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Homework Help: Help me please

  1. Sep 30, 2007 #1
    i need to solve 3 problems and i can't because i don' understand this can anyone help me.
    sorry for bad english and some bad expressions.
    i'm portuguese and i left the school 18 years ago.
    now i need some help to begin.
    thks a lot

    One wave packet wich represent the movement of one free particle in one dimension in unit h=c=1, is given for the expression:

    [tex]\Psi[/tex](x,t)= 1/[tex]\sqrt{2\pi}[/tex] [tex]\int-\infty[/tex][tex]\infty[/tex] dk [tex]\varphi(k)[/tex]exp {i(kx-w(k)t)}


    [tex]\varphi(k)[/tex] = 1/[tex]\sqrt{2\Delta k}[/tex] [tex]\theta[/tex](([tex]\Delta k[/tex])[tex]^{2}[/tex] - (k-[tex]\bar{}k[/tex])[tex]^{2})[/tex] =

    1/[tex]\sqrt{2\Delta k}[/tex] , |k-[tex]\bar{}k[/tex] | [tex]\leq[/tex] [tex]\Delta k[/tex]

    0 , |k-[tex]\bar{}k[/tex] | > [tex]\Delta k[/tex]

    and w(k) = k[tex]^{2}[/tex]/2m

    a) show in instant t=0 the wave function is given by:

    [tex]\Psi(x,t=0)[/tex]= 1/[tex]\sqrt{\pi\Delta k}[/tex] e[tex]^{i\bar{k}xsin(\Delta k x)}[/tex]/x

    and do one graphic of | [tex]\Psi[/tex](x, t=0) |[tex]^{2}[/tex] in function of x

    b) do graphicaly [tex]\Delta x[/tex] and [tex]\Delta x[/tex][tex]\Delta k[/tex] and compare result with heisenberg principle of uncertainty.

    c) do another graphic of | [tex]\Psi (x,t=1)[/tex] |[tex]^{2}[/tex] and | [tex]\Psi (x,t=2)[/tex] |[tex]^{2}[/tex] in the aproximation.

    w(k) = k[tex]^{-}[/tex][tex]^{2}[/tex]/2m + k[tex]^{-}[/tex]/m (k-k[tex]^{-}[/tex])

    in function of x and express the conclusion about the speed of the wave packet

    d) show that wave packet is solution of the following wave equation.

    i [tex]\partial[/tex]/[tex]\partial t[/tex] [tex]\Psi (x,t)[/tex]= -1/2m [tex]\partial[/tex][tex]^{2}[/tex]/[tex]\partial[/tex]x[tex]^{2}[/tex] [tex]\Psi(x,t)[/tex]

    e) now with w(k) = [tex]\sqrt{k^{2}+m^{2}}[/tex] Einstein Relation

    show the wave packet is solution of the following equation ( equation of Klein and Gordon)

    [tex]\partial ^{2}[/tex]/[tex]\partial t^{2}[/tex] [tex]\Psi (x,t)[/tex] = ([tex]\partial ^{2}[/tex]/[tex]\partial x^{2}[/tex] - m[tex]^{2}[/tex]) [tex]\Psi (x,t)[/tex]
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 30, 2007 #2


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    Staff Emeritus
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    Gold Member

    We need to see some effort from you before we can help. Do you have any thoughts?
  4. Sep 30, 2007 #3
    Unfortunely no

    as i said i leave school 18 years ago and now i'm very confused and need help.

    if anyone can help i apreciate.

  5. Oct 1, 2007 #4
    anyone can help please
  6. Oct 2, 2007 #5


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    Science Advisor
    Homework Helper

    what do you need help with? we cant do the whole problem for you. Ask a specific question and we can answer you so you can proceed. WHAT is confusing you?

    I mean, many of the things you are supposed to do are jusr basically putting in values in the original expression and see that it works, and show that the wave package fulfil some differential equations.

    for example:

    a) have you tried to just put in t=0 and to the integration?
    Last edited: Oct 2, 2007
  7. Oct 2, 2007 #6
    yes i try that.

    but i don't know what happens with [tex]\varphi[/tex](k)
  8. Oct 2, 2007 #7


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    Science Advisor
    Homework Helper

    okey: first stop mixing LaTeX and usual text, looks very strange..

    second; show what you did. Then we can see what you have done wrong, or if you are roght but are not aware of it.
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