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Electric field momentum

  1. Oct 11, 2011 #1
    Is it fair to say that any changing electric field (hence electrodynamic) has momentum?
  2. jcsd
  3. Oct 11, 2011 #2
    I don't think so.
  4. Oct 11, 2011 #3


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    No it isn't. A changing E-field alone does not carry momentum. You have to consider with it the induced B-field and the Poynting vector.

    The momentum density stored in the electromagnetic field is given by:

    [tex]\rho_{EM}=\mu_0\epsilon_0 \vec{S}=\mu_0\epsilon_0 (\vec{E}\times \vec{B})[/tex]

    There's a few pieces of information we can gain from this equation:

    1. Momentum can only be stored in a field that has non zero E and B.

    2. The fields do not need to be time varying. Even a static field can have momentum, as long as its Poynting vector is not zero.
  5. Oct 11, 2011 #4
    Thank you very much GO1.

    So in the example I had the Poynting vector would be non zero. But that is not the case for all electric fields and all conditions. If either E or B are zero then you have no momentum.

    GO1 do you use a tool for creating latex equations or are you just entering the codes?
  6. Oct 11, 2011 #5


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    Your welcome!

    Physics Forums, being the great place that it is, has Latex built in:

    https://www.physicsforums.com/showthread.php?t=386951 [Broken]

    You can also click on the [itex]\Sigma[/itex] box on the reply to page to pull up a handy latex code library.
    Last edited by a moderator: May 5, 2017
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