# Exam-style Questions: Transformers and Rectification

1. A transformer cannot be used to run a 230 V, 100 W mains lamp directly from a 12 V car battery.

Which one of the following is the correct reason for this?

a) The internal resistance of the battery will not allow enough current in the primary coil.

b) Eddy currents in the iron core will heat up the iron core.

c) The current from the battery will produce a steady flux in the secondary coil.

(1 Mark)

2. An alternating supply delivers a current of 0.025 A at 12 V to the primary coil of a transformer. A 20 Ω resistor is connected to the secondary coil.

a) The current in the secondary circuit is 0.110 A.

the power input

power input = .................. W

(1 Mark)

the power output

power output = ................ W

(2 Marks)

the efficiency

efficiency = .................... %

(2 Marks)

b) The frequency of the supply is increased. The power input is kept constant. The current in the secondary coil falls to 0.105 A.

Calculate the new efficiency of the transformer.

efficiency = ................... %

(2 Marks)

(Marks available: 7)

Answer outline and marking scheme for question:

C The current from the battery will produce a steady flux in the secondary coil.

(1 Mark)

2. a) power input = 0.025 x 12 = 0.30 W

(1 Mark)

power output = I2R

= 0.1102 x 20 = 0.242 W

(2 Marks)

efficiency = (output power / input power) x 100

= 0.242 / 0.30 x 100 = 80.7 % (e.c.f)

(2 Marks)

b) New efficiency = (0.1052 x 20 / 0.03) x 100

= 73.5 % (e.c.f)

(2 Marks)

(Marks available: 7)