qc = {2,-1} * 1.6 * 10^-19;

xc = {-2,0} * 10^-10;

yc = {1,0} * 10^-10;

framesize = 3*10^-10;

eps0 = 8.854*10^-12;

V = Sum[qc[[b]]/Sqrt[(x-xc[[b]])^2+(y-yc[[b]])^2],
 {b,Length[qc]}] / (4 Pi * eps0);


ContourPlot[V,{x,-framesize,framesize},{y,-framesize,framesize},
	ContourShading->False,Contours->15,
	Frame->False,PlotPoints->40];