4.6.4 Transformers

What is a Transformer?

• A transformer is an electrical device that can be used to increase or decrease the voltage of an alternating current

(Transformers only work with a.c.)

A transformer consists of two coils of wire wrapped around a soft iron core

• A step-up transformer increases the voltage of a power source
• A step-up transformer has more turns on the secondary coil than on the primary coil
• A step-down transformer decreases the voltage of a power source
• A step-down transformer has fewer turns on the secondary coil than on the primary coil

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How a Transformer Works

• When an alternating current is supplied to the primary coil, a changing magnetic field is produced by the primary coil
• This field passes through the soft iron core and through the secondary coil
• The changing field in the secondary coil induces an EMF
• This EMF is also alternating and has the same frequency as the original current

The Transformer Equation

• The output voltage of a transformer depends on the number of turns on each of the coils as well as the input voltage:

(The ratio of the voltages is equal to the ratio of the number of turns)

Transformer calculation worked example

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Power

• Although transformers can increase the voltage of a power source, due to the law of conservation of energy, they cannot increase the power output
(Increasing the voltage decreases the current, and vice-versa)
• If a transformer if 100% efficient:

(Where IP and IS are the currents in the primary and secondary coils)

• Hence, if the voltage is increased by some factor, the current must decrease by the same factor

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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