JEE Main & Advanced JEE Main Paper (Held On 12-Jan-2019 Morning)

  • question_answer
    A particle of mass m moves in a circular orbit in a central potential field \[U(r)=\frac{1}{2}k{{r}^{2}}.\]If Bohr's quantization conditions are applied, radii of possible orbitals and energy levels vary with quantum number n as [JEE Main Online Paper Held On 12-Jan-2019 Morning]

    A) \[{{r}_{n}}\propto {{n}^{2}},{{E}_{n}}\propto \frac{1}{{{n}^{2}}}\]            

    B) \[{{r}_{n}}\propto \sqrt{n},{{E}_{n}}\propto n\]

    C) \[{{r}_{n}}\propto \sqrt{n},{{E}_{n}}\propto \frac{1}{n}\]         

    D)   \[{{r}_{n}}\propto n,{{E}_{n}}\propto n\]

    Correct Answer: B

    Solution :

    Force due to this field \[F=-\frac{\partial U}{\partial r}\] \[F=\frac{-\partial }{\partial r}\left( \frac{1}{2}k{{r}^{2}} \right)=-kr\] For circular orbit, \[\frac{m{{v}^{2}}}{r}=-kr\Rightarrow v\propto r\]                    ...(i) Also, by Bohr?s quantization condition\[mvr=\frac{nh}{2\pi }\]        ?(ii) For eqn. (i) and (ii),\[{{r}_{n}}\propto {{n}^{1/2}}\] \[U(r)=\frac{1}{2}k{{r}^{2}}\Rightarrow {{E}_{n}}=-\frac{1}{2}U(r)=-\frac{1}{4}k{{r}^{2}}\]\[\Rightarrow {{E}_{n}}\propto n\]


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