Question

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

• A. $30\%$
• B. $50\%$
• C. $90\%$
• D. $10\%$

Answer 1

Answer: (C)

Given : Output power $P=V_{S}I_{S}=100\,W$
Voltage across primary $V_{p}=220\,V$
Current in the primary $I_{p}=0.5\,A$
Voltage across secondary $= 110 \,V$
Efficiency of a transformer $\eta=\frac{\text{outputpower}}{\text{Inputpower}}\times100$
$\eta=\frac{P}{V_{p} I_{p}}\times100$
$=\frac{100}{220\times0.5}\times100$
$=90\%$