1. Mar 13, 2013

1. The problem statement, all variables and given/known dat
Say I have a closed surface

is the flux across this closed surface only zero if the force originates from inside the surface?

What about if it originates outside the closed surface and can penetrate through the closed surface, would it still be zero?

Is it only zero for a magnetic force?

2. Relevant equations

3. The attempt at a solution

Last edited: Mar 13, 2013
2. Mar 13, 2013

Dick

I'm not sure exactly what you mean by 'originates from'. Define it more clearly and use vector calculus ideas and Maxwell's equations if you talking about electromagnetism.

3. Mar 14, 2013

What I mean is what if the force originates from outside the closed surface and passes through it, assuming the surface is made of a penetrable material.

4. Mar 14, 2013

Dick

Depends on what kind of field you are talking about. Sounds you are talking about sources of electric and magnetic fields. Formulate the question in terms of those, like if you have a charge outside the sphere, does it create a net flux? What about inside? Use theorems!

5. Mar 18, 2013

rude man

For electrostatic flux, yes, the surface integral of flux is zero if and only if there is no net charge inside the closed surface. That's what Gauss's theorem says. Charges external to the closed surface generate flux that goes in & out of the closed surface but their net contribution to flux = ∫∫E*dA is zero.

For magnetic fields of any kind, div B = 0 always so the integral of magnetic flux over any closed surface is always = 0.

cf. the Divergence theorem.

2. Relevant equations

3. The attempt at a solution[/QUOTE]