This mini-lesson walks you through the whole of Edexcel Topic 12: magnetic poles & fields, permanent vs induced magnets, the field around a wire and a solenoid, electromagnets, and the motor effect — including F = BIL, Fleming's left-hand rule, the d.c. motor and the loudspeaker.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Screens marked Higher Tier are HT-only. Press Start when you're ready.
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:
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.
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.
Common mistake: an induced magnet loses most/all of its magnetism as soon as it is removed from the field. Permanent magnets keep theirs.
A magnetic field is the region where another magnet or magnetic material feels a force. We draw it with field lines:
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.)
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 the Earth's core is magnetic — its iron core produces a field like a giant bar magnet, which is evidence that the core must be magnetic.
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.
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.
Tap whether each change makes an electromagnet's field stronger or weaker.
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.
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.
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):
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).
A wire of length 0.20 m carries a current of 3.0 A at right angles to a field of flux density 0.50 T.
F = B × I × L = 0.50 × 3.0 × 0.20 = 0.30 N
Remember: F = BIL only applies when the wire is at 90° to the field. Parallel to the field, F = 0.
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.
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.
Tap a term on the left, then its matching description on the right.
Poles: like repel, unlike attract — a non-contact force.
Magnetic materials: iron, steel, cobalt, nickel.
Field lines: N → S outside the magnet; strongest (closest) at the poles.
Induced magnet: magnetic only in a field; loses it when removed.
Wire: circular field (right-hand grip); Solenoid: field like a bar magnet.
Electromagnet: solenoid + iron core = a switchable magnet.
Motor effect (HT): wire in a field feels a force; max at 90°, zero when parallel.
Fleming's LHR (HT): thuMb = Motion, First = Field, seCond = Current.
Equation (HT): F = B I L (B in tesla, T).
Uses: the d.c. motor and the loudspeaker.
You've covered the whole of Edexcel Topic 12 — Magnetism and the motor effect. Press Finish to see your score.
You've worked through Magnetism & the Motor Effect for Edexcel GCSE Physics. 🎉
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Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.