(excluding electromagnetism)

9.3 Magnetisation and making magnets - induced-temporary and permanent magnets

There are two types of magnet we need to consider.

Permanent magnets retain their magnetism and create their own magnetic field.

These are made from 'hard' magnetic materials like steel do not readily lose their magnetism and produce their magnetic fields all the time.

You can test whether a material/object is magnetic by seeing if it is attracted to a permanent magnet.

Induced or temporary magnets are materials that only become magnetic when placed in a magnetic field and will only then produce their own magnetic field.

Temporary magnets are made from 'soft' magnetic materials like iron and some iron-nickel (steel) alloys.

For example when a permanent magnet is placed in contact with another magnetic material like iron, they will always be attracted to each other AND a north and south pole are induced in the other magnetic material.

e.g. If you place a permanent steel magnet in contact with a piece of (soft) iron, the piece of iron becomes magnetised and the two induced poles match the S-N attraction.

In fact, the nearest pole on the iron to the magnet will always be the opposite of the pole on the permanent magnet, hence the attraction.

In other words a permanent north pole induces a temporary south pole in the magnetised material or  permanent north pole induces a temporary south pole.

This explains why a permanent magnet can pick up any other magnetisable material - it induces the opposite poles and attraction immediately follows.

In the case of iron, when you remove the permanent magnet you remove the source of induced magnetism and the iron bar gradually loses its magnetism.

The iron bar would be described as a temporary magnet and the iron as a soft magnetic material.

This magnetic pole induction produces some quite interesting effects!

e.g. a bar magnet will pick up a whole chain of iron nails or paper clips as each iron/steel item becomes an induced magnet and can so attract another one.

The magnetic field effect becomes weaker down the chain and eventually the weight of the chain becomes greater than the attractive force of the magnetic field and either no more 'stick on' or the whole lot fall off.

 

You can use the principle of induction to make a permanent magnet from another permanent magnet.

All you have to do is stroke the un-magnetised steel bar with a permanent bar magnet.

You repeatedly stroke the steel bar with the magnet repeatedly in the same direction.

All the 'atomic iron magnets' all line up to produce a permanent magnet with its own independent magnetic field - its still magnetic induction, but the effect is permanent.

 

The Earth's magnetic field can induce magnetism in materials

Even the weak Earth's magnetic field magnetises steel structures like bridges, especially if the magnetisable object is subjected to vibration - its as if you are shaking the atomic iron magnets into alignment turning the object into a weak magnet.

Know the difference between hard and soft magnetic materials.

Describe methods of how to induce magnetism in various ways to make a permanent magnet.

Described with diagrams how to convert steel or nickel into a permanent magnet.

Describe how iron can be temporarily magnetised.

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INDEX for physics notes on magnetism