# Gauss' Law (imaginary surface)

1. Dec 17, 2007

### geronimo8

1. The problem statement, all variables and given/known data
A certain region of space bounded by an imaginary closed surface contains no charge.
Is the electric field always zero on the surface? If not, under what circumstances is it zero on the surface?

2. Relevant equations
Φ = Qenclosed÷εo = EA

3. The attempt at a solution
Since the imaginary surface need no any material object. I think the E refres to the inside surface, so it is not zero...

2. Dec 17, 2007

### Staff: Mentor

Imagine the field produced by a single positive point charge at the origin, you know what the field looks like. Now, draw your surface as a sphere that does not include the origin. What does the field look like on the surface? Is it zero?

3. Dec 20, 2007

### geronimo8

I can't imagine that clearly... *sigh* sorry...

Last edited: Dec 20, 2007
4. Dec 20, 2007

### student85

One question related to this problem: When you get asked about the electric field on the surface is it the average electric field over the surface?? I mean, in the case of a sphere, only when having the charge in the center will you have the same electric field all throughout the surface...