Induction, generators and transformers

If a wire or coil moves relative to a magnetic field, or the field around it changes, a potential difference is induced across it; if the circuit is complete, a current flows. This is electromagnetic induction. The induced p.d. is larger if the movement is faster, the field stronger or the coil has more turns. An alternator generates a.c. by rotating a coil in a field; a dynamo uses a commutator to give d.c. A microphone uses induction to turn sound into an electrical signal.

A transformer has two coils wound on an iron core. An alternating current in the primary coil makes a changing field, which induces an alternating p.d. in the secondary coil. Vp ÷ Vs = Np ÷ Ns. A step-up transformer has more turns on the secondary. If it is 100% efficient, Vp × Ip = Vs × Is. Transformers only work with a.c.