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Homework Help: Schrodinger's Equation, Potential Energy Barrier U>E

  1. Apr 8, 2010 #1
    1. The problem statement, all variables and given/known data
    Consider particles incident on a potential energy step with E<U.
    (That is, a particle with total energy E travels along one dimension where U=0, then crosses, at point x=0 into a region where U>E.) (The particle is incident on the potential energy step from the negative x direction.)

    Starting with the wave functions,

    x < 0: Ψ0 = A’e^(ikx) + B’e^(-ikx), k = 2mE/h_bar2
    x >= 0: Ψ1 = Ce^(k1x) + De^(-k1x), k1 = 2m(U-E)/h_bar2

    Apply the boundary conditions for Ψ and dΨ/dx and show that the full wave intensity is reflected at the step [i.e., |A'|^2 = |B'|^2].


    2. Relevant equations
    Ψ0(x=0) = Ψ1(x=0)
    dΨ0(x=0)/dx = Ψ1(x=0)/dx


    3. The attempt at a solution
    I set C=0, or else the wave function Ψ1 may become infinity.

    The boundary conditions are stated above. They become
    A' + B' = D
    and
    ikA' - ikB' = -k1D

    How, from this, do I find that
    |A'|^2 = |B'|^2
    ?

    Thanks.
     
  2. jcsd
  3. Apr 9, 2010 #2
    you have two equations and three unknowns..

    my hint for your:

    write A and B in terms of D .. and i noted something in your solution (you may not need it, but i will say it anyway) when you wrote k = 2mE/h_bar2, and k1 = 2m(U-E)/h_bar2, they should actually be k^2 and k1^2 .. finally after you find A and B in terms of D find AA* (which is |A|^2) and BB* (which is |B|^2) you need to write them in that form since you will have to find a complex conjugate of both of them ..

    good luck with this .. and tell us what you get .. :)
     
  4. Apr 9, 2010 #3
    Yay, I got it! I didn't have enough faith in algebra, was my problem. I often don't.

    Thanks!
     
  5. Apr 9, 2010 #4
    :) u r welcome .. next time dont give up so fast..
     
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