Magnets and Magnetic Fields

Every magnet has two ends called poles: a north pole and a south pole. The poles are the regions where the magnetic forces are strongest.

When two magnets are brought near each other, the poles push or pull on one another without touching. This makes the magnetic force a non-contact force, like gravity or the electrostatic force.

Like poles repel, unlike poles attract.

North–north and south–south push apart. North–south pull together.

The force gets stronger as the magnets get closer and weaker as they move apart. Because the magnets never touch, you can feel this force acting through the empty gap between them.

Pole combination	Force
North and north	Repulsion
South and south	Repulsion
North and south	Attraction

There are two kinds of magnet, and the exam expects you to tell them apart.

A permanent magnet produces its own magnetic field all the time. A bar magnet, a fridge magnet and a horseshoe magnet are permanent magnets. Their magnetism does not switch off.

An induced magnet is a piece of magnetic material that becomes a magnet only when it is placed in an existing magnetic field. When you remove it from the field, it loses most or all of its magnetism.

An induced magnet is always attracted to the permanent magnet that induces it. Induced magnetism can never cause repulsion.

This is why a permanent magnet picks up a steel paperclip: the field of the magnet turns the paperclip into a temporary induced magnet, with the pole nearest the magnet becoming the opposite pole, so the two attract. Chain several paperclips and each becomes an induced magnet for the next.

Feature	Permanent magnet	Induced magnet
Produces its own field?	Yes, always	Only when in a field
Magnetism after removal	Kept	Lost (most or all)
Can it repel a magnet?	Yes	No — attraction only

Only certain materials are magnetic, meaning they are attracted to a magnet and can be made into an induced magnet.

The magnetic materials you must recall are iron, steel, cobalt and nickel. (Steel is mostly iron.)

The force on a magnetic material placed in a field is always attraction, never repulsion. This is different from two permanent magnets, which can either attract or repel depending on which poles face each other.

Most everyday metals, such as aluminium, copper and gold, are not magnetic and are not attracted to a magnet at all. A quick test for whether an object is a permanent magnet is to see if it can repel another magnet: a magnetic material like an iron nail will only ever be attracted, so it cannot be a permanent magnet.

A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material. We picture the field using magnetic field lines.

Two rules define how field lines are drawn:

• Direction: outside the magnet, field lines run from the north pole to the south pole. The direction of the field at any point is the direction of the force that would act on the north pole of a compass placed there.
• Strength: the closer together the lines are, the stronger the field. Because the lines are most crowded at the poles, the field is strongest at the poles and gets weaker with distance.

The field lines never cross. Each is a complete loop: it leaves the north pole, curves around the outside of the magnet to the south pole, and continues back through the magnet to the north pole. The lines bunch tightly at each pole and spread out in the gap between the poles, which is exactly why the poles feel the strongest force.

A plotting compass contains a tiny bar magnet, free to turn, that lines up with whatever field it sits in. This lets you map a real magnetic field, and it is the standard required method.

Method to plot the field of a bar magnet:

1. Place the bar magnet on a sheet of paper and draw around it.
2. Put a plotting compass near one pole and mark a dot at each end of the needle.
3. Move the compass so the tail sits where the head just was, and mark the new position.
4. Repeat all the way round, then join the dots into a smooth line with an arrow.
5. Start again from a different point on the pole to build up the whole pattern.

The arrows always point the way the compass north end points, so the finished field lines run from north to south. This is how you produce the looping pattern shown for a bar magnet.

A compass does not need a nearby magnet to work. Left alone, its needle settles pointing roughly north–south. This is powerful evidence that a magnetic field fills the space around us.

A compass gives evidence that the core of the Earth is magnetic, because the needle lines up with the Earth's own magnetic field.

The Earth behaves as though a giant bar magnet sits inside it. The needle's "north-seeking" pole is pulled towards the Earth's geographic north, which is why it points that way. Because a compass simply follows the field it is in, moving it around lets you map the Earth's field just as you would map a bar magnet's.

This is the same principle used for navigation for centuries: as long as no other magnets or magnetic materials are nearby to distort the field, a compass reliably shows direction.

1. Saying induced magnets can repel

An induced magnet is only ever attracted to the magnet inducing it. If a question describes repulsion, both objects must be permanent magnets. Repulsion is the true test of a permanent magnet.

2. Drawing field lines pointing the wrong way

Outside the magnet, arrows go from north to south. Drawing them south to north, or leaving off the arrowheads, loses the mark.

3. Forgetting which materials are magnetic

Only iron, steel, cobalt and nickel are magnetic. Copper, aluminium and gold are not, even though they are metals.

4. Getting field strength backwards

Lines that are close together mean a strong field. Widely spaced lines mean a weak field. The field is strongest at the poles, where the lines are most crowded.

5. Confusing the compass explanation

A compass points north because its needle is a small bar magnet lining up with the Earth's field. The exam wants this linked back to the idea that the Earth's core is magnetic, not just "it points north".