Gauss's law derivation using solid angle

AI Thread Summary
The discussion focuses on the rigorous mathematical derivation of Gauss's law using the concept of solid angles, referencing Jackson's proof. A key question arises regarding the relationship between the expressions Eda and dΩ, specifically how (1/r^2)cosθda equals dΩ. The need for a clear definition of solid angle is emphasized, as the poster seeks to understand this concept from the ground up. Additionally, the poster inquires about proving that total flux is zero when the charge lies outside the surface, questioning whether a solid angle approach or Stokes' theorem would be more effective. The thread highlights the importance of foundational understanding in advanced electromagnetic theory.
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So, I was trying to find a rigorous mathematical derivation of gauss's law(please I don't want to hear again any field lines nonsense) and I stumbled upon jackson's proof which uses the solid angle concept and seems a solid enough proof(stupid joke:smile:).The problem is that it's the first time I come in contact with solid angles and I don't get a certain part

Eda=(q/4πε)(1/r^2)cosθda=(q/4πε)dΩWhy
(1/r^2)cosθda=dΩ? (intuitively it makes sense but what is the actual proof)

I'd appreciate if someone gave a clear from zero definition of solid angle

Say we've proven Φ=Qen/ε
if the charge lies outside of the surface how do you prove that the total flux is zero?
Is there a solid angle approach or stoke's theorem is the easiest way?
Forgive me for not using latex it's my first post
 
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There is a good derivation in Section 1.4 of Franklin "Classical Electromagnetism".
 
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