JEE Main & Advanced JEE Main Paper (Held On 09-Jan-2019 Evening)

  • question_answer
    At a given instant, say\[t=0\], two radioactive substances A and B have equal activities. The ratio \[\frac{{{R}_{B}}}{{{R}_{A}}}\] of their activities after time t itself decays with time t as \[{{e}^{-3t}}\]. If the half-life of A is ln2, the half-life of B is- [JEE Main Online Paper (Held On 09-Jan-2019 Evening]

    A) \[\frac{In\,2}{4}\]          

    B) \[4\ln 2\]

    C) 2ln2                 

    D)               \[\frac{In\,2}{2}\]

    Correct Answer: A

    Solution :

    \[\,\because \,\,\,\,\,A\,\,=\,\,\lambda N\] \[\frac{{{R}_{B}}}{{{R}_{A}}}=\frac{{{A}_{0}}{{e}^{-{{\lambda }_{B}}t}}}{{{A}_{0}}{{e}^{-{{\lambda }_{A}}t}}}={{e}^{-3t}}\]  \[{{e}^{-({{\lambda }_{B}}-{{\lambda }_{A}})t}}={{e}^{-3t}}\] \[\Rightarrow {{\lambda }_{B}}-{{\lambda }_{A}}=3\] \[\because {{\lambda }_{A}}=1\] \[\Rightarrow {{\lambda }_{B}}=4\] \[{{({{T}_{1/2}})}_{A}}\,=\,\,\ln 2\,\,\,\,=\,\,\,\frac{\ln 2}{{{\lambda }_{A}}}\] \[\Rightarrow {{({{T}_{1/2}})}_{B}}=\frac{\ln 2}{{{\lambda }_{B}}}=\frac{\ln 2}{4}\]                          


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