Electric field of a point near a dipole

[jack343]

Homework Statement

Two equal and opposite charges form a dipole, as shown in the figure. What is the magnitude of the electric field at point P, in terms of k,q,d, and x? If point P is VERY far away, show that the result is approximately E = kqd/x^3.

http://imageshack.us/photo/my-images/403/img20111215161448.jpg/

please help :(

Homework Equations

F = kq1q2/r^2
E= F/q

The Attempt at a Solution

I don't know how to start? Does the dipole basically act like a wire?
I did some research on dipoles and nothing was related.

 

[Spinnor]

Maybe this will help.

 

[jack343]

Maybe this will help.

Thanks, now I know where the forces are going. But why is sin(theta) = d/2L? shouldn't it be cos(theta) since L is the adjacent side?

Can't we use the Pythagorean theorem to get that r = sqrt(L^2 + (d/2)^2) ?

 

[Spinnor]

Thanks, now I know where the forces are going. But why is sin(theta) = d/2L? shouldn't it be cos(theta) since L is the adjacent side?

Can't we use the Pythagorean theorem to get that r = sqrt(L^2 + (d/2)^2) ?

My mistake, should be sin(theta) = d/2r. As r gets large sin(theta) gets small as required, the forces tend to cancel to a greater extent.

 

[jack343]

THANK YOU SO MUCH! I FIGURED IT OUT :)

Unfortunately, I bombed my electrostatics test today... great way to start winter vacation. :(

 

[Spinnor]

You lost a battle but you can still win the "war".

Good luck!

 

[jack343]

I lost another battle... deffered from my top school :/

anyhow, thanks for the help, in return I will help someone else.