Q. A transformer is used to light $140$ Watt $24$ Volt lamps from $240$ Volt $AC$ mains, the current in the main cable is $0.7$ amps. The efficiency of the transformer is

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A

$63.8 \%$

B

$84 \%$

C

$83.3 \%$

D

$48 \%$

Solution:

Out power $=140\, Watt$
Input power $=240 x 0.7=168$ Watt
Efficiency $=(\text { out put power } / \text { input power }) \times 100$
$=(140 / 168) \times 100=83.3 \%$

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