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Old Sep 21st 2016, 04:40 AM   #11
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Originally Posted by ohm View Post
I think there may be charges inside the box. Equal no of +ve and -ve charges may be inside the box. In effect net charge will be zero.
If so then there would be no flux through the surface.

Originally Posted by ohm View Post
My question is Why INSIDE the box why not Surface of the box.
I think you're trying to read too much into the question. The point is this: if there is a flux through any closed boundary, there must be a charge within that boundary.
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Old Sep 22nd 2016, 10:10 AM   #12
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Ohm, do you feel like you're getting the hang of Gauss's Law problems? You've got some kind of material (maybe conductor, maybe insulator, sometimes both) which may or may not have a net charge. Then you construct some kind of shell around it, usually a box or spherical shell or cylindrical shell in these problems. This shell is typically imaginary! It's just there to enclose the material which may or may not be charged. You shouldn't think of the shell as something that might conduct the charge. It's just a surface to help you solve the problem. In your problem, don't think of the box as touching the charged material inside. It's just there to surround it so that you can ask hypothetical questions and employ Gauss's Law.

Also, there may be some positive and negative charges in the real material that sits within the imaginary shell, but we're only worried about the net charge. In your problem, you mention an equal number of +ve and -ve charges. That tells you immediately that the net charge adds up to zero.

You're absolutely right regarding charge being forced to the surface of a conductor. When envisioning a solid metal sphere (which is the real material and not the imaginary enclosing surface) with a net negative charge for instance (extra electrons), you can "feel" the electrons repelling each other and pushing each other to the surface of the sphere. They'll stop there because they can go no farther, although the "push" is still present as they sit on the surface. Now let's try some imaginary spherical shells. One that resides completely within the surface of the metal sphere will have no net charge inside (we know it's all on the surface of the real sphere and outside of our imaginary shell). So no charge within = no flux. Now put another imaginary shell totally outside the real metal sphere. The net negative charge is all inside it. So there will be flux per the equation given.

Quite the long-winded answer. Make sense though?
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