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Homework Help: D and E at boundary (dielectrics)

  1. Jun 12, 2013 #1
    1. The problem statement, all variables and given/known data
    Electromagnetics, Kraus, 4th edition problem 4.7.3
    The y-z plane is the boundary between 2 dielectrics of relative permittivities εr = 2 and εr = 5. For negative values of x, E = (3,0,2) V/m. Find D (magnitude and direction) for positive values of x.

    2. Relevant equations
    The one that is similar to Snell's Law, not sure of name:
    i) ε2 tan (incident angle to normal) = ε1 tan (refracted angle to normal)

    which is derived from boundary conditions for 2 dielectrics with the absence of surface charges

    ii)D1(normal) = D2(normal)
    iii)E1(tangent) = E2(tangent)

    3. The attempt at a solution
    The x-axis is parallel to the normal of the boundary. So, I first found the angle the electric field vector makes with the x-axis to be 0.588 radians.

    incident angle to normal (x-axis) = inverse tan (2/3)

    This value was substituted into equation i) along with 5 and 2 for ε1 and ε2 respectively. Solving for the refracted angle, I obtained 1.03037 radians. I believe all of this is correct.

    What I am having trouble with is now getting the magnitude of D2. If I just use equations ii) and iii) with pythagorean theorem I get an answer different than the book, which says: D2 = ε0(6,0,10).
  2. jcsd
  3. Jun 12, 2013 #2
    I figured it out, I had originally put the relative permittivity of 5 on the negative x side, but making it positive gives the correct result. Also, the "snell's law"-like equation isn't necessary at all.
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