Simple Domain Theory Of Magnetism

It’s no coincidence that the three common magnetic materials, iron, nickel and cobalt  are next to each other in the Periodic Table. This suggests that the number and configuration of their electron structure might be responsible for magnetic behaviour.

We believe that magnetic materials such as iron have inside them small molecular magnets. These small magnets are contained in tiny ‘cells’ called domains. There might be millions of molecules in a domain. Within each domain all the molecular magnets point in the same direction.  The molecular magnets are pointing in different directions in an unmagnetised sample

If we stroke the iron using a magnet – like stroking a cat, always in the same direction – the magnet will attract all the opposite poles so that they point in the same direction. The iron becomes a magnet. In this case, we have stroked the iron with a S pole in the anticlockwise direction, so a N pole is formed on the right hand side.

Once all the domains have lined up, our iron bar is completely magnetised. It won’t make any difference if we keep on stroking it.

If we cut a magnet in half, it’s easy now to see that we now have two magnets –   the original S-N now makes two magnets, S-N, S-N

Iron is magnetically SOFT – easy to magnetise or demagnetize.

Steel is magnetically HARD because impurities make it more difficult for the domains to line up, so it’s hard to magnetise or demagnetise

Heating or hammering messes up the domain directions and causes a magnet to lose its magnetism.

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