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Magnitude of electric force

  1. Mar 23, 2008 #1
    1. The problem statement, all variables and given/known data
    Four point charges are placed at the four corners of a square. Each side of the square has a length L.
    Find the magnitude of the electric force on q2 due to all three charges q1 , q2 , q3
    and q4. Given L=1 and q= 1.38 μC. Answer in N


    http://rs209gc2.rapidshare.com/files/101850597/81d74d53637084fd2c6b7bf5e75220df.jpg
    Photo of problem ^

    2. Relevant equations



    3. The attempt at a solution
    http://rs220l32.rapidshare.com/files/101852192/Snapshot_2008-03-23_19-17-38.jpg
    Photo of work ^
     
  2. jcsd
  3. Mar 24, 2008 #2

    dynamicsolo

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    Since you give units of N and microcoulombs in your statement, you are presumably working in SI units. Right off, I'd say check your value for k: I believe it is off by three orders of magnitude... (In your statement, I think you want the net force on q2 from the other three charges, so you don't want to type 'q2' again...)

    Also, watch your forces and components. For starters, what is the distance along the diagonal of the square? What is the force between the diagonally opposed charges?
     
  4. Mar 25, 2008 #3
    Well the diagonal is also 1. K should = 8.99E9. I still get the wrong answer.
     
  5. Mar 25, 2008 #4

    dynamicsolo

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    If the side of a square is 1, how long is the diagonal?
     
  6. Mar 25, 2008 #5
    A^2 + B^2 =c^2
    1^2+1^2=c^2
    2=c^2
    1=c
     
  7. Mar 25, 2008 #6
    Square root of 2 is 1?
     
  8. Mar 25, 2008 #7
    hmm...good point, guess i shouldn't do this in my head. well, its 1.41421. Where Do i need to use it? I didn't have r in my final formula? where did i go wrong?
     
  9. Mar 25, 2008 #8

    dynamicsolo

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    The value for c you just calculate is what you will use in Coulomb's Law for the force between the diagonally opposite charges. Since the formula calls for r^2, you could use r^2 = 2 there.
     
  10. Mar 25, 2008 #9

    ??????I'm sorry, I'm physics illiterate
     
  11. Mar 26, 2008 #10

    dynamicsolo

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    You have the equation for the force

    kqq/(r^2) , which is Coulomb's Law.

    In the image you show for your work, the force for the "southeast" diagonal should be using 2 in the denominator, instead of 1, since the distance squared between the two charges q2 and q4 is 2 .
     
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