Transformers are used to change the voltage of an a.c. supply.

At power stations, (step up) transformers are used to produce very high voltages before the electricity is transmitted to where it is needed through power lines (National Grid).

Local (step-down) transformers reduce the voltage to safer levels before the electricity is supplied to consumers. You should understand how a transformer works by electromagnetic induction, and know why they are used for power transmission (that the higher the voltage, the smaller the current needed to transmit energy at the same rate therefore less energy is wasted by heat loss to the atmosphere).

A transformer consists of two separate coils wound around an iron core. When an alternating voltage is applied across one coil (the primary) an alternating voltage is induced across the other coil by electromagnetic induction (secondary).

The voltages across the primary and secondary coils of a transformer are related as shown:

where V is the potential difference across the coil and N is the number of turns of wire on the coil.

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