Fun with science

Press, 12 August 1980, Page 14

Fun with science

A magnet is a fascinating object to use for experiments with different materials. In general, the only things it attracts contain iron, or steel, but a moving magnet can produce electric currents in materials other than iron and steel to which it is attracted. • // .

You can prove this by a simple experiment using a strong,? horseshoe-shaped magnet, an aluminium foil dish about 20cm in diameter, a darning needle, a cork and a ball-of string. You'?will need to hang the magnet from a ceiling or rafter, and so the best place in which to set up the experiment may be a garage or shed. Before' you start, try out the magnet over the foil dish and see what happens. ■ ' '■ Set up the experiment in this way: 1. Cut a length of string and tie it in a suitable place above a work bench or table. 2. Push the needle through the cork so that the . point sticks up through the top, and place it on the bench under the string. 3. Balance the dish carefully on the needle point, i 4. Tie the end of the string to the magnet. The string should be long enough to allow the magnet to hang about half a centimetre above the dish, before the string is twisted. 5. Twist the string about 50 times. Steady the magnet above the centre of the dish. Let the string go-

As the string unwinds, the magnet will begin to turn and the foil dish should begin to turn with it.

The reason is that when the magnet moves over the dish it causes eddy currents in the metal. These currents produce a new magnetic force. The action of the hanging magnet on this magnetic force drags the dish round after it.

One of the important uses of magnetism is in the production of electric currents.

Clever balloons We are told that most ordinary materals contain electricity of two kinds in equal quantities — positive and negative. Usually the two kinds counteract each other, but you can increase the amount of one kind of electricity in an object by rubbing it with a different type of material. The object then becomes charged because it contains a greater quantity of one kind of electricity than the other.

This simple experiment with two balloons will show you how one charged balloon reacts to light uncharged,, objects, and how the two balloons react to each other when both are charged. ? You will need two long balloons, a piece of' knitted woollen ‘ material, , a table tennis ball, small scraps.of paphr, and a few grains of.- breakfast cereal such as cornflakes or rice flakes./ ? ‘.'/ ’ - ; Wh.at to do

1. Blow "'up the two balloons to about the same size.’Tie the mouth of Teach balloon with a length of wool. -' /

2. Tie a piece of sewing thread round the tabletennis, ball and hang it in a convenient place. 3. Charge one of the balloons by rubbing it briskly with the woollen material.

Experiments Try these simple ex-, periments, and watch the results: < 1.. Place the charged balloon on the scraps of paper and the flakes of breakfast cereal. 2. Hold the charged balloon near the suspended table-tennis ball. 3. Remove the tabletennis ball and hang the charged balloon in its place. : '••

4. Charge the second balloon by rubbing it with the woollen cloth. 5. Hold the second balloon- near the suspended one.

' The charged balloon should pick up the light uncharged objects, and pull the table-tennis ball towards it. When ' you move the second charged balloon towards the suspended one, the suspended balloon should pull away from it.