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  4. A transformer is used to light 140 W, 24 V lamp from a 240 V ac mains. If the main current is 0.7 A, the efficiency of the transformer is

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Q. A transformer is used to light $140\, W$, $24\, V$ lamp from a $240\,V$ ac mains. If the main current is $0.7\, A$, the efficiency of the transformer is

Alternating Current

Solution:

Output power $= 140\,W$
Input power $= 240 \times 0.7 = 168 \,W$
Efficiency $=\frac{output \,power}{input\, power}\times100$
$=\frac{140}{168}\times100$
$=83.3\%$