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QM - Find <E>

  1. Sep 26, 2005 #1

    quasar987

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    There's this question ngyah (Liboff 5.33) that says

    "We consider a particle in an infinite 1-dimensional well. The particle is described by an arbitrary wave function [itex]\psi(x)[/itex]. (a) For this particle, show that [itex]<E> \geq E_1[/itex]. (b) What is the condition on [itex]\psi(x)[/itex] such that [itex]<E> = E_1[/itex]?"

    My solution: First of all, I'm assuming that by <E> he means <H>. But <H> = E. And E is the energy associated with the superposition of the sine waves that make up [itex]\psi(x)[/itex]. So

    [tex]E = \sum_{i=1}^{\infty}c_i E_i[/tex]

    where the c_i are either 1 or 0 depending on wheter or not [itex]\psi_i(x) = sin(k_ix)[/itex] is present in the Fourier expansion of [itex]\psi(x)[/itex]. This said, if [itex]\psi_1(x)[/itex] is in the expansion of [itex]\psi(x)[/itex], then [itex]E = E_1 +... \geq E_1[/itex]. If not, then [itex]E = 0\cdot E_1 + ... + 1\cdot E_j +... \geq E_1[/itex].

    And for (b), the condition on psi is that psi be exactly psi_1.

    That looks pretty neat to me. The only point I'm unsure of is wheter or not, it is true that the energy of a psi made up of a linear combination of other psi_i is the sum of the energy of the psi_i. Actually, this seems FALSE to me. For suppose psi_1 and psi_2 are solution of the time ind. SE with respective energy eigenvalues E_1 and E_2. Then let's see if psi = psi_1 + psi_2 is a solution with eigenvalue E_1 + E_2.

    [tex](E_1+E_2)\psi = (E_1+E_2)(\psi_1 + \psi_2) = E_1\psi_1 + E_2\psi_2 + E_1\psi_2 + E_2\psi_1 = H\psi_1 + H\psi_2 + E_1\psi_2 + E_2\psi_1 = H\psi + E_1\psi_2 + E_2\psi_1 \neq H\psi [/tex]

    So the energy of psi_1 + psi_2 is not E_1 +E_2. Correct? :frown: Hopefully I'm missing something.
     
  2. jcsd
  3. Sep 26, 2005 #2

    quasar987

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    Arg, it's assuredly wrong, for any function that requires an infinity of sine waves to be constructed will have an infinite energy. :frown:
     
  4. Sep 27, 2005 #3

    CarlB

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    Yes. If you have a wave function that is not an eigenstate of the Hamiltonian, it's energy is not a precise number. What the instructor was asking for was <H>, which means the average of the energy.

    If c_n are the (complex) amount of \psi_n that you have to use to make up your (normalized) wave function \psi = \sum \psi_n, then


    [tex]\langle E \rangle = \langle \psi |E| \psi \rangle[/tex]
    [tex] = \sum_n \sum_m c_n^* c_m \langle E_n| H |E_m\rangle[/tex]
    [tex]= \sum_n \sum_m c_n^* c_m E_m \langle E_n | E_m\rangle[/tex]
    [tex]= \sum_n \sum_m c_n^* c_m E_m \delta_n^m[/tex]
    [tex]= \sum_n c_n^* c_n E_n[/tex]
    [tex]= \sum_n |c_n|^2 E_n < \infty[/tex]

    Since [tex]\sum_n |c_n|^2 = 1[/tex], you can see that the maximum that <E> could be would be the highest of the E_n, which is less than infinity.

    Hope this helps.

    Carl
     
  5. Sep 27, 2005 #4

    quasar987

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    I'm not used to the notation you used. What is <psi|E|psi> ?
     
  6. Sep 27, 2005 #5

    CarlB

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    I don't know what book you're being taught out of, but here goes:

    [tex]\psi(x) = \langle x|\psi\rangle[/tex]

    That is, [tex]|\psi\rangle[/tex] is just the ket associated with the state [tex]\psi[/tex].

    It could be that in your book / class, they haven't covered the bra-ket formalism yet. In that case,

    [tex]\langle \psi | M| \psi \rangle = \int_{-\infty}^\infty \psi^*(x) M \psi(x) dx[/tex]

    for wave functions in one dimension. In the above, M is an operator.

    Did this help?

    Carl
     
  7. Sep 27, 2005 #6

    quasar987

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    Now we're talking the same language! :biggrin:
     
  8. Sep 27, 2005 #7

    quasar987

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    Your way seems kinda complicated CarlB with double sums and stuff. Here's mine..

    [tex]<E> = \sum_{n=1}^{\infty}E_n|c_n|^2[/tex]
    [tex]<E> = E_1|c_1|^2 + E_2|c_2|^2+... \geq E_1|c_1|^2+E_1|c_2|^2+... = E_1 \sum_{n=1}^{\infty}|c_n|^2 = E_1[/tex]
     
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