JEE Main & Advanced JEE Main Paper (Held On 16 April 2018)

  • question_answer
    At some instant, a radioactive sample\[{{S}_{1}}\] having an activity\[5\mu Ci\] has twice the number of nuclei as another sample\[{{S}_{2}}\]which has an activity of \[10\mu Ci.\]The half-lives \[{{S}_{1}}\]of and \[{{S}_{2}}\]are        [JEE Main 16-4-2018]

    A)  10 years and 20 years, respectively

    B)  5 years and 20 years, respectively

    C)  20 years and 10 years, respectively

    D)  20 years and 5 years, respectively

    Correct Answer: B

    Solution :

     For sample 1 we can write,                 \[{{N}_{1}}={{N}_{o1}}{{e}^{-{{\lambda }_{1}}t}}\]                 \[{{N}_{2}}={{N}_{o2}}{{e}^{-{{\lambda }_{2}}t}}\] Given that at a particular time t\[{{N}_{1}}=2{{N}_{2}}\] Also given that For sample 1, \[{{A}_{1}}=-\frac{d{{N}_{1}}}{dt}={{N}_{o1}}{{\lambda }_{1}}{{e}^{-{{\lambda }_{1}}t}}=5\mu {{C}_{i}}\] \[{{A}_{2}}=-\frac{d{{N}_{2}}}{dt}={{N}_{o2}}{{\lambda }_{2}}{{e}^{-{{\lambda }_{2}}t}}=10\mu {{C}_{i}}\] Solving above equations we get, \[\frac{{{\lambda }_{1}}}{{{\lambda }_{2}}}=\frac{1}{4}\] \[\frac{half\,life\,{{e}_{1}}}{half\,life\,{{e}_{2}}}=\frac{\ln 2}{{{\lambda }_{1}}}\times \frac{{{\lambda }_{2}}}{\ln 2}=4\]


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