Q. A transformer is used to light 140 W, 24 V lamp from 240 V AC mains. The current in the mains is 0.7 A. The efficiency of transformer is nearest to
A) 90%
B) 80%
C) 70%
D) 60%
Explanation
(b) $P_{i} = 240\ \times 0.7 = 168\ $W, $P_{0} = 140$ W
$\eta\ = \frac{P_{0}}{P_{i}} \times 100 = \frac{140}{168} \times 100\ \approx 80\%$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.