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Homework Help: Flux across parametrized surface

  1. Jul 27, 2008 #1
    1. The problem statement, all variables and given/known data
    Use a parametrization to find the flux across the surface in the given direction:

    F = (2xy)i + (2yz)j + (2xz)k upward across the portion of the plane x + y + z = 2a that lies above the square 0 <= x <= a, 0 <= y <= a, in the xy-plane.

    2. Relevant equations
    Flux = Double integral of F dot n d(sigma),

    where d(sigma) = |(r_u x r_v)| du dv

    x = cross product

    3. The attempt at a solution
    I need some help coming up with a parametrization for the plane x + y + z = 2a. Since it lies in the xy-plane, I figured I can use the parametrization x = u and y = v, so I get u = 2a - v, and v = 2a - u, and z = 2a - u - v, and use order of integration du dv. However, I end up with a rather large integral with this parametrization, so I just want to make sure I'm doing this right before trying to solve it. Thanks for any suggestions!
  2. jcsd
  3. Jul 27, 2008 #2


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    Homework Helper

    Yeah the parametrisation seems all right. Make sure your normal vector points in the correct direction, though.
  4. Jul 27, 2008 #3
    Oh, thanks. Isn't the unit normal vector just n = (r_u x r_v) / |(r_u x r_v)| ? How do I know if it's pointing in the right direction?
  5. Jul 27, 2008 #4


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    Homework Helper

    You can sketch the plane right? Just visualise the normal vector and see if it's the same direction as that required by the question. And yes, the normal vector is that, except I don't think it needs to be of unit length.
  6. Jul 28, 2008 #5


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    Science Advisor

    The problem referred to the flow upward make sure you normal vector points upward: i.e. has positive z component.
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