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Homework Help: Energy of a proton using de broglie formula

  1. Aug 18, 2010 #1
    I am trying to help my son in his year 12 physics. I know very little of this
    can someone please assist.

    find in eV (electron volts) the energy of a proton with a de broglie wavelength of 40 pm (picometres)

    What would the correct formula be ?

    A good chance for me to learn as well ;)

  2. jcsd
  3. Aug 18, 2010 #2


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    Wave length λ = h/p or

    p = h/λ.

    And energy E = p^2/2m.
  4. Aug 18, 2010 #3
    Thanks but I am obviously still way off beam
    (my Sons gone to bed but I am trying this on my own)

    de brognie wave length = 4x10^-11
    plancks constant = 6.63x10^-34
    mass of proton = 1.673x10^-27

    first part

    p= h/λ
    .: p = (6.63x10^-34)/(4 x 10^-11)
    .: p = 1.6575x10^-23

    2nd part
    E = p^2/2m
    .: E = (1.6575x10^-23)^2/(2*1.673x10^-27)
    .: E = 2.747x10^-46/3.46x10^-27
    .: E = 8.2x10^-20

    The answer should be E =0.51eV

    So what did I do wrong :(

  5. Aug 18, 2010 #4


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    E = 8.2x10^-20 J.

    To convert it into eV, divide it by 1.6x10^-19.
  6. Aug 19, 2010 #5
    So from this I gather E = Energy in Joules?
  7. Aug 19, 2010 #6


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    Energy in eV = 8.2x10^-20/1.6x10^-19
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