What transformers do
A transformer changes an alternating voltage from one value to another.
Figure to be added
Transformers are used in many places: in distributing mains electricity, in power supplies, and in equipment such as computers and radios.
A mobile phone charger is a good example. The socket gives 230 V, but a phone battery needs about 5 V. Inside the charger, a small transformer steps the 230 V down, and a rectifier of the kind you met in chapter 11 turns the result into direct current for the battery. At the other end of the scale, the transformers at a power station are the size of a room, stepping the generator's voltage up to hundreds of thousands of volts to send it across the country.
Why a transformer needs AC
A transformer is two coils on one iron core, with no electrical connection between them. Energy passes from one coil to the other only through the magnetic field — by electromagnetic induction. From the last pages, induction needs a changing field. An alternating current in the first coil makes a field that is always changing, so the second coil always has an emf induced in it. A steady direct current makes a steady field, which induces nothing. The next page shows this directly with two coils on an iron bar.
A transformer does not make energy
Stepping the voltage up does not give more energy. As the power page shows, when the voltage goes up the current comes down.
Transformers change AC voltages
A transformer changes an alternating voltage from one value to another, by induction between two coils on an iron core.