1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    Science Advisor
    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!!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted