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Homework Help: Point charges

  1. Jan 30, 2012 #1
    1. The problem statement, all variables and given/known data

    Two point charges, 3q and q lie along x-axes. 3q is at 0.00 m and q is at 3.0m.
    Find the point between the 2 charges at which the net force on charge q is zero.

    2. Relevant equations


    3. The attempt at a solution
    Really can't figure this one out. Missed the first lesson.
  2. jcsd
  3. Jan 30, 2012 #2
    are they talking about net electric field or net electric force?
  4. Jan 30, 2012 #3
    I'm pretty sure they mean net electric field, since there are only 2 charges. Otherwise, the electric force would be the same so long as the two charges are in the same place and if a point charge is placed somewhere between. I mean, in that case, the net force on the point charge anywhere between the two charges would be zero, but that the charge on q would be the same.
  5. Jan 30, 2012 #4
    The answer is 1.9m.

    I still don't understand it because since both the charges are positive, wouldn't the forces be in the other direction (no forces in between both the charges) since they are going to repel each other?
  6. Jan 30, 2012 #5
    If they are talking about the net electric field being that of zero, then here's what you do:

    make K(3q)/((3-r)^2)=k(q)/((r)^2)



    0=9 -6r -3r^2 +r^2

    use quadratic formula to solve

    6 +/- sqrt(36 + 4(2)(9))/2(-2)=r

    (6+/- 10.39)/-4=1.1meters
    and 3-1.1=1.9 AH! :D theres your answer!
  7. Jan 31, 2012 #6
    Oh wow.. thanks!
    That makes so much more sense than what my TA taught me!

    Totally made my day!! Thank you so much!! :D
  8. Jan 31, 2012 #7
    I'm also in physics 2 and I had a bit of trouble understanding why this is true. I mean, it has everything to do with distance and electric field.
  9. Feb 1, 2012 #8
    I guess my lecturer just worded it weirdly.. =/
  10. Feb 1, 2012 #9
    My professor is funny guy, but he and I talk a lot. So, It's cool. The best thing to do is if you get a better professor that fits your style of learning. I'm the kind of person who learns if he feels free in the classroom. So, if a professor is extremely strict and too authoritarian, I'd hate the professor.
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