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Homework Help: Two Infinite Non-Conducting Planes

  1. Jan 28, 2009 #1
    1. The problem statement, all variables and given/known data
    The electric field equals 19j for points above plane 'r', -31j between the planes, and -19j below plane 's', where j is the unit vector in the +y direction and the fields are in V/m. Calculate σr, the suface charge density for plane 'r'.

    positive y- straight up
    positive x-to the right.

    2. Relevant equations


    3. The attempt at a solution

    i tried adding the 19j + -31j then multiplying that by Epsilon 0. that didnt work. not sure how to add or apply the electric fields. any pointers on how to solve that?
  2. jcsd
  3. Jan 28, 2009 #2
  4. Jan 28, 2009 #3
    anyone? i dont know which field to add or subtract. any help?
  5. Jan 28, 2009 #4


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    Homework Helper

    You're using the wrong relationship between charge and E. While your relationship is strictly correct, what you need is


    where [itex]E>0[/itex] means that it points away from the plane where [itex]\sigma[/itex] is (and [itex]\sigma>0[/itex]), and [itex]E<0[/itex] means that it points toward the plane where [itex]\sigma[/itex] is (and [itex]\sigma<0[/itex]). Then, you need to realize that the E in each of the three regions is the sum of the E's from the individual planes, and you have to be careful about the + and - signs. So, you can write 3 equations for the three regions in terms of the three E's and two [itex]\sigma[/itex]'s.
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