JEE Main & Advanced Physics Nuclear Physics And Radioactivity JEE PYQ-Nuclear Physics

  • question_answer
    A radioactive nucleus A with a half-life T, decays into a nucleus B. At t = 0, there is no nucleus B. At some time t, the ratio of the number of B to that of A is 0.3. Then, t is given by: [JEE Main 2017]

    A)

    B)

    C)

    D)

    Correct Answer: D

    Solution :

    [d] A time t
                \[\frac{{{N}_{B}}}{{{N}_{A}}}=.3\Rightarrow {{N}_{B}}=.3{{N}_{A}}\]
    also let initially there are total N0 number of nuclei
    \[{{N}_{A}}+{{N}_{B}}={{N}_{0}}\]
    \[{{N}_{A}}=\frac{{{N}_{0}}}{1.3}\]
    Also as we know
    \[{{N}_{A}}={{N}_{0}}{{e}^{-\lambda t}}\]
    \[\frac{{{N}_{0}}}{1.3}={{N}_{0}}{{e}^{-\lambda t}}\]
    \[\frac{1}{1.3}={{e}^{-\lambda t}}\ell n(1.3)=\lambda t\]or \[t=\frac{\ell n(1.3)}{\lambda }\]
    \[t=\frac{\ell n(1.3)}{\frac{\ell n(2)}{T}}=\frac{\ell n(1.3)}{\ell n(2)}T\]


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