Electric Field: Deriving Equations and Understanding Charge Density

[musicfairy]

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.

 

[LowlyPion]

This should give you a good start:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elelin.html#c2

 

[musicfairy]

Thanks. That will go on my favorites list. But it doesn't have a semi circle. Any ideas where I can find that?

 

[LowlyPion]

I think the examples there should suggest ways to calculate it.