Given that you mention the divergence theorem, and that part (b) is asking you to find the downward flux through the disk
, I think it's same to assume that the hemisphere referred to in part (a) is the upper half of the sphere
.
a. Let
denote the hemispherical <u>c</u>ap
, parameterized by
![\vec r(u,v)=\sqrt3\cos u\sin v\,\vec\imath+\sqrt3\sin u\sin v\,\vec\jmath+\sqrt3\cos v\,\vec k](https://tex.z-dn.net/?f=%5Cvec%20r%28u%2Cv%29%3D%5Csqrt3%5Ccos%20u%5Csin%20v%5C%2C%5Cvec%5Cimath%2B%5Csqrt3%5Csin%20u%5Csin%20v%5C%2C%5Cvec%5Cjmath%2B%5Csqrt3%5Ccos%20v%5C%2C%5Cvec%20k)
with
and
. Take the normal vector to
to be
![\vec r_v\times\vec r_u=3\cos u\sin^2v\,\vec\imath+3\sin u\sin^2v\,\vec\jmath+3\sin v\cos v\,\vec k](https://tex.z-dn.net/?f=%5Cvec%20r_v%5Ctimes%5Cvec%20r_u%3D3%5Ccos%20u%5Csin%5E2v%5C%2C%5Cvec%5Cimath%2B3%5Csin%20u%5Csin%5E2v%5C%2C%5Cvec%5Cjmath%2B3%5Csin%20v%5Ccos%20v%5C%2C%5Cvec%20k)
Then the upward flux of
through
is
![\displaystyle\iint_C\vec F\cdot\mathrm d\vec S=\int_0^{2\pi}\int_0^{\pi/2}((2\sqrt3\cos v+2)\,\vec k)\cdot(\vec r_v\times\vec r_u)\,\mathrm dv\,\mathrm du](https://tex.z-dn.net/?f=%5Cdisplaystyle%5Ciint_C%5Cvec%20F%5Ccdot%5Cmathrm%20d%5Cvec%20S%3D%5Cint_0%5E%7B2%5Cpi%7D%5Cint_0%5E%7B%5Cpi%2F2%7D%28%282%5Csqrt3%5Ccos%20v%2B2%29%5C%2C%5Cvec%20k%29%5Ccdot%28%5Cvec%20r_v%5Ctimes%5Cvec%20r_u%29%5C%2C%5Cmathrm%20dv%5C%2C%5Cmathrm%20du)
![\displaystyle=3\int_0^{2\pi}\int_0^{\pi/2}\sin2v(\sqrt3\cos v+1)\,\mathrm dv\,\mathrm du](https://tex.z-dn.net/?f=%5Cdisplaystyle%3D3%5Cint_0%5E%7B2%5Cpi%7D%5Cint_0%5E%7B%5Cpi%2F2%7D%5Csin2v%28%5Csqrt3%5Ccos%20v%2B1%29%5C%2C%5Cmathrm%20dv%5C%2C%5Cmathrm%20du)
![=\boxed{2(3+2\sqrt3)\pi}](https://tex.z-dn.net/?f=%3D%5Cboxed%7B2%283%2B2%5Csqrt3%29%5Cpi%7D)
b. Let
be the disk that closes off the hemisphere
, parameterized by
![\vec s(u,v)=u\cos v\,\vec\imath+u\sin v\,\vec\jmath](https://tex.z-dn.net/?f=%5Cvec%20s%28u%2Cv%29%3Du%5Ccos%20v%5C%2C%5Cvec%5Cimath%2Bu%5Csin%20v%5C%2C%5Cvec%5Cjmath)
with
and
. Take the normal to
to be
![\vec s_v\times\vec s_u=-u\,\vec k](https://tex.z-dn.net/?f=%5Cvec%20s_v%5Ctimes%5Cvec%20s_u%3D-u%5C%2C%5Cvec%20k)
Then the downward flux of
through
is
![\displaystyle\int_0^{2\pi}\int_0^{\sqrt3}(2\,\vec k)\cdot(\vec s_v\times\vec s_u)\,\mathrm du\,\mathrm dv=-2\int_0^{2\pi}\int_0^{\sqrt3}u\,\mathrm du\,\mathrm dv](https://tex.z-dn.net/?f=%5Cdisplaystyle%5Cint_0%5E%7B2%5Cpi%7D%5Cint_0%5E%7B%5Csqrt3%7D%282%5C%2C%5Cvec%20k%29%5Ccdot%28%5Cvec%20s_v%5Ctimes%5Cvec%20s_u%29%5C%2C%5Cmathrm%20du%5C%2C%5Cmathrm%20dv%3D-2%5Cint_0%5E%7B2%5Cpi%7D%5Cint_0%5E%7B%5Csqrt3%7Du%5C%2C%5Cmathrm%20du%5C%2C%5Cmathrm%20dv)
![=\boxed{-6\pi}](https://tex.z-dn.net/?f=%3D%5Cboxed%7B-6%5Cpi%7D)
c. The net flux is then
.
d. By the divergence theorem, the flux of
across the closed hemisphere
with boundary
is equal to the integral of
over its interior:
![\displaystyle\iint_{C\cup D}\vec F\cdot\mathrm d\vec S=\iiint_H\mathrm{div}\vec F\,\mathrm dV](https://tex.z-dn.net/?f=%5Cdisplaystyle%5Ciint_%7BC%5Ccup%20D%7D%5Cvec%20F%5Ccdot%5Cmathrm%20d%5Cvec%20S%3D%5Ciiint_H%5Cmathrm%7Bdiv%7D%5Cvec%20F%5C%2C%5Cmathrm%20dV)
We have
![\mathrm{div}\vec F=\dfrac{\partial(2z+2)}{\partial z}=2](https://tex.z-dn.net/?f=%5Cmathrm%7Bdiv%7D%5Cvec%20F%3D%5Cdfrac%7B%5Cpartial%282z%2B2%29%7D%7B%5Cpartial%20z%7D%3D2)
so the volume integral is
![2\displaystyle\iiint_H\mathrm dV](https://tex.z-dn.net/?f=2%5Cdisplaystyle%5Ciiint_H%5Cmathrm%20dV)
which is 2 times the volume of the hemisphere
, so that the net flux is
. Just to confirm, we could compute the integral in spherical coordinates:
![\displaystyle2\int_0^{\pi/2}\int_0^{2\pi}\int_0^{\sqrt3}\rho^2\sin\varphi\,\mathrm d\rho\,\mathrm d\theta\,\mathrm d\varphi=4\sqrt3\pi](https://tex.z-dn.net/?f=%5Cdisplaystyle2%5Cint_0%5E%7B%5Cpi%2F2%7D%5Cint_0%5E%7B2%5Cpi%7D%5Cint_0%5E%7B%5Csqrt3%7D%5Crho%5E2%5Csin%5Cvarphi%5C%2C%5Cmathrm%20d%5Crho%5C%2C%5Cmathrm%20d%5Ctheta%5C%2C%5Cmathrm%20d%5Cvarphi%3D4%5Csqrt3%5Cpi)