I haev a hw problem where I need to find the magnitude and direction of the electric field from a thin, uniformly charged 18cm rod bent into a semicircle. The test point is at the center of the semicircle. It seems pretty basic but I am stuck on a point (assuming I am even on the right track). Because of symmetry I assume Ex = 0 so I just need to find Ey. the y component of dE is k*(dq/r^2) * sin (theta). So the sum of the little dE's can be found by integrating across the semicircle. The integral would look something like 2k*dq/r^2 *[integral from 0 to pi/2]sin (theta). I know dq=lambda/dL where L is the length of the bent rod. How do I go from dL to d(theta)? Thanks!