JEE Main & Advanced Physics Nuclear Physics And Radioactivity Question Bank Radioactivity

  • question_answer
    The activity of a sample of a radioactive material is A, at time \[{{t}_{1}}\] and \[{{A}_{2}}\] at time \[{{t}_{2}}\] \[({{t}_{2}}>{{t}_{1}}).\] If  its mean life T, then                                                                  [BHU 2002]

    A)            \[{{A}_{1}}{{t}_{1}}={{A}_{2}}{{t}_{2}}\]                      

    B)            \[{{A}_{1}}-{{A}_{2}}={{t}_{2}}-{{t}_{1}}\]

    C)            \[{{A}_{2}}={{A}_{1}}{{e}^{({{t}_{1}}-{{t}_{2}})/T}}\]            

    D)            \[{{A}_{2}}={{A}_{1}}{{e}^{({{t}_{1}}/{{t}_{2}})T}}\]

    Correct Answer: C

    Solution :

                       \[A={{A}_{0}}{{e}^{-\lambda t}}={{A}_{0}}{{e}^{-t/\tau }};\] where \[\tau =\]mean life                    So \[\Rightarrow \Delta L=\frac{h}{2\pi }({{n}_{2}}-{{n}_{1}})\]Þ\[{{A}_{0}}=\frac{{{A}_{1}}}{{{e}^{-{{t}_{1}}/T}}}={{A}_{1}}{{e}^{{{t}_{1}}/T}}\]                    \[\therefore {{A}_{2}}={{A}_{0}}{{e}^{-t/T}}=({{A}_{1}}{{e}^{{{t}_{1}}/T}})\,{{e}^{-{{t}_{2}}/T}}\Rightarrow {{A}_{2}}={{A}_{1}}{{e}^{({{t}_{1}}-{{t}_{2}})/T}}\]


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