JEE Main & Advanced JEE Main Paper Phase-I (Held on 08-1-2020 Evening)

  • question_answer
    An electron (mass m) with initial velocity \[\vec{v}={{v}_{0}}\,\hat{i}+{{v}_{0}}\,\hat{j}\] is in an electric field \[\vec{E}=-{{E}_{0}}\hat{k}\]. If \[{{\lambda }_{0}}\] is initial de-Broglie wavelength of electron, its de-Broglie wavelength at time t is given by  [JEE MAIN Held on 08-01-2020 Evening]

    A) \[\frac{{{\lambda }_{0}}}{\sqrt{1+\frac{{{e}^{2}}E_{0}^{2}{{t}^{2}}}{{{m}^{2}}v_{0}^{2}}}}\]          

    B) \[\frac{{{\lambda }_{0}}\sqrt{2}}{\sqrt{1+\frac{{{e}^{2}}{{E}^{2}}{{t}^{2}}}{{{m}^{2}}v_{0}^{2}}}}\]

    C) \[\frac{{{\lambda }_{0}}}{\sqrt{1+\frac{{{e}^{2}}{{E}^{2}}{{t}^{2}}}{2{{m}^{2}}v_{0}^{2}}}}\]

    D) \[\frac{{{\lambda }_{0}}}{\sqrt{2+\frac{{{e}^{2}}{{E}^{2}}{{t}^{2}}}{{{m}^{2}}v_{0}^{2}}}}\]

    Correct Answer: C

    Solution :

    \[v={{v}_{0}}\,\hat{i}+{{v}_{0}}\hat{j}+\frac{e{{E}_{0}}}{m}t\,\,\hat{k}\] \[\therefore \,\,\,\,\,\,\,\left| {\vec{v}} \right|=\sqrt{2v_{0}^{2}+\left( \frac{e{{E}_{0}}t}{m} \right)}\] Initially, \[{{\lambda }_{0}}=\frac{h}{m{{v}_{0}}\sqrt{2}}\] Now, \[\lambda =\frac{h}{m\sqrt{2v_{0}^{2}+\left( \frac{e{{E}_{0}}t}{m} \right)}}\] \[\frac{\lambda }{{{\lambda }_{0}}}=\frac{h}{m\sqrt{1+{{\left( \frac{e{{E}_{0}}t}{\sqrt{2}m{{v}_{0}}} \right)}^{2}}}}\] \[\lambda =\frac{{{h}_{0}}}{\sqrt{1+\frac{{{e}^{2}}E_{0}^{2}{{t}^{2}}}{2{{m}^{2}}v_{0}^{2}}}}\]


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