Energy of an uniformly charged bar of length L

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Discussion Overview

The discussion revolves around the challenges of calculating the electric potential energy of a uniformly charged bar of length L and total charge Q, comparing it to the case of a uniformly charged sphere. Participants explore the reasons behind the difficulties encountered in the integration process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant expresses difficulty in calculating the electric potential energy of a uniformly charged bar, suggesting it should be simpler like that of a uniformly charged sphere.
  • Another participant attributes the complexity to the reduced symmetry in the bar compared to the sphere.
  • A follow-up question is raised regarding how the symmetry argument relates to the divergence in the energy integral computation.
  • It is noted that the zero radius of the line charge leads to an infinite electric field, resulting in infinite energy calculations.
  • A participant suggests that if the charged bar were modeled as a uniformly charged cylindrical surface, the energy calculation would not yield an infinite result.

Areas of Agreement / Disagreement

Participants generally agree on the challenges posed by the geometry of the charged bar compared to the sphere, but the discussion includes differing views on the implications of these geometrical considerations on energy calculations. The discussion remains unresolved regarding the specifics of the energy calculations.

Contextual Notes

Participants mention the need for finite fields to achieve finite energies, indicating a dependence on the definitions and assumptions regarding charge distributions and their geometries.

DaTario
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Hi All

I would like to know why is it so difficult to calculate by integration the electric potential energy of an uniformily charged bar of length L and total charge Q. I have tried hard, thinking it would be as easy as the case of a uniformly charged sphere but my efforts failed.

Why these two systems are so different ?

Best Regards

DaTario
 
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Because there is less symmetry in the bar case than in the sphere case.
 
Ok with respect to the simmetry, but how this argument relates to divergence in the computation of the energy's integral?

Thanks,

DaTario
 
The radius of your line charge is zero. This means an infinite E field at the wire.
The energy calculation is infinite because of this. The energy of a point charge is also infinite. You need finite fields to get finite energies.
 
ok. good.

thanks

DaTario
 
Let me just provoke a little further. If my bar (or rod) of length L were a cylinder surface uniformly charged, then no infinite would appear in the energy calculation. Is it?

best regards,

DaTario
 
That's right.
 

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