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Draw a transformer


desribe a transformer

Transformers are used to change the size of an ac voltage.
A transformer consists of two coils of wire wound on a metal core.

Transformers work on the principles of electromagnetism and electromagnetic induction.


An ac voltage is applied to one coil (the primary coil). This causes a changing (alternating) magnetic field to be set up in the core. The other coil (the secondary coil) is in this changing magnetic field and so it has an ac voltage induced in it.
The size of the secondary voltage depends on the number of turns on both the primary and the secondary coils and on the size of the ac voltage applied across the primary coil.


What is the formula for transformer

The relationship linking the number of turns on each coil to the voltages is:

V p         N p

---- -   =   ------

V s         N s


Vs is the voltage induced in the secondary coil in volts
Vp is the voltage applied to the primary coil in volts
ns is the number of turns on the secondary coil
np is the number of turns on the primary coil


Calculate the voltage obtained from the secondary coil of a mains (230 volt) transformer which has 11 500 turns on its primary and 600 turns on its secondary?

Vp =  Np

Vs      Ns


230 \times 600

11500} = 12\;volts
Did you use the correct relationship?
Did you substitute the numbers in the correct places?
And did you remember to include the unit in your final answer?
Well done!


explain why a transformer is not 100% efficient.

because not all of the input electrical energy is transferred to electrical energy at the output. There is some heat generated and also a little bit of sound - transformers get hot and hum!

Energy is lost in the wires in the coils and in the iron core itself. Small eddy currents are induced in the iron core, which waste energy heating up the core. Eddy currents can be reduced by laminating the core - for instance, by making the core out of thin slices of metal, which are 'glued' together. This reduces the area the eddy currents can flow through, hence increasing the resistance and reducing the size of the eddy currents.


what is  a step up transformer

On a step-up transformer there are more turns on the secondary coil than the primary coil. The induced voltage across the secondary coil is greater than the applied voltage across the primary coil or in other words the voltage has been “stepped-up”.