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Doubt in plotting Vector Fields !

by iamnotageek
Tags: vector field plot
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iamnotageek
#1
Dec6-12, 06:22 AM
P: 5
Hi,

I have a doubt in plotting the vector field.

In the post http://www.physicsforums.com/showthread.php?t=155579 it is mentioned that a vector field could be plotted for F (x,y) by, marking the (x,y) as the tail and F(x,y) as the head portion.

If so, then consider the function, F(x,y)=(x,y)

The, if the input is (2,4) then output is (2,4)

Then, if it plotted, there will be only points everywhere right? Because, the head and the tail portion is marked at the same point.

But, when I tried the same using a online plotter (http://cose.math.bas.bg/webMathemati...on/VectorField) , I got a different result, which I have attached.
Attached Images
File Type: bmp vector field for f(x,y)=(x,y).bmp (147.3 KB, 4 views)
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Mark44
#2
Dec6-12, 08:41 AM
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Quote Quote by iamnotageek View Post
Hi,

I have a doubt in plotting the vector field.

In the post http://www.physicsforums.com/showthread.php?t=155579 it is mentioned that a vector field could be plotted for F (x,y) by, marking the (x,y) as the tail and F(x,y) as the head portion.

If so, then consider the function, F(x,y)=(x,y)

The, if the input is (2,4) then output is (2,4)

Then, if it plotted, there will be only points everywhere right? Because, the head and the tail portion is marked at the same point.
Right.
Quote Quote by iamnotageek View Post

But, when I tried the same using a online plotter (http://cose.math.bas.bg/webMathemati...on/VectorField) , I got a different result, which I have attached.
It might be that the online graphing software doesn't handle zero vectors correctly.
HallsofIvy
#3
Dec6-12, 08:46 AM
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Quote Quote by iamnotageek View Post
Hi,

I have a doubt in plotting the vector field.

In the post http://www.physicsforums.com/showthread.php?t=155579 it is mentioned that a vector field could be plotted for F (x,y) by, marking the (x,y) as the tail and F(x,y) as the head portion.
No, that's not what I said. I said:
Choose some point (x,y), Caculate the vector F(x,y)= -yi+ xi, draw that vector starting at (x,y) (with its "tail" at (x,y)).
If so, then consider the function, F(x,y)=(x,y)

The, if the input is (2,4) then output is (2,4)
You are confusing points and vectors. If you use (x, y) to mean both the point (x,y) and the vector from point (0,0) to (x,y) then you are going to be confused! Since you learned to use (x, y) to mean a point way back in "pre-Calculus", it is better to use either <x, y> or xi+ yj to denote the vector. Then F(x, y)= <x, y> or, better, F(x, y)= xi+ yj. F(2, 4)= 2i+ 4j. With its "tail" at (2, 4), its head would be at (2+2, 4+ 4)= (4, 8).

Then, if it plotted, there will be only points everywhere right? Because, the head and the tail portion is marked at the same point.

But, when I tried the same using a online plotter (http://cose.math.bas.bg/webMathemati...on/VectorField) , I got a different result, which I have attached.


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