Frequently asked in IGCSE Physics are questions on experiments using thermistor and light-dependent resistor.

Thermistor

- Resistance of thermistor decreases as temperature increases.

- Can be used as temperature sensors

In an electric circuit, the thermistor symbol looks like this:

Designing Experiments to Study the Relationship Between Temperature and Resistance of a Thermistor

1. Place thermistor inside a beaker of water with a thermometer. The beaker of water acts as a water bath so that the temperature increases more gradually when heated and also helps to prevent the thermistor from getting heated directly.
2. The thermistor should be connected to electrical circuit. Connect the ammeter in series and the voltmeter in parallel with the thermistor.
3. At fixed temperature, read the values on the ammeter and voltmeter to get the current and voltage across the thermistor.
4. Calculate the resistance at a specific temperature by using the equation R = V / I.
5. Calculate the resistance for different temperatures (0°C ~ 100°C). For temperatures below room temperature, put ice in the beaker to lower the temperature. For temperatures above room temperature, heat up the water in the beaker.
6. For each temperature, repeat the experiment a few times and get the average.

If you plot the resistance-temperature graph, it will look something like this...

Light-Dependent Resistor

- Resistance of LDR decreases as light intensity increases.

- Can be used in street lamps

The LDR symbol looks like this in an electric circuit:

Designing Experiments to Study the Relationship Between Light Intensity and Resistance of LDR

1. Place LDR so that light can be shone through a circular hole in the black card.
2. The LDR should be connected to electrical circuit. Connect the ammeter in series and the voltmeter in parallel with the LDR.
3. At a fixed hole size, read the values on the ammeter and voltmeter to get the current and voltage across the LDR.
4. Calculate the resistance at a specific hole size by using the equation R = V / I.
5. Calculate the resistance for different hole sizes. The larger the hole size, the larger the light intensity. Keep the distance between the LDR and black card constant.
6. For each light intensity, repeat the experiment a few times and get the average.

If you plot the resistance-light intensity graph, it will look like this:

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