[SOLVED] Electric potential 1. The problem statement, all variables and given/known data The charge on the rod of the figure (length 2l, center at the origin) has a nonuniform linear charge distribution, λ = ax. Determine the potential V at: (a) points along the y-axis. (b) points along the x-axis. (Assume x > l) (express all answers in terms of a, x, l, ε0 and appropriate constants) 2. Relevant equations dQ = λdx dV = dQ/(4*pi*ε0*r) 3. The attempt at a solution For part a, V = 0 because dV = dQ/[4*pi*ε0*(x2 + y2)1/2] dx with limits of integration from -l to l. For part b, I'm having a really hard time determining the limits of integration and what "r" in dV is (ie, is it x or is it r, a segment of the rod?). I tried a lot of things, none of which produced the correct answer. Right now, I have Let k = 1/(4*pi*ε0) I'm treating "x" as a fixed distance from the rod, while calling r a segment or distance along the rod starting at x - l. dV = [k(ar)]/(x - l + r) dr with integration limits from x - l to x + l (one end of the rod to the other) With change of variables, u = x - l + r dx = du r = u - x + l integration limits become 2x - 2l to 2x dV = [ka(u - x + l)] /u du = ka (1 + (l - x)/u) du V = ka(u + (l - x)ln(u)) V = ka(2x - (2x - 2l) + (l - x)ln(2x) - (l - x)ln(2x - 2l)) V = ka(2l + (l - x)ln(2x / 2x - 2l)) V = ka(2l + (l - x)ln(x / (x - l))) I have a feeling that this is wrong, especially because I still don't completely understand what I'm supposed to integrate along, etc. Can someone explain how to go about solving this problem and point out what I am doing incorrectly? Thank you!