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Homework Help: Charges: Electirc forces

  1. Apr 6, 2010 #1
    1. The problem statement, all variables and given/known data
    Three point charges lie in a straight line along the y-axis. A charge of q1 is -9 microcouloumbs at y = 6 m and a charge of q2 is -8 microcoulombs at y = -4m. The net electric force on the third point charge is zero. Where is this charge located.

    2. Relevant equations

    Fe = k (q1*q2/r^2)

    3. The attempt at a solution

    Fe = k (q1*q2/r^2)
    = (9*10^9)((9*10^-6)(8*10^-6)/(10m)^2))
    = 6.5*10^-3 N

    Then i attempted to plug in this value for Fe to receive the r, and the location, but
    i don't know which two charges i had to use.


  2. jcsd
  3. Apr 6, 2010 #2


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    Homework Helper

    What's the equation for the electric field? Do you know how to add electric fields induced by different charges?
  4. Apr 6, 2010 #3
    I know how to add different electrical forces, but the problem i have is finding the location of it, instead of the force.
  5. May 3, 2010 #4


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    Gold Member

    So you've added the two equations for the electric fields of the first two charges together, giving you an equation for the overall electric field E as a function of y (considering both charges). What location along y produces a zero electric field? (Hint: you need to set something to zero and solve for y.)

    If a charge is in a zero electric field, what can you say about the net electric force acting on that charge? :wink:
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