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Measuring energy and then momentum

  1. Oct 10, 2012 #1
    Hi there,

    Im confused by something that came up in my quantum mechanics lecture. The lecturer posed us a question. What result do I get for a measurement of momentum if I have already measured the energy.

    I assumed after measuring energy that the system would be left in an energy eigenfunction. So I tried to do the momentum operator on this eigenfunction (the one with sin in) and I have something that looks like:

    [i [itex]\hbar[/itex]Cos([itex]n \pi x/\sqrt{a}[/itex])]/a

    Is that correct? If so how do I extract the eigenvalues from it?

  2. jcsd
  3. Oct 10, 2012 #2


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    Staff: Mentor

    Eigenfunction where?
    Your momentum measurement might depend on the position, if your potential is variable. A pure momentum measurement would not give a single, predictable value, but you could integrate over your whole wave function to get an expectation value.
  4. Oct 10, 2012 #3
    Sorry I meant in the well, where the potential is zero.
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