The units of this section

Section 6 uses three electrical units you should recognise by name and symbol:

Why these three? Generators induce a voltage (V), that voltage drives a current (A), and transformers move power (W) across the grid — so all three appear in this section.

Poles attract and repel

Every magnet has a north (N) and a south (S) pole. The rule for the forces between them:

• Like poles repel — N and N push apart; S and S push apart.
• Unlike poles attract — N and S pull together.
• A magnet also attracts magnetic substances (iron, steel, nickel, cobalt) even though they are not themselves magnets.

Watch out: attraction alone does not prove something is a magnet — an unmagnetised iron nail is attracted by either pole. Only repulsion proves you have a second magnet.

Magnet or not?

Permanent and induced magnets

Put a magnetic material in a magnetic field and magnetism is induced in it — it becomes a temporary magnet. What happens when you remove the field depends on the material:

• Magnetically hard (e.g. steel) — hard to magnetise but keeps its magnetism. Used for permanent magnets.
• Magnetically soft (e.g. soft iron) — easy to magnetise but loses its magnetism quickly once the field is removed. Used for electromagnet cores.

Memory hook: hard = holds on (permanent); soft lets go. That's exactly why an electromagnet's core is soft iron — you want it to switch off the instant the current stops.

Field lines run N → S

A magnetic field line shows the direction of the force on a north pole at that point. Two rules you must get right:

• Direction: outside the magnet, field lines point from N to S (arrows leave the N pole, enter the S pole).
• Strength: where lines are closer together the field is stronger — so they bunch up at the poles.

Common mistake: drawing arrows pointing into the N pole. Outside the magnet they always go N → S; field lines never cross and never have gaps.

Which way do the arrows point?

Making a uniform field

Place two bar magnets with opposite poles facing (N facing S) a small gap apart. Between them the field lines run straight, parallel and evenly spaced from the N pole of one to the S pole of the other — a uniform field (same strength and direction everywhere in the gap).

A current makes a magnetic field

Whenever an electric current flows in a conductor, it creates a magnetic field around it. For a straight wire the field is a set of concentric circles centred on the wire.

The right-hand grip rule gives the direction: point your right thumb along the conventional current, and your curled fingers show the way the field circles round.

Solenoids and electromagnets

Wind the wire into a coil (solenoid) and the circular fields add up to give a field just like a bar magnet — with a clear N end and S end. Wrap that coil around a soft-iron core and you have an electromagnet you can switch on and off, make stronger, or reverse.

Why soft iron? It magnetises strongly while the current flows but loses its magnetism the instant you switch off — letting the electromagnet release whatever it was holding.

Field shape → source

Tap a field description on the left, then its source on the right.

A force on moving charges

A moving charge is a current. So a charged particle moving through a magnetic field feels a force — but only if it is not moving parallel to the field.

• Move across the field lines → there is a force (it gets deflected).
• Move parallel to the field lines → no force at all.

Force on a current-carrying wire

Put a current-carrying wire in a magnetic field and it experiences a force. The wire's own circular field and the magnet's field combine — strong on one side, weak on the other — pushing the wire. This is the motor effect.

Fleming's left-hand rule predicts the force direction (when current is perpendicular to the field). Hold the left hand with thumb and first two fingers at right angles:

Application (6.12): this same force runs d.c. motors and loudspeakers — a coil in a field is pushed, so a motor spins or a speaker cone vibrates to make sound.

Apply the left-hand rule

What changes the force?

The size and direction of the force on the wire depend on the current and the field:

• Bigger current → bigger force.
• Stronger magnetic field → bigger force.
• Reverse the current or reverse the field → the force reverses direction. (Reverse both and it stays the same.)

Common mistake: assuming the force is always there. If the wire runs parallel to the field, there is no force — the motor effect needs the current to cut across the field lines.

Bigger, reversed, or no force?

Tap what happens to the motor-effect force in each case.

How a d.c. motor spins

A coil sits between the poles of a magnet. Current flows in opposite directions on the two sides of the coil, so by Fleming's left-hand rule the forces are opposite — one side pushed up, the other down. That couple makes the coil rotate.

The generator effect

Run the motor effect in reverse: move a conductor through a magnetic field (or change a field through a coil) and a voltage is induced. If the circuit is complete, that voltage drives a current. This is electromagnetic induction (the generator effect).

The induced voltage is bigger when you:

• move the conductor (or magnet) faster;
• use a stronger magnetic field;
• use more turns / a longer length of wire in the field.

Key idea: induction needs a changing field — relative motion between magnet and coil. A stationary magnet, no matter how strong, induces nothing.

When is a voltage induced?

Generating electricity

Keep that change going continuously: rotate a magnet inside a coil, or rotate a coil inside a magnetic field. As it turns, the field through the coil keeps changing, so a voltage — and current — is induced. Because the coil cuts the field one way then the other each half-turn, the output is alternating (an a.c. generator / alternator).

Transformers change voltage

A transformer has two coils — primary (Np turns) and secondary (Ns turns) — wound on a soft-iron core. An alternating voltage in the primary makes a changing magnetic field in the core; the core carries it to the secondary, where it induces an alternating voltage (this is mutual induction).

National Grid (6.18): step-up transformers raise the voltage for transmission — higher voltage means lower current, so far less power is wasted heating the cables. Step-down transformers then lower it to safe values for homes.

The transformer equation

The ratio of voltages equals the ratio of turns:

Crucial misconception: transformers only work on alternating current (a.c.). A steady d.c. supply gives a constant field in the core — nothing changes, so no voltage is induced in the secondary. No changing field, no transformer action.

Your turn — turns ratio

Your turn — find the turns

A 100% efficient transformer

If a transformer is 100% efficient, no power is lost, so the power in equals the power out:

This is why stepping voltage up lowers the current. If you raise Vs, then Is must fall to keep the product equal — and lower current is exactly what cuts transmission losses on power lines.

Your turn — power in = power out

Section 6 at a glance

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