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Why did they use these formulas for energy of electron.

  1. Nov 29, 2009 #1
    1. The problem statement, all variables and given/known data
    An electron microscope employs a beam of electrons to obtain an image of an object. What energy must be imparted to each electron of the beam to obtain a wavelength of 10.0pm?



    2. Relevant equations
    1. E = hv=hc/lambda
    2. lambda = h/(mv)
    3. KE = .5mv^2



    3. The attempt at a solution
    In this problem I thought you could use the first equation to find the energy with the specific wavelength of 10pm. However, the workbook goes through equations 2 and 3 to first find the velocity and then find the kinetic energy. The answers come out very different, so I must be thinking wrong. Can anyone help me understand this?
    Since I am solving for energy, why do I need to go through equations 2 and 3?
     
  2. jcsd
  3. Nov 29, 2009 #2

    Doc Al

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    Staff: Mentor

    This only applies to photons.
    For an electron, you'll need these two equations.
     
  4. Nov 29, 2009 #3
    So whenever they say "electrons" I need to use them? That makes sense, I figured the electrons were being emitted as photons so I thought of them as photons.

    Thank you for your help, I see why I got the wrong answer on a couple of other problems now too:smile:
     
  5. Nov 29, 2009 #4

    Doc Al

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    Staff: Mentor

    For any kind of massive particle. (A photon has no mass.)
    Yikes, don't do that. :wink:
     
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