Physics 12.1 - 12.3 - Magnetic materials and induced magnets
Magnetic materials behave differently when a magnetising field is removed. Use pole interactions and magnetic retention to choose materials and explain why attraction alone cannot prove that an object is a permanent magnet.
Poles and magnetic materials
Every magnet has two poles, called north (N) and south (S). The interaction depends on the pair of poles brought together:
- unlike poles, N and S, attract;
- like poles, N and N or S and S, repel.
Iron, nickel, cobalt and many steels are magnetic materials: an ordinary magnet can attract them and they can be magnetised. This does not mean that every metal is magnetic. For example, copper and aluminium are not attracted strongly by an ordinary magnet.
The best material depends on what the magnet must do.
| Material choice | Magnetic behaviour | Why and where it is useful |
|---|---|---|
| Soft iron | Easy to magnetise and loses most of its magnetism when the magnetising field is removed | A temporary magnetic core is useful in a device that must pick up and then release iron or steel, such as a switchable lifting magnet. |
| Suitable steel | Harder to magnetise but retains magnetism well | Permanent compass needles, magnetic catches and other objects that must remain magnetised. |
| Alloys containing cobalt or nickel | Can retain strong magnetisation; alnico is one example containing aluminium, nickel and cobalt | Permanent magnets in devices such as loudspeakers and sensors. |
"Permanent" and "temporary" describe how well the material retains magnetisation. Permanent does not mean impossible to demagnetise, and temporary does not mean that the object itself is short-lived.
A material is selected for its magnetic behaviour: retention is useful for a permanent magnet, while easy magnetisation and demagnetisation are useful when the magnetic effect must be temporary.
Permanent and induced magnets
Permanent magnet
A magnet that produces its own persistent magnetic field and remains magnetised when no inducing magnet is nearby.
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