# Homework Help: Divergence Theorm example for 28 Nov 12:00

1. Nov 27, 2012

### debian

1. The problem statement, all variables and given/known data

Let S be a smooth surface enclosing the volume V, and let $\vec{n}$ to be the unit outward normal. Using the Divergence Theorm show that:

∫∫ x $\vec{r}$ ° $\vec{n}$ dS = 4 * ∫∫∫ x dV,

where $\vec{r}$=(x,y,z)

2. Relevant equations

Divergence theorm

http://www.math.oregonstate.edu/home...rg/diverg.html [Broken]

3. The attempt at a solution

I tried to change the form of the those two equations to the form stated in divergence theorm and then to compare the u (or F as in link above), but the u (F) on the left hand side is never equal to this on the right.

Last edited by a moderator: May 6, 2017
2. Nov 27, 2012

### clamtrox

Your link is broken. But let's suppose the divergence theorem says $$\iint (\vec{F} \cdot \vec{n}) dS = \iiint \nabla \cdot \vec{F} dV.$$ Now you are given $\vec{F}= x \vec{r}$. Can you calculate $\nabla \cdot \vec{F}$?

3. Nov 27, 2012

### debian

$\nabla$ $\cdot$ $\vec{F}$ = (d/dx, d/dy, d/dz) $\cdot$ (x^2,xy,xz) = 2x+x+x=4x

Last edited: Nov 27, 2012
4. Nov 27, 2012

Good job. :)