Magnetisation and Demagnetisation

Theory of Magnetism and Magnetic Domains

A popular theory of magnetism considers the molecular alignment of the material. This is known as Weber's theory. This theory assumes that all magnetic substances are composed of tiny molecular magnets.



Any unmagnetized material has the magnetic forces of its molecular magnets neutralized by adjacent molecular magnets, thereby eliminating any magnetic effect. A magnetized material will have most of its molecular magnets lined up so that the north pole of each molecule points in one direction, and the south pole faces the opposite direction. A material with its molecules thus aligned will then have one effective north pole, and one effective south pole.

An illustration of Weber's Theory is shown in figure 1-11, where a steel bar is magnetized by stroking. When a steel bar is stroked several times in the same direction by a magnet, the magnetic force from the north pole of the magnet causes the molecules to align themselves.

Ways of Making magnets

1. ‘Stroke’ method

A piece of magnetic material can be turned into a magnet if it is stroked by a magnet. As the magnet moves along the magnetic material, it causes the magnetic dipoles in the magnetic material to become aligned in one direction and give rise to a magnetic field.

2. Electrical method using a direct current

When a large direct current is passed through the solenoid, the unmagnetised steel bar will become magnetized after a while. This is because when an electric current flows through the solenoid, it produces a strong magnetic field which magnetizes the steel bar.

The poles of the magnet can be determind by a simple method known as Right-hand grip rule.


Ways of demagnetizing magnets

1. Heating

Heating a piece of magnetized metal in a flame will cause demagnetization by destroying the long-range order of molecules within the magnet. By heating a magnet, each molecule is infused with energy. This forces it to move, pushing each molecule out of order within the magnet and leaving the piece of metal with very little or no magnetization.

2. Hammering

When a magnet is hammered or dropped, the vibrations caused by the impact on the magnet randomize the magnetic molecules within the magnet, forcing them out of order and destroying the long-range order of the unit magnet.

3. Alternating Current (AC) Field

Using an AC current produces a magnetic field which can be moved and reduced to demagnetize materials. The field created by the AC current drags the magnetic molecules of the magnet in different directions. When the AC current is altered or reduced, the molecules within the magnet do not all return to previous positions, causing randomization of the molecules and reducing the force of the magnet.