Conceptual question about electric flux

[faller217]

I am studying electric flux through both a disk, cube, and sphere. I understand how the flux is calculated in a disk, but I don't understand how the flux through a cube with the point charge outside the cube is equal to zero when the disk is not. Is this because the disk is 2-D?

Thank you for your help

 

[Hootenanny]

I am studying electric flux through both a disk, cube, and sphere. I understand how the flux is calculated in a disk, but I don't understand how the flux through a cube with the point charge outside the cube is equal to zero when the disk is not. Is this because the disk is 2-D?

Thank you for your help

You've got the right idea. The disc is an open surface whereas the cube and sphere are closed surfaces, this means that they enclose a volume. Now since there are no charges inside these surfaces, any electric field lines that pass through one of the faces of the cube, must also pass out of another face. The same is true of the sphere. Therefore, there is as much flux entering the cube and sphere as there is leaving it, hence the net flux through those surfaces is zero.

Do you follow?

 

[faller217]

I follow perfectly. Thanks very much for explaining! :)