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Homework Help: Momentum density vector of a plane em wave

  1. Oct 26, 2011 #1
    1. The problem statement, all variables and given/known data

    Show that in vacuum the magnitude of the momentum density
    vector of a plane electromagnetic wave is equal to the energy density
    divided by the speed of light, c.

    2. Relevant equations

    [itex]\vec{S}[/itex] = [itex]\vec{E}[/itex]x[itex]\vec{H}[/itex]

    3. The attempt at a solution

    This doesn't seem too hard if you know what a momentum density vector is, so if this should be in the introductory section I apologize but it is a hw question from an upper division course.

    The only thing I'm really looking for is how to find/ what is a momentum density vector. I have no idea how to go about finding it.
  2. jcsd
  3. Oct 27, 2011 #2


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    Science Advisor
    Homework Helper

    Do you know how the 16 components of the energy-momentum tensor look like ?
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