Q136 / 180 · COMEDK 2015 · Physics
A transformer is used to light a $100 \mathrm{~W}, 110 \mathrm{~V}$ lamp from a $220 \mathrm{~V}$ supply. If the supply current is $0.6 \mathrm{~A}$, then the efficiency of the transformer is
- A$66 \%$
- Correct answerB$76 \%$
- C$86 \%$
- D$96 \%$
Solution
For a transformer,
Input voltage, $V_{1}=220 \mathrm{~V}$
Input current, ${I}_{1}=0.6 \mathrm{~A}$
Output power, $P_{2}=100 \mathrm{~W}$
From, input power, $\quad P_{1}=V_{1} I_{1}$
$$
\begin{aligned}
&=220 \times 0.6 \\
&=132 \mathrm{~W}
\end{aligned}
$$
We know that, efficiency of transformer,
$$
\begin{aligned}
\eta &=\frac{P_{2}}{P_{1}}=\frac{100}{132}=0.7576 \\
&=75.76 \% \simeq 76 \%
\end{aligned}
$$
Hence, the correct answer is option: 'B'.