MATLAB: Does anyone know why not all of the streamlines complete themselves

electric field linesstreamline

close all; clear;
x = -100:.05:100; y = -100:.05:100;
[X,Y] = meshgrid(x,y);
%positive charge at x =0 y = 0
u = 25*X./(X.^2 + Y.^2).^1.5; %x-hat
v = 25*Y./(X.^2 + Y.^2).^1.5; %y-hat
%negative charge at x=0 y=-5
u2 = -30*X./(X.^2 + (Y+5).^2).^1.5; %x-hat
v2 = -30*(Y+5)./(X.^2 + (Y+5).^2).^1.5; %y-hat
%creating streamline starting points
%in a circle about the positive charge
theta = 0:pi/9:2*pi;
startx = 1*cos(theta);
starty = 1*sin(theta);
%plot the streamlines in the X-Y Plane
figure;
streamline(X,Y,u+u2,v+v2,startx,starty)

Best Answer

If you check out the documentation for the streamline function, you'll see that you can specify the maximum number of vertices the streamline will have (i.e. how long each streamline is).
By changing the last line of your code to this:
streamline(X,Y,u+u2,v+v2,startx,starty,[0.1,20000])
where 0.1 is the step size (default) and 20,000 is the max number of vertices, the plot looks like this:
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