Question

A transformer is used to light 100 W 25 volt lamp from 250 Volt ac mains. The current in the main cable is 0.5 A. Calculate the efficiency of the transformer.

(a) 50%

(b) 60%

(c) 90%

(d) 80%

The question was posed to me in a national level competition.

Asked question is from Alternating Current in portion Alternating Current of Physics – Class 12

Answer 1

The correct option is (d) 80%

The explanation: Given: Output power (Po) = 100 W; Ip = 0.5 A; Vp = 250 V

Input power (Pi) = Vp × Ip

Pi = 250 × 0.5 = 125 W

Efficiency = \((\frac {Output \, power}{Input \, power})\) × 100%

Efficiency = \((\frac {100}{125})\) × 100

Efficiency = 80

Therefore, the efficiency of the transformer is 80%.