12th Class Physics Alternating Current / प्रत्यावर्ती धारा Question Bank MCQ - Alternating Currents

  • question_answer
    The current drawn by the primary of a transformer, which step down 200 V to 20 V to operate a device of resistance \[20\,\Omega \] is (Assume the efficiency of the transformer to be 80%):

    A) 0.125 A

    B)                         0.225 A

    C) 0.325 A                        

    D) 0.425 A

    Correct Answer: A

    Solution :

    (a) 0.125 A Given: \[{{V}_{P}}=200\,\,V,\,R=20\,\Omega \,{{V}_{S}}=20\,V,\,\eta =80%\] Current through the secondary coil is             \[{{l}_{S}}=\frac{{{V}_{S}}}{R}=\frac{20\,V}{20\,\Omega }=1\,A\] Efficiency of transformer, \[\eta =\frac{Output\,\,power}{Input\,\,power}=\frac{{{V}_{S}}{{l}_{S}}}{{{V}_{P}}{{l}_{P}}}\] Or         \[{{l}_{P}}=\frac{{{V}_{S}}{{l}_{S}}}{{{V}_{P}}\eta }=\frac{20\times 1\times 100}{200\times 80}=0.125\,\,A\]


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