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Question

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%.

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