Magnets and Magnetic Fields

So far in Topic P3 we have followed charge as it flows round circuits. The second half of this topic turns to magnetism — and, as you will see over the next three lessons, magnetism and electricity are two sides of the same coin. Bring two bar magnets slowly together and you can feel something invisible at work: sometimes a gentle pull that snaps them together, sometimes a stubborn push that no amount of squeezing will overcome. That invisible influence is magnetism, and the region around a magnet where it acts is the magnetic field. This lesson sorts out permanent and induced magnets, names the magnetic materials, states the rules of attraction and repulsion, and maps the magnetic field of a bar magnet — including the Earth's own field that a compass responds to.

By the end of this lesson you should be able to distinguish permanent magnets from induced magnets, name the magnetic materials, state the rules for attraction and repulsion between poles, describe and draw the magnetic field around a bar magnet, explain how a plotting compass reveals the field, and explain how the Earth's magnetic field acts on a compass.

This lesson is mostly AO1 recall and understanding of magnets, magnetic materials and field patterns, with some AO2 application when you use a plotting compass to reveal a field and explain how the Earth's field acts on a compass.

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Permanent Magnets and Magnetic Poles

A permanent magnet is an object that produces its own magnetic field all the time, without any help. A bar magnet, a horseshoe magnet and a fridge magnet are all permanent magnets. Every magnet has two poles, called the north pole (strictly the "north-seeking" pole) and the south pole. The poles are where the magnetic field is strongest — which is why iron filings cluster most thickly at the ends of a bar magnet.

A single magnet always has both a north and a south pole; you cannot have a magnet with only one pole. If you snap a bar magnet in half, you do not get a separate north piece and a separate south piece — instead each half becomes a complete magnet with its own north and south pole.

Exam Tip: Every magnet has two poles, north and south. Cutting a magnet in two gives two smaller magnets, each with a full pair of poles — never a single isolated pole.

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Attraction and Repulsion

The way two magnets behave when brought together depends entirely on which poles face one another. The rule is short and must be known exactly:

• Like poles repel — two north poles, or two south poles, push apart.
• Unlike poles attract — a north pole and a south pole pull together.

This is a non-contact force: the magnets push or pull on each other without touching, through the magnetic field in the gap between them. The force gets stronger as the magnets are brought closer together, because the field is more concentrated near the poles, and weaker as they are moved apart. (Notice the close parallel with electric charges from earlier in this topic, where like charges repel and unlike charges attract — both are non-contact forces acting through a field.)

Poles facing each other	Force	Effect
North – North	Repulsion	Push apart
South – South	Repulsion	Push apart
North – South	Attraction	Pull together
South – North	Attraction	Pull together

Exam Tip: "Like repel, unlike attract." A test of whether something is a magnet is repulsion: a magnet can attract an unmagnetised piece of iron, but only another magnet facing it the wrong way will repel it. Repulsion is the sure sign that both objects are magnets.

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Induced Magnets and Magnetic Materials

Not everything is magnetic. A magnetic material is one that is attracted to a magnet. The magnetic materials you must know are the metals iron, steel (which is mostly iron), cobalt and nickel. Most other materials — aluminium, copper, plastic, wood, glass — are not magnetic and are not attracted to a magnet at all.

When a magnetic material such as a piece of iron is placed in a magnetic field (for example, next to a permanent magnet), it becomes a magnet itself. This is called an induced magnet. Induced magnetism is what lets a magnet pick up a chain of paper clips: the first clip becomes an induced magnet, which then induces magnetism in the next clip, and so on.

The crucial difference between a permanent and an induced magnet is what happens when the field is taken away:

• An induced magnet loses most or all of its magnetism when it is removed from the magnetic field. This is why the chain of paper clips falls apart the moment you take the top clip away from the permanent magnet.
• A permanent magnet keeps its magnetism whether or not another magnet is nearby.

Induced magnetism always causes attraction: the induced pole nearest the permanent magnet is always the opposite pole, so the two are pulled together. An induced magnet can never repel the magnet that is inducing it.

Feature	Permanent magnet	Induced magnet
Produces its own field?	Always	Only while in another magnetic field
When field removed	Stays magnetic	Loses most/all magnetism
Example	Bar magnet, fridge magnet	Iron nail held near a magnet, paper clips in a chain
Force produced	Attraction or repulsion	Attraction only

Exam Tip: The give-away of an induced magnet is that it loses its magnetism when removed from the field and can only attract, never repel. A material that stays magnetic after the field is taken away is a permanent magnet.

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The Magnetic Field of a Bar Magnet

A magnetic field is the region around a magnet where a magnetic material or another magnet feels a force. We picture the field using magnetic field lines (lines of force), which have a set of fixed rules:

• Field lines always run from the north pole to the south pole outside the magnet (and from south back to north inside it, completing a loop).
• The lines have arrows on them showing this N → S direction outside the magnet.
• The lines never cross one another.
• Where the lines are close together the field is strong; where they are spread apart the field is weak. The lines are most crowded at the poles, so the field is strongest there.

The diagram below shows the classic looping pattern of a bar magnet's field.

The closely spaced lines bunched at each end show that the field is strongest at the poles, while the widely spaced lines further out show a weaker field. This idea — line spacing tells you field strength — applies to every field diagram in this topic, magnetic and electric alike.