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MathGroup Archive 2000

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Re: Question:Polar Field Plot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg21618] Re: Question:Polar Field Plot
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Tue, 18 Jan 2000 02:35:09 -0500 (EST)
  • Organization: Universitaet Leipzig
  • References: <85s1ag$aj5@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

ZEE MEANT NG wrote:
> 
> I am trying to plot a 2D vector field. Equations which i have are in
> polar form ie r and theta.
> r^2= x^2+y^2
> Er=f(r,Theta)
> Ep=g(r,Theta)
> 
> I have changed them to cartesian form ie Ex, Ey .
> and put Theta= arctan[y/x]
> My plot covers the region {x,-1,1} and {y-1,1}
> 
> Question 1:
> The values of theta produced by mathematica are
> -90<Theta<90 degree but what i required is theta which gives the
> value from 0 to 360 degree.
> I have used the if statement as follows but it does not work.
> If[x >= 0, [Theta] = ArcTan[y/x], [Theta] = ArcTan[y/x] + \[Pi]];
> PlotVectorField[{Ex1, Ey1},  {x, - R, R},
>  {y, - R, R }]

ArcTan[x,y]  ? Notice that x and y are exchanged to the C-function
atan2(y,x)

> 
> Question 2:
> Using the built-in function to plot the vector field, it plots all
> the field for the entire region. How do i set some constraints so
> that it only plots at the desired region (eg. plot where x^2+y^2 < R)
> ?
> 

No.

> I am having the same problem in the contour plot as well. I will be
> very grateful if someone can advice me on how to solve the above
> problems.

The book by Tom Wickham-Jones has some functions to constrain a contour
plot
to a region. The Matheamtica code is on MathSource

http://www.mathsource.com/Content/Enhancements/Graphics/3D/0208-976

Hope that helps
  Jens


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