Question

A transformer is used to light a $120W$, $24V$ lamp from $240V$ a.c. mains. The current in the main cable is $0.6A$. The efficiency of the transformer is

• A. $48\%$
• B. $63.8\%$
• C. $83.3\%$
• D. $90\%$

Answer 1

The correct option is C $83.3\%$

Answer is C.

The efficiency of a transformer can be calculated using Equation,
$Efficiency=\frac{Outputpowerofthetransformer}{Inputpowerofthetransformer}=\frac{P_s}{P_p}\times 100$
In this case, the output power is 120W.
The input power is calculated as follows.
The input voltage is 220 V and current is 5 A
$Power=VI=240\times 0.6=144W$.
So, $\eta =\frac{120}{144}\times 100=83.3$.
Hence, the efficiency of the transformer that lights the bulb is 83.3%.