Evaluate the surface integral ∫∫s F · dS for the given vector field F and the oriented surface S. In other words, find the flux of F across S. For closed surfaces, use the positive (outward) orientation. F(x, y, z) = x i + y j + 10 k. S is the boundary of the region enclosed by the cylinder x2 + z2 = 1 and the planes y = 0 and x + y = 8.

Respuesta :

Use the divergence theorem.

[tex]\vec F(x,y,z)=x\,\vec\i\math+y\,\vec\jmath+10\,\vec k\implies\nabla\cdot\vec F(x,y,z)=\dfrac{\partial(x)}{\partial x}+\dfrac{\partial(y)}{\partial y}+\dfrac{\partial(10)}{\partial z}=2[/tex]

The div theorem says the integral of [tex]\vec F[/tex] across [tex]S[/tex] is equal to the integral of [tex]\nabla\cdot\vec F[/tex] over the region with boundary [tex]S[/tex] (call it [tex]R[/tex]):

[tex]\displaystyle\iint_S\vec F(x,y,z)\cdot\mathrm d\vec S=\iiint_R(\nabla\cdot\vec F(x,y,z))\,\mathrm dx\,\mathrm dy\,\mathrm dz[/tex]

[tex]=\displaystyle2\iiint_R\mathrm dx\,\mathrm dy\,\mathrm dz[/tex]

Convert to cylindrical coordinates:

[tex]\begin{cases}x=r\cos\theta\\y=y\\z=r\sin\theta\end{cases}\implies\mathrm dx\,\mathrm dy\,\mathrm dz=r\,\mathrm dr\,\mathrm d\theta\,\mathrm dy[/tex]

Then the integral is

[tex]=\displaystyle2\int_0^{2\pi}\int_0^1\int_0^{8-r\cos\theta}r\,\mathrm dy\,\mathrm dr\,\mathrm d\theta=\boxed{16\pi}[/tex]