Why Charge Accumulates at Edges/Corners?

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Discussion Overview

The discussion centers around the phenomenon of charge accumulation at the edges and corners of objects, particularly focusing on plate capacitors and human fingers. Participants explore the underlying reasons for this behavior in the context of electrostatics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that charge accumulates at edges and corners due to the repulsion between like charges, which causes them to "try" to maximize their distance from one another.
  • Another participant introduces the concept of "field enhancement," explaining that the electric field strength is greater at sharp corners due to the geometry of the metallic surface, leading to increased charge accumulation in those areas.
  • A mathematical expression for surface charge density near the edge of a conductor is proposed, indicating that it varies inversely with the square root of the distance from the corner.

Areas of Agreement / Disagreement

Participants present multiple viewpoints regarding the mechanisms of charge accumulation, with no consensus reached on a singular explanation. The discussion remains open to further exploration of the topic.

Contextual Notes

The discussion involves assumptions related to the geometry of conductors and the nature of electric fields, which may not be fully resolved. The dependence on material properties (conductors vs. dielectrics) is also noted but not conclusively addressed.

tommyers
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Hi,

Could someone help me understand why charge accumulates at the edges / corners of items - such as our fingers... but more importantly on the edges / corners of plate capacitors!

Thank you in advance.

Regards

Tom
 
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Because like charges repel. If you don't mind the anthropomorphism, charges "try" to get as far as possible from other like charges.

- Warren
 
tommyers said:
Hi,

Could someone help me understand why charge accumulates at the edges / corners of items - such as our fingers... but more importantly on the edges / corners of plate capacitors!

Thank you in advance.

Regards

Tom

What you are asking about is what is known as the "field enhancement". When you put a metallic object in an electric field, the strength of the field is enhanced at the sharp corners. To be able to show this, one has to solve the electrostatic Poisson's equation with the appropriate boundary condition reflecting the geometry of the metallic surface.

When you solve this, you'll notice that the electric field "lines" tend to be "bunched up" at sharp edges and corners. This means that the external field that has been applied to the object is enhanced at these locations. The amount of enhancement depends on the degree of sharpness.

Hence, since the electric field is stronger at these locations, there will be more charges accumulating there.

This is also true, somewhat, for dielectric and insulators, but the field enhancement isn't as pronounced as metals under the same field amplitude.

Zz.
 
You can calculate the surface charge density at a corner of a conductor.
Near the edge of a conducting sheet, it goes like
\sigma~1/\sqrt{r}.
 

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