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Electric field of a point near a dipole

  • Thread starter jack343
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  • #1
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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.
 

Answers and Replies

  • #2
Spinnor
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Maybe this will help.
 

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  • #3
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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) ?
 
  • #4
Spinnor
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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.
 
  • #5
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THANK YOU SO MUCH! I FIGURED IT OUT :)

Unfortunately, I bombed my electrostatics test today... great way to start winter vacation. :(
 
  • #6
Spinnor
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You lost a battle but you can still win the "war".

Good luck!
 
  • #7
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I lost another battle... deffered from my top school :/

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

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