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Ohm's Law in Scalar Form

A transformer...

Question

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 will be

63.8%

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74%

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83.3%

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48%

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Solution

The correct option is B 83.3%
Output power = 140 W
Input power = 240 $\times$ 0.7 = 168 W
Efficiency = $\frac{o u t p u t p o w e r}{i n p u t p o w e r} \times 100 = \frac{140}{168} \times 100 = 83.3$

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