Investigating Resistance in Thermistors & LDRs

Thermistors and Resistance

A thermistor is a type of resistor whose resistance changes with temperature. Specifically, the resistance of a thermistor decreases as the temperature increases. This happens because heating the thermistor gives more energy to the charge carriers (usually electrons), allowing them to move more freely through the material.

Thermistors are used in temperature sensing devices, such as digital thermometers, thermostats, and temperature-controlled circuits. Their resistance change is non-linear, meaning the resistance does not change at a constant rate with temperature.

Light Dependent Resistors (LDRs) and Resistance

An LDR is a resistor that changes its resistance based on the intensity of light falling on it. The resistance of an LDR decreases as the light intensity increases. This is because light provides energy to free up charge carriers, reducing resistance.

LDRs are commonly used in light sensing circuits such as automatic street lights, night lights, and light meters. Like thermistors, the resistance change in an LDR is non-linear.

Investigating Resistance in Thermistors and LDRs

To investigate how resistance changes in thermistors or LDRs, you set up a simple circuit including:

• A power supply (e.g., a battery or variable power supply)
• An ammeter to measure current through the component
• A voltmeter connected in parallel to measure potential difference (voltage) across the component
• The thermistor or LDR whose resistance you want to investigate

Resistance $R$ is calculated using Ohm’s law as $R = \frac{V}{I}$, where $V$ is the voltage across the component and $I$ is the current through it. This can also be shown as a block equation:

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

Measuring Resistance of a Thermistor

To investigate how resistance changes with temperature:

• Set up the circuit with the thermistor, ammeter, and voltmeter.
• Measure the current and voltage at room temperature and calculate resistance.
• Carefully change the temperature of the thermistor (e.g., by placing it in warm water or near a heat source).
• Record the new current and voltage readings at each temperature.
• Calculate resistance for each temperature.

You will observe that as the temperature rises, the resistance decreases. Remember to allow the thermistor to reach thermal equilibrium at each temperature before taking measurements.

Measuring Resistance of an LDR

To investigate how resistance changes with light intensity:

• Set up the circuit with the LDR, ammeter, and voltmeter.
• Measure current and voltage in a dark or dimly lit room and calculate resistance.
• Gradually increase light intensity on the LDR (e.g., by moving a lamp closer or using a variable light source).
• Record the current and voltage at each light level.
• Calculate resistance for each light intensity.

You will find that as light intensity increases, the resistance decreases.

Example: Calculating Resistance of a Thermistor

If the voltmeter reads 3.0 V across a thermistor and the ammeter reads 0.02 A, the resistance is calculated as:

$R = \frac{3.0}{0.02} = 150 \, \Omega$

This shows the thermistor has a resistance of 150 ohms at that temperature.

Practical Considerations

• Safety: When heating a thermistor, avoid overheating or direct contact with flames.
• Accuracy: Use a digital multimeter for more precise voltage and current readings.
• Control Variables: Keep other factors constant, such as the power supply voltage, to ensure valid results.
• Non-linear Graphs: The resistance vs temperature or light intensity graphs are curved, not straight lines, due to the non-linear behaviour of thermistors and LDRs.
• Thermal Equilibrium: Allow the thermistor to reach a stable temperature before taking readings to ensure accuracy.