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Finding the constants in an Electric Field Equation

  1. Sep 27, 2013 #1
    1. The problem statement, all variables and given/known data
    The problem calls for finding the relationship between the constants A and B in the following equation of an instantaneous electric field:


    The details of the medium in which the field exists are that it is homogenous, isotropic, linear, and is source-free.


    2. Relevant equations

    3. The attempt at a solution

    I really have no idea how to go about solving this problem. Initially I was thinking I was just playing around with the vector equation by seeing what it looks like when I plug in different values of x and y and when t=0 or 1. Here is what that looked like:


    Can someone please help point me in the right direction?
  2. jcsd
  3. Sep 27, 2013 #2


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    Have you studied the properties of the "divergence" of the electric field?
  4. Sep 28, 2013 #3


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    The only question is, what an "instantaneous" electric field might be. I think it's just an electric field, right?

    Then just use the Maxwell and constitutive equations for the given situation to constrain the functions [itex]A[/itex] and [itex]B[/itex] as much as you can.
  5. Sep 28, 2013 #4

    rude man

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    And how about the curl, since the field is apparently time-invariant.
  6. Sep 28, 2013 #5
    Using Maxwell's equation

    [itex]\nabla \cdot E[/itex]=[itex]\frac{q_{ev}}{ε_{0}}[/itex]

    where [itex]q_{ev}[/itex] is the electric charge density and noting that [itex]q_{ev}[/itex]=0 for a source free medium, I get:

    [itex]\nabla \cdot E[/itex]={[itex]\frac{\partial}{\partial_{x}}[/itex][itex]\widehat{a}_{x}[/itex][itex]\cdot[/itex][[itex]\widehat{a}_{x}[/itex]A(x+y)]+[itex]\frac{\partial}{\partial_{y}}[/itex][itex]\widehat{a}_{y}[/itex][itex]\cdot[/itex][[itex]\widehat{a}_{y}[/itex]B(x-y)]}cos(ωt)=0



    Which is true when ωt=[itex]\frac{\pi}{2}[/itex], or when A=B.

    I am not really sure how to interpret this result.
    Last edited: Sep 28, 2013
  7. Sep 28, 2013 #6
    Sorry I had some mistakes in there, I was making a few changes!
  8. Sep 28, 2013 #7


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    You might want to start with what you know about ##\vec{\nabla} \cdot \vec{D}## and then, as vanhees71 suggested, use the consitutive relations to relate ##\vec{D}## to ##\vec{E}##.

    Using all four of the assumptions: (1) source free (2) homogeneous (3) isotropic (4) linear medium, you can deduce what ##\vec{\nabla} \cdot \vec{E}## must equal.
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