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Two infinite sheets with charges

  1. Jul 16, 2013 #1
    Two infinite-plane non-conducting, thin sheets of uniform surface charge p1 = 12.30 uC/m2 and p2 = -3.30 uC/m2) are parallel to each other and d = 0.615 m apart. What is the electric field between the sheets? (Note: the field is positive if it is parallel to the vector x).

    Hi, I've tried this problem through the equation

    σ/2ε of sheet 1 + σ/2ε of sheet 2= E

    but that didnt seem to work. I also thought that maybe all the electric field lines were concentrated in the area between the two sheets since opposite charges attract. So i multiplied by two and that didnt work. Anyone got a clue to this problem?
  2. jcsd
  3. Jul 16, 2013 #2


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    I think you have the right idea. Since you didn't show the details of your calculation, it's hard to tell where you made a mistake. Could be an error associated with the signs or directions of the electric fields from the plates.

    If you are at a point between the plates, does the electric field, E2, at that point from the negatively charged plate point in the same direction or the opposite direction as the field E1 from the positive plate?
  4. Jul 16, 2013 #3
    The field from the two sheets is a vectorial sum of each sheet's field. The equation you have is generally correct, but, depending on how exactly the sheets are positioned along the x-axis (and I assume it is perpendicular to both sheets), you may have to mind the signs. That, and a numerical error, are all that I think could go wrong here.
  5. Jul 16, 2013 #4
    Wouldn't you just add the absolute value of both electric fields since the electric fields from both of the plates are headed towards the same direction?
  6. Jul 16, 2013 #5
    I got 880957 N/C as my answer
  7. Jul 16, 2013 #6
    sorry, solved... direction mattered
  8. Jul 16, 2013 #7


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    That looks correct to me. (You might want to round off your answer to the appropriate number of significant figures.)
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