Appropriate coordinates for a given electric field

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Discussion Overview

The discussion revolves around the appropriate coordinate systems for analyzing an electric field and its divergence, particularly focusing on the differences between Cartesian, cylindrical, and spherical coordinates. Participants explore the implications of coordinate choice on the calculations of divergence and charge density.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant mentions discrepancies in results when using different coordinate systems, specifically noting that both Cartesian and spherical coordinates yielded the same output.
  • Another participant asserts that the divergence of a field at a point is independent of the coordinate system, suggesting that a zero divergence in spherical coordinates should also be zero in cylindrical coordinates.
  • A participant expresses confusion regarding the calculations in cylindrical coordinates, particularly due to the absence of a component in the theta direction, while noting that the z component results in zero and the r component yields a non-zero value.
  • There is a query about the divergence in cylindrical or spherical coordinates, hinting at the complexity of the divergence operator in different systems.
  • One participant provides an expression for the electric field and its divergence, indicating a calculation that results in zero, while also apologizing for issues with their LaTeX formatting.

Areas of Agreement / Disagreement

Participants express differing views on the implications of coordinate choice for divergence calculations, with some asserting independence from the coordinate system while others highlight specific challenges encountered in cylindrical coordinates. The discussion remains unresolved regarding the exact nature of the calculations and their outcomes.

Contextual Notes

Participants note potential issues with coordinate transformations and the specific forms of divergence in different coordinate systems, but do not resolve these complexities.

kirito
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TL;DR
I was facing a problem where I have an electric field and I have to find the charge density , I noticed that the field outside is that of a dipole and because I decided to use spherical coordinates and reached an appropriate answer yet using divergence in cylindrical lead me to different outputs
Screenshot 2024-10-13 at 16.19.16.png

this is the field I was provided
and this is the charge density that I have reached
Screenshot 2024-10-13 at 16.21.52.png

Screenshot 2024-10-13 at 18.40.38.png

I tried to use this yet the output was different
I also used Cartesian it gave me the same output as the spherical ones
 
Last edited:
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The value of the divergence of a field at a point is independent of the coordinate system. So, if you get zero in spherical coordinates you should also get zero in cylindrical coordinates.
 
TSny said:
The value of the divergence of a field at a point is independent of the coordinate system. So, if you get zero in spherical coordinates you should also get zero in cylindrical coordinates.
thats actually why I even made this post , I got weirded out by the calculation using cylindrical coordinates since there is no component in the theta direction whereas the z results in 0 and the component in the r direction results in something other than zero unless I had to do something to the coordinates I changed from cylindrical to spherical so changed the z hat appropriately in the first case I may have missed something but got no clue what it may be
 
kirito said:
since there is no component in the theta direction whereas the z results in 0 and the component in the r direction results in something other than zero
What is the divergence in cylindrical (or spherical) coordinates? Hint: it is not the same as for Cartesian replacing variable names.
 
It's easier to do it without coordinates:
$${\bf E}=\frac{3{\bf (r\cdot \mu)r}}{r^5}-\frac{\mu}{r^3}$$
$$\nabla\cdot{\bf E}=\frac{3[\mu\cdot{\bf r}+3{\bf (\mu\cdot r)]}}{r^5}-\frac{15\mu\cdot{\bf r}}{r^5}+\frac{3\mu\cdot{\bf r}}{r^5}=0.$$
Sorry. I don't know what's wrong with my latex.
 
Last edited:
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