Sunday, December 26, 2010

Magnetic Field

A magnetic field is a region in which a magnetic object, placed within the influence of the field, experiences a magnetic force.

A pattern of this directional force can be obtained by performing an experiment with iron filings. A piece of glass is placed over a bar magnet and the iron filings are then sprinkled on the surface of the glass. The magnetizing force of the magnet will be felt through the glass and each iron filing becomes a temporary magnet.

If the glass is now tapped gently, the iron particles will align themselves with the magnetic field surrounding the magnet just as the compass needle did previously. The filings form a definite pattern known as the magnetic field pattern, which is a visible representation of the forces comprising the magnetic field. The magnetic field is very strong at the poles and weakens as the distance from the poles increases. It is also apparent that the magnetic field extends from one pole to the other, constituting a loop about the magnet.

Magnetic field lines between two magnets


When two magnets or magnetic objects are close to each other, there is a force that attracts the poles together.

Magnets also strongly attract ferromagnetic materials such as iron, nickel and cobalt.


When two magnetic objects have like poles facing each other, the magnetic force pushes them apart.

Magnets can also weakly repel diamagnetic materials.

Temporary and Permanent magnets

• Permanent magnets — are able to retain their magnetism for long periods. They can be found around us as fridge magnets, bar magnets or button magnets used in games, or lodestones (natural magnets).

• Temporary magnets — are sometimes called induced magnets. They refer to magnetic materials that have been placed within a strong magnetic field and become magnets. These magnets lose their magnetism once they are removed from the magnetic field. Temporary magnets can be found in telephones, electric motors, and cranes at refuse dumps.

• Materials that are more easily magnetised tend to lose their magnetism more quickly. They are referred to as ‘soft’ magnetic materials. Examples include iron and alloys like MumetalTM (a nickel–iron alloy). ‘Hard’ magnetic materials, on the other hand, are much less easily magnetised, but they retain their magnetism for a longer time, e.g. steel.

• The Earth behaves like a giant magnet. Just like any magnet, it has two magnetic poles — North and South. These poles are not the same as the geographic North and South Poles that we see on world maps. The north-pole of a freely suspended bar magnet, such as that in a compass, points to the Earth’s magnetic North, which is near to its geographic North.

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