Magnets and magnetic fieldsIB MYP Physics: Revision notes

Every magnet has two ends called poles: a north pole and a south pole. The magnetic force is strongest at the poles.

• Like poles repel (N and N, or S and S).
• Unlike poles attract (N and S).

If you cut a bar magnet in half, you get two smaller magnets, each with its own north and south pole. You cannot have a north pole on its own.

Only a few metals are magnetic: iron, steel (which is mostly iron), nickel and cobalt. A magnet attracts these but not copper, aluminium, plastic, glass or wood.

A permanent magnet keeps its magnetism and can attract and repel. Steel is used because it keeps its magnetism.

An induced magnet is a magnetic material that becomes a magnet when it is placed near or touching a magnet. It is always attracted by the magnet and, in a material like soft iron, it loses most of its magnetism when the magnet is taken away.

A magnetic field is the region around a magnet where a magnetic material feels a force. We show it with field lines.

• Field lines go from the north pole to the south pole outside the magnet.
• The lines are closest together at the poles, where the field is strongest, and spread out further away, where it is weaker.
• Lines never cross.
• Between a north and south pole facing each other the lines are straight, parallel and equally spaced, which shows a uniform field of the same strength.
• Between two like poles the lines push apart and there is a point in the middle with no field.

An arrow on each line shows the direction a free north pole would move.

The Earth has a magnetic field, like a giant bar magnet in the centre of the planet, which is caused by its liquid iron core. A compass contains a small magnet on a pivot. The needle lines up with the Earth's field, so its north-seeking end points towards geographic north.

A compass will also line up with the field of any magnet or electric current near it, which is why a compass can be used to show the direction of a magnetic field.

To plot the field of a bar magnet:

1. Place the magnet in the middle of a sheet of paper.
2. Put a small plotting compass next to one pole and mark the positions of both ends of the needle with pencil dots.
3. Move the compass so that one end of the needle is on the second dot, and mark the new position.
4. Repeat until you reach the other pole, then join the dots with a smooth line and add an arrow.
5. Repeat from other starting points to draw more field lines.

Alternatively, scatter iron filings on paper over the magnet and tap it. They line up along the field lines and show the pattern, but not the direction.

To magnetise a piece of steel, stroke it many times with one pole of a magnet. Always stroke in the same direction and lift the magnet away at the end of each stroke. Steel keeps its magnetism, so it becomes a permanent magnet.

To demagnetise a magnet, you can hit it repeatedly, drop it, or heat it strongly. This shakes the tiny magnetic regions into random directions. You can also place it in a coil carrying alternating current and slowly pull it out.

Soft iron is easy to magnetise and demagnetise, so it is used for temporary magnets. Steel is harder to magnetise but keeps its magnetism.