Is the Net Force on a Charge at the Center of a 13-Sided Polygon Zero?

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Homework Help Overview

The problem involves determining the net force on a test charge placed at the center of a regular 13-sided polygon, where equal charges are located at each vertex. The discussion centers around the application of symmetry and the principle of superposition in electrostatics.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore whether the net force is zero by comparing the situation to simpler cases, such as an equilateral triangle. Questions arise about the implications of symmetry in this context and whether the reasoning holds for a 13-sided polygon.

Discussion Status

Some participants have attempted calculations for simpler shapes and are discussing the implications of their findings for the 13-sided case. There is an ongoing exploration of symmetry arguments and the mathematical justification for the net force being zero, but no consensus has been reached yet.

Contextual Notes

Participants are considering the effects of symmetry and the need for mathematical proofs involving trigonometric series to support their reasoning. The original poster expresses uncertainty about the application of symmetry in this specific case.

aaaa202
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Homework Statement


13 equal charges are placed in the corners of a regular 13-sided polygon. What is the force on a test chrage at the center?


Homework Equations


principle of superposition
symmetry

The Attempt at a Solution


I am not sure if the net force is 0 or not. Had the polygon been 12-sided the symmetry would be obvious, but I'm not sure in this case. Can anyone explain if the net force is 0 or not?
 
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hi aaaa202! :smile:
aaaa202 said:
13 equal charges are placed in the corners of a regular 13-sided polygon. What is the force on a test chrage at the center?

try it first for an equilateral triangle (3 equal charges) …

is that 0 ?​

then use the same method for 13 (you'll need a formula for summing a trig series)
 
Did the calculations for a triangle as you said, where i fixed the coordinates such that one of the charges sat on the y-axis. From there it was easy to see that Fres = 0, and I think that this must hold for any rotation of the coordinate system. After all I think the length of a vector is more or less defined to be preserved on a rotation in the euclidean coordinate system. However, I'm not sure, so can you confirm this? :/
And can you perhaps come up with a symmetry argument that makes it easy to see, that rotation of the coordinate system should not change the lFresl? :)
 
aaaa202 said:
Did the calculations for a triangle as you said, where i fixed the coordinates such that one of the charges sat on the y-axis. From there it was easy to see that Fres = 0

yes!

presumably you did cos0 + cos2π/3 + cos4π/3 = 1 - 1/2 - 1/2 = 0 ?

ok you now need to prove cos0 + cos2π/13 + cos4π/13 + … cos24π/13 = 0 :smile:
 

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