The effect of temperature on a thermistor

Class practical

This experiment, for advanced level students, shows that the current through a thermistor increases with temperature, as more charge carriers become available.

Apparatus and Materials

• timer or clock
• Leads, 4 mm
• Crocodile clip holder
• Thermometer -10°C to 110°C
• Thermistor - negative temperature, coefficient, e.g. 100 ohm at 25°C (available from Rapid Electronics).
• Power supply, 5 V, DC or four 1.5 V cells
• Beaker, 250 ml
• Kettle to provide hot water
• Digital multimeter, used as a milliammeter
• Heat-resistant mat
• Power supply, low voltage, DC, continuously variable or stepped supply with rheostat (>1 A)

Health & Safety and Technical Notes

Read our standard health & safety guidance

A thermistor may be described as:

• ntc negative temperature coefficient: its resistance decreases as the temperature increases
• ptc positive temperature coefficient: its resistance increases as the temperature increases

If you have both types available, students may be interested in comparing them.

Procedure

1. Set up the circuit as shown below.

2. Pour boiling water into the beaker and take readings of the current through the thermistor as the temperature falls. Record the results.
Analysis
3. Plot a graph of current/ mA (y-axis) against temperature/ °C (x-axis).
4. Assuming that the voltage is constant, describe how the conductance or resistance varies with temperature.

Teaching Notes

• The thermistor is made from a mixture of metal oxides such as copper, manganese and nickel; it is a semiconductor. As the temperature of the thermistor rises, so does the conductance.

• The increase in conductance is governed by the Boltzmann factor. Whether or not your students need to understand Boltzmann, they should be able to grasp that
• as the temperature goes up, the resistance goes down
• in this case, it happens because more charge carriers are released to engage in conduction.

This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.