HookThe flick of a needle that built the electricity industry
On the evening of 29 August 1831, Michael Faraday wound two separate coils of wire onto opposite sides of an iron ring in the basement laboratory of the Royal Institution on London's Albemarle Street. One coil he wired to a battery; the other to a galvanometer — a needle that flicks when a current passes. He expected a steady current in the first coil to drive a steady current in the second. It did nothing of the sort. The needle kicked once at the instant he connected the battery, fell dead while the current flowed steadily on, then kicked the other way the moment he disconnected it. Faraday had discovered that it is not a magnetic field that induces a current but a changing one — and in that single twitch of a needle lay the electric generator, the transformer, and very nearly the whole of the modern electricity supply.
Everything in P7 grows from two mirror-image effects. Push a current through a wire and the wire becomes a magnet — the basis of electromagnets, electric motors and loudspeakers. Move a wire through a magnetic field, or change the field passing through a coil, and you generate a current — the basis of generators, microphones and transformers. Britain's National Grid uses the second effect twice over: spinning turbines induce the current, and transformers — Faraday's iron ring scaled up — step its voltage up to 400,000 V for transmission across the country and back down to 230 V for the socket in your wall. Get the direction of those two effects straight and the rest of this section is bookkeeping.
ModelPoles, permanent magnets and induced magnetism
Every magnet has two poles, north and south, where the field is strongest. The rule is the one you met with a pair of bar magnets on a desk: like poles repel, unlike poles attract. That force acts at a distance, without the magnets touching, so magnetism is a non-contact force — the same family as gravity and electrostatic attraction.
Only a short list of materials is magnetic: iron, steel, cobalt and nickel, plus a few alloys. Aluminium, copper and gold are not, however shiny and metallic they look. A permanent magnet produces its own field all the time. An induced magnet is different — an ordinary piece of a magnetic material becomes a magnet only while it sits inside another magnet's field, and induced magnetism is always attractive, which is why a paperclip clings to a fridge magnet and then dangles a second paperclip beneath it. Soft iron loses its induced magnetism almost the instant you take the field away; steel keeps it, which is how permanent magnets are made.
This hands you the one reliable test for a permanent magnet. Attraction proves nothing — an unmagnetised magnetic material is attracted just the same. Only repulsion is proof, because two things can push each other apart only if both are already permanent magnets.
ModelMagnetic fields and the plotting compass
A magnetic field is the region around a magnet where another magnet or a magnetic material feels a force. We draw it with field lines that run from the north pole round to the south pole on the outside of the magnet. Two conventions carry the marks: the line's arrow shows the direction the force would push the north pole of a tiny test magnet, and the spacing of the lines shows the field's strength — lines crowded close together (near the poles) mean a strong field, lines spread apart mean a weak one.
You map a real field with a plotting compass. Its needle is itself a tiny magnet, so it swings to line up with the field at whatever point you place it; dot round the arrow, shuffle the compass along, and you trace a field line. Do this well away from any magnet and the compass still settles — pointing roughly north. That is the evidence that the Earth generates its own magnetic field, produced deep in its molten iron core, which behaves as though a colossal bar magnet were buried inside the planet. A compass is not detecting north; it is obeying the Earth's field the same way it obeys a bar magnet on the bench.
MechanismTurning current into magnetism — wires, solenoids and electromagnets
Here is the bridge from electricity to magnetism, first spotted by Hans Christian Ørsted in 1820 when a compass twitched beside a live wire. A current-carrying wire produces a magnetic field. Around a straight wire the field lines are concentric circles, strongest close to the wire and fading with distance; a bigger current makes a stronger field, and reversing the current reverses the field's direction. The right-hand grip rule keeps it straight: point your right thumb along the conventional current and your curled fingers show the way the circular field points.
Bend the wire into a coil — a solenoid — and each turn's field adds to the next, concentrating the field into a strong, roughly uniform region down the middle. The finished field is identical in shape to a bar magnet's, with a north pole at one end and a south at the other. Slide a core of soft iron inside and the field becomes far stronger still: that is an electromagnet. Its killer feature is control — switch the current off and the magnetism vanishes, so a scrapyard crane can grab a car and drop it on command. The same switchable pull runs electric bells, relays that let a small current safely control a large one, the read-write heads in machines, and, scaled up, the magnets that levitate a maglev train.
MechanismThe motor effect — Fleming's left hand (Higher tier)
Put a current-carrying wire into an external magnetic field and something new happens: the wire's own circular field adds to the applied field on one side and cancels it on the other, so the wire is pushed from the strong side toward the weak side. This push is the motor effect, and it is a force on the wire itself, not on the charges alone.
To find which way the force points, use Fleming's left-hand rule. Hold the thumb and first two fingers of your left hand at right angles: the First finger points along the Field (north to south), the seCond finger along the conventional Current (positive to negative), and the thuMb then gives the Motion — the force. The size of the force follows \(F = BIl\), where \(B\) is the magnetic flux density in tesla, \(I\) the current in amps and \(l\) the length of wire in the field in metres. That equation gives the full force only when the wire is at right angles to the field; align the wire along the field and the force drops to zero.
Coil that wire and the motor effect spins it. In a simple dc motor the two sides of a coil carry current in opposite directions, so Fleming's rule pushes one side up and the other down — a turning moment. A split-ring commutator swaps the current's direction every half turn, so the push always drives the same way round and the coil keeps rotating. Reverse the same idea and you get a loudspeaker: feed an alternating current through a coil sitting in a magnet's field and the motor effect pushes the coil, and the paper cone glued to it, back and forth at the signal's frequency, moving the air as sound.
