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Electric fields two point charges

  1. Jan 30, 2008 #1
    i just need to get this question over with
    i tried and tried but i couldnt get the right answer plss anyone u.u

    1. The problem statement, all variables and given/known data

    point charges q1 = -6q and q2 = +3q are separated by distance d. Locate the point (measured from the origin at q1) at which the electric field due to the two charges is zero.
    <img src=http://www.webassign.net/hrw/23_30.gif>



    2. Relevant equations
    E=kQ/x^2


    3. The attempt at a solution

    well i did 0= E1+E2

    i got this

    0 = 3kQ/x^2 - 6kQ/(x+d)^2

    6kQ/(x+d)^2 = 3kQ/x^2

    cross multiplied

    2x^2 = x^2 + 2dx + d^2

    x^2 - 2dx - d^2 = 0

    and then i did quadratic formula and got d-+d(sqrt2)
    like 2.414d and .414 but as i put them in the webassign i got em wrong
    can anyone help or tell what i did wrong or any feedback would be apreciated u.u
     
  2. jcsd
  3. Jan 30, 2008 #2
    There is only one point where the charges cancel each other out. It is one of the two you have calculated for, but only one has the E field vectors pointing opposite directions, which give a net charge of zero.
     
  4. Jan 30, 2008 #3
    but something must be wrong because like i said earlier
    the webassign says its wrong :S u___U
     
  5. Jan 30, 2008 #4

    Shooting Star

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    Homework Helper

    > point charges q1 = -6q and q2 = +3q are separated by distance d. Locate the point (measured from the origin at q1) at which the electric field due to the two charges is zero.

    > got this

    0 = 3kQ/x^2 - 6kQ/(x+d)^2


    You origin is given at q1 = 6q, but you are measuring x from the other charge.
     
  6. Jan 30, 2008 #5
    true :O
    im gonna try it
    and see what comes out :O
     
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