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13. Nuclei — Physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 SciencePhysics13. Nuclei1 Mark
MCQ
Two radioactive materials $A$ and $B$ have decay constants $25 \lambda$ and $16 \lambda$ respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of $B$ to that of $A$ will be "$e$" after a time $\frac{1}{a \lambda}$. The value of $a$ is $......$
  • ✓
    $9$
  • B
    $8$
  • C
    $5$
  • D
    $6$
✓

Answer

Correct option: A.
$9$
a
$N = N _{0} e ^{-\lambda t }$

$\frac{ N _{ B }}{ N _{ A }}=\frac{ e ^{-\lambda_{2} t }}{ e ^{-\lambda_{1} t }}= e ^{-\lambda_{2} t } \cdot e ^{\lambda_{1} t }$

$e ^{1}= e ^{\left(\lambda_{1}-\lambda_{2}\right) t }$

$\left(\lambda_{1}-\lambda_{2}\right) t =1$

$t =\frac{1}{\lambda_{1}-\lambda_{2}}=\frac{1}{25 \lambda-16 \lambda}=\frac{1}{9 \lambda}$

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