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Homework Help: A particle in a box.

  1. Apr 23, 2009 #1
    1. The problem statement, all variables and given/known data

    A particle is in a box of width L. Calculate the probability to find the particle in the region [L/4, 3L/4] when the particle is a) in the ground state b) in the first excited state.

    2. Relevant equations

    (2/L)sin(n*π*x/L)^2 dx is the probability in [x, x+dx]

    3. The attempt at a solution

    Integrating that gives me 2/L[x/2-[L/(4π)]sin(2n*π*x/L)], boundaries being L/4 and 3L/4. For a) n=1 and b) n=2, right? After I plug in the values, I get value greater than 1. Where have I gone wrong?

    Hopefully this is readable, no LaTeX. :cry:
  2. jcsd
  3. Apr 23, 2009 #2


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    The probability density is (2/L)sin²(nπx/L) for even n and (2/L)cos²(nπx/L) for odd n.

    EDIT: Sorry, this is only if you take the boundaries of the box to be -L/2 and L/2.
    Last edited: Apr 23, 2009
  4. Apr 23, 2009 #3
    No, it's the same formula for all n!
    After integration i got 2/L[x/2-[L/(4π n)]sin(2n*π*x/L)]
    For n = 1 -> (2+π)/(2π)
    for n = 2 -> 1/2
  5. Apr 23, 2009 #4


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    Yes sorry, I took the boundaries to be -L/2 and L/2.
  6. Apr 24, 2009 #5
    Thanks for the replies, guys! I forgot the one n in my first post. I found out my error was in the easy stuff after the integration, I'd done a mistake in adding fractions. :redface:
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