KVPY Sample Paper KVPY Stream-SX Model Paper-8

  • question_answer
    The electric potential between a proton and an Electron is given by \[V={{V}_{0}}\ell n\frac{r}{{{r}_{0}}}\] , where r,, is a '0 constant. Assuming Bohr's model to be Applicable, write variation of r with n, n being the principal quantum number

    A) \[{{r}_{n}}{{\alpha }_{n}}\]                            

    B) \[{{r}_{n}}\alpha 1/n\]

    C) \[{{r}_{n}}\alpha \,{{n}^{2}}\]                       

    D) \[{{r}_{n}}\alpha \,1/{{n}^{2}}\]

    Correct Answer: A

    Solution :

    electric field, \[E=-\frac{dV}{dr}\]\[=-\frac{d}{dr}\left[ {{V}_{0}}\ln \frac{r}{{{r}_{0}}} \right]\]\[=-\frac{{{V}_{0}}}{r}\]
    Centripetal force, \[E\,e=\frac{m{{v}^{2}}}{r}\]
    \[\operatorname{or}\frac{{{V}_{0}}}{r}e\,\,\,=\frac{m{{v}^{2}}}{r}\]
    \[\operatorname{or}\,\,\,\,\,\,\,\,\,v\,\,\,\,=\,\,\,cr{}^\circ \]
    Now \[mvr=\frac{nh}{2\pi }\]
    \[\therefore \,\,\,\,\,\,\,r\,\,\,\,\,\,\,=\frac{nh}{2\pi mv}\]
    Or \[\,\,\,\,r\,\,\,\,\,\,\,\propto \,\,\,\,\,\,\,n\]


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