Charge-Dipole Derivation - Assumption That x >> a

Click For Summary

Discussion Overview

The discussion revolves around the assumption that the distance x from a dipole is much greater than the dipole size a (x >> a) in the context of deriving the electric field using Taylor expansion. Participants explore the implications of this assumption, its validity, and the consequences if it does not hold, focusing on theoretical and conceptual aspects.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions the validity of the assumption x >> a, asking why it is acceptable to consider the dipole as much smaller than the distance and what the implications are if this condition is not met.
  • Another participant explains that the assumption allows for a simplified formula for the electric field that behaves as 1/r³, noting that this is an approximation that improves with distance from the charges.
  • A third participant points out that the exact solution provided in the referenced document (equation 8) is applicable for any value of x, emphasizing that the simplification is particularly useful when considering dipoles of atomic size, where the fields are typically observed at much larger distances.
  • A later reply expresses appreciation for the discussion, highlighting a lack of opportunity to ask such questions in a classroom setting due to fast-paced coverage of material.

Areas of Agreement / Disagreement

Participants generally agree on the utility of the assumption x >> a for simplification purposes but express differing views on the implications and the necessity of the exact solution when this condition does not hold. The discussion remains unresolved regarding the process to find a solution when x is not much greater than a.

Contextual Notes

The discussion does not resolve the mathematical steps or assumptions involved in applying the Taylor expansion or the exact solution, leaving these aspects open for further exploration.

Who May Find This Useful

This discussion may be useful for students and researchers interested in electrostatics, particularly those studying dipole fields and the mathematical techniques used in their derivations.

kmcguir
Messages
5
Reaction score
0
In this derivation:

https://cpb-us-e1.wpmucdn.com/sites.../1599/files/2017/06/taylor_series-14rhgdo.pdf

they assume in equation (8) that x >> a in order to use the Taylor Expansion because a/x has difficult behavior. Why does that assumption work? Meaning, why can we assume the dipole is that much smaller? What happens if x is not much greater than a, then what would be the process to find a solution?
 
Physics news on Phys.org
They're simply saying that if you're far away from the charges (x >> a), then you can use the Taylor expansion to arrive at a nice simple formula for the field that goes like 1/r3. It's an approximation, but it gets closer to the truth, percentage-wise, as you go further from the charges.

If you're not far away from the charges, then you have to use the exact but somewhat messy solution, equation (8).
 
  • Like
Likes   Reactions: kmcguir
The exact solution that works for any value of x is given by Equation (8) in the reference you quoted. For example, when you are at x = 0, the electric field is zero as should be obvious and as predicted by equation (8). One considers the behavior at x >> a because in that limit the exact expression becomes simplified. This limit has its use when, for example, you consider dipoles of atomic size. The fields they generate are usually observed at distance much larger than the size of an atom in which case the approximation gives a very good description of the field.
 
  • Like
Likes   Reactions: kmcguir
Great! Thanks for those responses. There was never any time to ask questions like this in class. Always trying to cover too many chapters from the text every week, so they move too fast.
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 10 ·
Replies
10
Views
8K
  • · Replies 42 ·
2
Replies
42
Views
5K
  • · Replies 18 ·
Replies
18
Views
4K
  • · Replies 13 ·
Replies
13
Views
10K