# Where is my mistake? Calculating surface integral/Stoke's theorem

Let

$$F(x,y,z)= \begin{pmatrix} -y \\ 2x\\z \end{pmatrix}$$

be a vector field and $$A$$ a hemisphere

with $$x^2+y^2+z^2=9$$ , $$z>0$$

with a circular edge at the $$x,y$$- level with the unit normal vector $$n$$ showing outwards.

I want to determine

$$\int_A ( \nabla \times F) n\; do$$

1) as a surface integral
2) with Stoke's theorem.

1)

I used the parametrization $$\Phi(\phi, \theta) =\begin{pmatrix} R\sin \theta \cos\phi \\ R\sin\theta \sin \phi\\R\cos \theta \end{pmatrix}$$ with $$0 \leq \phi \leq 2 \pi$$ and $$0\leq \theta\leq \frac{\pi}{2}$$

for the unit normal:

$$\frac{ \delta \Phi }{ \phi} \times \frac{ \delta \Phi}{ \theta} = \begin{pmatrix} R^2\sin^2 \theta \cos \phi \\ R^2\sin^2 \theta \sin \phi \\R^2\sin \theta \cos\theta \end{pmatrix}$$

so integrate

$$\int_0^{ \frac{ \pi}{2}} \int_0^{2 \pi} \begin{pmatrix} -Rsin\theta \cos \phi \\ 2R\sin \theta \sin \phi \\R \cos\theta \end{pmatrix}\begin{pmatrix} R^2\sin^2 \theta \cos \phi \\ R^2\sin^2 \theta \sin \phi \\R^2\sin \theta \cos\theta \end{pmatrix} d\phi d\theta = 2 \pi R^3$$

2)

for Stoke's theorem I use as parametrization $$\Phi ( \phi) = \begin{pmatrix} -\sin \phi \\ \cos \phi \\ 0 \end{pmatrix}$$ because $$z=0 .$$

Then I get to calculate following:

$$\int_0^{2 \pi} \begin{pmatrix} -r \sin \phi \\ 2r\cos \phi \\ -r^2 \end{pmatrix}\begin{pmatrix} -\sin \phi \\ \cos \phi \\ 0 \end{pmatrix} d \phi =\int_0^{2 \pi} r \sin^2 \phi+ 2r \cos^2 \phi d \phi = 3\pi r,$$

and with $$r=3$$ follows $$9 \pi .$$

So, they are not equal. I dont see my mistake. Could not find one in the calculations, so there must be one in the process? I have lost perspective, I appreciate any help a loot !!

1) I don’t understand why didn’t you calculate the curl directly. I got $$\nabla \times F = (0,0,3)$$. I am sorry but — is the two $$R\sin\theta\cos\phi$$ a typo?

2) The parametrization part seems incorrect. In the context, the hemisphere has a radius 3. So you should probably go with $$\Phi ( \phi) = \begin{pmatrix} -R\sin \phi \\ R\cos \phi \\ 0 \end{pmatrix}$$ instead. Besides, it seems that you didn’t calculate the derivative in the last integration formula.

By the way, in both cases, my answer is $$3\times \pi R^2=27\pi$$.

EDIT: As a reply to the comment:

This is how I would work out the curve integral. First, find a parametrization such that it moves anti-clockwise as you see from the top. It has to rotate in that way because the Stokes’ theorem involves direction. For example, let $$x=R\cos t$$, $$y=R\sin t$$ and $$z=z$$, where $$t\in [0,2\pi]$$. Then use $$\oint_C F(x,y,z) \cdot\mathrm d l= \int_0^{2\pi} F(x,y,z) \cdot (x’(t), y’(t), z’(t)) \mathrm d t$$

Note that we have $$\mathrm d l = (x’(t), y’(t), z’(t)) \mathrm d t$$. In this case, $$\oint_C F(x,y,z) \cdot\mathrm d l = \int_0^{2\pi} (-R\sin t, 2R\cos t, 0)\cdot(-R\sin t, R\cos t, 0) \mathrm d x = R^2\int_0^{2\pi} (\sin^2 t + 2\cos^2 t) \mathrm d t = 3\pi R^2$$

• thank you ! okay, the 1) is clear..i don't know why I didnt see this. But what do you mean at 2) with the derivative? I get $R^2 \pi 3$ with $\int_0^{2 \pi} \begin{pmatrix} -Rsin \phi \\2Rcos \phi \\o \end{pmatrix} \begin{pmatrix} -Rsin \phi \\ Rcos \phi \\ 0 \end{pmatrix}$ Dec 12 '18 at 19:58
• @constant94 See my edit. Dec 13 '18 at 1:11
• Even easier, you can compute the line integral around the circle by applying Stokes's Theorem again, filling in the circle with a disk. So you get $3$ times the area of the circle of radius $R$, namely $3\pi R^2$. Dec 13 '18 at 1:35
• That’s true, and the easy way — projecting to the disk — can also be seen from the first method. Here I am just make some comparisons, in order to point out where he is mistaken. Dec 13 '18 at 1:46