JEE Main & Advanced AIEEE Solved Paper-2004

  • 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

    A) 4L                           

    B) 2L           

    C)        L/2         

    D)        L/4

    Correct Answer: C

    Solution :

    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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