and two -q charges aligned along the x axis as follows... -q__________q__________-q <--- the space between them being d a) Suppose the +q charge is displaced perpendicularly by a small vertical distance dy. What is the total force F acting on it? Taylor expand your answer to leading order in dy b) Next suppose instead the +q charge is displaced along the x axis by a small dx distance. What is the force acting on it? -------------- So for a) I'm kind of stuck at the Taylor expansion on the force. F = (q)(-q)/4piε0d2 yhat + (q)(-q)/4piε0d2 where d= sqrt(x^2+dy^2) I'm at a point where I'm trying to simplify something but feel that I"m doing something wrong in the math Same for b) Thanks!