From Griffiths' Intro to electrodynamics:
Now I'm confused about 3 things:
1) What is the 'mobile' surface charge density? Isn't the surface current density itself the 'mobile' surface charge density?
2) Is the small current $dI$ in the whole ribbon or only part of the ribbon? I'm guessing its the whole ribbon of width $dl$ but I want to make sure.
3) If I'm asked to find the surface current density as follows:
Is the current flowing only on the surface of the wire or is it flowing naturally through the whole volume of the wire and I need to only account for the current on the surface? , I'm able to derive $K$ will be equal to $I/2\pi a$ mathematically, but if the current is flowing throughout the volume of the wire, how can I visualize the unit length perpendicular to that flow? Thanks I hope I was clear about the questions