KVPY Sample Paper KVPY Stream-SX Model Paper-31

  • question_answer
    A radioactive substance is being produced at a constant rate of 200 nuclei/s. The decay constant of the substance is\[{{1}^{s-1}}\]. After what time the number of radioactive nuclei will become 100. Initially there are no nuclei present

    A) 1s        

    B) \[\frac{1}{ln\left( 2 \right)}s\]

    C) \[ln\,(2)\,s\]

    D) 2 s

    Correct Answer: C

    Solution :

    Let N be the number of nuclei at any time t. Then \[\frac{dN}{dt}=200-\lambda N;\]
    \[\therefore \,\,\,\,\,\int\limits_{0}^{N}{\frac{dN}{200-\lambda N}=\int\limits_{0}^{t}{dt}}\] or \[N=\frac{200}{\lambda }\left( 1-{{e}^{-\lambda t}} \right)\]
    Given that \[N=100\,\,\,\] and \[\lambda =1{{s}^{-1}}\]
    \[\therefore \,\,\,\,100=200\left( 1-{{e}^{-t}} \right)\]or \[{{e}^{-t}}=\left( \frac{1}{2} \right)\]
    \[\therefore \,\,\,\,t=1n\left( 2 \right)\sec \]


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