TOPIC 3: MAGNETISM – PHYSICS FORM TWO
Contents to be covered are;
i. Meaning of magnetism.
ii. History of magnetism.
iii. Origin of magnetism.
iv. Magnetisation and demagnetisation.
v. Magnetic field of a magnet.
vi. Earth’s magnetic fields.
WHAT IS MAGNETISM?
Magnetism is a phenomenon produced by the motion of electric charges that results in an attractive and repulsive force between objects. The earliest observation of magnetism was recorded in 600 BC by the Greek philosopher Thales.
He observed that pieces of iron were attracted to a natural mineral iron ore called magnetite. The lodestone is an example of a magnetised piece of the mineral magnetite.
Magnetic and non-magnetic materials
When a magnet is brought near different objects, some will be attracted or repelled, and others will not. Objects that are attracted are said to possess induced magnetism. The process by which a material becomes a magnet due to its closeness or contact with a magnet is called induction.
The induced magnetism can be temporary or permanent upon the removal of the magnet in contact. The nature of the material being magnetised determines how long the induced magnetism lasts.
For example, iron exhibits temporary magnetism while materials like steel tend to retain their magnetism for a long time. Materials which are not affected by a magnet are called non-magnetic materials.
Types of magnets
Magnets are categorised according to their sources of magnetism. They include:
1. Temporary magnets
These acquire magnetism due to an applied external magnetic field but lose their magnetism when the external field is removed.
2. Permanent magnets
These retain some magnetism even after the external magnetic field is removed.
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