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Angular momentum in ElectroMagnetic fields(Feynman's Disk Paradox)

  1. May 4, 2013 #1
    In Griffiths book, "Introduction to Electrodynamics" example 8.4 he calculates the angular momentum density for a set up that is a version of Feynman disk paradox. His answer for the angular momentum points in the z direction. But if we you assume that the r vector has component in the s direction and z direction(I am almost sure this is correct) [itex]\vec{r}[/itex] = s[itex]\hat{s}[/itex]+ z[itex]\hat{z}[/itex], then the angular momentum density has both a z component and s component. The s component is not constant. The total angular moment on the other hand has to end up with only z component or the cylinders would tip over. Where is the error in my reasoning?
  2. jcsd
  3. May 4, 2013 #2
    It seems to me you've made no error. The angular momentum density should in fact have an [itex]\hat{s}[/itex] component for [itex]z \neq 0[/itex]. It seems Griffths neglected this. However, there is no xy component of the total angular momentum of EM field; it cancels out in via integration.
    Last edited: May 4, 2013
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