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
- 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:
- 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
Circuit diagram for investigating the resistance of a component
Examiner 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)
