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Homework Help: Quantum physics problem

  1. Jun 15, 2009 #1
    1. The problem statement, all variables and given/known data

    This is a problem I did during a test, and I am looking it over to see what I did wrong..

    1. An Electron is accelerated by a potential difference of 75V. What is the de Broglie wavelength

    2. Relevant equations

    P = h/λ
    Ek = 1/2mv^2
    eV = 1.6E-19

    3. The attempt at a solution

    P = h/λ
    λ = h/p (1)

    Ek = 1/2mv^2
    (2Ek/m)^1/2 (2)

    Sub (1) into (2)
    λ = h/p
    λ = h/m((2Ek/m)^1/2)

    I figured 75V = 75*eV = 75*(1.6E-19) = 1.2E-17J
    So 1.2E-17J = Ek

    I Plugged in all the known values into
    λ = h/m((2Ek/m)^1/2)
    and got the answer as: 1.4E21

    Any clues as to what I did wrong? I assume it has something to do with my volt conversion, any help would be greatly appreciated.
  2. jcsd
  3. Jun 15, 2009 #2
    I think its just a math error. The energy is fine.

    Mass of a electron is 9.10938188*10^-31.

    Try putting it into your calculator again.
  4. Jun 15, 2009 #3


    User Avatar
    Homework Helper

    There's nothing wrong with your equations. You probably just calculated the numbers incorrectly. Try again, and you should get 1.4e-10 m.
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