# Derivation of gauss's law for electrical fields please help

1. Feb 3, 2007

### Ragnar

This is a very stupid question. extremely stupid. In fact I'm extremely embarassed.

I was reading a text on electromagnetism, and it said that since the flux due to a charge does not depend on the radius of the sphere then the formula, q/permitivitty applies to all closed surfaces. This is where i got confused. why does that make it apply to all closed surfaces?

please don't laugh at me.

Please I need an answer!

Last edited: Feb 3, 2007
2. Feb 4, 2007

### Hootenanny

Staff Emeritus
Try calculating the electric flux of spheres with various radii using $\phi = \int\vec{E}\cdot d\vec{A}$ and see where that gets you. Intuitively, one can consider the definition of electric flux (number of electric field lines passing through a unit area) and the definition of the electric field and how it relates to the density of electric field lines...

3. Feb 4, 2007

### robphy

After calculating for spheres, try (say) a "northern" hemisphere at radius r1 and a "southern" hemisphere at radius r2 with an http://mathworld.wolfram.com/Annulus.html" [Broken] joining their "equators". Note the flux through this annulus.

Then, approximate ANY closed surface by portions of spheres and portions of annuli. ...take limits.

Last edited by a moderator: May 2, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook