# Gauss's Law and electric flux

1. Jan 25, 2010

### Keithkent09

Four closed surfaces, S1 through S4, together with the charges -2Q, Q, and -Q are sketched in the figure below. (The colored lines are the intersections of the surfaces with the page.) Find the electric flux through each surface. (Use Q for the charge Q and epsilon_0 for 0.)
(Picture Attached)

2.
Electric Flux= the integral of EdA=q/epsilon_0

3.
All that I could think to do was set q=the charge of the inside of the desired surface. I was not sure how I could quantitatively define how the different charges within the surfaces affected each other.

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2. Jan 25, 2010

### dacruick

Gauss law is your best friend. the flux through a surface will only be non zero if there is a charge inside that surface. think about putting an imaginary sphere in the middle of a river. All the water that runs into the sphere also runs out of it. Having a charge inside the surface is like putting a sprinkler inside your imaginary sphere. Now the amount of water in is 0, and the amount of water out is this analogys version of flux.

So any surface with no charge inside, has 0 flux.

This principle is displayed simply in your equations for flux. where flux is = to
charge enclosed / epsilon naught.

So your flux for the red surface with Q and -2Q, it will have a flux of -Q/epsilon naught