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Q.
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
The efficiency of transformer
$=\frac{\text { Energy obtained from the secondary coil }}{\text { Energy given to the primary coil }}$
or $\eta=\frac{\text { Output power }}{\text { Input power }}$
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, $\eta=\frac{100}{220 \times 0.5}=0.90=90 \%$