Practical activities designed for use in the classroom with 11- to 19-year-olds.
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# 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

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

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

a Set up the circuit as shown below.

b Pour boiling water into the beaker and take readings of the current through the thermistor as the temperature falls. Record the results.

Analysis
c Plot a graph of current/ mA (y-axis) against temperature/ °C (x-axis).

d Assuming that the voltage is constant, describe how the conductance or resistance varies with temperature.

#### Teaching notes

1 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.

2 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 PhysicsIt has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.