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Homework Help: Can you check this? - Charges and Forces

  1. Sep 23, 2013 #1
    Can you check this? -- Charges and Forces...

    1. The problem statement, all variables and given/known data
    A +3-mC charge and a -4-mC charge are fixed in position and separated by 5 meters.
    A)Where could a +7-mC charge be placed so that the net force on it is zero?
    B)Where could a -7-mC charge be placed so that the net force on it is zero?

    I came up with this diagram:
    q[1] q[2] q[3]

    2. Relevant equations

    3. The attempt at a solution
    F[1]=F[2]<---Because they are equal
    K(q[1]*q[2])/r[1]^2=K(q[3]*q[2])/(5-r[1])^2<---We get (5-r[1])=r[2] from r[1]+r[2]=5 solved for r[2])
    Then after we solve for r[1] we get this:

    So that means that both the +7-mC charge and the -7-mC charge are in the same position. The number I got was r[1]=2.32m and r[2]=2.68m. Did I set this up, and execute this correctly?
    Thank you very much for your time.
  2. jcsd
  3. Sep 23, 2013 #2
    With a +3 on the left and a-4 on the right, is there any position between the two where the net force will be zero? Maybe the position is to the left or right of both charges.
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