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Part of the surface, S, is: $z=x^2+y^2$ above the disk $ \ x^2+y^2 = 1 \ $ oriented in the $\vec k$ direction. I need to set up an integrated integral to calculate the flux of $\vec F = yz\vec i+xz\vec j-y^2\vec k$ through S.

I am wanting to make sure I am setting up the flux integral properly before I begin to calculate it.

$$\int_S \vec F \cdot dA = \int_S \vec F(x,y,f(x,y)) \cdot dA $$

First I found dA:

$$dA = (-f_x\vec i-f_y\vec j+\vec k)d xd y=(-2x\vec i-2y\vec j+\vec k)d xd y$$

Then found $\vec F(x,y,f(x,y))$: $$\vec F(x,y,f(x,y)) = yz\vec i+xz\vec j-y^2\vec k=y(x^2+y^2)\vec i +x(x^2+y^2)\vec j-y^2\vec k$$

Then I changed to polar coordinates:

$$dA=(-2r\cos\theta\vec i-2r\sin\theta\vec j+\vec k)rd rd \theta$$ $$\vec F(r,\theta)=r^3\sin\theta\vec i +r^3\cos\theta\vec j-r^2\sin^2\theta\vec k$$

Did the dot product of the two vectors obtaining:

$$(-4r^4\cos\theta \sin\theta-r^2\sin^2\theta)$$

Thus, $$\int_{\theta=0}^{\theta=2\pi}\int_{r=0}^{r=1} (-4r^4\cos\theta \sin\theta-r^2\sin^2\theta)r dr d\theta$$

Does this seem right? I'm working off the example in the book, and as usually not very helpful with intermediate steps.

colormegone
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Student
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  • Your $dA$ does not look very infinitesimal... I think that's actually the normal vector field... but in the end it looks right. I haven't checked the arithmetic. – James S. Cook Nov 25 '13 at 21:12
  • I made a small edit to the statement of the problem, since it did not indicate the radius of the disk, which only finally appeared in the integration limit near the end of the post. Note, incidentally, that $ \ \nabla \cdot \vec{F} \ = \ 0 \ $ . – colormegone Apr 27 '14 at 05:14

1 Answers1

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I think you have your thoughts in the right direction. However, I would be careful about a couple of things:

1) Generally we abuse notation by writing $d \vec{S} = \vec{n} \cdot dS$ denoting the oriented infinitesimal surface element, with orientation given by the unit outward normal $\vec{n}$. Therefore, your $dA$ should been written different. Also, do not write $\delta x \, \delta y$ for $dx \, dy$.

2) I would switch to polar coordinates only after I've completely set up the double integral in the plane. Then you exploit the circular symmetry by switching into polar coordinates.

The rest looks okay. Like James, I haven't really checked your substitutions but I considered these points relevant enough to write an answer.

Mark Fantini
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  • Is it incorrect to switch to polar coordinates before setting up the double integral completely? or is it just better practice to help reduce mistakes on future problems? thanks for your input also – Student Nov 25 '13 at 22:45