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Homework Help: Intro to Quantum Physics questions

  1. Jan 20, 2007 #1
    Hello everybody, I got two questions on my assignment that I am stuck on...it'd be great if you guys can give me some hints to get me in the right direction. :smile:

    1) Using the uncertainty principle, find the minimum value (in MeV) of the kinetic energy of a nucleon confined within a nucleus of radius [tex]R=5x10^-15 m.[/tex]

    2) Show that the electron orbits in the semi-classical Bohr model are not real. Do this by showing that any attempt to measure the orbit radius to an accuracy [tex] \Delta{x}<<R_{n+1}-R_{n}[/tex] is the radius of the electron in the hydrogen atom, would cause an uncertainty in the energy [tex]E_{n}[/tex] which is larger than the binding energy in that orbit. (Hint: This problem requires that you make suitable approximations).

    Of the two questions, I am more desperate for number 2 :cry:...I don't know where to begin!!
    Last edited: Jan 20, 2007
  2. jcsd
  3. Jan 20, 2007 #2


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    According to the posting guidelines, we can't help you unless you start.

    What do you know about the Uncertainty Principle?
  4. Jan 20, 2007 #3
    About the uncertainty principle...I just know the basics...that if [tex]\Delta{x}[/tex] and [tex]\Delta{p}[/tex] are the uncertainties in position and momentum, respectively, then their product must be at least [tex]\hbar /2[/tex]. Do I set the radius equal to [tex]\Delta{X}[/tex]??
    Last edited: Jan 20, 2007
  5. Jan 20, 2007 #4
    Correct, you would set the radius to [tex]\Delta x[/tex].

    If you solved the uncertainty principle for the minimum momentum in this case, you could easily find the minimum kinetic energy.
  6. Jan 20, 2007 #5
    Thank you for the help. Here's what I got after setting the radius equal to [tex]\Delta{x}[/tex]...please check my work. :)

    [tex]\Delta{p}\geq{\hbar /2\Delta{x}}[/tex]

    Then I used the minimum value of momentum and the mass of the nucleon to find the minimum kinetic energy...
    Last edited: Jan 20, 2007
  7. Jan 20, 2007 #6
    [tex]\hbar[/tex] should be [tex]1.05*10^{-34} Js[/tex]
  8. Jan 20, 2007 #7
    yes, that was a typo there. [tex]\hbar[/tex] should be [tex]1.05*10^{-34} Js[/tex]. The final value I get is [tex]E_{k}\geq1.65*10^-14J[/tex].
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