A transformer is used to light a 140 W, 24 V lamp from 240 V AC mains. The current in mains cable is 0.7 A, find the efficiency of transformer.

Options

A. `63.8% `
B. `84% `
C. `83.3% `
D. `48% `

Answer

C

Explanation

Mains voltage is V = 240 V and mains current is J = 0 .7 A input power will be
`P_("input") = 240 xx 0.7 = 168 ` W
But output power is given as
`P_("output ") = 140 `W
Therefore efficiency
`= ("output ")/("input") xx 100 % = 83.33% `