Poles attract and repel — without touching

Every magnet has two poles, a north and a south. The force between them is a non-contact force — it acts at a distance, like gravity and electrostatic forces:

• Unlike poles attract (N–S pull together).
• Like poles repel (N–N or S–S push apart).

Test for a magnet: two materials only repel if both are magnets. A magnet will attract an unmagnetised magnetic material, but repulsion is the only sure proof that something is itself a magnet.

Attract or repel?

What is magnetic?

Only a few metals are magnetic materials — they are attracted to a magnet and can be magnetised. The four you need are:

Soft vs hard: iron is magnetically soft — easy to magnetise but it loses its magnetism quickly, ideal for electromagnet cores. Steel is magnetically hard — harder to magnetise but it keeps it, ideal for permanent magnets.

Two kinds of magnet

• A permanent magnet produces its own magnetic field all the time — it always has poles (used in compasses, fridge magnets, loudspeakers).
• An induced magnet is a magnetic material that becomes a magnet only when it is placed in a magnetic field. The force between a magnet and an induced magnet is always attraction.

Common mistake: an induced magnet loses most/all of its magnetism as soon as it is removed from the field. Permanent magnets keep theirs.

Permanent or induced?

The field around a bar magnet

A magnetic field is the region where another magnet or magnetic material feels a force. We draw it with field lines:

• Field lines always point from north to south OUTSIDE the magnet.
• The field is strongest where the lines are closest together — at the poles.
• Lines never cross, and arrows show the direction the force on a free north pole would act.

Watch out: the arrows go N → S on the outside. (Inside the magnet they actually run S → N, but you draw and arrow only the field outside.)

Uniform fields, compasses & the Earth

Between two opposite poles the field is almost uniform — the lines are straight, parallel and evenly spaced, running from the N pole straight across to the S pole.

A plotting compass is a tiny magnet that lines up with the field, so it maps the shape and direction of field lines. A compass also points (roughly) north because it lines up with the Earth's magnetic field. That field has the same shape as the field of a giant bar magnet, which is evidence that the Earth's core must be magnetic.

Which way do the lines point?

Why does a paperclip never get pushed away?

Where does the magnetism come from?

A current makes a magnetic field

When a current flows through a long straight wire, it creates a magnetic field of concentric circles around the wire. Use the right-hand grip rule: point your right thumb along the current (conventional, + to −) and your curled fingers show the field direction.

A dot ⊙ means current coming out of the page (towards you); a cross ⊗ means current going into the page (away). Reverse the current and the field reverses too.

The solenoid — a switchable bar magnet

Coil the wire into a solenoid and the fields from the loops add together to make a strong, almost uniform field along the centre — and cancel to give a weak field outside. The result looks just like a bar magnet, with a N and a S end.

Electromagnet = solenoid + iron core. An electromagnet is like a switchable magnet: turn the current on and it is magnetic; turn it off and it stops.

Make it stronger by: increasing the current, adding more turns (coils), or adding a soft iron core. Uses: scrapyard cranes, electric bells, relays, MRI scanners and maglev trains.

Stronger or weaker?

Tap whether each change makes an electromagnet's field stronger or weaker.

Where does the magnetism come from?

Which way does the wire jump?

The motor effect

Put a current-carrying wire into a magnetic field and the two fields interact. The wire experiences a force and gets pushed — this is the motor effect.

• The wire's circular field and the magnet's field overlap; the combined field pushes the wire out of the gap.
• By Newton's third law, an equal and opposite force acts on the magnet.
• Magnetic forces arise from the interaction between two magnetic fields.

Force is biggest when the wire is at 90° to the field and zero when the wire is parallel to the field. Reverse either the current or the field and the force reverses.

Fleming's left-hand rule

Use your left hand to find the direction of the force when the force, field and current are mutually perpendicular (each at 90° to the others):

• thuMb → Motion (the Force on the wire)
• First finger → Field (points N → S)
• seCond finger → Current (conventional, + to −)

Which finger is which?

Which way does the wire jump?

Calculating the force: F = BIl

For a wire at right angles to the field, the size of the force is:

Magnetic flux density, B, measures how strong the field is — how concentrated the field lines are. Its unit is the tesla (T), equal to N/(A·m).

Remember: F = BIl only applies when the wire is at 90° to the field. Parallel to the field, F = 0.

Your turn — force on a wire

Your turn — find the flux density

The simple d.c. motor

A coil of wire sits between the poles of a magnet. Current flows round the coil, so by the motor effect one side is pushed up and the other is pushed down — turning the coil. A split-ring commutator swaps the current direction every half-turn, so the coil keeps spinning the same way.

Make it spin faster: increase the current, use a stronger magnet, or add more turns to the coil.

The moving-coil loudspeaker

A loudspeaker is another use of the motor effect. An a.c. signal passes through a coil that sits in the field of a permanent magnet. The motor-effect force pushes the coil back and forth in time with the current; the coil is attached to a cone, which pushes the air to make sound waves.

Because the current is alternating, the force keeps changing direction, so the cone vibrates in and out — the frequency of the signal sets the pitch of the sound.

When is the force biggest?

Match the term to its meaning

Tap a term on the left, then its matching description on the right.

The key facts & the equation

You've covered the key ideas in Edexcel Topic 12 — Magnetism and the motor effect. Press Finish to see your score.