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Magnetism

Permanent magnets can be used in:
a compass, magnetic holders for knives, magnetic recording tape (video and audio), computer floppy discs, computer hard discs, loudspeakers, bicycle dynamos, magnetic ink on cheques, magnetic catches, magnetic strip or letters, magnetic soap holders, magnetic toys, removing metal splinters from eyes, checking car body repairs, testing the quality of cutlery, holding pads for window cleaning etc.

Magnetism was discovered by the Greeks in about 600 BC. They noticed that a substance called lodestone had some strange properties. If a piece of lodestone was hung on a thread it would always point in the same direction. Lodestone - we now call it magnetite - is an oxide of iron and it was the first known magnetic material. Lodestone will also attract other magnetic materials.

The following materials can be magnetized and attracted by a magnet:

iron steel cobalt nickel

Not all materials can be magnetized, the following are examples of the many materials that cannot:
Lead    aluminium    brass    copper    tin    wood    plastic    glass

Many new kinds of magnetic materials have been made since the discovery of lodestone. One of these is ferrite - a ceramic material made from magnetic powder (a mixture of iron oxide and barium oxide). Ferrites are used in radio aerials and you will probably have used one type - magnadur - in your school. These ceramic magnets are magnetsied when the material is soft and then fired to produced strong but rather brittle magnetic materials.

Other materials used are Alnico (a mixture of iron, nickel, cobalt and aluminium giving a permanent magnet) and Mumetal (nickel, iron and copper) giving a material that can be magnetised and demagnetised easily.

A magnet is said to have two POLES, these are the places where its magnetism is greatest; in a bar magnet the poles are close to the ends of the magnet. If a magnet is suspended, then one pole always points northwards while the other points southwards.
(You will see that it is MAGNETIC, north and MAGNETIC, south.)
The pole that points north is called the north seeking pole or simply the north pole of the magnet. The pole that points south is called the south seeking pole or simply the south pole of the magnet.

The effect of one type of pole on the other can be found by hanging one magnet up and bringing another one up to it and seeing if it is repelled or attracted.

This will show that:

A north pole repels another north pole.
A south pole repels another south pole.
A north pole and a south pole will attract each other.

These results can be summarised as:

Like poles repel each other
Unlike poles attract each other

The magnetism is a property of the whole magnet because if a magnet is cut in half each half will be a small magnet. You can go on doing this as shown in the diagram. See if you can find out if there is a limit to this.


Inside a magnet

Inside a bar of magnetic material there are millions of tiny magnets. When the bar is not magnetised these are joined in closed loops (i). As the bar is magnetised these split up (ii) and (iii) and when it is fully magnetised they are arranged in an orderly way, all pointing in one direction (iv).




It is important to keep all credit cards and other magnetic cards away from magnets. I was on holiday, staying the hotel in the photograph, and was wearing a pair of trousers which had a small pair of magnets making a magnetic catch to the back pocket. I couldn't understand why the magnetic-card door key to our room kept failing. My wife (she is not the physics teacher) then suggested it was because I was putting the card in my back pocket. I tested it with another card she was right the magnets were corrupting the information on the card. So be careful!

 
 
 
© Keith Gibbs 2011