Investigating Resistance in Thermistors & LDRs (AQA GCSE Combined Science: Trilogy: Physics): Revision Note

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:

• For the LDR circuit:

  • Begin with the lamp turned off in a dark room

  • Record the reading on the voltmeter and ammeter

  • Measure the light intensity with a light meter, or record the distance between the lamp and the LDR as a measure of intensity

  • Record the temperature or light intensity alongside each voltmeter and ammeter reading

• For the thermistor circuit:

  • Begin with the heater turned off

  • Record the reading on the voltmeter and ammeter

  • Measure the temperature next to the thermistor with a thermometer (or place the thermistor in a beaker of water heated gradually and read the water temperature)

  • Record the temperature or light intensity alongside each voltmeter and ammeter reading

• 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:

$R=\frac{V}{I}$

• 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, otherwise a large current heats the component and changes its resistance

Circuit diagram for investigating the resistance of a component