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  4. A transformer of efficiency 90 % draws an input power of 4 kW. An electrical appliance connected across the secondary draws a current of 6 A. The impedance of the device is

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Q. A transformer of efficiency $90\%$ draws an input power of $4\,kW$. An electrical appliance connected across the secondary draws a current of $6\,A$. The impedance of the device is

KEAMKEAM 2009Alternating Current

Solution:

$ \eta =\frac{output\text{ }power}{input\text{ }power}=\frac{{{V}_{S}}{{I}_{S}}}{4000}=\frac{Z{{({{I}_{S}})}^{2}}}{4000} $
$ \therefore $ $ \frac{90}{100}=\frac{Z{{(6)}^{2}}}{4000} $
or $ Z=100\,\Omega $