# The Field of an Electric Dipole

## Homework Statement

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

See question 1.

## 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...

#### Attachments

• Electric field of P.jpg
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marcusl
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?

Yes it does, thank you very much!!