HookThe compass needle that twitched in the middle of a lecture
On the evening of 21 April 1820, the Danish physicist Hans Christian Ørsted was lecturing on electricity in Copenhagen when he closed a switch and noticed something nobody had ever recorded: a magnetic compass lying near the wire flicked sideways, every time the current flowed. Electricity and magnetism had been treated as two separate sciences for centuries; a needle's twitch, seen live by a room of students, welded them into one. News crossed Europe in weeks. Within about a year, Michael Faraday in London had used the effect to make a current-carrying wire circle endlessly around a magnet — the first electric motor, a spinning wire in a dish of mercury with no obvious use at all.
Two centuries later, the descendants of that twitch outnumber people. A modern family car carries dozens of small electric motors — window winders, mirror adjusters, pumps, fans, wipers; your phone buzzes with a motor the size of a pea; and a scrapyard crane hoists a whole car on a magnet that lets go the instant someone cuts the current. P7 walks the exact path Ørsted opened: magnets and their fields, the field a current creates, the force a magnet exerts back on that current — and the trick of turning that force into rotation, which is all a motor is.
ModelPoles — and the difference between permanent and induced magnets
The poles of a magnet are the places where the magnetic forces are strongest, named north and south. The interaction rule is the one you know — like poles repel, unlike poles attract — and both attraction and repulsion here are non-contact forces, acting across a gap.
The distinction the exam leans on is between two kinds of magnet. A permanent magnet produces its own magnetic field, all the time. An induced magnet is a material that only becomes magnetic while it sits inside another magnetic field — the field turns an ordinary iron nail into a temporary magnet, with its induced polarity arranged so that it is always attracted, never repelled. Pull the nail away and it loses most or all of its magnetism. Only a handful of materials play this game at all: iron, steel, cobalt and nickel are the magnetic materials the specification names. Copper coins and aluminium foil ignore a magnet completely.
Induced magnetism has a sharp logical consequence that examiners adore: attraction proves nothing. If a bar attracts a known magnet, the bar might be a permanent magnet — or just a lump of iron being induced. But induced magnets can only attract, so if the bar repels one pole of a known magnet, it must be a permanent magnet itself. Repulsion is the only conclusive test, and 'describe how you would test whether this bar is a magnet' is a stock three-marker built entirely on that sentence.
ModelMagnetic fields — mapping the invisible, including the planet-sized one
A magnetic field is the region around a magnet in which a force acts on another magnet or on magnetic material. You cannot see it, but you can map it. The field's strength is greatest at the poles and falls off as distance from the magnet grows; its direction at any point is defined as the direction of the force that would act on a north pole placed there. Follow that rule around a bar magnet and the field lines run from north to south outside the magnet, bulging out in the familiar looped pattern. Where the lines crowd together the field is strong; where they spread, it is weak. Arrows on every line are not decoration — a field diagram without arrows loses its mark.
The mapping instrument is a plotting compass: a small compass whose needle — itself a tiny bar magnet — swings to line up with the field wherever it is placed. Put it near a pole, mark dots at each end of the needle, move the compass so the tail sits where the head was, and repeat: joining the dots traces out one field line, and starting from different points builds the whole pattern.
Take the compass away from the magnet and it still points somewhere: roughly geographic north. A needle can only do that if it is sitting in a field — which is the evidence that the Earth's core is magnetic. The end of the needle we call 'north' is properly the north-seeking pole. And the planet's field is livelier than the diagrams suggest: the magnetic north pole has been drifting from Arctic Canada towards Siberia at up to roughly 50 km a year in recent decades — fast enough that the World Magnetic Model used by navigation systems needed an out-of-cycle update in 2019.
MechanismElectromagnetism — a current makes its own field
Here is Ørsted's twitch, stated properly: when a current flows through a conducting wire, a magnetic field is produced around the wire. The field lines are concentric circles centred on the wire, and the field's strength depends on two things — it grows with the current, and it weakens with distance from the wire. Reverse the current and the field direction reverses too; that is why Ørsted's needle flicked one way, then the other, as he switched.
A single straight wire's field is feeble, so engineering bends the wire into a coil. Wind it into a solenoid — many turns side by side — and the little circular fields of every turn add together inside the coil: the result is a field that is strong and uniform inside the solenoid, while the outside field looks exactly like a bar magnet's, one end behaving as a north pole and the other as a south. Slide an iron core down the middle and the field strengthens dramatically, because the iron becomes an induced magnet and adds its own field to the coil's. A solenoid with an iron core is an electromagnet.
The electromagnet's superpower is control — everything a permanent magnet cannot do. Switch the current off and the magnetism dies (the scrapyard crane releasing the car); turn the current up and the field strengthens; reverse the current and the poles swap. To make one stronger: more current, more turns, add the iron core. Electric bells, relays that let a small safe current switch a large dangerous one, and magnetic door locks all run on that switchability.
MechanismThe motor effect and Fleming's left-hand rule (Higher tier)
Put the two field-makers together. A current-carrying wire sits between the poles of a magnet: the wire's circular field and the magnet's field interact, and the result is a force on the wire — and, by Newton's third law, an equal and opposite force on the magnet. This is the motor effect. The force is largest when the wire is at right angles to the field, and falls to zero when the wire runs parallel to it.
