# NEET Physics Nuclei Questions Solved

NEET - 2017

Radioactive material A has decay constant 8$\lambda$ and material B has decay constant $\lambda$ Initially, they have same number of nuclei. After what time, the ratio of number of nuclei of material B to that A will be $\frac{1}{e}$?

(a) $\frac{1}{\lambda }$

(b) $\frac{1}{7\lambda }$

(c) $\frac{1}{8\lambda }$

(d)$\frac{1}{9\lambda }$

(b) Let initial number of nuclei in A and B is ${N}_{O}$

Number of nuclei of A after time t is

${N}_{A}={N}_{0{e}^{-8\lambda t}}$                          ...(i)

Similarly, number of nuclei of A after time t is

${N}_{B}={N}_{0}{e}^{-\lambda t}$                          ...(ii)

It is given that

Now,from Eqs.(i)  and (ii)

$\frac{{e}^{-B\lambda t}}{{e}^{-\lambda t}}=\frac{1}{e}$

Rearranging

Difficulty Level:

• 12%
• 66%
• 17%
• 7%
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