Understanding the Effect of Distance on Dipole Behavior: A Scientific Inquiry

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SUMMARY

The discussion centers on the treatment of a system of three charges as a dipole by omitting the middle charge under the condition that the distance (r) is significantly greater than the separation (a) between the charges. Participants clarify that while the middle charge contributes to the electric field, it can be ignored for dipole approximation when r>>a. The net electric field at point P is derived from the contributions of the dipole formed by the outer charges, validating the dipole approximation in this context.

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gracy
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in this video from time 1:20 to 1:24

he says we can treat the system of three charges as a dipole ignoring middle charge .I want to know why ?I mean just because r>>a how we can ignore the middle charge ?How is it relevant?
dipole.png
 
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From your diagram, it appears you may be getting confused about the charge geometry. In the video, the rightmost charge is positive.
 
jedishrfu said:
From your diagram, it appears you may be getting confused about the charge geometry. In the video, the rightmost charge is positive.
No.I know it is positive.
Let me correct it

dipole.png
 
In computing the electric field at P, the video considers the middle charge and says it points upward. The rightmost charge points upward and to the left. The leftmost charge the electric field points downward and to the left so if you draw that out you'll be able to find force on P.
 
But my question is regarding dipole.
 
During the time interval you mentioned, he temporarily omitted the middle charge because he is calculating the contribution from the other two charge, which turns out to form a dipole. You should see toward the end of the video he stated that the net field would be the total field from the middle charge and the dipole.
 
blue_leaf77 said:
he stated that the net field would be the total field from the middle charge and the dipole.
yes.But we can consider (the two charges the right most and the leftmost )as a dipole ,right?
 
gracy said:
But we can consider (the two charges the right most and the leftmost )as a dipole ,right?
Isn't that what the video demonstrates?
 
blue_leaf77 said:
Isn't that what the video demonstrates?
Wanted to verify :smile:
 
  • #10
Yes we can.
 
  • #11
blue_leaf77 said:
Yes we can.
Any conditions for doing that?I mean is it necessary r>>a ?
 
  • #12
No, it applies for any value of ##r##. The condition of ##r>>a## was imposed in order for the expression of the field due to the dipole to be able to be written in terms of ##r##, instead of the distance from the right (or left) most charge to the observation point.
 
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