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Rotating Frames: charges in a magnetic field

  1. Jan 14, 2010 #1
    I've got a problem understanding a line of proof in my lecture notes.

    Given that you have a charge of +Q and mass m orbiting a fixed particle of charge -Q' in the presence of a magnetic field B. The particle is moving slowly enough for relativistic effects to be ignored.

    Given that:

    [tex]m\hat{a_{I}}=-\frac{QQ'}{4\pi\epsilon_{0}r^{2}}\hat{r}+Q\hat{v_{I}} \times \hat{B}[/tex]

    where [tex]\hat{v_{I}}[/tex] is the particle's velocity in the inertial frame.


    [tex]\hat{a_{I}}=\hat{a_{R}}+2\hat{\omega} \times \hat{v_{R}}+\hat{\omega} \times (\hat{\omega} \times \hat{r})[/tex]

    into the first equation along with [tex]\hat{v_{I}}=\hat{v_{R}}+\hat{\omega} \times \hat{r}[/tex]


    [tex]\hat{a_{R}}+2\hat{\omega} \times \hat{v_{R}}+\hat{\omega} \times (\hat{\omega} \times \hat{r})[/tex] = [tex]-\frac{QQ'}{4\pi\epsilon_{0}mr^{2}}\hat{r}+(\frac{Q}{m})[\hat{v_{R}}+(\hat{\omega} \times \hat{r})] \times \hat{B}[/tex]

    Apparently the terms porportional to [tex]\hat{v_{R}}[/tex] cancel if [tex]\hat{\omega}=-\frac{Q}{2m}\hat{B}[/tex]

    Why is this so? Visualizing the situation will probably help a lot.
    Last edited: Jan 14, 2010
  2. jcsd
  3. Jan 14, 2010 #2


    User Avatar
    Science Advisor
    Gold Member

    What is \omega \times r? Are these scalars or vectors? You are mixing scalars and vectors here.
  4. Jan 14, 2010 #3
    I've corrected it now. The \times is supposed to be the cross product. I didn't know what the latex is for cross product.
  5. Jan 14, 2010 #4


    Staff: Mentor

    This is just a guess. To me that looks like the cyclotron frequency in the presence of a net charge. So the motion is purely radial in a reference frame which is rotating at the cyclotron frequency.
  6. Jan 14, 2010 #5
    Well, the omega frequency is known as the larmor frequency. It is half the cyclotron frequency. The charge +Q is precessing around the charge -Q. I don't understand how the velocity in the rotating frame cancels, giving that it is orbiting in the rotating frame (which means there must be a v subscript R).
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