JEE Main & Advanced Physics Atomic Physics Question Bank Self Evaluation Test - Atoms

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

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

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

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

    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 \[I=\frac{1}{2}\left( {{m}_{1}}r_{1}^{2}+{{m}_{2}}r_{2}^{2} \right)\] Thus, \[E=\frac{1}{2}\left( {{m}_{1}}r_{1}^{2}+{{m}_{2}}r_{2}^{2} \right){{\omega }^{2}}\]                      ?.(i) \[E=\frac{1}{2}\left( {{m}_{1}}r_{1}^{2}+{{m}_{2}}r_{2}^{2} \right)\frac{{{L}^{2}}}{{{I}^{2}}}\]                          ?(ii) \[L=n\,\hbar \] (According Bohr's Hypothesis) \[E=\frac{1}{2}\left( {{m}_{1}}r_{1}^{2}+{{m}_{2}}r_{2}^{2} \right)\frac{{{L}^{2}}}{{{\left( {{m}_{1}}r_{1}^{2}+{{m}_{2}}r_{2}^{2} \right)}^{2}}}\] \[E=\frac{1}{2}\frac{{{L}^{2}}}{\left( {{m}_{1}}r_{1}^{2}+{{m}_{2}}r_{2}^{2} \right)}=\frac{{{n}^{2}}{{h}^{2}}}{8{{\pi }^{2}}{{r}^{2}}{{m}_{1}}m{{ & }_{2}}}\]\[E=\frac{1}{2}\frac{{{L}^{2}}}{\left( {{m}_{1}}r_{1}^{2}+{{m}_{2}}r_{2}^{2} \right)}=\frac{{{n}^{2}}{{h}^{2}}}{8{{\pi }^{2}}{{r}^{2}}{{m}_{1}}m{{ & }_{2}}}\] \[E=\frac{\left( {{m}_{1}}+{{m}_{2}} \right){{n}^{2}}{{h}^{2}}}{8{{\pi }^{2}}{{r}^{2}}m{{ & }_{1}}{{m}_{2}}}\]


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