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Homework Help: Electrodynamics - relativistic generalization of a formula

  1. Jun 3, 2010 #1
    [PLAIN]http://img46.imageshack.us/img46/1699/dsfadsfas.png [Broken]

    The formula holds in the rest frame.
    Why is this particular extension 'the' extension?
    (Context is Electrodynamics/SR. It's just a piddly 16 lecture course that outlines Electrodynamics and SR and a bit of Quantum phenomena at the end. Ohm's law hasn't been mentioned and I don't think you're expected to know a lot about Ohm's law itself)

    If you use a "taylor series in U" as in,
    [tex]\sigma F^{ab}U_b = J^a + \lambda U^a[/tex]
    and solve for lambda, it does turn out as what it's meant to be.

    But why is this the correct thing to substitute in? Or is it a naughty question that demands us to be psychic?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jun 5, 2010 #2


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    Gold Member

    Why not just use the first equation you are given (supposed to show)? In the rest frame of the ohmic medium, you have:

    [tex]\sigma F^{ab}U_{b}=(0,\sigma\textbf{E})=(0,\textbf{j})[/tex]

    What do you get when you apply a Lorentz boost to this 4-vector?
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