The divergence of F:
Implement the integral of over the interior of the cylinder in cylindrical coordinates:
=
To compute the flux through the cylinder, note that there are three boundaries of , the "wall" of the cylinder, and its circular top and bottom. Hence, there are three flux integrals to calculate.
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Flux through the "wall" of the cylinder:
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Describe this surface with the position vector so that requires the calculation of
= = N
and . Using the same parametrization for F, on the wall of the cylinder becomes
=
Consequently, the flux through the wall of the cylinder is given by
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Flux through the top of the cylinder:
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Take so on the top of the cylinder. Use polar coordinates to integrate this over the unit disk, obtaining
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Flux through the bottom of the cylinder:
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Take so on the bottom of the cylinder. Clearly, the flux integral on this bounding surface is zero.
In the Divergence theorem, the volume integral on the left has value and the surface flux integral(s) on the right, have values , which also sum to .