The poles of a magnet

Magnetic poles

Supposing you take a bar magnet and suspend it freely at its mid-point on a length of thread. Once the magnet stops spinning it takes up a position such that one end points roughly towards the Earth's North Pole and the other towards the South Pole. Set the magnet spinning again and allow it to settle down. The same outcome results, the magnet (and any other magnet) aligns itself along the north–south direction.

Good use of this property of magnets is made in compasses, where the compass needle is simply a freely suspended bar magnet. If you are caught out on the moors when a mist descends, it is still possible to find your way by using a compass which always points along the north–south line.

The end of the compass which points towards the north is called the north-seeking pole or simply the north pole. The end of the compass which points towards the south is called the south-seeking pole or simply the south pole. In fact the compass does not point directly towards the geographical North Pole, but aligns with what is called the magnetic north. We shall return to this point a little later in discussing the Earth's magnetism.

All magnets have two poles

Do all magnets have two poles? Look at these examples, which cover those you are most likely to come across.

So in the lab you'll never get just one pole. But the universe is a much larger place – what about there?

Cutting a magnet in half

Karen: But what happens if you cut a magnet in half? Do you get a north pole and a south pole?

Well you know that is not the case. If the physics lab bar magnet is cut in half then there will be two magnets, each with a north and a south pole. But then there is the next question:

Karim: What happens if you cut it in half loads of times so you are left with just one atom?

The answer is that the atom will still behave as a magnet with a north and a south pole. Although it is possible to know this answer the fundamental question that the pupil is asking is a very good one: why can't you get a north pole or south pole on its own? This seems all the more strange since positive and negative charge don't always accompany one another. This difference between electric and magnetic fields is one of the oldest puzzles in physics. Why is it possible to isolate positive and negative electrically charged particles, but not north and south magnetic poles?

In fact finding magnetic poles on their own, or magnetic monopoles, is an idea that was suggested first in 1931 by physicist Paul Dirac. The idea has also been put forward in some modern theories that try to connect all the forces together (grand unified theories). However so far all experimental searches for these elusive particles have proved fruitless.

So, if the question of monopoles is bothering pupils in class you can assure them it bothers physicists too.