AC generators and transformers (SACE Stage 2 Physics)

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 $P = VI$). Halving the current quarters the line loss. 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, whereas DC cannot be transformed simply.

How SACE assesses this

SACE Stage 2 transformer questions are direct substitutions into $\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$ to find a missing voltage or turns count, often followed by a current calculation using $V_p I_p = V_s I_s$ for an ideal transformer. The reliable sanity check is the direction of the change: a step-up transformer (more secondary turns, higher voltage) draws a higher primary current, while a step-down transformer does the reverse, because power is conserved. Generator questions are usually qualitative, asking you to explain why the output is alternating and where in the rotation the EMF peaks. State that the EMF is greatest when the coil is parallel to the field (flux changing fastest) and zero when perpendicular, and name slip rings as the component that delivers an alternating output.