BHU PMT BHU PMT (Screening) Solved Paper-2006

  • question_answer
    The de-Broglie wavelength of an electron, \[\alpha -\]particle and a proton all having the same kinetic energy is respectively given as\[{{\lambda }_{e}},{{\lambda }_{\alpha }}\] and\[{{\lambda }_{p}}\]. Then which of the following is not true?

    A)  \[{{\lambda }_{e}}<{{\lambda }_{p}}\]                 

    B) \[{{\lambda }_{p}}<{{\lambda }_{\alpha }}\]

    C)  \[{{\lambda }_{e}}<{{\lambda }_{\alpha }}\]                      

    D) \[{{\lambda }_{\alpha }}<{{\lambda }_{p}}>{{\lambda }_{e}}\]

    Correct Answer: A

    Solution :

                     de-Broglie postulated that the wavelength associated with a moving material particle is in analogy with light, given by \[\lambda =\frac{h}{p}\] where p is the momentum of the particle. A particle of (reladvistic) mass m and moving with a velocity v has a momentum mv. Therefore,                                         \[\lambda =\frac{h}{mv}\]                           ?? (i) Or           \[\lambda \propto \frac{1}{m}\] Therefore,\[{{\lambda }_{e}}\propto \frac{1}{{{m}_{e}}},{{\lambda }_{\alpha }}\propto \frac{1}{{{m}_{\alpha }}}\]and\[{{\lambda }_{p}}\propto \frac{1}{{{m}_{p}}}\] As we know that                 \[{{m}_{e}}<{{m}_{p}}<{{m}_{\alpha }}\] \[\therefore \]  \[{{\lambda }_{e}}<{{\lambda }_{p}}<{{\lambda }_{\alpha }}\] Or           \[{{\lambda }_{e}}<{{\lambda }_{p}}\]or\[{{\lambda }_{p}}>{{\lambda }_{\alpha }}\]or\[{{\lambda }_{e}}>{{\lambda }_{p}}\] Note: The relation\[(i)\]connects wavelength (a characteristic of wave) with momentum (a characteristic of particle).


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