RAJASTHAN PMT Rajasthan - PMT Solved Paper-2010

  • question_answer
    In an L-R circuit, time constant is that time in which current grows from zero to the value (where \[{{I}_{0}}\]is the steady state current)

    A)  0.63\[{{I}_{0}}\]                              

    B)  0.50\[{{I}_{0}}\]

    C)  0.37 \[{{I}_{0}}\]             

    D)         \[{{I}_{0}}\]

    Correct Answer: A

    Solution :

    Current at any instant of time \[t\] after closing an \[L-R\] circuit is given by\[I={{I}_{0}}\left[ 1-{{e}^{\frac{-R}{L}t}} \right]\] Time constant\[t=\frac{L}{R}\] \[\therefore \]  \[I={{I}_{0}}\left[ 1-{{e}^{\frac{-R}{L}\times \frac{L}{R}}} \right]={{I}_{0}}(1-{{e}^{-1}})\]                    \[={{I}_{0}}\left( 1-\frac{1}{e} \right)\]                 \[={{I}_{0}}\left( 1-\frac{1}{2.718} \right)=0.63{{I}_{0}}=63%\,\,of\,\,{{I}_{0}}\]


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