1. The problem statement, all variables and given/known data Find the electric field a distance z above the midpoint of a straight line segment of length 2L, which carries a uniform line charge λ. 2. Relevant equations 1) my textbook says : E(r) = 1/4πεo ∫V λ(r')/r2 r dl' 2) and this also works? : E(r) = 1/4πεo ∫V λ(r')/r3 r dl' both where r is the unit vector from the charge to the point we are caluclating the field at 3. The attempt at a solution I can integrate once I get the setup down just fine. I just wanted to know when and why this two are the same. For example, in my book it says for the setup for this problem : E(r) = 2 * 1/4πεo ∫V(λdx/r2)cosθz For one thing, how did that cos appear and since it equals z/r couldn't 2) have just been used from the start? And if 2) can be used could I apply it to surface and volume integrals?