Transformers

Introduction. In the UK we receive electricity as 230 V a.c. This is far too high a voltage for sensitive electronics: ipod Games console Laptop Toothbrush charger etc etc.

You will need to use an adapter like this.

Or these…

All of these mains adapters contain a transformer. What is a transformer? It changes the voltage either up or down. Step-up transformer  Increases V Step-down transformer  Decreases V

Iron core, loops of wire. One side has more turns of wire than the other

Transformers only work with alternating current (a.c.) It consists of two electromagnets not physically connected.

The changing magnetic field on one side induces an a. c The changing magnetic field on one side induces an a.c. current on the other side.

Step-up transformer If there are more turn on the secondary side than the primary side it is a step-up transformer.

The ratio of turns decide how much the voltage steps-up e.g. a transformer with 100 turns on the primary side and 200 turns on the secondary side will double the input voltage.

Step-down transformer 5 times fewer turns on the secondary side  voltage is 5 times smaller on secondary side.

Power lines – National Grid Electricity produced at power stations cannot be sent directly to towns across long power lines. The power lines would heat up and cause too much energy to be lost as heat!

Power lines Transformers are used at either end to step-up and then step-down the voltage.

Example The power generated at this power station is 25000V, 1000A = ? W

How much does the current fall to? Vp = 25 000, Ip = 1000, Vs = 132 000, Is =?

Answer Is = 189 A

Answer Vp x Ip = Vs X Is 25 000 X 1000 = 132 000 x Is 189.4 A = Is The current on the secondary side is 189 Amps.

Power loss in a cable.. …depends on current squared. Therefore we want the current to be as low as possible to avoid wasting energy as heat!

True or false Transformers only work with d.c. A transformer that increases the voltage is ‘step-up’. The ratio of turns on the primary to secondary side dictates how the voltage changes.

True or false 4. Power loss in a long wire will be twice as great if the current is doubled. 5. A transformer with more turns on the secondary side than the primary side is step-down. 6. Transformers are very efficient (low power loss.)