JCECE Engineering JCECE Engineering Solved Paper-2009

  • question_answer
    Activity of a radioactive sample decreases to \[{{(1/3)}^{rd}}\] of its original value in \[3\] days. Then, in \[9\] days its activity will become

    A) \[(1/27)\] of the original value

    B) \[(1/9)\] of the original value

    C) \[(1/18)\] of the original value

    D) \[(1/3)\]of the original value

    Correct Answer: A

    Solution :

    \[R={{R}_{0}}{{e}^{-\lambda t}}\] \[\Rightarrow \]               \[\left( \frac{1}{3} \right)={{e}^{-\lambda \times 3}}={{e}^{-3\lambda }}\]                            ? (i) Again,   \[\frac{R'}{{{R}_{0}}}={{e}^{-\lambda \times 9}}={{e}^{-9\lambda }}={{({{e}^{-3\lambda }})}^{3}}\]                       \[={{\left( \frac{1}{3} \right)}^{3}}\]  [from Eq.(i)]                        \[=\frac{1}{27}\] \[\Rightarrow \]               \[R'=\frac{{{R}_{0}}}{27}\] Hence, in \[9\] days activity will become\[\left( \frac{1}{27} \right)\]of the original value.


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