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

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Discussion Overview

The discussion centers around the treatment of a system of three charges as a dipole, specifically addressing the conditions under which the middle charge can be ignored. Participants explore the implications of distance on dipole behavior and the relevance of charge geometry in calculating electric fields.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question the reasoning behind ignoring the middle charge when the distance is much greater than the separation between the charges.
  • Others clarify the charge geometry, noting that the rightmost charge is positive and discussing the direction of the electric fields produced by each charge.
  • One participant points out that the video suggests the middle charge is omitted to focus on the dipole formed by the outer charges.
  • There is a proposal that the two outer charges can indeed be treated as a dipole, with some participants seeking confirmation of this interpretation.
  • Participants discuss whether the condition of distance being much greater than separation (r >> a) is necessary for treating the outer charges as a dipole.
  • One participant asserts that the dipole approximation can apply for any distance, while another emphasizes that the condition is relevant for expressing the dipole's field in terms of distance from the dipole rather than individual charges.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of the condition r >> a for treating the outer charges as a dipole. While some agree that it can be applied generally, others emphasize the importance of the condition for specific calculations.

Contextual Notes

There are unresolved aspects regarding the assumptions made about charge interactions and the implications of distance on the validity of the dipole approximation.

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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