NEET Sample Paper NEET Sample Test Paper-75

  • question_answer
    The activity of a radioactive substance decreases to one-third of the original activity number \[{{N}_{0}}\] in a period of 9 yr. After a further lapse of 9 yr, its activity will be

    A)  \[\frac{3}{4}{{N}_{0}}\]                     

    B)  \[\frac{{{N}_{0}}}{9}\]

    C)  \[{{N}_{0}}\]                          

    D)  None of these

    Correct Answer: B

    Solution :

    As,        \[N={{N}_{0}}{{e}^{-\lambda t}}\] \[\frac{{{N}_{0}}}{3}={{N}_{0}}{{e}^{-\,9\lambda }}\] \[\Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{1}{3}={{e}^{-\,9\lambda }}\] Now, after further 9 y \[{N}'\,\,=\,\,{{N}_{0}}{{e}^{-\,18\lambda }}\] \[{N}'\,\,=\,\,{{N}_{0}}{{\left( {{e}^{-\,9\lambda }} \right)}^{2}}\] \[{N}'\,\,=\,\,{{N}_{0}}{{\left( \frac{1}{3} \right)}^{2}}\,\,=\,\,\frac{{{N}_{0}}}{9}\]


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