A basic transformer consists of a primary coil and a secondary coil wound on an iron core. • Iron is used as it is easily magnetised. • Knowledge of laminations and eddy currents in the core is not required. • The ratio of the potential differences across the primary and secondary coils of a transformer Vp and Vs depends on the ratio of the number of turns on each coil, np and ns. Vp/Vs = np/ns (potential difference, Vp and Vs in volts, V). • In a step-up transformer Vs > Vp. In a step-down transformer Vs < Vp. • Students should be able to: explain how the effect of an alternating current in one coil in inducing a current in another is used in transformers • explain how the ratio of the potential differences across the two coils depends on the ratio of the number of turns on each.