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Wavelenght associated with electron

  1. Jan 2, 2012 #1
    1.given mass of electron = 9.11*10^-31 ... kinetic energy=1mega electron volt

    2. Relevant equations:: k.e=1/2mv^2 and λ=h/mv

    3. The attempt at a solution :: am unable to get it....can any one ry...
  2. jcsd
  3. Jan 2, 2012 #2

    You have KE and also expression for it
    So you can find v?
    and then mv

    PS: for energy conversion use: 1eV = e J (e=1.6*10-19)
  4. Jan 2, 2012 #3


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    Big caution here: if you use classical mechanics to compute the velocity of the electron, you get v > c. This is not correct.

    I'm afraid that here, you have to use the relativistic formulae ([itex]\Delta{E} = \Delta{m}c^2[/itex] and [itex]m = \frac{m_0}{\sqrt{1 - \frac{v^2}{c^2}}}[/itex]) to get the answer. You can't use [itex]E_k = \frac{1}{2}mv^2[/itex].
    Last edited: Jan 2, 2012
  5. Jan 2, 2012 #4
    Nice catch, i missed that. But its use depends on the level of question ... in basic physics, relativity eqns are usually not used ... Let the OP decide weather he is supposed to use them or not
  6. Jan 2, 2012 #5


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    I suppose, but frankly, if the student is expected to understand and apply the de Broglie equation, he/she should be expected to discern when SR should be used rather than Classical Mechanics.
  7. Jan 3, 2012 #6


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    Usually true, unless the class is learning introductory relativity.

    If this is to be solved without relativity, it is the responsibility of the teacher or professor to keep energies and speeds in the nonrelativistic regime. This teacher didn't do that, so the correct way to solve this is to consider relativity.
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