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Homework Help: De Broglie wavelength of an electron and speed

  1. Jul 22, 2008 #1
    1. The problem statement, all variables and given/known data
    An electron has a de Broglie wavelength of λ = 6.4e-10m.
    What is its speed? What voltage was needed to accelerate to this speed?

    2. Relevant equations

    KE = 1/2mv^2

    3. The attempt at a solution
    I already determined that the speed is 1.136e6 m/s. Finding the voltage is what has me stumped. Using KE = 1/2mv^2, I found KE is 5.883e-19 J. I converted this to eV, so KE = 3.677 eV.

    With sig figs taken into account, the answer (I think) is supposed to be 3.7 eV but the online homework isn't accepting it. I feel like it shouldn't be hard, but I can't see what I did wrong. Any help would be appreciated!
  2. jcsd
  3. Jul 22, 2008 #2


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    Science Advisor
    Homework Helper

    I get pretty much the same thing you did. But did you really put 3.7 eV in? Because that's the kinetic energy. The question is asking for the voltage to accelerate it. That's just 3.7 V not eV. Did you put the wrong units in?
  4. Jul 22, 2008 #3
    Awesome. Thank you so much for your help!
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