I need help with the electric field of a line of charge, a ring, semicircle, a line of charge at a point that bisects the line of charge, and a uniform disc. The teacher showed us how to derive those equations but he does it so fast that I didn't have time to copy them down. Even if I did I won't understand why he made certain moves. So can someone please show me how to derive the equations for the electric field and how everything works? I also don't understand the role charge density plays. In each one it comes up.