Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Surface integral and divergence theorem over a hemisphere

  1. Dec 10, 2012 #1
    1. The problem statement, all variables and given/known data

    Please evaluate the integral [itex]\oint[/itex] d[itex]\vec{A}[/itex][itex]\cdot[/itex][itex]\vec{v}[/itex], where [itex]\vec{v}[/itex] = 3[itex]\vec{r}[/itex] and S is a hemisphere defined by |[itex]\vec{r}[/itex]| [itex]\leq[/itex]a and z ≥ 0,

    a) directly by surface integration.

    b) using the divergence theorem.

    2. Relevant equations

    -Divergence theorem in spherical coordinates

    3. The attempt at a solution

    Another one where the [itex]\vec{r}[/itex] messes me up. Simple enough if it was in regular xyz. Plus the [itex]\vec{v}[/itex]... and I don't really know where to start.
  2. jcsd
  3. Dec 10, 2012 #2
    Well, [itex] \mathbf{r} = x \mathbf{e}_x + y \mathbf{e}_y + z \mathbf{e}_z [/itex] shouldn't be too confusing...

    In this case, the shape of the integration area suggests that you might want to use spherical coordinates instead of x, y and z. Do you know what the area element is in spherical coordinates? (or even better, can you calculate it?) What about volume element?
  4. Dec 10, 2012 #3
    Ok so attempt at a solution:

    ∫∫ 3[itex]\vec{r}[/itex] r[itex]^{2}[/itex]sinθdθd[itex]\phi[/itex]

    limits are 0 to 2∏ for θ, and 0 to ∏/2 for [itex]\phi[/itex], or I could just do 3r^3 time the surface area of a hemisphere, which is 2*∏*r^2, so, 6*∏*a^5?

    for divergence... do I just take the divergence in spherical coordinates and multiply by the volume of a hemisphere, which is 2/3*∏*r^3?
  5. Dec 10, 2012 #4
    Okay, couple of things

    1) The surface area element is a vector.
    2) Be careful with the angles. Right now your definitions do not work.
    3) As you noticed, the integrand does not depend on the angles. Be careful about jumping over the integral though; now you're getting too many factors of r.

    Is the divergence constant? If it is, then it should of course work.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook