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Homework Help: Speed with a de Broglie wavelength.

  1. May 6, 2008 #1
    1. At what speed is an electron's de Broglie wavelength:
    (a) 1.0 pm
    (b) 1.0 nm
    (c) 1.0 [tex]\mu[/tex]m
    (d) 1.0 mm

    2. [tex]\lambda[/tex] = [tex]\frac{h}{mv}[/tex]

    3. I have solved for v, and I plugged in values, it gives me, for a = 4.54 x 10^(27) m/s, the ANSWER is: 2.77 x 10^(8) ms, it's way off, I tried converting energy into Js it also did not work, what am I overlooking?
  2. jcsd
  3. May 6, 2008 #2
    Is it possible that my teacher is wrong? I think she copied down her answer wrong on my paper...
  4. May 6, 2008 #3
    I am not sure on this one, should you perhaps take into account the relativistic speed?
    [tex]\lambda = \frac{h}{m \gamma v}[/tex]

    For a I get:
    [tex]v = \frac{h}{m \lambda} = \frac{6.63 \times 10^{-34}}{(9.109 \times 10^{-31})(1\times 10^{-12})} = 7.28 \times 10^8 \text{m/s}[/tex]
    This is higher than the speed of light and can therefore never be correct.

    Your answer of 4.54 x 10^27 m/s is ridiculously high. The answer can NEVER be more than the speed of light which is approximately 3.0 x 10^8 m/s.

    Using 2.77 x 10^8 as the speed, you get a wavelength of 2.63 x 10^-12 m or 2.63 ps. Something wrong with the question maybe?
    Last edited: May 6, 2008
  5. May 7, 2008 #4


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    Homework Helper

    The formula with the relativistic momentum in Nick89's post will give the correct answer.
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