Question

A transformer is used to light a $140W,24V$ bulb from a $240V$ a.c mains. The current in the main cable is $0.7A$. The efficiency of the transformer is

• A. $63.8$%
• B. $83.3$%
• C. $16.7$%
• D. $36.2$%

Answer 1

Answer: (B)

$83.3$%

$\text{Power source}=EI=240\times 0.7=166$
$\Rightarrow \eta =\frac{\text{Output}}{\text{Input}}=\frac{140}{166}$ $\Rightarrow \eta =83.3\%$