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Homework Help: Photoelectric Effect: Which Answer is Correct?

  1. Jun 10, 2009 #1
    1. The problem statement, all variables and given/known data

    A photon of wavelength 250 nm ejects an electron from a metal. The ejected electron has a de Broglie wavelength of 0.85 nm.

    (a) Calculate the kinetic energy of the electron.

    2. Relevant equations

    KE = .5mv^2

    3. The attempt at a solution

    Which answer is correct?

    ANSWER1: E = .5mv^2 and v = h/m[tex]\lambda[/tex]
    Therefore, E = .5m(h/[tex]\lambda[/tex])^2

    Substituting values (including [tex]\lambda[/tex] = .85*10^(-9)), you get 3.34*10^(-19) J

    ANSWER2: wavelength=h/p=h/rad(2mK)
    K=6.87e-4 Joules
  2. jcsd
  3. Jun 10, 2009 #2


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    Homework Helper

    The first one is correct, I think. But the second method will also give you the correct answer if you do it properly - you just made a couple of little mistakes (one algebraic, one plugging in the wrong number).
  4. Jun 10, 2009 #3
    Ah! Thank you very much!
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