A short example of Green's theorem.
An Example
Consider F = 3xy i +
2y^{2} j and the
curve C given by the quarter circle of radius 2 shown to the
right. We are taking C to have positive orientation:
that is, we are traversing it in the counter-clockwise direction.
We could evaluate the line integral of F.dr along C directly, but it is almost always easier to use Green's theorem. Thus we have
In this case the domain D is most easily described in polar coordinates, for which 0 <= r <= 2 and 0 <= theta <= pi/2, so that the integral becomes
Some Related Strands
Another Example
Green's Theorem only works for simple, closed curves.
Further, we assume a positive orientation. The following
illustrates what happens when one or both of these are not the case.
Consider the same vector field we used above, F = 3xy i + 2y^{2} j, and the curve C_{1} shown in figure 2, which is the quarter circle starting at the point (0,2) and ending at (2,0). To find the line integral of F on C_{1} we can't apply Green's Theorem directly, but can do it indirectly.
First, note that the integral along C_{1} will be the negative of the line integral in the opposite direction. Thus by reversing signs we can calculate the integrals in the "positive" direction and get the integral we want.
Second, note that, calling C_{2} the curve shown in figure 3, we can write the integral we did in the first example in terms of the integrals on C_{1} and C_{2}:
(be sure you see where the negative sign on the left-hand side of this equation comes from!) so that to find only the line integral on C_{1} we can subtract:
We already know the first term on the right-hand side from above (it's 8), so that the line integral we want is equal to 8 minus the line integral on C_{2}, which is easy to find. On the section along the x-axis, y is zero, and so F is zero and there is no contribution to the line integral. On the section along the y-axis, x is zero, and F = <0, 2y^{2}>. We can parameterize the curve with r(t) = <0, 2-2t> (with 0 <= t <= 1), so
And we have