theory of magnetism

Theory of Magnetism
If a magnetic piece of steel rod is cut into smaller pieces, each piece is a magnet with a N or a S pole.
Therefore a magnet can be said to be made of lots of "tiny" magnets all lined up with their N poles pointing in the same direction. At the ends, the "free" poles of the "tiny" magnets repel each other and fan out so the poles of the magnet are round the ends.

Magnetized Bar Unmagnetized

In an unmagnetised bar the "tiny" magnets point in all directions - the N pole of one neutralized by S pole of another. Their magnetic effects cancel out and there are no "free poles near the ends.
This theory explains:
• the breaking of a magnet
• limit to strength of magnet
• demagnetization

Destroying magnetism
This can be done by:
• hammering a magnet
• heating
• dropping a magnet
• demagnetizing by using reduced alternating current through a coil of wire wrapped round a magnet



• Magnetic Fields
• A magnetic field is the area around a permanent magnet or a wire carrying a current in which a force is experienced.
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• The direction of the field at any point should be the direction of the force on a N pole.
• The direction is shown by arrows - these point away from N pole towards S pole.
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• Magnetic field of two magnets with like poles facing.
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• Magnetic field of two magnets with unlike poles facing.
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• The Earth s Magnetic Field
• The Earth behaves as if there is a bar magnet in it
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• In the diagram above there is an imaginary magnet drawn, note the south pole of the magnet is in the northern hemisphere in order to attract the north pole of a compass.



What Do Magnets Do?
• They attract certain materials eg. Iron, steel, nickel and cobalt.
• Magnetic materials are attracted to the poles of the magnet.
On a bar magnet the poles are at either end of the magnet. Poles occur in pairs of equal strength.
• One pole - North seeking pole - N pole- pointing roughly to the Earth s North pole.
Other pole - South - S pole
• Like poles repel Unlike poles attract
• Force between magnetic poles decreases as their separation increases.
Test for a Magnet
A permanent magnet causes repulsion with one pole when the poles are brought in turn near a suspended magnet.
An unmagnetised magnet material would be attracted to both poles.
Repulsion is the only sure test for a magnet
• Magnetism can work over a distance and magnets can exert a force (push or pull) on objects without making contact with them.
Just think about the effect of the Earth s magnetic field on a compass needle.

Induced Magnetism
When a piece of unmagnetised magnetic material touches or is brought near to the pole of a permanent magnet, it becomes a magnet itself. The magnetism is induced.

A North pole induces a North pole in the far end.









Magnetic Properties of Iron and Steel
Chains of paper clips can be hung from a magnet.
Each paper clip magnetises the one below it by induction and the unlike poles so formed attract.
If top paper clip is removed the chain collapses -
Magnetism induced in
iron is temporary (SOFT)
steel chain it does not
collapse -
Magnetism induced in
steel is permanent (HARD)

Strings of papers clips

Storing Magnets
Magnets become weaker with time (due to ‘free’ poles) near the ends repelling each other and upsetting alignment of tiny magnets). To prevent this bar magnets are stored in pairs with unlike poles opposite and pieces of soft iron - keepers across the ends.
The keepers become induced magnets and their poles neutralise the poles of the bar magnets.
Making a Magnet
• By stroking (i) Single touch method (ii) Double touch method is a better method

• Electrically
Place magnetic material in a solenoid (cylindrical coil of wire) which is connected to 6-12 V d.c. supply. Switch on current for a second and then off. Remove material from solenoid - material will be a magnet.
Polarity of magnet produced depends on direction of the current -
Right Hand Grip Rule
= if the right hand grips the solenoid - place
fingers on one by one in the direction of the
current - (positive to negative) the thumb
points to the north pole.