Question

A transformer is used to light 100 W – 110 V lamp from 220 V mains. If the main current is 0.5 A, the efficiency of the transformer is

• A. 96$\%$
• B. 90 $\%$
• C. 99 $\%$
• D. 95 $\%$

Answer 1

The correct option is B

90 $\%$

$\eta =\frac{P_{output}}{P_{input}}=\frac{100}{220\times \frac{1}{2}}=\frac{100}{110}=\frac{10}{11}\approx 90\%$