# The effect of temperature on conductivity

##### Class practical

The conductivity of a wire decreases as it is heated.

#### Apparatus and materials

Power supply, low voltage, DC, continuously variable or stepped supply with rheostat (>1 A)

Coil of bare Eureka wire (1 metre, SWG 28)

Coil of bare copper wire (1 metre, SWG 32)

Ammeter, 0 to 1 A, DC

Bunsen burner

Means of supporting the wire coils so that they can be heated in the flame

#### Health & Safety and Technical notes

It is probably safer to use a clamped coil and move the flame than vice versa. However, some clamps are made from an alloy with a low melting point!!

#### Procedure

a Set up simple series circuit with the variable power supply, the copper coil and the ammeter. Adjust the power supply until about 0.8 amp flows. A very low voltage is needed.

b Warm the coil very gently in a low Bunsen flame and observe the ammeter reading.

c Replace the copper coil by the coil of Eureka wire and repeat the experiment. A greater potential difference will be necessary.

#### Teaching notes

1 The emphasis in this experiment is on the change in the current when the temperature of the material is changed. The wider the temperature range the better. The potential difference remains constant. The variation of current with temperature is noted. It is possible to adapt this experiment so that both current and temperature can be measured. But a qualitative understanding of the concept might be all that is needed.

The experiment can be extended by immersing the coil in a mixture of ice and salt. Alternatively, it can be put in some solid CO2. However, there is a high probability of shorting out some of the coils and this is likely to mask the true current change. An aerosol freezer spray would be a safer option.

2 The conductance, G, of a device or component is a measure of how well it conducts electricity and is the ratio of current to potential difference, I/V = G. This is obviously the inverse of resistance; use whichever concept seems appropriate in a given context. Conductivity is the inverse of resistivity; both of these are properties of a material.

3 Compared to its resistance at room temperature, copper's resistance is higher when it is red hot and lower when it is very cold. This effect can be made use of in constructing a resistance thermometer. The best thermometers using this effect are made from platinum.

4 The current passing through Constantan or Eureka wire does not vary much when its temperature is increased and the potential difference kept constant. Constantan wire is designed to behave like this.

5 How Science Works Extension: This experiment can be the basis of an investigation of the effect of temperature on resistance, between 0°C and 100°C, using a water bath. It can be extended over a wider range if you have a means of cooling the wire below 0°C (aerosol freezer spray, or obtain a flask of liquid nitrogen, boiling pint 77 K) and above 100°C (use an oven). If students are familiar with the experimental observation of Charles’ law, you could ask them to extrapolate their graph of resistance against temperature to find the approximate temperature at which the wire’s resistance would be zero. For a pure metal, resistance decreases approximately linearly towards a temperature close to 0 K. (The temperature coefficient of resistance of many pure metals is close to 0.004 K-1, so the resistance-temperature graph will extrapolate back to 1/0.004 = 250 K.) You could link this to the idea that the resistance of a pure metal at room temperature is dominated by the vibration of ions, and this will reduce to zero close to 0 K.

This experiment was safety-checked in October 2006