Explain the operation of an AC generator and apply the transformer equation to step voltage up or down.

What this dot point is asking

You need to explain how an AC generator produces alternating voltage and apply the transformer turns-ratio equation.

The AC generator

A generator is the reverse of a motor: instead of using a current to produce motion, it uses motion to produce a current.

A coil is rotated in a magnetic field. As it turns, the flux through it changes continuously (because the angle $\theta$ in $\Phi = BA\cos\theta$ changes). By Faraday's law this changing flux induces an EMF.

A generator uses slip rings (not a split-ring commutator), which keep the connection to each end of the coil fixed, allowing the output to alternate naturally.

The transformer

A transformer changes the size of an alternating voltage using mutual induction. Two coils - primary and secondary - are wound on a shared iron core.

• AC in the primary creates a continuously changing magnetic flux in the core.
• The core channels this flux through the secondary coil.
• The changing flux induces an alternating EMF in the secondary (Faraday's law).

If the secondary has more turns it is a step-up transformer (higher voltage, lower current); fewer turns makes a step-down transformer (lower voltage, higher current).

A transformer only works with AC. A steady DC current produces no changing flux, so no EMF is induced in the secondary.

Power transmission

Power lost in transmission lines is $P_{\text{loss}} = I^2 R$. To minimise this loss, power is sent at very high voltage and therefore low current (since $P = VI$). Step-up transformers raise the voltage at the power station for transmission, and step-down transformers reduce it to safe levels for homes. This is the main reason the grid uses AC - it can be transformed; DC cannot be transformed simply.