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Electric Charge

  1. Jul 4, 2006 #1
    Hello -

    I worked out this problem but I got a wrong answer.


    First, I used the Pythagorean theorem to find the radial distance between A and each charges. So 1.2m divided by 2 (= 0.6), then the square root of .6 squared + .6 squared = .849, which is the radius.

    Then I used Coulomb's Law to calculate the net force:

    kqAq1/R2 + kqAq2/R2 + ... and so forth.

    I took out the kqA/R2, which is the same for all, and came up with:

    kqA/R2 (q1 + q2 + q3 + q4).

    But it so happens that the numbers inside the parenthesis turns out to be 0.

    What did I do wrong?
  2. jcsd
  3. Jul 4, 2006 #2
    I think you're forgetting the vector nature of force. Simply adding up the numbers won't do any good. You have to add them vectorially.
  4. Jul 4, 2006 #3


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    First your pythagorean theorem is a bit off. it should be:

    [tex] a^2=b^2+c^2 [/tex]

    EDIT: You got it right I misinterpreted what you had done originally.

    Then you have neglected to take any directions when working out the force so try setting up a reference freame and adding the directions into your equation.
  5. Jul 4, 2006 #4


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    Hello sophzilla,

    I think you don't take into account that the forces are vectors.

    You can start applying Coulomb's law for one diagonal at a time. For instance [tex]Q_4, q, Q_2[/tex]. Do you agree that the 2 forces will add up to a net force pointing from [tex]q[/tex] towards [tex]Q_2[/tex]?


  6. Jul 4, 2006 #5

    I would appreciate any help. Thanks.
  7. Jul 4, 2006 #6


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    Those are not vectors. That is why it is not working. Consider the unit vectors i and j and how they would add up to pointin the directions you require to the charges from the centre. The magnitudes are then as you have calculated.
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