# Gauss' Law help

1. Jul 26, 2005

### misogynisticfeminist

I wonder if i could compute resultant E-fields using Gauss' law and finding the field from the flux. I have a few difficulties, the first is of course, finding the E-field from the flux and the second is regarding the closed surface. how should i choose what surface to use, especially if the question just want me to find the resultant E-field alone? Thanks....

2. Jul 26, 2005

### OlderDan

Gaussian surfaces are chosen based on the known symmetry of the charge distribution. For any spherically symmetric charge distribution, you know that the electric field must be radial with constant magnitude at any specific distance from the center of symmetry, so you use a spherical surface for the Gaussian integral. For a uniform plane of charge, or just outside the surface of a metal, you know the field must point directly away from the surface and be constant at any given distance from it. The Gaussian "pillbox" exploits this symmetry to make the integral trivial to evaluate. Inside a metal, the field must be zero, so a closed surface everywhere within a metal yields a zero Gaussian integral. When there is no symmetry, Gauss' Law is still valid, but not particularly useful.