A transformer is used to light a 120 W, 24 V lamp from 240 V a.c. mains. The current in the main cable is 0.6 A. The efficiency of the transformer is :
A
48%
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B
63.8%
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C
83.3%
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D
90%
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Solution
The correct option is C 83.3% Answer is C.
The efficiency of a transformer can be calculated using Equation, Efficiency=OutputpowerofthetransformerInputpowerofthetransformer=PsPp×100 In this case, the output power is 120W. The input power is calculated as follows. The input voltage is 220 V and current is 5 A Power=VI=240×0.6=144W. So, η=120144×100=83.3. Hence, the efficiency of the transformer that lights the bulb is 83.3%.