Question

A transformer is used to light a $100\, W$ and $110\, V$ lamp from a $220\, V$ mains. If the main current is $0.5\, A$, 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, $V_{s} I_{s}=100\, W,\, V_{p}=220\, V,\, I_{p}=0.5\, A$
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 $100 \%$).