Question

A transformer is used to light $140W$, a $24V$ lamp from $240VAC$mains. The current input to the mains is $0.7A$. The efficiency of the transformer is nearest to

• A. $90$%
• B. $80$%
• C. $70$%
• D. $60$%

Answer 1

The correct option is B $80$%

Here, voltage across primary $V_p=240V$ and primary current $I_p=0.7A$ and output power $P=140W$

The efficiency of a transfer is $\eta =$ (output power /input power) $\times 100=\frac{P}{V_p{I}_p}\times 100=\frac{140}{240\times 0.7}\times 100=83.3\sim 80$ %