Physics Theory : Magnets

Magnetic poles are the points (N for north and S for south) slightly within the ends of a magnet where most of its magnetic power (influence) appears to be concentrated.
Both the north pole and south pole are of equal strength.
Unlike poles attract each other, whereas like poles repel each other.

Magnetic axis is defined as the imaginary line NS which passes through the magnetic north and south poles of any bar magnet.

Length of magnet is defined as the distance between one of the magnetic poles and the centre of the magnet (ON = OS = L).

Effective length is defined as the distance between the magnetic north pole and the magnetic south pole of a bar magnet (NS = 2L).
Thus, the effective length is twice the length of magnet.

Magnetic equator is defined as the imaginary line WE perpendicular to the magnetic axis NS and bisecting the effective length of the magnet.

Magnetic meridian is defined as the imaginary vertical plane passing through the magnetic axis NS of a freely suspended bar magnet.

Equatorial meridian is defined as the imaginary vertical plane passing through the magnetic equator WE of a freely suspended bar magnet.

Magnetic substances (or magnetic materials) are those which can be influenced by a magnet.
For example, iron, steel, cobalt and nickel are magnetic substances.

Soft iron can be easily and highly magnetized.
However, the magnetism is temporary because it loses its magnetism as soon as the inducing magnet is removed.
Thus, it is used for making only temporary magnets (electromagnets).

Steel cannot be magnetized very highly.
However, the magnetism is permanent, i.e., it does not lose its magnetism when the inducing magnet is removed.
Thus, it is used for making permanent bar magnets and horse-shoe magnets.

Repulsion is the surest test for magnetism.
This is so because attraction can occur between two magnets (unlike poles) as well as between a magnet and a magnetic material, whereas repulsion can occur only between two magnets (like poles).

Magnetism can act through non-magnetic materials.
For example, the unlike poles of two bar magnets will still attract each other even if plastic (non-magnetic material) is placed between them.

Loss of magnetism occurs if magnets are strongly heated, or even knocked or hammered.