E-field between two concentric cylinders (not homework)

1. Oct 18, 2012

JDStupi

Hi, I have a question. Suppose you have two concentric cylinders of radii a and b with opposite uniform surface charges, line charges or something to that effect. Now, I know (or was told) that the E-field between the two, that is for a<r<b, is equal to the field from the inner cylinder (radius a). This is because if you draw a Gaussian surface in the region between a and b the only enclosed charge is that of the inner cylinder and, as such, the field produced will be solely due to that cylinder. Now, my question is what is physically going on? I understand the mathematical explanation in terms of Gauss Law, but I do not understand why, physically, the outer cylinder would not contribute. the way I see it there would be charges emitting a field from both and being equal and opposite they would cancel leaving nothing in that region. Why does the presence or lack thereof of a charge not make a difference wthin that region? I understand that the flux has to be zero from the outer cylinder as whatever goes in the cylinder comes out the other side thus making a net flux of zero, but I don't know, something about the physical field picture is confusing me. Simply looking at the E-field it seems counterintuitive that the field, though proximal, wouldn't make a difference.

2. Oct 18, 2012

Staff: Mentor

You might want to do the exercise of directly calculating the field just due to the outer cylindrical shell of charge for all points within the shell. You'll find that its contribution to the field inside the shell is exactly zero. (At least for an infinitely long cylinder.)