20e-lecture23

20e-lecture23 - Lecture 23 - Wednesday May 27th...

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Unformatted text preview: Lecture 23 - Wednesday May 27th jacques@ucsd.edu Key words: Curl, Conservative vector field, formal determinant 23.1 Conservative vector fields Recall that f : R n R n is conservative if f = F for some F : R n R . A test for conservativity of a given vector field in R 2 or R 3 is done using the curl of the vector field. Definition. The curl of a vector field ( f,g,h ) in R 3 , denoted curl( f ) or f , is the vector ( h y- g z ,f z- h x ,g x- f y ). An easy way to remember the formula for the curl is to write it as a formal determinant: f = fl fl fl fl fl fl i j k x y z f g h fl fl fl fl fl fl . Here we use the notation i = (1 , , 0), j = (0 , 1 , 0) and k = (0 , , 1), and when we multiply x by g for example, we formally obtain g x . As an exercise, verify that this gives the above formula from the definition. We now come to a simple test for a vector field f : R 3 R 3 to be conservative. An oriented curve in R 3 is simple if it the image of a one-to-one piecewise C 1 map r : [ a,b ] R 3 . It is closed if it is one-to-one on ( a,b ] and r ( a ) = r ( b ). Test for Conservativity. 1. A vector field f is conservative if and only if f = 0. 2. A vector field f is conservative if and only if for any two simple oriented curves and with the same endpoints, R f dr = R f dr . 3. A vector field f is conservative if and only if for any simple oriented closed curve , R f dr = 0. Example 1. Let f ( x,y,z ) = ( yz,xz,xy ). We saw that this is conservative since f = F where F = xyz . Now we check it using the test f = fl fl fl fl fl fl i j k x y z yz xz xy fl fl fl fl fl fl = i xy y- xz z - j xy x- yz z + k xz x- yz y = ( x- x,y- y,z- z ) = 0 ....
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20e-lecture23 - Lecture 23 - Wednesday May 27th...

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