calculating flux through a surface

**TheRekz** · Apr 21st 2009, 12:20 PM

My question is:

Compute the flux of the vector field, $\vec{F}$ , through the surface, S.
$\vec{F} = 7\vec{r}$ and S is the part of the surface $z = x^2 + y^2$ above the disk $x^2 + y^2 \leq 4$ oriented downward.

My answer so far is

$\int\limits_R (7x\vec{i} + 7y\vec{j} + (x^2 + y^2)\vec{k}) \cdot (2x\vec{i} + 2y\vec{j} - k) dA$
$\int\limits_R (14x^2 + 14y^2 - (x^2 + y^2)) dA$
$\int\limits_R (13x^2 + 13y^2) dA$
$13 \int\limits_R (x^2 + y^2) dA$
$13 \int^{2\pi}_0\int^2_0 r^3 dr d\theta$
$13 \int^{2\pi}_0 4 d\theta$

$104\pi$

is this correct?

**Calculus26** · Apr 21st 2009, 02:28 PM

yes but for one cosmetic change

in you next to last integral you should have replaced dA with drdtheta since dA is rdrdtheta

--in your last integral dA should be dtheta

**TheRekz** · Apr 21st 2009, 02:30 PM

ok my bad , edited my original post, however my final result that I got is:

$1664\pi$ , and this is a web assign question and it doesn't accepts it (marks it as wrong)

**Calculus26** · Apr 21st 2009, 02:34 PM

Aah-- I didn't see it either r varies from 0 to 2 not 4

The radius of the disk is 2

**TheRekz** · Apr 21st 2009, 02:36 PM

I tried $104\pi$ and it doesn't work as well

**Calculus26** · Apr 21st 2009, 02:38 PM

In your first integral you have 7x +7y +z --should be 7x+7y+7z

**TheRekz** · Apr 21st 2009, 02:41 PM

thanks I got it now

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