A transformer is used in the National Grid to step up the voltage from a power station before electricity is transmitted through overhead cables. Explain why a transformer only works with alternating current and not with direct current, and describe what happens inside the transformer when alternating current flows through the primary coil.

A transformer requires a changing magnetic flux to induce a voltage in the secondary coil. Alternating current continuously changes direction, so the current in the primary coil varies sinusoidally. This produces a continuously changing magnetic field in the iron core. The changing flux links the secondary coil and, by Faraday’s law, induces an electromotive force (e.m.f.) that steps the voltage up or down depending on the turns ratio. Direct current, however, produces a constant magnetic field; a steady flux does not change, so no e.m.f. is induced in the secondary and the transformer gives no output with DC.