Inducing a potential difference

Move a magnet and a conductor relative to one another — so the conductor experiences a changing magnetic field — and a potential difference is induced across it. If the conductor is part of a complete circuit, that p.d. drives an induced current. This is the generator effect.

• It works in the lab (pushing a magnet into a coil) and on a large scale (power-station generators). [13.1P]
• The induced p.d. is bigger if you increase the speed of movement, the strength of the magnetic field, or the number of turns on the coil. [13.2]
• The induced current always flows so that the magnetic field it produces opposes the change that caused it (Lenz's idea) — pushing the magnet in is resisted. [13.2]

Watch out: there must be relative motion / a changing field. Hold the magnet still inside the coil and the field is steady — no p.d. is induced, even though the magnet is right there.

When is a p.d. induced?

Generating a.c. and d.c.

Spin a coil in a magnetic field and the generator effect produces a current. The difference between an alternator and a dynamo is only how the coil connects to the circuit. [13.3P]

Key contrast: slip rings keep each brush on the same end of the coil, so the output swaps direction (a.c.). A split-ring swaps the connections every half-turn, so the output never reverses (d.c.).

Generator features

Tap a feature on the left, then tap the generator it belongs to.

Sound → induced current

A moving-coil microphone uses the generator effect to turn sound into a varying electrical signal. [13.4P]

• Sound waves are pressure variations in the air. They make a thin diaphragm vibrate.
• The diaphragm is attached to a coil that sits around a permanent magnet, so the coil moves through the magnetic field.
• This relative motion induces a varying potential difference in the coil — an electrical copy of the sound.

How a microphone works

Mutual induction

A transformer has two coils — a primary and a secondary — wound on the same iron core. It can change the size of an alternating voltage. [13.5, 13.6]

How it works: the a.c. in the primary coil makes a continually changing magnetic field. The iron core channels this changing flux through the secondary coil, where it induces an alternating p.d. — this is mutual induction.

Misconception alert: a transformer needs a changing field, so it works on a.c. only. Steady d.c. gives a constant flux → nothing is induced in the secondary (except a brief blip when you switch on or off).

The transformer equation

The voltages and the numbers of turns are linked by the turns-ratio equation: [13.7P]

• A step-up transformer has more turns on the secondary (N_s > N_p) → it increases the voltage.
• A step-down transformer has fewer turns on the secondary (N_s < N_p) → it decreases the voltage.

Analogy: a transformer is like gears for voltage. Just as gears trade speed for turning force, a transformer trades voltage for current — turn the voltage up and the current goes down (and vice-versa), so the power stays (nearly) the same.

Your turn — turns ratio

Finding a number of turns

The same equation can be rearranged to find a missing number of turns instead of a voltage:

Your turn — number of turns

The power relationship

For an ideal (100%-efficient) transformer, no power is lost, so the power going in equals the power coming out (remember P = I × V): [13.10]

This is why a step-up transformer that raises the voltage must lower the current by the same factor — and a step-down does the reverse.

Your turn — Vp Ip = Vs Is

Moving electricity around the country

The National Grid carries electrical energy from power stations to homes. Transformers change the voltage along the way. [13.8, 13.9]

• A step-up transformer raises the voltage to transmit across the country efficiently.
• A step-down transformer lowers it to a safe ~230 V for local domestic use.

Reducing transmission losses

Transmission cables have resistance, so a current heats them and wastes energy. The power wasted as heat is given by P = I²R. [13.11P]

Because the loss depends on the current squared, even a small drop in current makes a big drop in wasted power. From P = I × V, sending the power at a high voltage means a low current — which is exactly what a step-up transformer does before transmission.

Misconception alert: the high voltage is not there to "push harder" — its job is to make the current low, because it is the I²R heating in the wires (not the voltage) that wastes energy. Halving the current cuts the cable loss to a quarter.

Why transmit at high voltage?

Step-up or step-down?

Tap a statement, then tap the box it belongs in.

The key ideas & equations

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