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Homework Help: Getting Magnetic Field from Electric Field

  1. Dec 14, 2014 #1
    1. The problem statement, all variables and given/known data
    Hi! There is a formula in Griffiths that relates the electric field and the magnetic field:

    Question: Is this formula applicable for plane waves only? Can I use this for spherical waves? Thank you very much.

    2. Relevant equations
    See above.

    3. The attempt at a solution
    I was told this is just for plane waves, but I want to be sure. Thank you! :)
  2. jcsd
  3. Dec 15, 2014 #2


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    Gold Member

    I know what the k vector is for a plane wave. And with a little re-reading notes on this stuff I could reproduce the construction that gets you your relation between B, K, and E for a plane wave.

    What would be the equivalent of a k vector for a spherical wave? Can you write down the E and B as functions of position and time for a spherical wave? It's going to have to be some kind of function of radius. How can you write something that looks like the k dot E or k dot B in a plane wave but that gives you something that is a function of radius? Hint: It's going to involve the radius vector. Can you demonstrate your solution obeys Maxwell's equations? Does your solution have a k vector? Hint: plane waves have a single axis of travel and so there is a single k vector for all positions and time. That clearly can't be true for a spherical wave.

    And, having done all of that, can you then write k x E and compare it to B?
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