KVPY Sample Paper KVPY Stream-SX Model Paper-20

  • question_answer
    A particle of mass m is confined to a narrow tube of length L. If \[L=0.5\text{ }nm,\] then possible energies of electrons are

    A) 1.5 eV, 6 eV, 13.5 eV......, etc.

    B) 2 eV, 4 eV, 6 eV ....... etc.

    C) 1.5 eV, 3 eV, 6 eV....... etc.

    D) 3eV, 5 eV, 7eV......, etc.

    Correct Answer: A

    Solution :

    de-Broglie waves will resonate with a node at each end of tube.
    \[\therefore \]\[L=\frac{{{\lambda }_{1}}}{2},\]\[{{\lambda }_{2}},\frac{3}{2}{{\lambda }_{3}}...\frac{n{{\lambda }_{n}}}{2},\]etc
    So, \[{{\lambda }_{n}},\frac{2L}{n},n=1,2,3...\] As, \[{{\lambda }_{n}}=\frac{h}{{{p}_{n}}}\]and \[{{(KE)}_{n}}=\frac{{{p}^{2}}}{2m}\]
    We have, \[{{(KE)}_{n}}=\frac{{{n}^{2}}{{h}^{2}}}{8{{L}^{2}}m},n=1,2,3...,\text{etc}\]
    With \[m=9.1\times {{10}^{-\,31}}\]and \[L=5\times {{10}^{-10}}\,m\]
    \[{{(KE)}_{n}}=2.4\times {{10}^{-\,19}}{{n}^{2}}J=1.5{{n}^{2}}eV\]
    \[\therefore \]\[KE=1.5eV,6eV,13.5eV,...,\text{etc}\]


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