Putting the model to work
Physics Narrative for 11-14
How to make a magnet: magnetising by stroking
Supposing you have an iron nail. The nail is made from iron, a magnetic material, but it is not a magnet at present. How could you turn the nail into a magnet?
Perhaps the easiest way of turning the nail into a magnet is to take an existing bar magnet and use it to stroke along the length of the nail.
As the permanent magnet is stroked down the side of the nail the mini-magnets inside the nail are gradually forced into a line with their north and south poles all pointing in the same direction.
You could flip the polarity of the stroking magnet, swapping north and south poles.
You could also stroke from right to left.
And finally, you could keep this direction of stroking, but flip the polarity back again.
Magnetising by placing in a solenoid with a direct current
If you were making magnets for a living, the stroking technique would not be very economical. Just imagine all of those people stroking bars of iron with a magnet! A far better technique is to place the bar of iron inside a solenoid (a cylindrical coil of wire) and then to send a steady or
direct (DC) electric current through the turns of wire.
The direct electric current through the solenoid creates a strong, steady magnet field (see episode 02) and the mini-magnets line up in that field.
How to break a magnet: demagnetising
Any vibration or rough treatment, such as dropping onto a hard surface or tapping with a hammer, will cause weakening of the magnet.
Hitting the magnet makes the mini-magnets become disordered. This, of course, is the reason why physics teachers hate the sound of magnets being dropped on the floor. It's not the clumsiness of the pupils, or the noise produced, but the image of demagnetisation which creates alarm!
If a magnet is strongly heated and allowed to cool, it will become demagnetised.
As the magnet is heated there is increased vibration of the molecules and the mini-magnets become disordered.
The best way to demagnetise a magnet is to place it inside a solenoid and then to send an alternating (AC) electric current through the coils.
The alternating current is such that it continuously reverses the direction of flow of charge; the charge passes first in one direction through the coils and then in the opposite direction. As this happens the magnetic field created within the solenoid keeps reversing direction, and this changing field disorders the mini-magnets and so the magnet becomes demagnetised.
Why are magnetic materials attracted to magnets?
Supposing you have a bar magnet. You move it towards a paperclip (which is made from a magnetic material) and the clip is attracted towards the magnet. You turn the magnet around and try again. The paperclip is attracted to the magnet in exactly the same way. The clip is never repelled (as would be the case with two magnets). How can we explain this?
Our simple model can be used: when the magnet is moved towards the magnetic material (in this case the paperclip), the magnetic material becomes temporarily magnetised. Furthermore, it is magnetised in such a way that the pole nearest the permanent magnet becomes an opposite pole – hence the attractive force.
This process is referred to as magnetic induction: an opposite pole is induced as the magnet is brought near.