# Math Help - Stokes Theorem

1. ## Stokes Theorem

Verify Stokes Theorem ∬(∇xF).N dA where surface S is the paraboloid z = 0.5(x^2 + y^2) bound by the plane z=2, C is its boundary, and the vector field F = 3yi - xzj + yzk

I had found (∇xF) = (z+x)i + (-z-3)k

r = [u, v, 0.5(u^2 + v^2)]
Therefore N= ru X rv = -ui -uj +k
Therefore (∇xF).N = [(z+x), 0, (-z-3)].[-x, -y, 1]
After that I substitute x = rcos(θ), y = rsin(θ), z = 0.5r^2
Thus ∫(0-2)∫(0-2pi) (∇xF).N r dθdr

But I cant seems to get the answer. Can anyone help? Help would greatly appreciated

2. [QUOTE=HeheZz;510745]Verify Stokes Theorem ∬(∇xF).N dA where surface S is the paraboloid z = 0.5(x^2 + y^2) bound by the plane z=2, C is its boundary, and the vector field F = 3yi - xzj + yzk

I had found (∇xF) = (z+x)i + (-z-3)k

r = [u, v, 0.5(u^2 + v^2)]
Therefore N= ru X rv = -ui -uj +k[/tex]
First this is wrong because this N is not a unit vector.

Therefore (∇xF).N = [(z+x), 0, (-z-3)].[-x, -y, 1]
After that I substitute x = rcos(θ), y = rsin(θ), z = 0.5r^2
Thus ∫(0-2)∫(0-2pi) (∇xF).N r dθdr

But I cant seems to get the answer. Can anyone help? Help would greatly appreciated
It seems to me simpler to use $u= r \cos(\theta)$, $v= r \sin(\theta)$ from the start: $\rho(r, \theta)= r \cos(\theta)\vec{i}+ r\sin(\theta)\vec{j}+ (1/2)r^2\vec{j}$ so that $\rho_r= \cos(\theta)\vec{i}+ \sin(\theta)\vec{j}+ r\vec{k}$ and $\rho_\theta= -r \sin(\theta)\vec{i}+ r \cos(\theta)\vec{j}$.

Now, $\rho_r\times\rho_\theta= -r^2 \cos(\theta)\vec{i}- r^2 \sin(\theta)\vec{j}+ r\vec{k}$.

There is no need to calculate "N" and "dS" separately since both would involve calculating the length of that vector which would then cancel. Instead $\vec{N}dS= d\vec{S}= (-r^2 \cos(\theta)\vec{i}- r^2 \sin(\theta)\vec{j}+ r\vec{k})dr d\theta$.

$\nabla F= (z+ x)\vec{i}+ (-z- 3)\vec{k}= ((1/2)r+ r \cos(\theta))\vec{i}- ((1/2)r- 3)\vec{j}$ so $\nabla F\cdot d\vec{S}= (-r^2((1/2)\cos(\theta)+ \cos^2(\theta)- ((1/2)r- 3)r^2 \sin(\theta))dr d\theta$.

The parts involving only $\sin(\theta)$ and $\cos(\theta)$ are easy- the integral with respect to $\theta$ is 0.

That leaves only $-\int_{r= 0}^2 \int_{\theta= 0}^{2\pi} r^2 \cos^2(\theta) d\theta dr$.

By the way, notice the negative sign in front of that. Doing a vector integration over a surface involves a choice of orientation. I chose to orient with "z component upward". Choosing z component downard will reverse the sign. Of course, in using Stoke's theorem, you also have to choose an orientation for the path integral around the boundary- which direction you will integrate around the boundary. An orientation for one implies a corresponding orientation for the other.