KVPY Sample Paper KVPY Stream-SX Model Paper-20

  • question_answer
    If we apply potential difference so that an electron is accelerated continuously in a vaccum tube such that a decrease of 10% occurs in its de Broglie wave length, hi such a case the change observed in kinetic energy of electron will be approximately

    A) A decrease of \[11\text{ }%\]

    B) An increase of \[11.1%\]

    C) An increase of \[10%\]

    D) An increase of \[\text{23}\text{.4}%\]

    Correct Answer: D

    Solution :

    using the relation, \[\lambda \propto \frac{1}{K.E.},\]
    If \[{{\lambda }_{1}}=\lambda \]and \[\operatorname{K}.E{{.}_{1}}=E\](initial)
    Then \[{{\lambda }_{2}}=0.9\lambda \]
    [\[10%\]Decrease from \[{{\lambda }_{1}},\]
    \[{{\lambda }_{2}}=\lambda -\frac{10}{100}\lambda \]\[=\lambda \left( 1-0.1 \right)=0.9\lambda ]\]
    \[\therefore \]\[\operatorname{K}.E{{.}_{2}}=\frac{\lambda _{1}^{2}}{\lambda _{2}^{2}}\times \operatorname{K}.E{{.}_{1}}=\frac{{{\lambda }^{2}}}{{{\left( .9\lambda  \right)}^{2}}}\times \operatorname{K}.E{{.}_{1}}\]\[=\frac{{{\lambda }^{2}}\times 100}{81{{\lambda }^{2}}}\times \operatorname{E}\]
    \[=\frac{100}{81}\operatorname{E}\]   [more than\[\operatorname{K}.E{{.}_{1}}i.e.,\] increase]
    Now increase in \[K.E.=K.E{{.}_{2}}-K.E{{.}_{1}}\]\[a=\frac{100}{81}E-E\]
    Or \[%\]increase of \[\operatorname{K}.E.=\frac{\frac{100}{81}E-E}{E}\times 100\]            \[=\frac{19}{81}\times 100=23.4%\]


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