Question

Use the divergence theorem to find the outward flux of the vector field F(xyz)= 4x^2 i + 4y^2 j + 3z^2 k across the boundary of the rectangular prism: 0

Answer 1

$\vec F(x,y,z)=4x^2\,\vec\imath+4y^2\,\vec\jmath+3z^2\,\vec k\implies\nabla\cdot\vec F=8x+8y+6z$

Let $S$ be the surface of the rectangular prism bounded by the planes $x=0$, $x=a$, $y=0$, $y=b$, $z=0$, and $z=c$. By the divergence theorem, the integral of $\vec F$ over $S$ is given by the integral of $\nabla\cdot\vec F$ over the interior of $S$ (call it $R$):

$\displaystyle\iint_S\vec F\cdot\mathrm d\vec S=\iiint_R(\nabla\cdot\vec F)\,\mathrm dV$

$=\displaystyle\int_0^c\int_0^b\int_0^a(8x+8y+6z)\,\mathrm dx\,\mathrm dy\,\mathrm dz=\boxed{abc(4a+4b+3c)}$