Question

A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 amp, the efficiency of the transformer is approximately 

(a) 50% 

(b) 90 % 

(c) 10% 

(d) 30% 

Answer 1

Correct option  (b) 90 % 

Explanation:

Given : Output power P = 100 W

Voltage across primary V_p = 220 V

Current in the primary l_p = 0.5 A