Permanent and Induced Magnetism

There is a separate article that explains magnetic fields and forces so this article focuses on magnetic materials and the difference between permanent and induced magnets.

There are naturally occuring rocks that are capable of being magnetised (magnetite) and others that are already magnetised (lodestone). The important ingredient in both of these rocks is iron but two adjacent elements in the Periodic Table, cobalt and nickel, are also capable of being magnetised in their pure state. We say these elements are ferromagnetic, meaning they have magnetic properties that are like those of iron.

Stronger magnets can be made using alloys (combinations of metals): the best known of these are based on neodynium (also containing iron and boron) or samarium (also containing cobalt). Even the natural magnet, lodestone, contains iron bonded with another element (oxygen). Iron itself is not a permanent magnet: it is only capable of being magnetised.

When we talk about magnets we normally mean materials that exhibit permanent magnetism. These are materials are relatively difficult to magnetise but they retain their magnetism under a wide variety of conditions.

Other materials can be attracted to magnets but they are not magnets themselves. They become magnetised when they are in a magnetic field (close to a permanent magnet) but they lose that magnetism soon after they are removed from the magnetic field. This behaviour is known as induced magnetism.

There is a foolproof way to distinguish permanent magnets from induced magnets;

• permanent magnets can either attract and repel another permanent magnet, depending on how the two magnets are turned
• induced magnets only attract other magnetic objects, including permanent magnets, regardless of how they are turned

Effectively, induced magnets “pass on” magnetic fields whereas permanent magnets have magnetic fields of their own.

If you have ever done the experiment to pick up a paper clip using a magnet then you have seen both permanent magnetism and induced magnetism in action. The magnet you used was a permanent magnet and the paper clip became magnetised so that it could be attracted to the magnet. If you had tried to pick up a piece of aluminium foil, you would have failed. This is because aluminium is not ferromagnetic and therefore cannot become magnetised.

Note that (permanent) magnets do not “pick things up” on their own: there must be a two-way force of attraction between the objects for picking-up to become possible. If the second object cannot be magnetised then it cannot be picked-up.