Question

A transformer is an electrical device which is used for changing the a.c. voltage. It is based on the phenomenon of mutual induction. It can be shown that
`(E_(s))/(E_(P)) = (I_(P))/(I_(s)) = (n_(s))/(n_(P)) = K`
Where the symbols have their standard meaning. For a step up trasnformer,`K gt 1` and for a step down transformer, `K lt 1`. The above relations are on the assumption that efficiency to transformer is `100%` . In fact, efficiency
`eta = ("outpu power")/("input power") = (E_(s) I_(s))/(E_(P) I_(P))`
The number of turns in the primary and secondary coils of a transformer are 2000 and 50 respectively. The primary coil is connected to main supply of `120 V` and secondary to a night bulb `0.6 ohm`. The efficiency of transfomer is `80%`
Voltage across the secondary of transformer is
A. `125 V`
B. `360 V`
C. `40 V`
D. `3 V`

Answer 1

Correct Answer - D
Here, `n_(P) = 2000, n_(s) = 50, E_(P) = 120 V`,
`R_(s) = 0.6 ohm eta = 80%`
As `(E_(s))/(E_(P)) = (n_(s))/(n_(P))`
`:. E_(s) = (n_(s))/(n_(P)) xx E_(P) = (50)/(2000) xx 120 = 3 V`