Transformers change the voltage of an alternating current (ac) electrical supply.

Some times transformers make the output voltage bigger than the input voltage, this is a **STEP UP TRANSFORMER**. In a step up transformer there are more coils of wire on the secondary coil than on the primary coil.

Some times transformers make the output voltage smaller than the input voltage, this is a **STEP DOWN TRANSFORMER**. In a step down transformer there are more coils of wire on the primary coil than on the secondary coil.

Transformers are used in the national grid.

**How a transformer works**

Alternating current (ac) in the **primary coil** (input coil) produces a changing magnetic field in the **laminated soft iron core**.

The changing magnetic field induces a potential difference (voltage) in the **secondary coil** (output coil).

**Equations for transformers**

We can work out the transformer output voltage if we know the input voltage and the number of turns (coils) on the primary and secondary coils, using the equation below;

V_{p} = potential difference (voltage) input on the primary coil

V_{s} = potential difference (voltage) output on the secondary coil

n_{p} = number of turns (coils) of wire on the primary coil

n_{s} = number of turns (coils) of wire on the secondary coil

Transformers are very efficient devices, if we assume they are 100% efficient (no energy wasted), then the electrical input power will equal the electrical output power and that gives is a new equation to use;

Where I_{p}is the current in the primary coil and I_{s} is the current in the secondary coil.