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Electric Field Due to a Point Charge

  1. May 31, 2009 #1
    1. The problem statement, all variables and given/known data

    In below figure, the three particles are fixed in place and have charges q1 = q2 = +e and
    q3 = +2e. Distance a = 6.00 μm. What are the magnitude of the
    net electric field at point P due to the particles?

    2. Relevant equations

    |E| = k * |q| / r2

    3. The attempt at a solution

    Well, I've got as far as understanding how to set up my vectors, (blue, above) realizing that E1 and E2 cancel. So now I set up my Enet = E3 = k * 2e / r2.

    What is r in this case?! I think my real problem here could be geometry, and not physics, but who knows I could be missing something vital from the beginning of the E-fields chapter. I though r would be sqrt 2 * a = 8μm, making r = 4μm. But when I evaluate Enet = k * 2e / (4x10-6)2 I do not get the right answer.

    In the solutions manual the final eq. they evaluate is: k * 2e / (a/sqrt 2)2

    Where do they get r = a / sqrt 2 ?

    Is r not the blue line, but twice as long since the third point charge is twice the charge of the others?

    If this ends up being a geometry problem a) slap me b) accept my apologizes and move post.
  2. jcsd
  3. May 31, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Realize that "a" is the hypotenuse of a right triangle, so it must be the largest side of that triangle.
    Use the pythagorean theorem.

    (It's a geometry problem.)
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