• # question_answer 48) A transformer is used to light a $140\text{ }W,\text{ }24\text{ }V$ bulb from a $240\text{ }V$ a.c. mains. The current in the main cable is $0.7$ A. The efficiency of the transformer is A)  $63.8%$                     B)  $83.3%$C)  $16.7%$                     D)  $36.2%$

Power of source $=EI=240\times 0.7=166$$\Rightarrow$Efficiency $=\frac{140}{166}\Rightarrow \eta =83.3%$