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Homework Help: Electric Field (multi choice)

  1. Feb 22, 2009 #1
    1. The problem statement, all variables and given/known data
    Charge is distributed uniformly on the surface of a large flat plate. The electric field 2cm from the plate is 30 N/C. The electric field 4cm from the plate is:

    A. 120 N/C
    B. 80 N/C
    C. 30 N/C
    D. 15 N/C
    E. 7.5 N/C

    2. Relevant equations

    E=kq/r^2 (possibly)

    3. The attempt at a solution

    Well, the answer I got was E, 7.5 N/C. I got that by using the above equation, plugging in 30 for E and 2 for r and solving for q. Then, I used that same value of q and the new r to solve for E and got 7.5 N/C. Supposedly the correct answer is C, 30 N/C. I don't understand how they got that though. Why is the E field still the same? Shouldn't it be decreasing with the greater distance?

    Anyways, I think I might be wrong because I remembered afterwards that E=kq/r^2 is the E field due to a point charge and not a plate....maybe that's why I got the wrong answer?
  2. jcsd
  3. Feb 22, 2009 #2


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    I think you are on to something.

    What is the field of a sheet of charge?
  4. Feb 22, 2009 #3
    Oh. I believe it's in here: http://230nsc1.phy-astr.gsu.edu/hbase/electric/elesht.html

    So the eq looks like E=charge density/2*epsilon. So this is independent of distance. I see.
  5. Feb 22, 2009 #4


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  6. Feb 22, 2009 #5
    Yep. Thanks for the help!
  7. Feb 22, 2009 #6


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    Actually you solved it yourself.

    The best outcome of all.
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