigating the resistance of a component

Exam Tip

Make sure to draw all the circuit symbols accurately. Many of them are very similar with small differences denoting what they do:

  • Two arrows pointing towards a symbol mean that it is light-dependent (eg. LDR)
  • Two arrows pointing away mean that it is light-emitting (eg. LED)

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