Magnetism Unit 13

 Magnetism

 Magnets are useful things:

• Magnets can be used to hold cupboards doors shut.

• They are used in electric motors and generators.

• They are used in headphones and loudspeakers.

• They have fun uses - for example, magnetic letters.

1. Permanent Magnet

• A permanent magnet is an object that is magnetised for a long time. It doesn't stop being a magnet after it has been used.

• Permanent magnets come in different shapes and sizes. 

B. Magnetic Material

• A material which is attracted by a magnet is called a magnetic material.

• Iron is a magnetic material and also steel, nickel and cobalt (most of steel are also magnetic).

• Nowadays many small and strong magnets are made using neodymium.

• Authors metal such as aluminium and tin are non-magnetic.

C. Magnetic Poles

• Magnets attract magnetic material because there is a magnetic force which pulls magnetic object when magnet attracts it.

• The ends of the magnet are called its magnetic pole.

• The poles are called the north pole(N) and the south pole(S).

• When you hang up a bar magnet so that it is free to turn around, it will turn until one end points towards the north. The south pole point towards the south.

D. Magnet Rules

• When the poles of two magnets are close together they may attract or repel each other.

• Likes poles repel

• Unlike poles attract

          like poles’ means both poles are the same. ’unlike’ poles’ means different poles. Sometimes  

           people simply remember the phrase opposites attract.

 

E. Making a magnet

• You can use a permanent magnet to magnetise a piece of iron or steel. Here is how:

• Lay the piece of iron steel of iron or steel on the bench.

• Using one pole of magnet, stroke it gently from one end to the other.

• Stroke it several times, using the same pole. Make sure you always stroke in the same direction.     

F. Magnetic Field Lines

• A magnet field is invisible. Here are two ways to show up the shape of the magnetic field around a bar magnet:

• Use iron filings. These tiny pieces of iron cluster together and line up to show the pattern of the field.

• Use small compasses  called plotting compasses. They show the direction of the field.

• We can represent the magnetic field of a magnet by drawing magnetic field lines.

                     These are imaginary lines.

• Magnetic field lines start from a north pole to south pole and end up at a south pole.

G. The Magnetic Earth

• People use a compass to help them find their way around. A compass has a needle which is magnetised.

• The needle can turn around so it always points north-south.

• A compass needle always points north-south because the Earth has a magnetic field.

• A compass points along the lines of the Earth’s magnetic field.

H. Making an Electromagnetic

• Permanent magnet works using electricity.

• All you need is a coil of wire and a battery or low-voltage power supply to make an electric current flow through the coil.

• When an electric current flows, the coil becomes magnetised.

I. Stronger Electromagnet

• Make a coil with more turns of wire.

• Make a bigger electric current flow in the coil of wire.

J. The Field of an Electromagnet

• The magnetic field lines come out of one end of the electromagnet. This is the north pole.

• The field lines go around and back into the other end of the electromagnet. This is the south pole.

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