Let M be the closed surface that consists of the hemisphere
M1: x2 + y2 + z2 = 1, z = 0,
and its base
M2: x2 + y2 = 1, z = 0 .

Let E be the electric field defined by E = (18 x, 18 y, 18 z). Find the electric flux across M. Write the integral over the hemisphere using spherical coordinates, and use the outward pointing normal.


(( a c
))M1 E ·dS = ((
)) f(?, f) d? df
b d
where

a = pi/2, b =0 , c =2pi , d =0 ,

Using t for ? and p for f,
f(?, f) = ??

((
))M1 E ·dS = ????

((
))M2 E ·dS = 0 , so

((
))M E ·dS = ????.

Respuesta :

Since [tex]M[/tex] is closed, you can use the divergence theorem: The flux of [tex]\vec E(x,y,z)[/tex] across [tex]M[/tex] is

[tex]\displaystyle\iint_{\partial M}\vec E\cdot\mathrm d\vec S=\iiint_M(\nabla\cdot\vec E)\,\mathrm dV=54\iiint_M\mathrm dV[/tex]

which is 54 times the volume of the hemisphere centered at (0, 0, 0) with radius 1, [tex]\boxed{36\pi}[/tex].

Judging by the question content, you're supposed to find this value by computing the the integral of [tex]\vec E[/tex] across [tex]M_1[/tex] and [tex]M_2[/tex].

  • Across [tex]M_1[/tex]:

Parameterize the hemisphere by

[tex]\vec r(u,v)=(\cos u\sin v,\sin u\sin v,\cos v)[/tex]

with [tex]0\le u\le2\pi[/tex] and [tex]0\le v\le\frac\pi2[/tex]. Take the normal vector to [tex]M_1[/tex] to be

[tex]\dfrac{\partial\vec r}{\partial v}\times\dfrac{\partial\vec r}{\partial u}=(\cos u\sin^2v,\sin u\sin^2v,\sin v\cos v)[/tex]

The flux of [tex]\vec E[/tex] across [tex]M_1[/tex] is

[tex]\displaystyle\iint_{M_1}\vec E\cdot\mathrm d\vec S=18\int_0^{\pi/2}\int_0^{2\pi}(\cos u\sin v,\sin u\sin v,\cos v)\cdot\left(\dfrac{\partial\vec r}{\partial v}\times\dfrac{\partial\vec r}{\partial u}\right)\,\mathrm du\,\mathrm dv[/tex]

[tex]=\displaystyle18\int_0^{\pi/2}\int_0^{2\pi}\sin v\,\mathrm du\,\mathrm dv=36\pi[/tex]

  • Across [tex]M_2[/tex]:

Parameterize the disk by

[tex]\vec s(u,v)=(u\cos v,u\sin v,0)[/tex]

with [tex]0\le u\le1[/tex] and [tex]0\le v\le2\pi[/tex]. Take the normal to [tex]M_2[/tex] to be

[tex]\dfrac{\partial\vec s}{\partial v}\times\dfrac{\partial\vec u}{\partial v}=(0,0,-u)[/tex]

Then the flux across [tex]M_2[/tex] is

[tex]\displaystyle\iint_{M_2}\vec E\cdot\mathrm d\vec S=18\int_0^{2\pi}\int_0^1(u\cos v,u\sin v,0)\cdot\left(\frac{\partial\vec s}{\partial v}\times\frac{\partial\vec s}{\partial u}\right)\,\mathrm du\,\mathrm dv=0[/tex]

Then the total flux across [tex]M[/tex] is [tex]36\pi[/tex], as expected.