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Finding KE when given de Broglie wavelength

  1. Apr 20, 2008 #1
    [SOLVED] Finding KE when given de Broglie wavelength

    1. The problem statement, all variables and given/known data
    What is the kinetic energy, in MeV, of a proton with a de Broglie wavelength of 10fm?
    1 MeV = 10^6 eV
    1 fm = 10^-15m
    1 eV = 1.602 x 10^-19 J
    h = 6.63 x 10^-34 Js
    wavelength = 10 x 10^-15 m = 10^-5 nm
    Mass proton = 1.673 x 10^-27 kg


    2. Relevant equations

    wavelength = h/mv
    KE = (1/2)(m)(v^2)


    3. The attempt at a solution

    I feel like I did this right, but I still got the incorrect answer and I can't seem to notice what I did wrong.

    v = (6.63 x 10^-34) / (10^-5)(1.673 x 10^-27) = .03962 m/s
    K = .5(1.673 x 10^-27)(.03862^2) = 1.316 x 10^-30 J = 8.2 x 10^-12 eV
    = 8.2 x 10^-18 MeV
     
  2. jcsd
  3. Apr 20, 2008 #2

    Hootenanny

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    Welcome to PF,
    Shouldn't that be 10*10-15? :wink:
     
  4. Apr 20, 2008 #3
    Thanks.



    I tried that the first time I did the problem, and realised 10 x 10^-15 is in meters. Wouldn't I need the wavelength to be in nm? So it would be 1 x 10^-5 nm.
     
    Last edited: Apr 20, 2008
  5. Apr 20, 2008 #4

    Hootenanny

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    Why would you want it in nm? If you want your output velocity to be in m/s, then all your inputs must be in SI units.
     
  6. Apr 20, 2008 #5
    Ah, you're right, silly me. I was looking at another example which was basically the same question, and to me it looked like they converted from angstroms to nanometers for wavelength. It was actually converted to meters.

    Thanks!
     
  7. May 14, 2011 #6
    Re: [SOLVED] Finding KE when given de Broglie wavelength

    Hey thanks both of u :) i made the same mistake :p
     
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