Question

A transformer is used to light a $100W$ and $110V$ lamp using a $220V$ main supply. If the supply current is $0.5A$, then the efficiency of the transformer is

• A. $100$%
• B. $99$%
• C. $90.1$%
• D. $87.7$%
• E. $85$%

Answer 1

Answer: (C)

$90.1$%

Given, $V=220V$ and $i=0.5A$
$\therefore$ Resistance $R=\frac{220}{0.5}$
$R=440\Omega$
$P=\frac{V^2}{R}=\frac{220\times 220}{440}$
$P=110W$
Efficiency of transformer
$\eta$% $=\frac{P_{out}}{P_{in}}\times 100=\frac{100}{110}\times 100$
$=\frac{1000}{220}=90.1$