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Homework Help: Flux of Vector Field Across Surface

  1. Mar 24, 2008 #1
    1. The problem statement, all variables and given/known data

    Compute the flux of vector field (grad x F) where F = (xz+x^2y + z, x^3yz + y, x^4z^2)
    across the surface S obtained by gluing the cylinder z^2 + y^2 = 1 (x is > or eq to 0 and < or eq to 1) with the hemispherical cap z^2 + y^2 (x-1)^2 = 1 (x > or eq to 1) oriented in such a way that the unit normal at (1,0,0) is given by i.

    My approach to find the flux is to take the triple integral of the gradient of F * dV. I found the gradient to be <z + 2xy, x^3z +1, 2zx^4> and I set up the integral but I am a bit confused as to what the limits should be. Should I be even taking a triple integral??
  2. jcsd
  3. Mar 24, 2008 #2
    also... can I use cylindrical coordinates for the volume of the cylinder then use spherical coordinates for the volume of the cap and add them together?
  4. Mar 25, 2008 #3


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    How are you taking the triple integral? The "gradient of F" is a vector quantity and dV is not. I thought perhaps you were using the divergence theorem but you say nothing about the divergence of grad F (which would be [itex]\nabla^2 F[/itex]).

    If you are doing this directly (not using the divergence theorem), you need to integrate grad F over the surface of the figure. Yes, you can do the cylinder and hemisphere separately.
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