# Flux

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• Jul 27th 2008, 11:06 PM
asi123
Flux
I need to find the flux of the vector field F through S (in the pic), when S represent the edges of a cube.
My question is, how do I find N (normal)? Do I need to split the curb and to find the flux through each face?
• Jul 28th 2008, 03:17 AM
mr fantastic
Quote:

Originally Posted by asi123
I need to find the flux of the vector field F through S (in the pic), when S represent the edges of a cube.
My question is, how do I find N (normal)? Do I need to split the curb and to find the flux through each face?

Yes. And you know the normal to each face, right?
• Jul 28th 2008, 04:11 AM
asi123
Quote:

Originally Posted by mr fantastic
Yes. And you know the normal to each face, right?

Is this right?
• Jul 28th 2008, 04:13 AM
mr fantastic
Quote:

Originally Posted by asi123
Is this right?

Yes.
• Jul 28th 2008, 06:32 AM
asi123
Another thing, why the fluxes aren't canceling one another?
• Jul 28th 2008, 07:16 PM
mr fantastic
Quote:

Originally Posted by asi123
Another thing, why the fluxes aren't canceling one another?

Why should they cancel? The vector field is not constant ...... eg. For the flux through faces in the plane z = 0 and z = 1, F is different at those two faces .....
• Jul 29th 2008, 10:20 PM
asi123
Still a bit stuck
Lets say I want to calculate the flux which goes through the upper face.
I know the vector field is F, and I know the normal, so I came up with this thing in the pic.
My question is, how do I find ds?
• Jul 30th 2008, 02:33 AM
asi123
I think I found a better way using Gauss' law, is this right?
• Jul 30th 2008, 04:15 AM
mr fantastic
Quote:

Originally Posted by asi123
Lets say I want to calculate the flux which goes through the upper face.
I know the vector field is F, and I know the normal, so I came up with this thing in the pic.
My question is, how do I find ds?

For a cube:
If you're in the plane z = a, dS = dx dy.
If you're in the plane y = a, dS = dx dz.
If you're in the plane x = a, dS = dy dz.

Quote:

Originally Posted by asi123
I think I found a better way using Gauss' law, is this right?

Looks fine.