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GCSE Level Questions: Transformers

Q2.

(a) The basic structure of a transformer is a primary coil of insulated wire, an iron core and a secondary coil of insulated wire.

(i) Why is the core made of iron?

Iron is a soft magnetic material so it (quickly) becomes magnetised when placed in a magnetic field and (quickly) loses its magnetism when the field is removed

Any reference to conduction of electricity/heat nullifies the mark

Iron is a good conductor, both of electricity and heat. BUT the function of the transformer does NOT depend on this property. In fact the heating of the core due to eddy currents causes energy loss.

The core is there to 'link the flux' - to become magnetised and demagnetised by the primary so that the secondary experiences that change. This induces a varying potential difference across the secondary coil which makes an alternating current flow in it. To do this efficeiently it must be 'soft' iron.

(Max 1 mark)

(ii) Explain how a transformer works.

any four from:

insulation prevents electricity/current flowing through the iron/core

alternating current/a.c. in the primary (coil)

produces a changing magnetic field (in the iron/core)

and hence the secondary (coil) experiences this changing magnetic field

inducing an alternating potential difference/p.d./voltage across the secondary (coil)

(this results in) alternating current/a.c. in the secondary (coil)

(4 marks)

(b) A small step-down transformer is used in the charger for an electric screwdriver. The input to the transformer is 230 V a.c. mains supply and the output is 5.75 V a.c. There are 3200 turns on the primary coil.

Calculate the number of turns on the transformer's secondary coil.

Show clearly how you work out your answer

Ns = VsNp/Vp

= 5.75 x 3200/230

= 80

number of turns number of turns on secondary = 80

Note that for a number of turns there is NO unit. If you add 'V' after your answer you will not get the mark!

(2 marks)

(Total 7 marks)