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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.

Question

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

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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