How do transformers work?

Electrical energy is transmitted at very high voltages. You should remember the equation       Power = Voltage x Current

Imagine a power station needs to deliver 1 kW of power to a factory. What voltage should it be transmitted at? There are many options.

This shows that to send a certain amount of power you could either do it using a small voltage and a high current or a high voltage and a small current. However if the current is greater the a lot more energy is lost as heat in the transmission lines. This is why electrical power is transmitted at very high voltages, typically 400,000V. The higher the voltage used the lower the current needed so the less energy is lost in transmission.

Step-Up and Step-Down Transformers

Before electricity is put on the National Grid its voltage is stepped up, i.e. made bigger, by a step up transformer for the reason explained above.

Before it comes to our homes, for safety reasons mostly, it is stepped down to 240V by several step-down transformers.

Step-Up               Makes an a.c. voltage bigger

Step-Down         Makes and a.c. voltage smaller

 Many of the appliances we use in our homes need much less than 240V to work so they use a transformer, either as an adapter or inside the device itself.

How do transformers work?

We have seen how a changing magnetic field produces an induced voltage in a conductor.

If two coils are linked together by an iron core then the magnetism that either of them produces flows round the iron core. They are linked magnetically.

Now if an alternating current flows in one of the coils, call it the primary coil, this will produce an alternating magnetic field in both of the coils. An alternating voltage is therefore induced in the secondary coil.

So, we put an alternating potential difference across the primary coil and an alternating potential difference is induced across the secondary coil. The coils are not linked electrically but magnetically.

The Transformer Equation

We can calculate the potential difference we get out of a transformer using this equation

N is the number of turns on the primary or secondary coil.

Remember that if the number of turns gets bigger or smaller then the voltage gets bigger or smaller by the same ratio, e.g. if the secondary coil has twice the turns as the primary then the voltage will double.

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Revision Questions

1. The primary coil of a transformer has 2000 turns. The secondary coil has 75 turns. If 240V a.c. were put across the primary coil what p.d. would be produced across the secondary?

2. Why do transformers only work with a.c., not d.c.?

3. Why are a.c. voltages "stepped up" before they are transmitted?

4. Why are they stepped down before we use them?