Transformer

A transformer consists of a primary coil and a secondary coil wound on a shared soft iron core.

The transformer equation:

$\frac{V_p}{V_s} = \frac{N_p}{N_s}$

Step-up transformer: $N_s > N_p$, increases voltage. Step-down transformer: $N_s < N_p$, decreases voltage.

For an ideal (100% efficient) transformer:

$V_p I_p = V_s I_s$

Transformers only work with alternating current because a changing magnetic field is needed to induce a voltage in the secondary coil.