Copyright © 2007 Dorling Kindersley
Magnetism is an invisible force that attracts or repels some materials, such as iron and steel, but not others, like plastic and silver. In a magnetic material, the atoms line up in groups or regions called MAGNETIC DOMAINS.
Every magnet has a north magnetic pole and a south magnetic pole. These are the places where the magnetic force is strongest. The laws of magnetism are that like (the same) poles repel each other, and unlike (opposite) poles attract each other.
The elements iron, nickel, and cobalt are magnetic materials—they can be magnetized by another magnet. But in their pure form they easily lose their magnetism by heat or hammering. Permanent magnets are made from mixtures of these elements with others, such as steel (iron and carbon), for example.
Earth’s core acts as a gigantic magnet with a vast MAGNETIC FIELD. In a magnetic compass, the northern end of the compass needle always points in the direction of Earth’s north magnetic pole. Its other (southern) end always points south.
Every atom in a piece of iron is a tiny permanent magnet. These tiny magnets group together in magnetic regions called domains. If the north–south magnetic poles of these domains all point in different directions, they cancel out each other’s magnetism.
In a permanent magnet, the magnetic poles of the domains point in the same direction, so their magnetic fields reinforce one another. Magnetic material can be magnetized by stroking it with a magnet to line up the domains. Heat or hammering shakes the domains out of position, and the material loses its magnetism.
A magnet creates a force in the space around it. The area in which the force operates is the magnetic field. A magnetic field can be imagined as lines of force that spread out from the magnet’s poles.
If iron filings are sprinkled around a magnet, they will line up along the lines of force of its magnetic field. The pattern made by the filings always shows the lines of force looping outward between the magnet’s north and south poles. The magnetic field gets weaker as it gets farther away from the magnet.