KVPY Sample Paper KVPY Stream-SX Model Paper-10

  • question_answer
    A small particle of mass m moves in such a way that P.E.\[=-\frac{1}{2}mk{{r}^{2}}\], where k is a constant and r is the distance of the particle from origin. Assuming Bohr's model of quantization of Angular momentum and circular orbit, r is directly proportional to:

    A) \[{{n}^{2}}\]                          

    B) n

    C) \[\sqrt{n}\]                                

    D) None of these

    Correct Answer: C

    Solution :

    relation between potential energy and kinetic energy of electron,
    \[\operatorname{P}.E=-2K.E\]
    \[-\frac{1}{2}\times P.E.=K.E\]
    \[-\frac{1}{2}\left( -\frac{1}{2}{{\operatorname{mkr}}^{2}} \right)=\frac{1}{2}{{\operatorname{mv}}^{2}},\]
      \[\frac{1}{2}{{\operatorname{kr}}^{2}}={{\operatorname{v}}^{2}}..(i)\]
    As we know,\[\operatorname{mvr}=\frac{\operatorname{nh}}{2\pi }\]
    \[\therefore \,{{\operatorname{v}}^{2}}=\frac{{{\operatorname{n}}^{2}}{{h}^{2}}}{4{{\pi }^{2}}{{\operatorname{m}}^{2}}{{r}^{2}}}\]
    Put this value in equation (i);
    \[{{\operatorname{r}}^{4}}=\frac{{{n}^{2}}{{h}^{2}}}{2{{\pi }^{2}}{{\operatorname{m}}^{2}}K}\]
    \[\operatorname{or}\,r\propto \sqrt{\operatorname{n}}\]


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