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!!

Read our standard health & safety guidance

Procedure

1. 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.
2. Warm the coil very gently in a low Bunsen flame and observe the ammeter reading.
3. Replace the copper coil by the coil of Eureka wire and repeat the experiment. A greater potential difference will be necessary.

Teaching Notes

• 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.
• 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.
• 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.
• 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.
• 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-tested in October 2006

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.

Demonstration

These two materials show contrasting effects when heated.

Apparatus and Materials

• Power supply, low voltage, variable
• Demonstration ammeter, 1 A
• Rheostat
• Small crucible
• Pipe-clay triangle
• Tripod
• Bunsen burner
• Common salt
• Paraffin wax
• Copper wire, stiff
• Leads and crocodile clips

Health & Safety and Technical Notes

The melting point of sodium chloride is -800°C. Wear eye protection when heating the crucible and salt and use a safety screen to protect observers.

Adjust the circuit resistance so the current flow is no more than about 100 mA., and ensure sensible levels of ventilation to disperse chlorine gas given off.

The quantity of chlorine released should be very low, but be aware of any pupils with asthma.

There will be a tiny amount of metal sodium on the negative electrode that will need care when disposing of the waste.

Read our standard health & safety guidance

Procedure

1. Place a small amount of salt in the bottom of the crucible. Support two stiff copper wires so that they reach the bottom of the crucible and make electrical contact with the salt. Connect up the circuit.

2. After noting that solid salt does not conduct electricity, remove the electrodes from the salt (they are excellent thermal conductors and will remove too much energy), heat the crucible strongly until the salt melts.
3. Replace the electrodes and adjust the rheostat to show a current of about 1 amp. Remove the burner and allow the salt to cool; current rapidly falls to zero.
4. Repeat the experiment using paraffin-wax instead of salt.

Teaching Notes

• Sodium chloride behaves as an insulator until it is nearly at its melting point, and then it readily conducts. Once it is melted, it behaves like an electrolyte and sodium and chlorine ions drift in opposite directions to the electrodes. This provides a method for producing pure sodium from sodium chloride.
• Paraffin-wax remains an insulator even when it has melted. It is made up of long-chain molecules consisting of carbon and hydrogen atoms. Both ends of the molecule are inactive and so are unlikely to carry an electrical charge.

This experiment was safety-tested in December 2006

Demonstration

A slice of germanium conducts when heated.

Apparatus and Materials

• Cells, 1.5 V, with holders, 3
• Lamp in holder, 6 V 3 W approx
• Mounted slice of germanium, see technical note
• 25 W soldering iron or beaker of boiling water

Health & Safety and Technical Notes

Read our standard health & safety guidance

Typically, the germanium slice (n type) should be approximately 5 mm square, 1-2 mm thick and with leads soldered as shown; its resistance cold is of the order of 300 Ω..

If the method of mounting the slice without using solder is adopted, then heating with a match becomes possible. If not, the match is liable to melt the solder.

Procedure

1. Connect the mounted slice in series with the three cells and the lamp. The lamp does not light.
2. Heat the slice by touching it with a small soldering iron (25 watt) or by immersing in boiling water. The lamp then lights.

Teaching Notes

• You will need to check in advance that the cells and lamp are correctly matched to your germanium slice to show the desired effect.
• Pure germanium belongs to a type of semiconductor which behaves as an insulator until the rise in temperature suddenly allows it to conduct. This temperature is much lower than for a true insulator. It is called an intrinsic semiconductor. When it is heated, more charge carriers are released and so the current increases.