Let $T$ be the area lying in the first octant where $x\geq0,y\geq0,z\geq0$ limited by the surfacs $z=a^2-x^2$ and $y=a^2-x^2$.
Calculate $\iint_S \vec{F}\cdot\hat{N}dS$ where $\vec{F}=(x,y,z)$ for $(x,y,z)\in\mathbb{R^3}$, $S$ is the part of the boundary $\partial T$ to $T$ that lies on the surface $z=a^2-x^2$ and $\hat{N}$ is the unit normal vector pointing out from $T$.
I have discussed the problem with a couple of friends, and we are all stuck. We where thinking about calculating $Div\vec{F}$ but then again we struggle with the limits of that triple integral. Any help would be really great! Thanks in advance
Best Answer
To make good use of the divergence theorem:
By the divergence theorem you find a total flux of $\tfrac{8}{5}a^5$ so the flux through the part $S$ is half of this: $\tfrac{4}{5}a^5$.
You can verify this by actually computing the surface integral. To do this, we parametrize $S$ (the paraboloid $z=a^2-x^2$ cut off by the paraboloid $y=a^2-x^2$) as follows: $$\vec r(u,v)=\left(u,v\left(a^2-u^2\right),a^2-u^2\right) \;,\; 0 \le u \le a\;,\; 0 \le v \le 1$$ Then: $$\vec N=\frac{\partial \vec r}{\partial u}\times\frac{\partial \vec r}{\partial v}=\left(2a^2u-2u^3,0,4-u^2\right)$$ and: $$\vec F \cdot \vec N=a^4-u^4$$ so: $$\iint_S \vec F \cdot \vec N\,\mbox{d}S = \int_0^1 \int_0^a \left(a^4-u^4\right)\,\mbox{d}u \,\mbox{d}v=\frac{4}{5}a^5$$