For the perpendicular case, the force is \[F = BIl\] — magnetic flux density × current × length of wire in the field. Magnetic flux density \(B\) is the proper measure of how strong a field is, in tesla (T): one tesla exerts one newton on each metre of wire carrying one amp. The equation is given on the equations sheet; the condition attached to it — current at right angles to the field — is the recall the examiner checks.
Three directions are in play — field, current, force — and they sit mutually at right angles, which is what Fleming's left-hand rule encodes. Left hand: thuMb = Motion (the force), First finger = Field (north to south), seCond finger = Current (positive to negative). Set any two and the hand tells you the third. Reverse either the current or the field and the force flips; reverse both and it stays put.
A wire carries \(I = 3.0\) A at right angles through a field of flux density \(B = 0.50\) T, with \(l = 0.10\) m of wire inside the field. \[F = BIl = 0.50 \times 3.0 \times 0.10 = 0.15\ \text{N}\] — about the weight of a couple of pound coins, easily enough to see the wire jump. Rearranged, what current doubles the force to 0.30 N? \(I = \dfrac{F}{Bl} = \dfrac{0.30}{0.50 \times 0.10} = 6.0\ \text{A}\): force is proportional to current, so doubling one doubles the other. The unit traps: length in metres (a '5 cm' wire must become 0.05 m), and if the wire is parallel to the field the answer is simply zero — no equation required.
CaseInside the dc electric motor (Higher tier)
A motor is the motor effect used twice at once. Take a rectangular coil of wire sitting between the poles of a magnet, free to spin on an axle, and pass a current through it. Look at the two long sides of the coil: the current flows up one side and down the other — opposite directions. Fleming's rule therefore gives them forces in opposite directions: one side is pushed up, the other pushed down. Equal and opposite forces on either side of an axle are a turning pair, and the coil rotates.
Left alone, the design defeats itself. After half a turn, the side that was being pushed up arrives where the pushed-down side used to be — and with everything else unchanged, the forces would now undo the rotation, rocking the coil to a halt. The fix is the split-ring commutator: the coil's ends connect to the circuit through two half-rings that the brushes slide over, and every half-turn the halves swap contacts, reversing the current direction around the coil. The reversal means whichever side of the coil is currently next to the north pole always carries current the same way — so the force on it always points the same way, and the rotation continues smoothly instead of oscillating.
Want it faster or stronger? Increase the current, use a stronger magnet (larger \(B\)), or wind more turns on the coil — each turn is another length of wire feeling \(F = BIl\). Want it to spin the other way? Reverse the current or swap the magnetic poles — but not both. That parts list — coil, field, commutator, brushes — is the machine inside a power drill, a hair-dryer fan and the pea-sized vibration motor in your phone.
Walk the classic six-marker once. Field runs left to right, from N pole to S pole. Say side X of the coil, nearest you, carries current away from you, and side Y carries it back. Left hand for X: First finger along the field, seCond finger along X's current — the thuMb points up, so X rises. For Y the current is reversed, so the force is reversed: Y is pushed down. X up, Y down: the coil turns. Half a turn later X and Y have swapped positions; at that exact moment the commutator gaps pass the brushes and the current around the coil reverses, so the side now nearest the N pole is pushed up again. Two applications of the rule plus one sentence on the commutator's half-turn reversal — that structure is the full-mark answer.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
P7 is the final topic on Physics Paper 2 and the shortest in the specification, which makes its marks unusually predictable. The AO1 core is drawing: a bar magnet's field needs looped lines from N to S, arrows on every line, symmetry, and wider spacing as distance grows — omitted arrows are the classic silent mark loss. The plotting-compass method is a stock 4–6 marker: write it as numbered steps (place compass at a pole, dot both ends of the needle, move the compass forward so the tail replaces the head, join the dots, repeat from new starting points). Foundation tier stops at electromagnets, so if you are on Higher, know that the motor effect, F = BIl and the dc motor are yours alone — and that describing an electromagnet's strength factors (current, turns, iron core) is wanted at both tiers.
For calculations, F = BIl is on the equations sheet; the examiner instead tests the conditions and conversions around it. State that the equation holds for a conductor at right angles to the field, convert lengths from centimetres to metres before substituting, and give force in newtons to a sensible number of significant figures. Proportionality shortcuts earn quick marks: doubling current doubles force, halving field halves it — say so rather than re-computing.
Use your actual left hand in the exam for Fleming's rule; examiners expect to see candidates doing it, and it beats visualising. Assign the fingers out loud in your working — First finger Field (N to S), seCond finger Current (positive to negative), thuMb the force — because writing the assignment is itself creditworthy. The motor six-marker has a fixed skeleton that AO2 questions reward: opposite currents in the two sides of the coil give opposite forces (apply the rule twice), the opposite forces create rotation, and the split-ring commutator reverses the current every half-turn so the rotation continues in one direction. Close with a control point — reversing current or field reverses spin; more current, stronger field or more turns speeds it up — and you have covered every marking point the level descriptors list.