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A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 amp, the efficiency of the transformer is approximately

1. 50%
2. 90%
3. 10%
4. 30%

Correct answer: 90%

Solution

The power input to the transformer is given by P_in = V_mains × I_mains = 220 × 0.5 = 110 W. The power output is P_out = 100 W (as the lamp consumes 100 W). Efficiency is η = (P_out / P_in) × 100 = (100 / 110) × 100 ≈ 90%.

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