Electromagnetic Induction

Squeeze the brakes on a bike with a dynamo light and the lamp glows — with no battery anywhere. Stand under a wind turbine, or beside a hydroelectric dam, or next to the roaring turbines of a power station, and it's the same trick, scaled up to light whole cities. None of these machines store electricity. They make it, on the spot, out of nothing but movement and magnets.

The trick has a name: electromagnetic induction. When a conductor (a wire, or a coil of wire) and a magnetic field move relative to each other — a magnet pushed into a coil, or a wire swept across field lines — a potential difference is induced across the conductor. Complete the circuit and that induced pd drives a current. Movement in; electricity out.

This is the generator effect, and it is the exact reverse of the motor effect. A motor is fed electricity and gives back movement; a generator is fed movement and gives back electricity. Same magnets, same coils — run the other way.

What actually induces the pd: change

Picture a bar magnet and a coil of wire connected to a sensitive meter (a galvanometer — a voltmeter whose needle can swing either side of zero). Push the magnet into the coil and the needle flicks one way. Hold the magnet still, even deep inside the coil, and the needle drops back to zero. Pull the magnet out and the needle flicks the other way.

Read that again, because it's the whole idea: what induces a pd is not the magnet being there — it is the magnetic field through the coil changing. As the magnet moves in, more of its field threads the coil; as it moves out, less does. A field that is changing induces a pd; a field just sitting there does nothing. The faster the change, the bigger the induced pd.

\text{no movement} \;\Rightarrow\; \text{no change in the field} \;\Rightarrow\; \text{induced pd} = 0.

Try it: the moving-magnet lab

Slide the magnet through the coil and watch the meter's needle. Then change how you move it. Three things make the induced pd bigger — so the needle swings further:

moving the magnet faster (a quicker change);
using a stronger magnet (more field to change);
winding more turns onto the coil (each turn adds its own induced pd).

Notice the two things the meter cares about: direction and size. Set the action to Hold still — or drop the speed to zero — and the needle falls to 0 no matter where the magnet is. Switch from Push in to Pull out and the needle flips to the opposite side. Crank up the speed or the number of turns and it swings harder.

\text{induced pd} \;\propto\; N \times \big(\text{how fast the field through the coil changes}\big).

Here N is the number of turns on the coil. There is nothing to plug numbers into at GCSE — it's a proportion: double the speed and you roughly double the induced pd; double the turns and you roughly double it again.

Worked reasoning: rank the flicks

A student pushes the same magnet through the same coil four different ways. Without any formula, put the meter readings in order — smallest to largest.

Magnet held still inside the coil. No change → induced pd = 0. Smallest.
Pushed in slowly. The field is changing, but gently → a small pd.
Pushed in quickly. Same change packed into less time → a bigger pd.
Pushed in quickly, with twice as many turns. Every turn adds its bit, so this is bigger still. Largest.

Order: (1) < (2) < (3) < (4). The lesson underneath it: the meter reads the rate of change multiplied by the number of turns — nothing else. A strong magnet sat motionless reads exactly zero, and a feeble magnet whipped through quickly can out-read it.

Which way? The induced current fights back

Here is the subtle, beautiful part. When you push a magnet's north pole into a coil, the induced current flows in whatever direction makes the coil's near face into a north pole too — so it pushes back against the incoming magnet. Pull the magnet out and the current reverses, turning that face into a south pole to pull the magnet back. Either way, the induced current sets itself up to oppose the very change that is making it. This is Lenz's law.

Why must nature be so contrary? Energy conservation. Because the coil resists you, you have to do work — push against the repulsion, or tug against the pull — to keep the magnet moving. That mechanical work you put in is exactly the electrical energy that comes out. If the induced current helped the magnet along instead, the magnet would accelerate on its own and pour out free electrical energy forever — a perpetual-motion machine. Nature says no. You get out only the energy you put in.

When a conductor and a magnetic field move relative to each other — or the field through a coil otherwise changes — a potential difference is induced across the conductor. If the circuit is complete, this pd drives an induced current.
The induced pd is larger when the field changes faster (faster movement or a stronger magnet) and when the coil has more turns.
Lenz's law: the induced current always flows in the direction that opposes the change that produced it. This is energy conservation — work must be done to generate electricity.
No change means no induced pd: a stationary magnet next to a coil induces nothing.

It's the CHANGE, not the magnet, that induces a pd. A magnet held motionless inside the coil — however strong — induces nothing. The needle only moves while the field is changing. Position doesn't matter; movement does.
The induced current OPPOSES the change — it doesn't help it. Lenz's law is the opposite of what feels natural. The coil pushes a north pole back out and drags it back in, so you always have to work against it. That resistance is the whole point — it's what makes you supply the energy.
Induction is the REVERSE of the motor effect — mind the hand. The motor effect (current in, force out) uses Fleming's left hand. The generator effect (movement in, current out) uses Fleming's right hand. Left for motoring, right for generating — don't swap them.

The generator: making a.c. on purpose

A bar magnet you shove back and forth by hand is a clumsy generator. The clever version spins a coil (or spins a magnet past a coil) so the change never stops. This is a generator, or alternator.

As the coil rotates in the field, its two sides sweep across the field lines, and the induced pd appears. But watch what happens over one turn: as a side sweeps up through the field the pd pushes one way; half a turn later that same side is sweeping down, so the pd reverses. Spin it steadily and the induced pd swings smoothly positive, back through zero, negative, and round again — an alternating voltage. That is why mains electricity and every power-station generator produce a.c. (alternating current): it falls straight out of a coil going round and round. The faster it spins, the bigger and the more frequent the swings.

The induced pd is biggest when the coil sides are cutting straight across the field lines, and momentarily zero when they are gliding along them (cutting no lines) — twice every turn. That in-and-out of cutting field lines is what carves the smooth a.c. wave.

Almost every watt that reaches your plug sockets was born this way. A power station's job is simply to spin a magnet inside coils of wire. A coal or gas station boils water into steam to push the turbine; a nuclear station uses heat from splitting atoms; a hydro dam uses falling water; a wind turbine uses the wind itself. Different sources of push — but the last step is always the same spinning coil and the same induction. Even the tiny dynamo on a bike is a magnet spun past a coil by the wheel, and wireless phone chargers use a changing field from a coil in the pad to induce a current in a coil in the phone — induction with no wires touching at all. Michael Faraday discovered the whole effect in 1831; when asked what use it could possibly be, he is said to have replied, "What use is a newborn baby?"

Induction at work: microphones and transformers

A moving-coil microphone is a generator small enough to sit on a stand. Sound waves wobble a thin diaphragm, which is attached to a little coil sitting around a magnet. As the sound pushes the coil back and forth past the magnet, it induces a tiny changing pd — an electrical copy of the sound wave. Louder sound moves the coil further and faster, so the induced signal is bigger. Your voice literally generates the electricity that carries it.

And the same idea, run continuously, gives the transformer — two coils sharing an iron core, where an alternating current in one coil makes a constantly changing field that induces a pd in the other. It's how the National Grid steps voltages up for efficient transmission and back down for your home — a whole concept of its own, built entirely on the induction you've just met.