Question
A 240 V a.c. mains supply is connected to the primary coil of the transformer shown in
Fig. 10.1. A lamp that gives full brightness with a 6 V supply is connected to the secondary coil.
(a) Name a suitable material from which the coils may be made.
…………………………………………………………………………………………………………………….. [1]
(b) State the name given to the part of the transformer that is made of soft iron
(see Fig. 10.1).
…………………………………………………………………………………………………………………….. [1]
(c) Calculate the number of turns of wire in the secondary coil that will enable the lamp to
light at full brightness.
number of turns =…………………………….. [3]
(d) State what would happen to the lamp if the number of turns in the secondary coil was
(i) much less than that calculated in (c),
……………………………………………………………………………………………………………… [1]
(ii) much more than that calculated in (c).
……………………………………………………………………………………………………………… [1]
[Total: 7]
Answer/Explanation
Ans: (a) copper
(b) core (c) 
in any form 8000/
= 240 / 6 OR 240 = 6 OR = 6 8000 Ns 8000 240
200
(d) (i) lamp less bright/less than full brightness/wouldn’t light (up properly)/ has less energy
(ii) lamp blows/bursts OR lamp too bright OR lamp overheats/burns out OR much brighter/has more energy
Question
A radio is connected to the mains supply using a step-down transformer.
(a) Draw a labelled diagram of the structure of a basic step-down transformer. [3]
(b) Explain the operation of a basic transformer. [3]
(c) The voltage of the mains supply is 230V. The output voltage of the transformer is 6.0V.
Calculate the value of the turns ratio \(\left ( \frac{N_{s}}{N_{p}} \right )\). Give your answer to two significant figures.
value of turns ratio = [2] [Total: 8]
Answer/Explanation
Ans:
(a) Labelled diagram showing
- labelled Iron (core)
- two coils, labelled primary and secondary, (around core and connected to separate circuits)
- Fewer turns on labelled secondary coil
(b) (Primary voltage causes) an alternating current in primary coil
produces a changing magnetic field
(changing) field induces pd / e.m.f. (in secondary coil)
(c) 0.026
(Ns / Np =) Vs / Vp in any form