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KVPY Sample Paper KVPY Stream-SX Model Paper-13

  • question_answer
    A sample contains number of stable nuclei equal to\[{{N}_{s}}\]and number of unstable nuclei equal to\[{{N}_{u}}.\] After a time T the activity of the sample decreased to one third of the initial activity, while the total number of nuclei (excluding decayed nuclei) became half. The ratio\[{{N}_{s}}/{{N}_{u}}\] initially is -

    A) 1/3

    B) 1/4  

    C) 3

    D) 2

    Correct Answer: A

    Solution :

    Let N do not the total no of nuclei. Initially \[N={{N}_{u}}+{{N}_{S}}.\] After a time T since the activity decreased to one third, number of unstable nuclei will also become one third, So after a time \[T,\] \[N/2={{N}_{u}}/3+{{N}_{S}}.\] Solving the two equation we get \[{{N}_{u}}/{{N}_{S}}=1/3.\]


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