Earnshaw's Theorem: Stability in Electrostatics?

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Discussion Overview

The discussion revolves around Earnshaw's theorem and its implications for the stability of charged particles in electrostatic systems. Participants explore the conditions under which a charged particle may be considered in stable equilibrium, particularly in the context of external forces acting on other charges.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants question whether a charged particle can be in stable equilibrium when placed between two equal positive charges held by external forces, suggesting that axial displacement leads to a restoring force.
  • Others argue that for equilibrium to be stable, it must restore the particle in all directions, not just axially, raising concerns about perpendicular displacements.
  • There is a discussion about the interpretation of Earnshaw's theorem, with some noting that it refers to a "charged particle" while others suggest it applies to a collection of charged particles.
  • One participant proposes that a single charged particle may be in equilibrium but not in stable equilibrium, suggesting it is in a neutral equilibrium instead.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of Earnshaw's theorem to single versus multiple charged particles, and whether the equilibrium described can be considered stable under the conditions outlined. The discussion remains unresolved with multiple competing interpretations.

Contextual Notes

Some limitations include the dependence on the dimensionality of the system and the specific conditions under which the charges are held. The discussion does not resolve the implications of these factors on the theorem's validity.

kini.Amith
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In Electrodynamics text by Griffiths there is the statement of Earnshaw's theorem "a charged particle cannot be held in a stable equilibrium by electrostatic forces alone." But if we consider the system in which a positive charge is placed midway(where E is zero) between two positive charges of equal magnitude which are held in position by external forces. If the charge in the middle is displaced axially , then the electrostatic force will force it back into the equilibrium position.So isn't the charge in stable equilibrium. Isn't this a violation of Earnshaw's theorem?
 
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In order for the equilibrium to be stable, it must force back any small displacement from the equilibrium, in every direction, not just axially.
Consider what happens when you move the charge in a perpendicular direction to the axis.
 
kini.Amith said:
In Electrodynamics text by Griffiths there is the statement of Earnshaw's theorem "a charged particle cannot be held in a stable equilibrium by electrostatic forces alone." But if we consider the system in which a positive charge is placed midway(where E is zero) between two positive charges of equal magnitude which are held in position by external forces. If the charge in the middle is displaced axially , then the electrostatic force will force it back into the equilibrium position.So isn't the charge in stable equilibrium. Isn't this a violation of Earnshaw's theorem?
Collection of charged particles, not "a charged particle". See: http://en.wikipedia.org/wiki/Earnshaw%27s_theorem
 
zoki85 said:
Collection of charged particles, not "a charged particle". See: http://en.wikipedia.org/wiki/Earnshaw%27s_theorem
Is this true? i have seen the wikipedia page, but the text specifically says"a charged particle". Is it not valid for a single particle?
Boorglar said:
In order for the equilibrium to be stable, it must force back any small displacement from the equilibrium, in every direction, not just axially.
Consider what happens when you move the charge in a perpendicular direction to the axis.
I see. So it is valid only in three dimensions.Thanks
 
kini.Amith said:
Is this true? i have seen the wikipedia page, but the text specifically says"a charged particle". Is it not valid for a single particle?

I guess the reason for the caveat here may be that a single charged particle is clearly in equilibrium.
 
Nabeshin said:
I guess the reason for the caveat here may be that a single charged particle is clearly in equilibrium.
But not in a stable equilibrium as stated in the theorem, just in a neutral equilibrium.
 

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