I'm trying to verify the divergence theorem for the vector field $F=(x+y)i+j(y+z)+k(z+x)$ in the region $$0\leq x^2+y^2 \leq 9\\
0\leq z\leq 5$$
I got the the divergence integral equal to $135\pi$ which is the book's answer.
But I got the wrong result for the flux across the cylinder.
Here are my calculations:
For the cylinder I projected the surface in the xz-plane
$\iint F \cdot n d\sigma= \iint \frac{(9+xy+yz)}{y} dz dx$
Where
$n=\frac{x\hat{i}+y\hat{j}}{\sqrt{x^2+y^2}}$,
$y=\pm \sqrt{9-x^2}$.
For each value of y I've evaluated
$\int_{-3}^{3} \int_{0}^{5} (\frac{9}{y}+x+z )dzdx$
I got the sum of the flux of the two surface integrals equal to
$45\pi+75+(-45\pi+75)$
I added this to the sum of the flux of the upper surface($z=5$) and lower surface($z=0$)
For the upper surface I got flux equal $45\pi$ and for lower surface I got flux equal to $0$ But the net flux doesn't equal to the divergence
Where was I wrong?
Best Answer
Your calculations of the flux through the upper and lower surfaces are correct. I couldn't follow your calculation of the flux through the cylindrical surface, but in any event, it's easier to change to cylindrical coordinates, I think. With $(x,y,z)=(3\cos{\theta}, 3\sin{\theta},z)$ we have $\vec{n}=\cos{\theta}\vec{i}+\sin{\theta}\vec{j}$ so that$$ \begin{align} F\cdot\vec{n}&= (3\cos{\theta}+3\sin{\theta})\cos{\theta}+(3\sin{\theta}+z)\sin{\theta} \\ &=3+3\sin{\theta}\cos{\theta}+z\sin{\theta} \end{align}$$ Since the surface element is $3\mathrm{d}z\mathrm{d}\theta$, the flux through the cylindrical surface is $$\int_0^{2\pi}\int_0^5(3+3\sin{\theta}\cos{\theta}+z\sin{\theta})3\mathrm{d}z\mathrm{d}\theta$$ which is easily evaluated to be $90\pi$.