So here's the question...
A cylindrical shell of radius 7.00 cm and length 240 cm has its charge uniformly distributed on its curved surface. The magnitude of the electric field at a point 19.0 cm radially outward from its axis (mesaured from the midpoint of the shell) is 36.0 kN/C. Find (a) the net charge on the shell and (b) the electric field at a point 4.00 cm from the axis, measured radially outward from the midpoint of the shell.
Flux = EA = q/Eo
The Attempt at a Solution
I was able to get the answer at (a) but I'm seriously questioning myself after taking a look at (b). So, basically (b) states that the E inside the cylindrical shell is 0 since all the charges reside on the surface of the spherical shell (so there's no charge inside right?). I solved (a) by ....
(3.6e4)(2r(pi)x) = q/Eo... where x=19cm since that's the distance from the center of the cylindrical shell to the point at where the E is 36.0 KN/c.
So that's the right way to do it but here's what confuses me. Why do we use 19cm instead of (19-7=12cm) when there are no charges inside the cylindrical shell? Isn't the charges ONLY coming from the surface of the shell? So basically shouldn't the distance from the charge to the point where we find the E be 12cm?
Any help will be appreciated :D