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Homework Help: 1D Particle in a box - locations at set probability

  1. Mar 18, 2009 #1
    1. The problem statement, all variables and given/known data

    An electron is confined to the region of the x axis between x = 0 and x = L (where L = 1nm). Given a state n = 3, find the location of the points in the box at which the probability of finding the electron is half it's maximum value

    2. Relevant equations

    [tex]\psi^2(x)=\frac{2}{L}\sin^2\left(\frac{n\pi x}{L}\right)[/tex]

    3. The attempt at a solution

    I understand that the wavefunction squared (above) gives the probability at location x, and its integration gives the probability over set regions between x = 0 and x = l. However, the only way I can see of finding x from a given probability is to assume:


    and try to manipulate the equation to give it in terms of x. Is this the right method? If so, how do I take out the sine term?

  2. jcsd
  3. Mar 18, 2009 #2


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    The maximum value of ψ2(x) is not 1.
  4. Mar 19, 2009 #3
    Ah, I see - half the maximum value. I'm guessing it'd only be 1 if we were considering the entire box, not one point.

    Alright, so you determine the maxima by differentiation?

    [tex]\frac{d\psi^2(x)}{dx}=2\sin\left(\frac{n\pi x}{L}\right)\cos\left(\frac{n\pi x}{L}\right)=0[/tex]
  5. Mar 19, 2009 #4


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    The rigorous way to determine the maxima would be by differentiation, as you said. However, ψ2 has a fairly simple formula and from what you know about sine functions, you should be able to see the maximum value by inspection.
  6. Mar 19, 2009 #5
    Hmmm.... okay, let's simplify this: 2/L is just a constant, and so's [tex]\frac{n\pi}{L}[/tex], so that gives us:


    Sine functions go to 1 when ax = 90, and since sin2(ax) is :

    [tex]\sin^2\left(ax) = \sin\left(ax)\sin\left(ax)[/tex]

    This means.... I'm seriously clutching at straws

    Hey, I'm a chemist - I'm amazed I've managed this much
  7. Mar 19, 2009 #6


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    What is the largest value sin(x) can give?

    Being a chemist is no excuse for not knowing math (unless of course, you're a biochemist :p)!
  8. Mar 20, 2009 #7
    The maximum of sin(x) is 1
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