Question

A transformer is used to light a $100W$ and $110V$ lamp from a $220V$ mains. If the main current is $0.5A$, the efficiency of the "transformer is approximately

• A. $30\%$
• B. $50\%$
• C. $90\%$
• D. $10\%$

Answer 1

Answer: (C)

The efficiency of transformer
$=\frac{\text{ Energy obtained from the secondary coil }}{\text{ Energy given to the primary coil }}$
$\eta =\frac{\text{ Output power }}{\text{ Inputpower }}$
or $\eta =\frac{V_s{I}_s}{V_P{I}_P}$
Given, $V_s{I}_s=100W,V_p=220V,I_p=0.5A$
Hence- $\eta =\frac{100}{220\times 0.5}=0.90=90$
Note The efficiency of an ideal transformer is $1$ (or $100\%$ ).
But in practice due to loss in energy, the efficiency of transformer is always less than $1$ (or less than $100\%$).