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Homework Help: Need help finishing Physics Problem, almost have answer.?

  1. Dec 18, 2007 #1
    Two Q = +5.85 mC charges are placed at opposite corners of a square d = 1.2 m on each side, and -Q = -5.85 mC charges are placed at the remaining corners, as shown in the figure.

    I labeled the square A, B, C, D from top left to bottem left.

    Fba = k • QA • QB / dBA2
    Fca = k • QA • QC / dCA2
    Fda = k • QA • QD / dDA2

    Fba= -.21365
    Fca= .10683
    Fda= -.21365

    I used the pythagorean therum to find Fba+Fda, and got .302, and then i subracted .10683 from that, but that was not the correct answer.

    Any suggestions?
     
  2. jcsd
  3. Dec 18, 2007 #2

    Doc Al

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    Staff: Mentor

    What are you asked to find?
     
  4. Dec 18, 2007 #3
    Yea, probably should have put that in,

    Determine the magnitude and direction of the force on each charge.
     
  5. Dec 18, 2007 #4

    Doc Al

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    What units are these in?
     
  6. Dec 18, 2007 #5
    They should be in Newtons
     
  7. Dec 18, 2007 #6

    Doc Al

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    Redo the calculations. (You are off by orders of magnitude.)
     
  8. Dec 18, 2007 #7
    Fba = k • QA • QB / dBA2
    Fba = (8.99*10^9 • (5.85)*10^-6 • (-5.85)*10^-6) / 1.2^2
    Fba = (-.30766) / (1.44)
    Fba = -.21365

    Are my numbers off, or just my sign, or both?
     
  9. Dec 18, 2007 #8

    Doc Al

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    Ah... you're using mC to mean microCoulombs. (Better to use [itex]\mu C[/itex].) In that case, the numbers look OK to me.
     
  10. Dec 18, 2007 #9
    I think you might be right, it should be milli. I will try that.
     
  11. Dec 18, 2007 #10
    I got it, 195872.2698.

    Thanks for your help.
     
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