MGIMS WARDHA MGIMS WARDHA Solved Paper-2007

  • question_answer
    In an L-R circuit, the current increases to three fourth of its maximum value in 4 s, then the S- time constant of the circuit is

    A) \[2{{\log }_{e}}\,2\]                                       

    B) \[\frac{4}{{{\log }_{e}}2}\]

    C) \[\frac{2}{{{\log }_{e}}2}\]                          

    D) \[\frac{{{\log }_{e\,}}2}{2}\]

    Correct Answer: C

    Solution :

    \[I={{I}_{0}}(1-{{e}^{-t/T}})\] where T = time constant \[\therefore \]  \[\frac{3}{4}{{I}_{0}}={{I}_{0}}(1-{{e}^{-4/T}})\] \[\Rightarrow \]               \[1-{{e}^{-4/T}}=\frac{3}{4}\] \[\Rightarrow \]               \[{{e}^{-4/T}}=\frac{1}{4}\] \[\Rightarrow \]               \[{{e}^{4/T}}=4\] \[\Rightarrow \]               \[\frac{4}{T}={{\log }_{e}}4\] \[\therefore \]  \[T=\frac{4}{{{\log }_{e}}4}=\frac{2}{{{\log }_{e}}2}\]


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