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A transformer with efficiency $80 \%$ works at $4 \mathrm{~kW}$ and $100 \mathrm{~V}$. If the secondary voltage is $200 \mathrm{~V}$, then the primary and secondary currents are respectively
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$40 \mathrm{~A}$ and $16 \mathrm{~A}$
$\begin{aligned} & \text { (a) : Here, } \eta=80 \% \\ & P_i=4 \mathrm{~kW}=4000 \mathrm{~W} \\ & V_p=100 \mathrm{~V}, V_s=200 \mathrm{~V} \\ & I_p=\frac{P_i}{V_p}=\frac{4000}{100}=40 \mathrm{~A} \\ & \eta=\frac{V_s I_s}{V_p I_p} ; \frac{80}{100}=\frac{200 I_s}{4000}=\frac{I_s}{20} \\ & \text { or } I_s=\frac{20 \times 80}{100}=16 \mathrm{~A}\end{aligned}$
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