For many purposes, it is necessary to change (or fransform) an alternating voltage from one to another of geater or smaller value. This is done with a device called a transformer.
When an alternating voltage is applied to the primary, the resulting current produces an alternating magretic flux which links the secondary and induces an emf in it. The value of this emf depends on the number of turns in the secondary. We consider an ideal transformer in which the primary has negligible resistance and all the flux in the core links both primary and secondary windings..
(i) A step-down transformer increases :
(a) Voltage
(b) Current
(c) Power
(d) Frequency
(ii) A step-up transformer has________ number of turns on primary winding and _______ number of turns on secondary winding.
(a) less, more
(b) more, less
(c) less, less
(d) more, more
(iii) If the primary coil of a transfonner has 100 turns and the secondary
has 200 turns, Ns/Np=2. Thus, a 220 V input at 10A will step-up :
(a) to 220 V output at 20 A
(b) to 440 V output at 10 A
(c) to 440 V ouput at 5 A
(d) to 110 V ouput at 10 A
(iv) A transforrner is used to reduce the mains supply of 220 V to 22 V. If
the currents in the primary and secondary are 2A and 15 A respectively, then the efficiency of the transformer is .
(a) 65%
(b) 75%
(b) 80%
(d) 90%
