Quiver lines on contour not showing

Question

Armando Marchena 2016-5-19

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https://ww2.mathworks.cn/matlabcentral/answers/285047-quiver-lines-on-contour-not-showing

评论： Star Strider 2016-5-21

Hi there, I have a contour plot which I successfully generated. However when I turn on the quiver lines, they do not show up on the plot. Below is my code:

 [x,y] = meshgrid([1:0.1:6],[1:0.1:6]); %Creates axis.
 f  = 8.9875517873681764*10^9;
 PC1 = 1e-6;     %Charge of point one.
 PC2 = 1e-6;     %Charge of point two.
 PC3 = 0.5e-6;   %Charge of point three.
 n = 20;         %N-number of levels.
 %Charge of individual points.
 Charge1 = (f.*PC1)./sqrt((x-3).^2+(y-3).^2);
 Charge2 = (f.*PC2)./sqrt((x-5).^2+(y-3).^2);
 Charge3 = (f.*PC3)./sqrt((x-4).^2+(y-4).^2);
 Sum = Charge1-Charge2+Charge3;
 figure(1)
 [px,py]=gradient(Sum);
 contour(x,y,Sum,n)
 hold on
 quiver(x,y,px,py)
 hold off

Does anyone have an idea why the quiver lines are not showing? Thanks.

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Answer 1

Star Strider 2016-5-20

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There seems to be a problem with the way you’re calculating the gradient. Making these changes will allow you to see the vectors and the contour plot (that I temporarily turned into contourf to make it visible). I will defer to you to determine what you need to do to fix it with the vector scaling turned back on:

   [px,py]=gradient(Sum, 1E+4, 1E+4);         % Set Derivative Intervals
   figure(1)
   contourf(x,y,Sum,n)
   hold on
   quiver(x,y,px,py,0)                        % Turn Off Scaling
   hold off