Electromagnetic Induction: The Generator Effect

Turning movement into electricity

In the motor effect you put a current into a wire in a magnetic field and got motion out. Electromagnetic induction is the exact reverse: you put motion in and get a current out. Move a magnet near a coil of wire and — astonishingly — a voltage appears across the coil, even though nothing is touching it. This single discovery, made by Michael Faraday in 1831, is how almost all of the world's electricity is generated, from power stations to wind turbines to the dynamo on a bicycle.

The basic effect

Take a coil of wire connected to a sensitive voltmeter (or a galvanometer). Now:

• Push a bar magnet into the coil → the needle flicks one way.
• Hold the magnet still inside the coil → the needle reads zero.
• Pull the magnet out → the needle flicks the other way.

The crucial lesson: a voltage is induced only while the magnetic field through the coil is changing. A stationary magnet, however strong, induces nothing. You can produce the change in two equivalent ways:

• move a magnet relative to a coil, or
• move a wire so it cuts through magnetic field lines.

Either way, the wire and the field must be in relative motion, and the wire must cut across (not slide along) the field lines.

Faraday's law: what makes the voltage bigger

The induced voltage depends on how quickly the magnetic field linking the coil changes. This is Faraday's law, and in practical terms it means the induced voltage increases if you:

• move the magnet faster (a quicker change of field),
• use a stronger magnet (more field to change),
• add more turns to the coil (each turn adds its own induced voltage),
• increase the area of the coil that the field passes through.

So a slow push of a weak magnet through a few turns gives a feeble flick; a fast thrust of a strong magnet through hundreds of turns gives a hefty voltage. Every design choice in a real generator comes back to these factors.

Lenz's law: nature pushes back

Which way does the induced current flow? Lenz's law answers this with a beautifully simple idea:

The induced current always flows in the direction that opposes the change producing it.

Push the north pole of a magnet towards a coil and the coil responds by becoming a north pole on the side facing the magnet, repelling it — fighting your push. Pull the magnet away and the coil becomes a south pole that tries to attract it back. Either way, the coil resists the change.

This is not a quirk; it is conservation of energy in action. If the coil helped your motion instead of opposing it, you would get electrical energy for free and the magnet would accelerate endlessly. Lenz's law guarantees you must do work to generate electricity — and that work is exactly the energy that ends up in the circuit. This is why a bicycle dynamo makes pedalling harder when its lights are on, and why a turbine is harder to spin under electrical load.

The AC generator

A practical generator (alternator) turns a coil continuously inside a magnetic field, rather than pushing a magnet in and out. As the coil spins:

• Each side of the coil sweeps up through the field for half a turn, then down through it for the next half-turn.
• Because the direction of motion through the field reverses each half-turn, the induced voltage reverses too.

The result is alternating current (AC) — a voltage that smoothly rises, falls, reverses and repeats, many times a second. The coil connects to the outside circuit through slip rings and brushes that keep contact while it spins. UK mains is AC at 50 cycles per second (50 Hz). To see how this is scaled up for the country, read how we generate electricity.

A dynamo is similar but uses a split-ring commutator (as in a motor) to flip the output every half-turn, so the current always comes out the same way — direct current (DC).

Worked example: a coil and a moving magnet

The induced voltage (electromotive force) for a coil is given by ε = N × (ΔΦ ÷ Δt), where N is the number of turns and ΔΦ ÷ Δt is the rate of change of magnetic flux (in webers per second).

Problem: A 200-turn coil sits in a field. When a magnet is moved towards it, the flux through each turn changes by 0.004 Wb in 0.1 s. What average voltage is induced?

Solution: ε = N × (ΔΦ ÷ Δt) ε = 200 × (0.004 ÷ 0.1) ε = 200 × 0.04 ε = 8 V.

Now move the magnet twice as fast, so the same flux change happens in 0.05 s: ε = 200 × (0.004 ÷ 0.05) = 200 × 0.08 = 16 V — double the voltage, exactly as Faraday's law predicts.

Try it yourself! 🧪

Generate your own electricity with a coil and a magnet — no power supply needed, because you are the power source.

You need: about 5 m of thin insulated copper wire, a cardboard tube, a strong bar or neodymium magnet that fits inside the tube, and a sensitive LED or a multimeter set to millivolts.

1. Wind the wire tightly around the tube to make a coil of 100+ turns, leaving two ends free. Strip the ends and connect them to the LED or meter.
2. Drop or push the magnet quickly through the tube. Watch the LED flicker or the meter needle jump.
3. Now move the magnet slowly — the reading is smaller. Move it fast — the reading is bigger.
4. Hold the magnet still inside the coil — the reading falls to zero.

You have just demonstrated Faraday's law: only a changing field induces a voltage, and faster changes induce more.

⚠️ Safety: This experiment generates only a tiny, harmless voltage from your own hand movement — that is the whole point and it is perfectly safe. Never attempt to "generate" or experiment with mains electricity or wall sockets, which carry lethal voltages. Strong neodymium magnets can pinch fingers and damage phones and cards, so handle them carefully.

What we learned

Moving a magnet relative to a coil induces a voltage — the generator effect, the reverse of the motor effect. A voltage appears only while the field is changing; faster changes, stronger magnets and more turns all increase it (Faraday's law). The induced current always opposes the change that caused it (Lenz's law), which is why generating electricity always takes work. Spinning a coil in a field gives alternating current, the form of electricity that powers the world.