JEE Main & Advanced Physics Alternating Current / प्रत्यावर्ती धारा JEE PYQ-Alternating Current

  • question_answer
    In an LCR circuit, capacitance is changed from C to 2 C. For the resonant frequency to remain unchanged, the inductance should be changed from L to                     [AIEEE 2004]

    A) 4L

    B) 2L    

    C) L/2

    D) L/4

    Correct Answer: C

    Solution :

    [c] At resonance, the net reactance of an AC circuit is zero. i.e., the circuit behaves like a purely resistive circuit with resonant frequency\[\frac{1}{2\pi \sqrt{LC}}\].
    At the condition of resonance,
    \[{{X}_{L}}={{X}_{C}}\]
    Or         \[\omega L=\frac{1}{\omega C}\]                                    ….(i)
    Since, resonant frequency remains unchanged.
    So, \[\sqrt{LC}=\]constant   (\[v=\frac{1}{2\pi \sqrt{LC}}=\]constant)
    \[\Rightarrow \]   \[LC=\]constant
    \[\therefore \]      \[{{L}_{1}}{{C}_{1}}={{L}_{2}}{{C}_{2}}\]
    \[\Rightarrow \]   \[L\times C={{L}_{2}}\times 2C\]                       \[(\because {{L}_{1}}=L)\]
    \[\Rightarrow \]   \[{{L}_{2}}=\frac{L}{2}\]


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