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How is mains electricity produced?


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Transformers - Higher

You should know how a transformer works and how to calculate the voltages [voltage: The potential difference of a cell, electrical supply or electric component. It is measured in volts, 'V'. ] involved.

How transformers work

A transformer needs an alternating currentalternating current: Also called 'AC'. This is an electric current that regularly changes its direction and size. that will create a changing magnetic field. A changing magnetic field also induces a changing voltage in a coil. This is the basis of how a transformer works:

  • The primary coil is connected to an AC supply.
  • An alternating current passes through a primary coil wrapped around a soft iron core.
  • The changing current produces a changing magnetic field.
  • This induces an alternating voltage in the secondary coil.
  • This induces an alternating current (AC) in the circuit connected to the secondary coil.

It's important to know that:

  • There is no electrical connection between the primary and the secondary coils.
  • Transformers only work if AC is supplied to the primary coil. If DC was supplied, there would be no current in the secondary coil.
  • As the current in the primary coil increases steadily or decreases steadily, there is a constant voltage induced in the secondary coil.
  • As the voltage in the primary coil reaches maximum strength the voltage induced in the secondary coil is at its weakest (zero volts).

Calculating voltages

The ratio between the voltages in the coils is the same as the ratio of the number of turns in the coils.

primary voltage / secondary voltage = turns on primary / turns on secondary

This can also be written as:

Vp/Vs = Np/Ns

Step-up transformers have more turns on the secondary coil than they do on the primary coil.

Step-down transformers have fewer turns on the secondary coil than they do on the primary coil.


A transformer has 20 turns on the primary and 400 on the secondary. What is the output voltage if the input voltage is 500V?

toggle answer


Vp/Vs = Np/Ns Therefore Vs/Vp = Ns/Np

Vs/500 = 400/20

Vs = 500 x (400/20)

Vs= 10,000 Volts

Check your understanding of this by having a go at the activity.

Now try a Test Bite - Higher


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