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Homework Help: The rotational analog of Ehrenfest's Theorem

  1. Feb 23, 2013 #1
    1. The problem statement, all variables and given/known data
    Show [itex] \frac{d}{dt}\langle\bf{L}\rangle = \langle \bf{N} \rangle[/itex] where [itex]\bf{N} = \bf{r}\times(-\nabla V)[/itex]

    2. Relevant equations.
    [tex]\frac{d}{dt}\langle A \rangle = \frac{i}{\hbar} \langle [H, A] \rangle [/tex]

    3. The attempt at a solution
    I get to this point: [itex]\frac{i}{\hbar}\langle [H,L] \rangle = \frac{i}{\hbar}\langle [H, r \times p] \rangle [/itex] and then I'm stuck. I know the next step is supposed to use something like [itex][H, r \times p] = [H,r] \times p + r \times [H,p][/itex] but I don't see how to get this. I don't understand how operators distribute over the cross-product. Can anyone help or point me to some online resource where I can study how to work with operators/commutators/ etc.? I've been searching the internet all day with little to show for it.
  2. jcsd
  3. Feb 23, 2013 #2
    Hey Bobbo Snap,

    [itex][H,L] = HL - LH = H(r\times p) - (r\times p)H = (Hr)\times p - (rH)\times p = [H,r]\times p [/itex]
  4. Feb 24, 2013 #3
    Thanks PhysicsGente, your expression is exactly what I keep getting. That is,
    [tex][H, r \times p] = [H, r]\times p.[/tex]

    Maybe my mistake is trying to follow a solution I found which includes an extra term. It begins with the commutator reversed though, like this:
    [tex][r \times p, H] = r \times [p, H] + [r, H] \times p[/tex]
    I don't get how they come up with this.
  5. Feb 24, 2013 #4


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    I find it least confusing to just write it out in Cartesian components. For the x-component of the identity that you want to prove, use Lx = (r x p)x = (ypz - zpy). Then simplify [H, Lx]. The y- and z-components of the identity then follow by "cyclic permutation" of x,y,z. Not very elegant, but it gets the job done.
  6. Feb 24, 2013 #5
    You can also write it as [tex][H, r \times p] = r\times [H, p].[/tex] I don't see why you'd need the second term though. Just calculate the commutator [itex][H, p][/itex] and you should be done.
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