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

  • question_answer
    Half-life of a radioactive substance is 20 minutes. Difference between points of time when it is 33% disintegrated and 67% disintegrated is approximately         [AIIMS 2000]                                    

    A)            10 min                                     

    B)            20 min

    C)            30 min                                     

    D)            40 min

    Correct Answer: B

    Solution :

               \[\lambda =\frac{0.693}{{{T}_{1/2}}}=\frac{0.693}{20}=0.03465\]                    Now time of decay \[t=\frac{2.303}{\lambda }\log \frac{{{N}_{0}}}{N}\]                    \[\Rightarrow {{t}_{1}}=\frac{2.303}{0.03465}\log \frac{100}{67}=11.6\min \]min                    and \[{{t}_{2}}=\frac{2.303}{0.03465}\log \frac{100}{33}=32min\]                    Thus time difference between points of time                    = t1 ? t2 =32 ? 11.6 = 20.4 min \[\approx \] 20 min.


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