JEE Main & Advanced Sample Paper JEE Main - Mock Test - 11

  • question_answer
    In a series LCR circuit \[R=200\Omega \] and the voltage and the frequency of the main supply is \[~220V\] and \[50\text{ }Hz\] respectively. On taking out the capacitance from the circuit the current lags behind the voltage by \[30{}^\circ \]. On taking out the inductor from the circuit the current leads the voltage by\[30{}^\circ \]. The power dissipated in the LCR circuit is

    A) \[305\text{ }W\]            

    B) \[210\text{ }W\]

    C) Zero W          

    D) \[242\text{ }W\]

    Correct Answer: D

    Solution :

    When capacitance is taken out, the circuit is LR.
    \[\therefore \,\,\tan \phi =\frac{\omega L}{R}\]
    \[\Rightarrow \,\,\,\omega L=R\,\tan \phi =200\times \frac{1}{\sqrt{3}}=\frac{200}{\sqrt{3}}\]
    Again, when inductor is taken out, the circuit is CR.
    \[\therefore \,\,\tan \phi =\frac{1}{\omega CR}\]
    \[\Rightarrow \,\,\frac{1}{\omega c}=R\tan \phi =200\times \frac{1}{\sqrt{3}}=\frac{200}{\sqrt{3}}\]
    Now,  \[Z=\sqrt{{{R}^{2}}+{{\left( \frac{1}{\omega C}-\omega L \right)}^{2}}}\]
    \[=\sqrt{{{(200)}^{2}}+{{\left( \frac{200}{\sqrt{3}}-\frac{200}{\sqrt{34}} \right)}^{2}}}=200\Omega \]
    Power dissipated \[={{V}_{rms}}{{I}_{rms}}\cos \phi \]
    \[={{V}_{rms}}.\frac{{{V}_{rms}}}{Z}.\frac{R}{Z}\]   \[\left( \because \,\,\cos \phi =\frac{R}{Z} \right)\]
    \[=\frac{{{V}^{2}}_{rms}R}{{{Z}^{2}}}=\frac{{{(200)}^{2}}\times 200}{{{(200)}^{2}}}=\frac{220\times 220}{200}=242W\]


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