JEE Main & Advanced Physics Alternating Current / प्रत्यावर्ती धारा Question Bank Self Evaluation Test - Alternating Current

  • question_answer
    An ac source of angular frequency co is fed across a resistor R and a capacitor C in series. The current registered is I. If now the frequency of source is changed to \[\omega /3\] (but maintaining the same voltage), the current in the circuit is found to be halved. Then the ratio of reactance to resistance at the original frequency \[\omega \]is     

    A) \[\sqrt{3/5}\]

    B) \[\sqrt{5/3}\]

    C) \[\sqrt{2/3}\]

    D) \[\sqrt{3/2}\]

    Correct Answer: A

    Solution :

    [a] According to given problem, \[I\,=\frac{V}{Z}\,=\frac{V}{{{[{{R}^{2}}+{{(1/C\omega )}^{2}}]}^{1/2}}}\]           ??.(i) and \[\frac{I}{2}\,=\frac{V}{{{[{{R}^{2}}+{{(3/C\omega )}^{2}}]}^{1/2}}}\]           ??.(ii) Substituting the value of I from eq. (i) in (11), \[4\,\left( {{R}^{2}}+\frac{1}{{{C}^{2}}{{\omega }^{2}}} \right)={{R}^{2}}+\frac{9}{{{C}^{2}}\,{{\omega }^{2}}}\] i.e., \[\frac{1}{{{C}^{2}}\,{{\omega }^{2}}}\,=\frac{3}{5}\,{{R}^{2}}\]       so that \[\frac{X}{R}\]


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