# Homework Help: Surface Integral over a Hemisphere (Work check please! I end up with zero!)

1. Apr 28, 2010

### NastyAccident

1. The problem statement, all variables and given/known data
Seawater has density 1025 kg/m^3 and flows in a velocity field v=yi+xj, where x, y, and z are measured in meters and the components of v in meters per second. Find the rate of flow outward through the hemisphere x^2+y^2+z^2=9, z≥0

2. Relevant equations
Surface integral of F over S is ∫∫ F • dS
In this case,

p * ∫∫s F • n dS

Where n = the cross product between rtheta and rphi.

3. The attempt at a solution
First, I parameterized the surface:

$$\vec r(\theta,\phi) = \langle 3\sin\phi\cos\theta,3\sin\phi\sin\theta,3\cos\phi \rangle$$
Where 0 < theta < 2pi and 0 < phi < pi/2.

Partial with respect to theta:
$$\vec r_{\theta}(\theta,\phi) = \langle -3\sin\phi\sin\theta, 3\sin\phi\cos\theta, 0 \rangle$$

Partial with respect to phi:
$$\vec r_{\phi}(\theta,\phi) = \langle 3\cos\phi\cos\theta, 3\cos\phi\sin\theta, -3\sin\phi \rangle$$

Cross:
$$\vec r_{\phi}(\theta,\phi) \times r_{\theta}(\theta,\phi) = \langle 9\sin^{2}\phi\cos\theta, 9\sin^{2}\phi\sin\theta, 9\cos\phi\sin\phi \rangle$$

Next, I look at the velocity field and grab the velocity vector:
$$\vec v = \langle 3\sin\phi\sin\theta,3\sin\phi\cos\theta,0 \rangle$$

I am now set to integrate:
$$\int\int_S \delta\vec v \cdot d\vec S = \int\int_{(\phi,\theta)} \delta\vec v \cdot \vec r_\phi \times \vec r_\theta\ d\phi d\theta$$

$$\int^{2\pi}_{0}\int^{\pi/2}_{0} (1025)* \langle 3\sin\phi\sin\theta,3\sin\phi\cos\theta,0 \rangle \cdot \langle 9\sin^{2}\phi\cos\theta, 9\sin^{2}\phi\sin\theta, 9\cos\phi\sin\phi \rangle d\phi d\theta$$

After the dot product I end up with....
27sin^3(phi)cos(theta)sin(theta) + 27sin^3(phi)cos(theta)sin(theta)
which, I simpify to:
54sin^3(phi)cos(theta)sin(theta)

Split the integral in two.
$$(1025)*54*(\int^{2\pi}_{0} \cos\theta\sin\theta d\theta) (\int^{pi/2}_{0} \sin_^{3}\phi d\phi$$

Trig Identity Substitution:
$$(1025)*27*(\int^{2\pi}_{0} -1/2\sin2\theta d\theta) (\int^{pi/2}_{0} (1-\cos\phi^{2})\sin\phi d\phi$$

So, I end up with...

$$1025*27*((-1/2\cos\theta)^{2\pi}_{0}) ( \cos^{3}\theta/3-\cos\theta)^{\pi/2}_{0})$$

Giving me...
$$1025*27*(0) ( \cos^{3}\theta/3-\cos\theta)^{\pi/2}_{0}) = 0$$

Thoughts?

Last edited: Apr 28, 2010
2. Apr 29, 2010

### HallsofIvy

Yes, that is correct (and nice work!), the total flow through the hemisphere is 0. The flow is "anti- symmetric" about the z- axis. The flow through a given point, (x, y, z), on the hemisphere is canceled by the flow through (-x, -y, z).