1. The problem statement, all variables and given/known data A circular arc of charge has a radius R and contains a total charge Q. If the angle of the arc is 90 degrees find: a) the charge density of the arc b) the electric field at point P in terms of the charge density L and the radius of the arc R L should really be lambda and P is the center of the arc. 2. Relevant equations E = (1/4pi Epsilon naught) s is the arc length 3. The attempt at a solution a) (I got this part right) Q = Ls, L = Q/s s = 1/4 circle = (1/2)pi R L = 2Q/pi R b) (I didn't get this part right) I said that because the arc is symmetric, the electric field in the x direction has the same magnitude as the electric field in the y direction, so I only solved for one of them. I decided to integrate dE = (1/4pi Epsilon) (dQ/R squared) * sin theta (for the x direction) from 0 to pi/2 I said that dQ = L * dS and that (this may very well be the part where I went wrong) ds = R*d theta for small theta so that I could integrate in terms of d theta. When I integrated I got Ex = L/(4 pi Epsilon R) *-cos theta evaluated from 0 to pi/2 That last part ended up being 1, so for my answer I got Ex = L/(4 pi Epsilon R), which was wrong. Hope you followed that. These are test corrections, so I might have made dumb math mistakes during my panic. I believe the correct answer is something like what I got but multiplied by square root of 2.