Confusion with the divergence of E fields

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Discussion Overview

The discussion revolves around the divergence of electric fields and the implications for charge density in different dimensional contexts. Participants explore the relationship between the direction of electric field components and the resulting charge density, considering both two-dimensional and three-dimensional scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant calculates the charge density for an electric field of the form ##\mathbf{E} = 3x\mathbf{i} + 3y\mathbf{j}## and finds it to be ##\rho = 6\epsilon_0##, but notes confusion when changing the direction of one component to ##\mathbf{E} = 3x\mathbf{i} - 3y\mathbf{j}##, resulting in a charge density of zero.
  • Another participant suggests considering the scenario from a three-dimensional perspective, questioning what the charge density must be to generate a similar electric field in 3D.
  • A participant challenges the notion of geometric similarity between the two electric fields presented, implying that they may not be as similar as initially thought.
  • Further clarification is provided regarding the divergence of the electric field in three dimensions, indicating that a constant surface charge density can produce a specific electric field configuration.
  • One participant mentions the relationship between electric flux from point sources in different dimensions, highlighting the differences in behavior across 1D, 2D, and 3D contexts.

Areas of Agreement / Disagreement

Participants express differing views on the geometric similarity of the electric fields and the implications for charge density. The discussion remains unresolved regarding the interpretation of the divergence results and the dimensional considerations involved.

Contextual Notes

The discussion involves assumptions about dimensionality and the nature of electric fields, which may not be fully articulated. The relationship between charge density and electric field divergence is also context-dependent and may require further clarification.

maNoFchangE
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Suppose I have electric field of the form ##\mathbf{E} = 3x\mathbf{i} + 3y\mathbf{j}##. Calculating the charge density gives me ##\rho = \epsilon_0 \nabla\cdot\mathbf{E} = 6\epsilon_0##.
But now if I turn one of the components of the field in the opposite direction, for example ##\mathbf{E} = 3x\mathbf{i} - 3y\mathbf{j}##, then the charge density vanishes. I am confused with this because the only difference between the first and the second fields is just the direction, geometrically they are similar. Where do I go wrong?
 
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You are on a 2D world. What must be ##\rho## on 3D world to make a field like ##\mathbf{E} = 3x\hat{\mathbf{i}}+3y\hat{\mathbf{j}} + 0\hat{\mathbf{k}} ##?
 
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Untitled-2.png

Do these look "geometrically similar" to you?
 
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theodoros.mihos said:
You are on a 2D world. What must be ##\rho## on 3D world to make a field like ##\mathbf{E} = 3x\hat{\mathbf{i}}+3y\hat{\mathbf{j}} + 0\hat{\mathbf{k}} ##?
The divergence of such a field is ##6\epsilon##, so this kind of charge distribution may generate that field.
@Fightfish ah I see so they are actually quite different.
 
A constant surface charge density ##\rho## make a field ##\mathbf{E} = c\mathbf{k}##, for infinity surface. Flux by point sources relates by ##1/r^2## for 3D, by ##1/r## for 2D and are constants for 1D. Just trust your calculation.
 

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