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Homework Help: Find an expression for the electrostatic energy of this charge distribution.

  1. Dec 14, 2012 #1
    1. The problem statement, all variables and given/known data
    electro.jpg



    2. Relevant equations
    work = kqqq/(distance)


    3. The attempt at a solution
    http://i33.photobucket.com/albums/d86/warnexus/electrodiag.jpg

    energyexpression.jpg
     
  2. jcsd
  3. Dec 14, 2012 #2

    gneill

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    Staff: Mentor

    Check the distance you've used for the diagonals. Substitute 'q' and 'q/2' for the appropriate charges and simplify.
     
  4. Dec 14, 2012 #3
    the distance I used for the diagonals is sqrt of a. I do not understand I do not have q/2.
     
  5. Dec 14, 2012 #4

    gneill

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    Staff: Mentor

    Is the length of a diagonal of a square of side length a equal to √a ?

    You have two sizes of charge: One is 'q', the other is 'q/2'. Make the appropriate symbolic substitutions.
     
  6. Dec 14, 2012 #5
    oops the diagonal is suppose to be sqrt 2a^2.

    heres the substitutions:

    k(qq/a + qq/sqrt(2a^2) + qq/a -1/2qq/a -1/2qq/a -1/2qq/sqrt(2a^2))
     
    Last edited: Dec 14, 2012
  7. Dec 14, 2012 #6

    gneill

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    Staff: Mentor

    Okay, but you can simplify further. For example, you can pull out the q2 from each term.
     
  8. Dec 14, 2012 #7
    heres the simplification

    kqq(1/a + 1/sqrt(2a^2) + 1/a - (1/a) - (1/2)/sqrt (2a^2)
     
  9. Dec 14, 2012 #8

    gneill

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    Staff: Mentor

    You can collect the terms, putting them over a common denominator...
     
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