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

  • question_answer
    When an AC source of voltage \[V={{V}_{0}}\sin \,100t\] is connected across a circuit, the phase difference between the voltage V and current l in the circuit is observed to be \[\pi /4\]. as shown in figure. If the circuit consists possibly only of RC or RL or LC in series, find possible values of two elements.

    A) \[R=1k\Omega ,\,C=10\,\mu F\] 

    B) \[R=1k\Omega ,\,C=1\,\mu F\]

    C) \[R=1k\Omega ,\,L=1\,0\,mH\]   

    D) \[R=10k\Omega ,\,L=1\,0\,mH\]

    Correct Answer: A

    Solution :

    (a) \[R=1k\Omega ,\,C=10\,\mu F\] Figure given in the question shows that current l leads the voltage V by a phase angle \[\pi /4\]. Therefore, the circuit can be RC circuit alone. \[\tan \,\phi =\frac{{{X}_{C}}}{R}=\frac{1}{\omega CR}\left( \because \,{{X}_{C}}=\frac{1}{\omega C} \right)\] \[\tan \frac{\pi }{4}=\frac{1}{\omega CR};1=\frac{1}{\omega CR}\]                     …(1) From \[V={{V}_{0}}\sin \,100\,t\]. we get, \[\omega =100\,rad\,{{s}^{-1}}\] \[\therefore CR=\frac{1}{\omega }=\frac{1}{100}\]                      (Using eq. (1)) When \[R=1\,k\Omega ={{10}^{3}}\Omega ,\,C=\frac{1}{{{10}^{5}}}={{10}^{-5}}F=10\,\mu F\]


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