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Homework Help: A protons Min KE

  1. Mar 28, 2007 #1
    1. The problem statement, all variables and given/known data

    A proton is confined in a uranium nucleus of radius 7.41 fm. Determine the proton's minimum kinetic energy K less than or equal to delta K according to the uncertainty principle if the proton is well approximated by a Gaussian wave packet confined by the nuclear diameter.

    2. Relevant equations

    change in momentum * change in x is greater than or equal to h(bar)/2
    I don't know

    3. The attempt at a solution

    I found the minimum change in momentum but can't seem to relate that to the minimum KE. I tried change KE= change in p*c but that was wrong.
    since it is confined by the diameter do I times the radius by 2? Thats how I did it.

    dx=14.82E-15 m
    5.273E-35/14.82E-15= 3.54E-21
    3.54E-21*3E8= 1.0674E-12 J

    But this is wrong and I don't have a clue as to why. Can someone help me out please?
  2. jcsd
  3. Mar 28, 2007 #2


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    First of all, try to put units on ALL of your quantities, ok? Otherwise, people can only guess what '3.54E-21' might be supposed to stand for. Having said that, it looks like you got delta(p) ok. But I am completely unable to guess what the '3E8' in the last line might be. How do you go from momentum to energy?
  4. Mar 28, 2007 #3


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    Oh, I get it. You really are using p*c=E!!! That only holds for massless particles. The proton is hardly massless.
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