MGIMS WARDHA MGIMS WARDHA Solved Paper-2011

  • question_answer
    In L-R circuit the current increases to three- fourth of its maximum value in 4s, then the 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 \[\frac{3}{4}{{I}_{0}}={{I}_{0}}(1-{{e}^{-4/T}})\] \[1-{{e}^{-4/T}}=\frac{3}{4}\] \[{{e}^{-4/T}}=\frac{1}{4}\] \[{{e}^{4/T}}=4\] \[\frac{4}{T}={{\log }_{e}}4\] \[T=\frac{4}{{{\log }_{e}}4}=\frac{2}{{{\log }_{e}}2}\]  


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