clear all; k = 9e9; q1 = 1.0e-6; q2 = -1.0e-6; ax = 1.0; ay = 0; bx = -1.0; by = 0; [X, Y] = meshgrid(-2:0.9:2,-2:0.9:2); V = 1./sqrt((X - ax).^2 + (Y-ay).^2 ) * k * q1 + 1./sqrt((X-bx).^2 + (Y-by).^2) * k * q2; surfc(X, Y, V); [Ex, Ey] = gradient(-V, 0.2, 0.2); figure contour(X, Y, V); hold on; quiver(X, Y, Ex, Ey);