A transformer is used to light a $140 W$ , $40 V$ lamp from $240 V$ $A C$ mains. The current in mains cable is $0.7 A$ , find the efficiency of transformer.

8.33 %

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50 %

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83.3 %

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25 %

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Solution

The correct option is C $83.3 %$
Efficiency of a transformer is given by,

$η = \frac{output power}{input power} = \frac{P_{S}}{P_{p}} = \frac{P_{S}}{V_{p} I_{p}}$

Given,

$P_{S} = 140 W, V_{P} = 240 V, I_{P} = 0.7 A$

$\Rightarrow η = \frac{140}{240 \times 0.7} = 0.8333 = 83.33 %$

Hence, option $(C)$ is correct.

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