Question

A transformer has an efficiency of 60% and works at 5kW. If the secondary voltage is 150 V, then what is the secondary current?

(a) 10 A

(b) 20 A

(c) 30 A

(d) 40 A

I have been asked this question in an international level competition.

This intriguing question comes from Alternating Current in section Alternating Current of Physics – Class 12

Answer 1

The correct choice is (b) 20 A

For explanation I would say: Given: efficiency (η) = 60%; Input power (Pi) = 5 kW = 5000 W; VS = 150 V

Efficiency (η)=\(\frac {Output \, power (P_o)}{Input \, power (P_i)}=\frac {V_s I_s}{V_p I_p}\)

\(\frac {60}{100}=\frac {P_o}{5000}\)

Po=\(\frac {60 \times 5000}{100}\)=3000 W

Thus,

Is=\(\frac {P_o}{V_s} = \frac {3000}{150}\)=20 A

Therefore, the current in the secondary coil is 20 A.