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Homework Help: Electric Field Question

  1. Jul 7, 2004 #1
    I need a little guidance in this problem...

    Two point charges lie along the y-axis. A charge of q1=-9 mu*C is at y=6.0m, and a charge of q2=-8.0 mu*C is at y=-4.0 m. Locate the point (other than infinity) at which the total electric field is zero.


    I made the statement

    E1y = -E2y


    Ke (q1/(r-4)^2) = Ke (q2/(r+4)^2)
    q1/(r-6)^2 = q2/(r+4)^2

    But that makes it very hard to solve for r. Is that equation set up correctly? If so, what is the easiest way, algebraicly, to solve for r.
  2. jcsd
  3. Jul 7, 2004 #2
    I think you have a typo in the equation above. Take the square root of both sides and simplify.
  4. Jul 7, 2004 #3
    I did have a typo....It was supposed to be a 6 instead of a 4 in the first equation.

    Let me try this and hopefully I can solve.
  5. Jul 7, 2004 #4
    Okay...so I took the square root of both sides of the equation, but that doesn't seem to get me very far.

    I end up with:

    sqrt(q1/q2) = (r-6)/(r-4)

    And frankly I don't know how to solve for r in this situation. I could use Maple or something, but it seems that this problem shouldn't require that. I think I might have got the initial set up wrong.
  6. Jul 7, 2004 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Two problems: (1) another typo, and (2) when you take square roots you better be sure your answer is positive.

    Your equation should be: sqrt(q1/q2) = (6-r)/(r+4)
    Start by multiplying both sides by (r+4). It's a simple linear equation.
  7. Jul 7, 2004 #6
    Of course it is. I just had a mental block. I wouldn't have caught the "(6-r)" though. Thanks for your help.
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