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Homework Help: Electric Field and the position of a charge

  1. Jul 13, 2009 #1
    1. The problem statement, all variables and given/known data

    A -10.0nC charge is located at position (x, y) = (2.0cm, 1.0cm) . At what (x, y) position(s) is the electric field - 225,000i N/C?

    2. Relevant equations
    E = (Kq/r^2)*r(hat)

    3. The attempt at a solution

    r = sqrt(Kq/E) = .02m = 2cm
    So coordinates are (0,-2)

    I tried putting this answer into mastering physics online and it says it is incorrect. I can't seem to find what I have done wrong. Any help please?
  2. jcsd
  3. Jul 13, 2009 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Right away without calculating anything, I can tell you that since the electric field of a negative point charge points radially inward in all directions, the only way for the field to be pointing in the x-direction is if the point at which you are measuring the field is somewhere along a line that runs parallel to the x-axis and passes through the charge. Your answer cannot be right, because your point lies on the y axis.

    Hint: r only tells you the distance of the point away from the charge...it does not specify in which direction.
  4. Jul 13, 2009 #3
    Yeah, you've got the right distance but the wrong location. You've even got the right idea, using the negative sign, but like cepheid says, the field points in the i direction.
  5. Jul 13, 2009 #4
    Ah, I get it now! Thank you very much!
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