# Electric flux through a surface

1. Sep 29, 2013

### Plasmosis1

1. The problem statement, all variables and given/known data
A point charge q is situated a distance d/2 above a square surface of side d as shown (see attached).
a) It cannot be determined
b) 2q/ε0d
c) q/4$\pi$ε0
d) q/$\pi$ε0d2
e) q/6ε0

2. Relevant equations

$\Phi$=qenc0=$\oint$E*dA

3. The attempt at a solution
Shouldn't the answer be zero because whatever enters the area also leave the area? If not I know I should use the integral of E*dA but I don't know what E would be.

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Last edited: Sep 29, 2013
2. Sep 29, 2013

### rude man

If the area were a thin volume you'd be correct, the net flux into the volume = 0, but flux thru a surface A = the integral over the surface of the dot-product of the electric field E and an element of surface area dA: flux = ∫∫ E*dA.

So yes, you need to integrate E*dA over the surface. dA is normal to the surface at every point on the surface.

3. Sep 29, 2013

### nasu

You can manage without integration.
Imagine the charge is in the middle of a cube with side d.
Due to symmetry, the flux through each face of the cube is the same.
And the total flux through the cubic, closed surface can be found from Gauss' law.

4. Sep 29, 2013

### rude man

Nasu has come up with the right and simple way to solve this.