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A transformer is used for a 100 W, 20 V electric bulb at a place where the AC mains potential is 200 V and the current drawn is 0.6 A. The efficiency of the transformer is nearly

1. 48%
2. 68%
3. 30%
4. 83%

Correct answer: 83%

Solution

The bulb uses 100 W at its rated condition, so output power is 100 W. The input power from the mains is 200 V × 0.6 A = 120 W. Hence efficiency = (100/120) × 100 ≈ 83%.

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