# 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:

• 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

#### 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)

### Author: Ashika

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.
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