1. The problem statement, all variables and given/known data Calculate the electric field at a given position due to two point charges +q and -q located on the z axis with a separation d. q=10^-8 c P(1 1 1) d=0.1m 2. Relevant equations E=1/4*pi*Eo*r^3*[(3*r'*p'*r'/r^2)-p'] p'=q*d' (d<<r) 3. The attempt at a solution I think +q charge on the positive z axis and -q charge on the negative z axis so r'=p-(0 0 0). Is that wrong ? The charges on the z axis with separation d so happy mean d/2 on the z direction. Can ı write r'=p-(0 0 d/2) ? Which one ı choose ?