Heating effect of a current

Class practical

Illustrates two ideas: electric current causes a heating effect; temperature affects the resistance of a wire.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Lamp with holder
• Crocodile clips, 2
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 5
• Eureka wire 34 SWG, 15 cm length

Health & Safety and Technical Notes

Read our standard health & safety guidance

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Procedure

1. Set up a series circuit of three cells and a lamp. Include two crocodile clips in the circuit.
2. Wind the length of bare Eureka wire into a coil (perhaps on a pencil). Clamp the ends of the wire into the two crocodile clips. (Make sure that the turns of wire do not touch each other.)
3. Stand back! Carefully, hold your hand above the wire coil. Can you feel hot air rising?

Teaching Notes

• When an electric current passes through a material, the material warms up. The open coil of wire will be warm to the touch. Blowing on the wire will reduce its temperature and the lamp will glow brighter. Try using an electronics freezer spray to reduce the temperature of the coil even more and the lamp will glow brighter still.
• It is important that the coils must not touch each other, or the coil will become a short circuit.
• This experiment can be demonstrated in order to explain how a filament lamp works. The filament is just a short piece of wire which gets so hot that it glows red for low currents, becoming whiter as the current increases, until the filament finally melts and the lamp lights no more.
• Students often ask why you can’t just use the filament; why does it have to be in a glass envelope? It is fairly easy to break the glass envelope of a torch lamp by squeezing the lamp in the jaws of a small clamp. (Cover the lamp in tissue paper or a fine cloth if necessary though the cloth might damage the filament.) The lamp, minus its glass envelope, can then be connected to a cell. The filament may disintegrate in a puff of smoke without there being time to see that it heated up. The air has oxidized the filament and it has broken up. So whatever the gas is in a lamp, it is not oxygen. (Frequently it is argon.)
• Some students may have noticed that, when a circuit consisting of a cell, a lamp and an ammeter is connected, the current is momentarily greater when the connection is made, and then the current settles down to a steady lower value. This is because the resistance of the cold wire is less than the resistance of the hot wire.

This experiment was safety-tested in April 2006