SITEMAP   School Physics Notes: Electricity-magnetism 9.1 Magnetism N & S poles

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Magnetism: 9.1 Introduction to magnetism and magnetic fields - north pole and south pole - rules on attraction or repulsion

Doc Brown's Physics exam study revision notes

9.1 Introduction to magnetism and magnetic fields

Most materials are non-magnetic, that is they cannot be magnetised to create their own independent magnetic field.

You might ask the question 'what is magnetism', the answer is the same for gravity, nobody is really quite sure and the answer will lie somewhere in some deep quantum theory, well above school physics level and my little grey cells!

However, the causes and effects of magnetic fields are well understood and can be described by 'working models' e.g. little atomic magnets to do with electrons in atoms like iron, magnetic fields from compass plotting and accurate mathematical models from very simple equations to the extremely complex!

Magnetism is a non-contact force because it causes objects to attract or repel without the two objects having to make contact.

Only relatively few materials display magnetic properties - that is, they are naturally magnetic, or more usually, capable of being magnetised.

Magnetic materials

Typical magnetic materials are iron and its alloys like steel, nickel and its alloys and cobalt and its alloys - the alloys, in particular, are used to produce very strong permanent magnets.

Other common structural metals like aluminium, copper and lead are not magnetic.

Jewellery metals like gold, platinum and silver are also not magnetic.

Actually most materials are NOT magnetic!

A bar or rod of magnetic material can be shown to have two poles, one at each end - a north seeking pole (north) and a south seeking pole (south).

e.g. a bar magnet is suspended by a fine thread, one end will point to the Earth's magnetic north and the other end to the Earth's magnetic south.

This is evidence that the metallic core of the Earth (lots of iron) is permanently magnetic.

As with electric charges, when you bring poles together, one of two things can happen.

Either the poles attract or they repel each other.

In other words any two magnetic objects exert a force on each other - attraction or repulsion.

Experiments demonstrating magnetic pole rules

The rule is quite simply like poles repel (<= N N => or <= S S =>) and unlike poles attract (N => <= S).

You can easily demonstrate these rules with two bar magnets (of known polarity) suspend on string and bringing the various ends together (as in diagram above).

=> <=   or   <= =>   or   <= =>

Previously magnetised materials will attract other magnetic materials whether they were or were not, already magnetised.

e.g. a magnet will always and readily attract an item of iron or steel.

In other words when a magnet is placed near a magnetic material the two objects always experience an attractive force - it doesn't matter which pole of the magnet is placed nearest the object - see induction later.

A magnetic force is a non-contact force, even if the objects eventually touch.

Electrical and gravitational forces are also non-contact forces.

Already magnetised materials are surrounded by their own magnetic field.

Keywords, phrases and learning objectives on magnetism

Know what is meant by magnetism.

Know that a magnetic field has a north pole and a south pole.

Know that unlike poles attract and like poles repel.

Be able to describe experiments that demonstrate the pole rules with freely moving suspended magnets.

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