KVPY Sample Paper KVPY Stream-SX Model Paper-4

  • question_answer
    A diatomic molecule is made of two masses \[{{m}_{1}}\] and \[{{m}_{2}}\] which are separated by a distance r. If we calculate its rotational energy by applying Bohr's rule of angular momentum quantization, Its energy will be given by: (\[n\] is an integer)

    A) \[\frac{{{({{m}_{1}}+{{m}_{2}})}^{2}}{{n}^{2}}{{h}^{2}}}{2m_{1}^{2}m_{2}^{2}{{r}^{2}}}\]       

    B) \[\frac{{{n}^{2}}{{h}^{2}}}{2({{m}_{1}}+{{m}_{2}}){{r}^{2}}}\]

    C) \[\frac{2{{n}^{2}}{{h}^{2}}}{({{m}_{1}}+{{m}_{2}}){{r}^{2}}}\]                       

    D) \[\frac{({{m}_{1}}+{{m}_{2}}){{n}^{2}}{{h}^{2}}}{2{{m}_{1}}{{m}_{2}}{{r}^{2}}}\]

    Correct Answer: D

    Solution :

    [d]
    The energy of the system of two atoms of diatomic molecule, \[E=\frac{1}{2}I{{\omega }^{2}}\]
    Where \[I=\]moment of inertia, \[\omega =\]Angular velocity\[=\frac{L}{I},\] \[L=\]Angular momentum
    \[I=\frac{1}{2}({{m}_{1}}{{r}_{1}}^{2}+{{m}^{2}}r_{2}^{2})\]
    Thus \[E=\frac{1}{2}({{m}_{1}}{{r}_{1}}^{2}+{{m}_{2}}r_{2}^{2}){{\omega }^{2}}\]
    \[E=\frac{1}{2}({{m}_{1}}{{r}_{1}}^{2}+{{m}_{2}}r_{2}^{2})\frac{{{L}^{2}}}{{{I}^{2}}}\]
    \[L=n\frac{nh}{2n}\](According Bohr?s Hypothesis)
    \[E=\frac{1}{2}({{m}_{1}}{{r}_{1}}^{2}+{{m}_{2}}{{r}_{2}}^{2})\frac{{{L}^{2}}}{{{({{m}_{1}}{{r}_{1}}^{2}+{{m}_{2}}r_{2}^{2})}^{2}}}\]
    \[E=\frac{1}{2}\frac{{{L}^{2}}}{({{m}_{1}}{{r}_{1}}^{2}+{{m}_{2}}{{r}_{2}}^{2})}=\frac{{{n}^{2}}{{h}^{2}}}{8{{\pi }^{2}}{{({{m}_{1}}{{r}_{1}}^{2}+{{m}_{2}}r_{2}^{2})}^{2}}}\]
    \[E=\frac{({{m}_{1}}+{{m}_{2}}){{n}^{2}}{{h}^{2}}}{8{{\pi }^{2}}{{r}^{2}},{{m}_{1}}{{m}_{2}}}\Rightarrow \frac{({{m}_{1}}+{{m}_{2}}){{n}^{2}}{{h}^{2}}}{2{{m}_{1}}{{m}_{2}}{{r}^{2}}}\]
    \[\left[ \because {{r}_{1}}=\frac{{{m}_{2}}r}{{{m}_{1}}+{{m}_{2}}};{{r}_{2}}=\frac{{{m}_{2}}r}{{{m}_{1}}+{{m}_{2}}} \right]\]


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