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The Field of an Electric Dipole

  1. Oct 12, 2013 #1
    1. The problem statement, all variables and given/known data
    Consider an electric dipole in the attached document. At point P, the fields E+ due to q+ and E- due to q- are:

    E+ = kq/(r-a)2, E- = -kq/(r+a)2


    Then total field at P is:

    E = E+ + E- = kq [ 1/(r-a)2 - 1/(r+a)2] y-hat
    2. Relevant equations

    See question 1.

    3. The attempt at a solution

    So this post is just trying to comprehend the example problem above. I don't believe I'm fully understanding electric field. In the problem above, they have a point P without a charge given. They say that the impact of the electric field of +q occurs at a distance of r-a which makes sense if P is a postive charge. But then for -q, it takes place at a distance of r+a. What I don't understand for this, shouldn't it take place at a distance of 2a+r? It seems like it'd be a distance of 2a from Point P. Or are we considering that the electric field is moving towards -q? I'm not sure I understand this problem...
     

    Attached Files:

  2. jcsd
  3. Oct 12, 2013 #2

    marcusl

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    Gold Member

    P is just the label for a point of interest where you are being asked to evaluate the electric field. There's no charge there because it's just a location--the location where an observation of the 2 charges will be made.

    Point P is located a distance r from the origin in your diagram. Since the + charge is a distance a from the origin in the same direction, the separation between +q and P is r-a. The - charge is located at -a, which is a distance a from the origin in the direction opposite from P. The distance from +q to P must be r+a as states. So far this is just simple use of a ruler.

    Does this help?
     
  4. Oct 12, 2013 #3
    Yes it does, thank you very much!!
     
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