We use transformers to change the size of a voltage. We can step the voltage down from a high voltage to a smaller one or we can step it up. The principles are the same.

A transformer has two coils - a primary coil and a secondary coil. An alternating current flows in the primary coil and this induces a voltage in the secondary coil.

Let's see why.

When If we close the switch in the primary circuit, the ammeter flicks to the right.
When we open the switch, the ammeter flicks to the left.

This is because the primary coil behaves like an electromagnet when we close the switch. It is as if we have pushed a bar magnet towards the secondary coil. And this induces a voltage in the secondary coil; but only while the 'magnet' is moving - i.e. whilst the current is changing. Once the magnetic field is steady, there is no induced EMF in the secondary coil.

When we open the switch, the electromagnet is switched off. This is like pulling a bar magnet away from the secondary coil. Again, this induces an EMF (in the opposite direction).

The primary coil produces a magnetic field in the secondary coil. It is only while this magnetic field is changing that we get an EMF induced in the secondary coil.

We can make the magnetic field carry on changing by using an alternating current in the primary coil.

This produces a magnetic field in the secondary coil. Since the current is alternating, the magnetic field also alternates back and forth. This changing magnetic field induces an alternating EMF in the secondary coil.

It is important to get as strong a magnetic field as possible in the secondary coil. We say we want a good flux linkage. To achieve this, we can put a soft iron core through the coils. This increases the induced EMF. Iron is a magnetic material so it guides the magnetic field from the primary coil to the secondary coil.

We get the best effect by mounting the coils on a closed loop of soft iron (picture 5.2). This is how we build transformers.

From Faraday's Law, we can deduce that the greater the rate at which the magnetic field changes, the greater the EMF induced. Also, the more turns there are on the secondary coil, the bigger the induced EMF. If we increase the turns on the secondary coil, the output voltage increases in proportion.

We find that the ratio of the induced EMF to the input voltage is the same as the ratio of the turns on the secondary coil to the turns on the primary coil.

If the number of turns on the secondary coil is less than the number on the primary coil, then the output voltage will be smaller than the input voltage. We call this type of transformer a step down transformer. We use a step down transformer to step the voltage down from a high voltage to a low voltage.

Pieces of electronic equipment (like T.V.s and radios) use a step down transformer to reduce the mains voltage from 230 V to the operating voltage of the electronics.

If the secondary coil has more turns than the primary coil then the output voltage is bigger than the input voltage. We call this a step up transformer.

At first it may seem that we are getting something for nothing. However, this is not the case when we consider the amount of energy going into and out of the transformer. We never get more energy out than we put in. The voltage may be bigger on the output, but the current will be bigger on the input. The power is voltage × current. Therefore, in an ideal transformer, the power in and the power out will be the same.