How transformers work

A transformer is made of two coils of wire which are not electrically connected but which are wrapped around the same soft iron core.

Transformer construction

The core is made from thin sheets of iron glued together (laminated) to avoid eddy currents in the core.

When a current is switched on in the primary coil a magnetic field is created in the core which then cuts through the secondary coil. This induces an EMF (Faradayʼs Law). When the current is switched of the field collapses again cutting through the secondary coil and again creating a pulse of electricity.

The magnetic field in the core changes direction as the current alternates.

As the primary current reverses the magnetic field direction reverses

A EMF is only induced in the second coil if the magnetic field is changing. To do this we use a constantly changing current in the primary coil - an alternating current.

The special thing about a transformer is that the voltage can be changed very simply. The ratio of the turns on the primary (input) coil to those on the secondary (output) coil control the voltage, so that:

That does not mean that there is an increase in energy, that cannot happen. If the voltage is increased (stepped up) then the current is stepped down by the same proportion. So, for example, if the voltage is stepped up five times the current is decreased to one fifth.

Step -up and step-down transformers

A step up transformer has more turns on the secondary than the primary.

step-up transformer

A step down transformer has the reverse, that is more turns on the primary than the secondary.

step-down transformer


In order to create a grid and to transmit electrical energy over long distances very high voltages have to be used. Power lines will convert some of the electrical energy from the current into heat, which is wasted. High voltages enable us to transmit relatively small currents and so wasting much less energy. The video below explains how transformers work and their special application to distributing electricity around the country.

Using transformers