Earnshaw's Theorem: Why Can't I Contain a Charged Particle Electrostatically?

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Discussion Overview

The discussion revolves around Earnshaw's Theorem and its implications for the equilibrium of charged particles under electrostatic forces. Participants explore the reasoning behind the theorem, its applicability to systems of charges, and comparisons to gravitational forces and other concepts like the hairy ball theorem.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants express confusion about why a charged particle cannot be held in equilibrium solely by electrostatic forces, particularly in configurations like a hexagon with a charge at the center.
  • It is noted that while the electric field may be zero at a point, this does not imply stable equilibrium, as any slight deviation would not necessarily return the particle to its original position.
  • One participant suggests that additional forces would be needed to maintain the positions of the charges in a hexagonal arrangement, which may extend beyond the scope of Earnshaw's Theorem.
  • Another participant argues that Earnshaw's Theorem applies to both the entire system of charges and to individual charges held in equilibrium by electrostatic fields.
  • A comparison is made to the hairy ball theorem, questioning whether the concepts are related.
  • Participants discuss the nature of equilibrium, distinguishing between stable and unstable equilibrium points, and emphasize that while equilibrium can exist, it does not guarantee stability.
  • One participant raises a hypothetical scenario involving a triangular arrangement of charges and questions the applicability of Earnshaw's Theorem to Lagrange points in gravitational systems.
  • Clarification is provided that Earnshaw's Theorem prohibits stable electrostatic equilibrium, but equilibrium with zero net force is possible.

Areas of Agreement / Disagreement

Participants generally agree that Earnshaw's Theorem applies to electrostatic forces and that stable equilibrium cannot be achieved under these conditions. However, there is disagreement regarding the implications of the theorem and the nature of equilibrium, with some participants expressing confusion and seeking further clarification.

Contextual Notes

There are unresolved assumptions regarding the definitions of equilibrium and stability, as well as the implications of the theorem in different contexts, such as gravitational systems.

MaxwellsDemon
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I'll be honest, I've never really understood the "why" of Earnshaw's Theorem. Why can't a charged particle be held in equalibrium with only electrostatic forces? If I have six charges arranged in at the corners of a hexagon and put a charge at the center, why can't it just stand there since the electric field is zero at that point? What is the reasoning/justification for this theorem? Does it apply to gravitostatic forces as well?
 
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MaxwellsDemon said:
I'll be honest, I've never really understood the "why" of Earnshaw's Theorem. Why can't a charged particle be held in equalibrium with only electrostatic forces? If I have six charges arranged in at the corners of a hexagon and put a charge at the center, why can't it just stand there since the electric field is zero at that point?
The particle would not be in stable equilibrium.
What is the reasoning/justification for this theorem?
The simplest way to get stable equilibrium is to arrange it so that the field always points inward toward the particle. Then, any slight deviation from its position will push it back. But that would imply a nonzero divergence of the field in free space, contrary to Gauss's law.
Does it apply to gravitostatic forces as well?
Yes. It applies to any inverse square force law.
 
MaxwellsDemon said:
I'll be honest, I've never really understood the "why" of Earnshaw's Theorem. Why can't a charged particle be held in equalibrium with only electrostatic forces? If I have six charges arranged in at the corners of a hexagon and put a charge at the center, why can't it just stand there since the electric field is zero at that point? What is the reasoning/justification for this theorem? Does it apply to gravitostatic forces as well?

As I see it you will need extra forces to hold the six charges at the corners of your hexagon and this goes beyond the scope of Earnshaw because as I understand it the theorem applies to the whole system of charges and to electrostatic forces only.
 
Dadface said:
As I see it you will need extra forces to hold the six charges at the corners of your hexagon and this goes beyond the scope of Earnshaw because as I understand it the theorem applies to the whole system of charges and to electrostatic forces only.
It certainly applies to the complete set of charges, but it also applies to an individual charge held in equilibrium by electrostatic fields only.
 
Isn't this an application of the hairy ball theorem?
 
Doc Al said:
It certainly applies to the complete set of charges, but it also applies to an individual charge held in equilibrium by electrostatic fields only.

Thanks for the clarification Doc Al.
 
Perhaps I'm a little slow but, I'm still not getting it... stable or unstable equilibrium point, its still an equilibrium point...which means that if I put a charge in that exact position it should just stay there. Suppose I simplify this a bit. Imagine I have a triangular pool ball rack with three positively charged pool balls in the corners. If I take another positively charged pool ball and put it in the exact center, I would think that the system could just sit there like that. The total electric force on the middle ball should be zero since the vector sum of the electric fields from the corner balls is zero, but Earnshaw's Theorem says my thinking is wrong, why? Since its true for gravity too, why can't I just stick a satillite at a Lagrange point for the Earth/Moon/Sun system and have it just stay there? Does it have something to do with the potential?
 
MaxwellsDemon said:
(stable or) unstable equilibrium point, its still an equilibrium point...which means that if I put a charge in that exact position it should just stay there.

That, precisely is the error.

You cannot balance a pencil on it's tip.
 
In post #2 everything is written very shortly Doc Al.
The first thing that only stable equilibrium state is considered.
The nonstable equilibrium state there of course exists.
May be you could find a situation when a stable equilibrium state exist?
That is when you slightly push a charge from this stable equilibrium state there appears
a force returning to this stable point. Of course this stable point is without charges.
 
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  • #10
MaxwellsDemon said:
Perhaps I'm a little slow but, I'm still not getting it... stable or unstable equilibrium point, its still an equilibrium point...which means that if I put a charge in that exact position it should just stay there. Suppose I simplify this a bit. Imagine I have a triangular pool ball rack with three positively charged pool balls in the corners. If I take another positively charged pool ball and put it in the exact center, I would think that the system could just sit there like that. The total electric force on the middle ball should be zero since the vector sum of the electric fields from the corner balls is zero, but Earnshaw's Theorem says my thinking is wrong, why? Since its true for gravity too, why can't I just stick a satillite at a Lagrange point for the Earth/Moon/Sun system and have it just stay there? Does it have something to do with the potential?
You misunderstand Earnshaw's theorem, which prohibits stable electrostatic equilibrium. Just plain old equilibrium--zero net force--is trivially possible.
 

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