KVPY Sample Paper KVPY Stream-SX Model Paper-30

  • question_answer
    At t=0 the no of active nuclei in radioactive is number is \[{{N}_{0}}\] if decay constant is \[\lambda \] and rate of formation of isotope is K. Then number of active nuclei-

    A) first increases then decreasing

    B) Goes on increasing

    C) Goes on decreasing

    D) Is \[\frac{K}{\lambda }\] after long time

    Correct Answer: D

    Solution :

    \[\xrightarrow[K\,nulei/sec]{\text{Production}}\,\,A\,\,\xrightarrow[\lambda ]{Decay}\,\,B\]
    Rate of change of number nuclei of \[A=\frac{dN}{dt}\]
    = Rate of formation of A- Rate of decay of \[A=K-\lambda N\] where N is number of nuclei of A at time t
    \[\frac{dN}{dt}=K-\lambda N\Rightarrow \int\limits_{{{N}_{0}}}^{N}{\frac{dN}{K-\lambda N}}=\int\limits_{0}^{t}{dt}\]
    \[N=\frac{K}{\lambda }(1-{{e}^{-\,{{\lambda }_{t}}}})+{{N}_{0}}{{e}^{-\,{{\lambda }_{t}}}}\]
    After long time \[t=\infty \] number of nucleus will become \[\frac{K}{\lambda }\]


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