Question

A transformer is used to light a $100W$ and $110V$ 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

The correct option is C $90\%$

Output power required, $P_0=100$ W

Input power given, $P_i=V_i{I}_i=220\times 0.5=110W$

$\therefore$ Efficiency of transformer, $\eta =\frac{P_o}{P_i}\times 100$

$\therefore$ $\eta =\frac{100}{110}\times 100=90.9$ %

Thus the efficiency of the transformer $\approx$ $90$ %