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Homework Help: Electric field of a point near a dipole

  1. Dec 15, 2011 #1
    1. The problem statement, all variables and given/known data
    Two equal and opposite charges form a dipole, as shown in the figure. What is the magnitude of the electric field at point P, in terms of k,q,d, and x? If point P is VERY far away, show that the result is approximately E = kqd/x^3.


    please help :(

    2. Relevant equations

    F = kq1q2/r^2
    E= F/q

    3. The attempt at a solution

    I don't know how to start? Does the dipole basically act like a wire?
    I did some research on dipoles and nothing was related.
  2. jcsd
  3. Dec 15, 2011 #2
    Maybe this will help.

    Attached Files:

  4. Dec 15, 2011 #3
    Thanks, now I know where the forces are going. But why is sin(theta) = d/2L? shouldn't it be cos(theta) since L is the adjacent side?

    Can't we use the Pythagorean theorem to get that r = sqrt(L^2 + (d/2)^2) ?
  5. Dec 15, 2011 #4
    My mistake, should be sin(theta) = d/2r. As r gets large sin(theta) gets small as required, the forces tend to cancel to a greater extent.
  6. Dec 16, 2011 #5

    Unfortunately, I bombed my electrostatics test today... great way to start winter vacation. :(
  7. Dec 16, 2011 #6
    You lost a battle but you can still win the "war".

    Good luck!
  8. Dec 16, 2011 #7
    I lost another battle... deffered from my top school :/

    anyhow, thanks for the help, in return I will help someone else.
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