A 5.0 cm length of wire sits at right angles to a magnetic field of flux density 0.30 T and carries a current of 4.0 A. Find the force on it.
Use \(F = BIl = 0.30 \times 4.0 \times 0.050 = 0.060\ \text{N}\).
Two cautions win the marks. First, the length must be in metres: 5.0 cm is 0.050 m, not 5.0 — a hundredfold slip that turns 0.060 N into 6.0 N. Second, \(F = BIl\) delivers this full force only because the wire is perpendicular to the field; swing the wire round until it lies along the field and the force falls to zero. Reverse either the current or the field on its own and the 0.060 N simply points the opposite way — which is exactly the switch a motor's commutator performs twice on every turn to keep the coil spinning one way.
MechanismThe generator effect — inducing a current (Higher tier)
Now run the motor effect backwards, which is what Faraday's needle was telling him. Move a conductor through a magnetic field so that it cuts across the field lines — or change the field passing through a coil — and you induce a potential difference across the conductor. Complete the circuit and that pd drives a current. Nothing is touching and no battery is involved; the movement itself is doing the work.
Four things make the induced pd bigger: moving faster, a stronger magnetic field, more turns on the coil, and a larger coil area. And the induced current always flows in the direction that opposes the change that caused it. Push a magnet into a coil and the induced current creates a magnetic field that pushes back against the magnet; that opposition is simply conservation of energy in disguise — you have to do work against the push, and that work is exactly the electrical energy you get out. It is why a bike dynamo makes pedalling harder the moment its lamp draws current.
Spin a coil steadily in a field and you have a generator. An alternator takes the coil's ends out through slip rings, so the output reverses every half turn: an alternating current whose graph is a smooth sine wave. A dynamo uses a split-ring commutator instead, flipping the connections each half turn so the output never goes negative: a direct current whose graph is a series of humps all on the same side. Spin either one faster and the peaks grow taller and crowd closer together. The moving-coil microphone is the same effect in miniature and is the exact reverse of a loudspeaker: sound waves vibrate a diaphragm with a coil attached, the coil moves in a magnet's field, and the generator effect induces a varying pd that copies the sound.
DataTransformers and the National Grid (Higher tier)
A transformer is two coils wound on the same iron core. An alternating current in the primary coil makes a constantly changing magnetic field in the core; the core carries that changing field through the secondary coil; and a changing field through the secondary induces an alternating pd across it. The word 'changing' is the whole story — feed a transformer steady direct current and the field is constant, nothing changes through the secondary, and the output is zero. That single fact is why the grid runs on ac.
The voltages follow the turns: \(\frac{V_p}{V_s} = \frac{n_p}{n_s}\). More turns on the secondary than the primary is a step-up transformer (higher output voltage); fewer is a step-down. A transformer cannot conjure energy, so an ideal one obeys \(V_p I_p = V_s I_s\): whatever it adds in voltage it takes back in current. The National Grid exploits this to move power efficiently. Power stations step the voltage up to 400,000 V for transmission because, for a given power, high voltage means low current — and the heat wasted in the cables is \(P = I^2 R\), which depends on the square of the current. Local substations then step the voltage back down in stages to the 230 V that is safe to bring into homes.
A step-up transformer at a power station has a 1,000-turn primary coil connected to a 25,000 V ac supply and a 16,000-turn secondary. Find the secondary voltage, then the secondary current when the station delivers 200 MW.
Turns first: \(V_s = V_p \times \frac{n_s}{n_p} = 25{,}000 \times \frac{16{,}000}{1{,}000} = 400{,}000\ \text{V}\) — the 400 kV that hums along the tallest pylons.
Now the current, using \(V_p I_p = V_s I_s\) with 200 MW flowing: \(I_s = \frac{P}{V_s} = \frac{2\times10^8}{4\times10^5} = 500\ \text{A}\).
Why go to the trouble? At the station's own 25,000 V the same 200 MW would demand \(I_p = \frac{2\times10^8}{2.5\times10^4} = 8{,}000\ \text{A}\). The cable heating loss is \(P = I^2 R\), so dropping the current from 8,000 A to 500 A cuts that loss by a factor of \((8000/500)^2 = 256\). Transmitting at high voltage and low current is the entire reason a national grid is possible — and it works only because a transformer runs on Faraday's changing field, which is why the grid must be alternating current, not direct.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Name the effect before you calculate. Examiners split P7 into two mirror worlds — current making magnetism (electromagnets, motors, loudspeakers) and movement making current (generators, dynamos, microphones) — and most lost marks come from answering with the wrong one. State which effect is at work in a sentence, then reach for the maths.
For \(F = BIl\), always convert the length to metres and state that the force is a maximum only when the wire is perpendicular to the field; a wire parallel to the field feels no force at all. When you use Fleming's left-hand rule in an answer, write out what each finger stands for so the examiner can follow your reasoning even if your sketch is rough.
On transformers, quote both equations and use them in the right order: \(\frac{V_p}{V_s} = \frac{n_p}{n_s}\) to get the missing voltage or turns, then \(V_p I_p = V_s I_s\) to get a current. Whenever you are asked why the grid transmits at high voltage, do not stop at 'to reduce energy loss' — say that high voltage means low current, and that the heating loss depends on \(I^2 R\), so the loss falls with the square of the current. And remember the single most common transformer trap: they only work on alternating current, because a steady field induces nothing.