J & K CET Engineering J and K - CET Engineering Solved Paper-2007

  • question_answer
    If a is radius of first Bohr orbit in hydrogen atom, the radius of the third orbit is

    A)  \[3a\]           

    B)  \[9a\]

    C)  \[27a\]          

    D)  \[81a\]

    Correct Answer: B

    Solution :

    Radius of Bohr orbit is given by \[{{r}_{n}}=\left( \frac{{{\varepsilon }_{0}}{{h}^{2}}}{\pi m{{e}^{2}}} \right){{n}^{2}}\] The quantities in the bracket are constant. \[\therefore \] \[{{r}_{n}}\,\propto \,{{n}^{2}}\] The expression gives the radius of nth Bohr orbit \[\therefore \] \[\frac{{{r}_{1}}}{{{r}_{2}}}=\frac{n_{1}^{2}}{n_{2}^{2}}\] \[\Rightarrow \] \[\frac{a}{{{r}_{2}}}=\frac{1}{{{3}^{2}}}\] \[{{r}_{2}}=9a\]


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