Two poles, a non-contact force

Every magnet has two poles: a north (N) and a south (S). The force between them is a non-contact force — it acts without the magnets touching:

• Like poles repel (N–N or S–S push apart).
• Unlike poles attract (N–S pull together).
• The force is strongest at the poles, where the field is most concentrated.

Magnetic materials: the magnetic metals are iron, steel, cobalt and nickel. Most other metals (like copper or aluminium) are not magnetic.

Which way is the force?

Fields, compasses and the Earth

A magnetic field is the region around a magnet where a force acts on another magnet or magnetic material. We draw it as field lines:

• Field lines always point from N to S (outside the magnet).
• The field is stronger where the lines are closer together — densest at the poles.
• A plotting compass needle lines up with the field, so it shows the field's direction at that point.

The classic slip: field lines point from N to S outside the magnet — never S to N. Always draw the arrowheads leaving the N pole and pointing into the S pole.

Two kinds of magnet

AQA wants you to tell these two apart precisely:

• Permanent magnet — produces its own magnetic field all the time. It can attract or repel (it has fixed poles).
• Induced magnet — a magnetic material that becomes a magnet only when placed in a magnetic field. It loses most of its magnetism when removed from the field. An induced magnet always attracts — never repels.

Common mistake: an induced magnet loses most of its magnetism the moment it leaves the field — unlike a permanent magnet, which keeps its own field. And if two objects can repel, both must be permanent magnets: an induced magnet can only ever be attracted.

Permanent or induced?

Tap whether each object is acting as a permanent magnet or an induced magnet.

A current makes a field

When a current flows through a wire, it creates a magnetic field around the wire — a set of concentric circles centred on the wire. The circles are closest together near the wire (strong field) and spread out further away (weaker field):

Solenoids & electromagnets

Wind the wire into a coil — a solenoid — and the fields from each turn add together. Inside, the field is strong and uniform (evenly spaced, parallel lines); outside it looks just like a bar magnet's field, with an N end and an S end.

Strengthen an electromagnet by: more turns on the coil, more current, or adding an iron core. Switching it off removes the field — handy for scrapyard cranes and circuit breakers.

Think of it as: an electromagnet is just a magnet with an on/off switch. Current on → it's magnetic; current off → the magnetism vanishes. A permanent magnet can never do that.

Make it stronger

A wire in a field feels a force

Put a current-carrying wire into a magnetic field and the two fields interact, pushing the wire — this is the motor effect. To find the direction, use Fleming's left-hand rule:

Direction matters: the force is greatest when the wire is at 90° to the field, and falls to zero when the wire lies parallel to the field (then the two fields don't interact). F = B I L only applies at 90°.

Your turn — the motor effect

Your turn — find the current

Electric motors & loudspeakers

The motor effect powers two devices AQA names directly:

• Electric motor — a current-carrying coil sits in a field. The forces on its two sides push in opposite directions, so the coil spins. A split-ring commutator swaps the current direction every half-turn to keep it rotating the same way.
• Loudspeaker — an a.c. current in a coil (around a permanent magnet) makes the coil and cone vibrate in and out, pushing on the air to produce sound waves.

Both convert energy in an electrical supply into kinetic energy of movement using the motor effect.

Movement makes a voltage

The motor effect run in reverse is the generator effect (electromagnetic induction): move a wire through a magnetic field — or change the field through a coil — and you induce a potential difference. If the circuit is complete, an induced current flows.

• Move the wire faster, use a stronger field or more turns → bigger induced p.d.
• Reverse the movement (or field) → the induced p.d. reverses.
• Alternators generate a.c.; dynamos use a split-ring to give d.c.
• A microphone uses it in reverse to a loudspeaker: sound moves a coil to induce a tiny signal current.

Note: the generator effect, transformers and the National Grid are Physics-only content (not on the Combined Science / Trilogy syllabus).

Reading the generator effect

Transformers change voltage

A transformer has two coils — a primary and a secondary — wound on a soft iron core. An a.c. in the primary makes a changing field in the core, which induces an a.c. p.d. in the secondary:

Transformers only work with a.c.! They need a constantly changing magnetic field in the core to induce a p.d. in the secondary. A steady d.c. makes a steady field — nothing changes, so nothing is induced. That is why transformers are used on the a.c. mains, not on batteries.

Think of it as gears: the turns ratio trades voltage for current just like gears trade speed for turning force. A step-up "gear" gives more voltage but less current; the power (the engine) stays the same.

Your turn — secondary voltage

Your turn — number of turns

An ideal transformer wastes nothing

For an ideal transformer, power in = power out (100% efficient). Since power = p.d. × current:

So if a transformer steps the voltage up, it must step the current down by the same factor — and vice versa.

Watch out: a step-up transformer raises the voltage but lowers the current — it does not create extra power. Power is conserved (V_pI_p = V_sI_s); you can never get more energy out than you put in.

Your turn — secondary current

The National Grid

The grid carries electricity from power stations to homes. It uses transformers to keep transmission efficient:

• Step-up transformer at the power station → very high voltage, which means a low current in the cables.
• Low current means less energy wasted as heat in the transmission cables (heating loss depends strongly on current).
• Step-down transformers near towns bring the voltage back down to a safe level (230 V) for use in homes.

Think of it as: sending energy at high voltage is like pushing water through a wide, low-friction pipe — you can move the same amount with only a trickle of current, so very little is wasted heating the cables on the long journey. Near towns the voltage is stepped back down to a safe 230 V.

Step-up or step-down?

Tap a feature, then tap the transformer it describes.

Why high voltage?

The key facts & equations

You've covered all three parts of AQA 4.7 — permanent & induced magnetism, the motor effect, and (Physics only) induced potential, transformers & the National Grid. Press Finish to see your score.