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Electromagnetism everyone's favorite!

  1. Feb 7, 2005 #1
    Hi again guys. I've got a test charge place a certain distance away from a large flat uniformly charged non-conducting surface. I'm told what the force charge is from this distance. Then the test charge is moved, and I'm supposed to say what the force on the test charge is now. I tried doing it the way I'd handle two charges,:

    E = (K*q1*q2)/r^2

    solving for q2 and then changing the distance, but that didn't work. So I'm a bit lost. A lil' help please?

    Edit: I'm told what the charge of the test charge is. Just in case.
    Last edited: Feb 7, 2005
  2. jcsd
  3. Feb 7, 2005 #2
  4. Feb 7, 2005 #3


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    Use gauss's law with a box straddling the surface to get it's field in terms of sigma (surface charge density), use this to get the field at the first location, and solve for sigma to get the field at the final location.
  5. Feb 7, 2005 #4
    I don't think I'm understanding the concept completely. I'll write out the problem just in case there was a misunderstanding. (I should just do this from the start, huh?)

    A test charge of 3.1microC is placed 8.3cm away from a large flat uniformly charged nonconducting surface. The force on the charge is 380N. The charge is now moved to 2.7cm away from the surface. What is the force on the test charge now?
    Last edited: Feb 7, 2005
  6. Feb 7, 2005 #5


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    The field of an infinite plane of charge is constant, so the answer is that the force is the same. To prove this, you can either integrate over the surface to get the field, or use gauss' law (if you know it) to make it a lot simpler. Maybe someone else can explain a way to prove the field of an infinite plane of charge is constant by some other method, but I can't think of one.
  7. Feb 7, 2005 #6
    Whoa, sweet, that was the answer.Thanks so much!

    I don't really understand why it's constant, though, so if someone could explain it to me, I'd appreciate it.
  8. Feb 7, 2005 #7


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    If you do the gauss's law on this, you'll find that the electric field is independent of the distance. This is because you are assuming that the plane charge is infinite (unbounded). Thus, the electric field lines (flux) is a constant and uniform - it doesn't "bow" outwards or inwards.

  9. Feb 7, 2005 #8
    Ahh, I get it now. Thanks you guys, you rock.
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