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Homework Help: Gauss' Law and magnitude

  1. Sep 13, 2008 #1
    Can someone explain the following?

    A positive charge Q is placed on a square sheet of nonconducting material of side x cm in the yz plane. What's the magnitude and direction of the electric field next to the sheet and proximate to the center of the sheet?

    I don't understand why the direction of E is into the sheet. If the sheet is positively charged, wouldn't the normal vector be directed outwards?

    Thank you.
  2. jcsd
  3. Sep 14, 2008 #2


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    The material is non-conducting, meaning to say that the charge won't spread out over the surface as it would on a conductor. Since it's placed on the sheet, it stays there and the resultant E field is through it.

    EDIT: Why is this titled Gauss law?
  4. Sep 14, 2008 #3
    Sorry, but if it's a positive charge, wouldn't the field be away from the charge and hence away from the sheet?

    You're right. It shouldn't be titled Gauss' law.

  5. Sep 14, 2008 #4


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    The question asked for the E-field next to and proximate to the centre of the sheet. So that doesn't include the point in space which is not in between the charge and the sheet.
  6. Sep 15, 2008 #5

    I'm sorry, but I still don't see. I know that the charge Q remains on the surface and doesn't move. So I would imagine a plate of charge. And a point in front of the plate of charge and near the plate's center. Since the plate is positively charged, wouldn't the field lines be moving out of the plate? And hence the direction of the E field would be out of the plate as well?

    Thank you.
  7. Sep 16, 2008 #6


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    Well given that the sheet is non-conducting, I'll take it to mean that the charge can't be pressed into the sheet, so you can't say the sheet is now positively charged. I'll think of it as pressing a marble onto a carpet; you don't expect the marble to end up within the carpet.
  8. Sep 16, 2008 #7
    Okay. Thanks.
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