In this section we are going to relate surface integrals to triple integrals. We will do this with the Divergence Theorem.
Divergence Theorem
Let E be a simple solid region and S is the boundary surface of E with positive orientation. Let be a vector field whose components have continuous first order partial derivatives. Then,
Let’s see an example of how to use this theorem.
Example 1 Use the divergence theorem to evaluate where and the surface consists of the three surfaces, , on the top, , on the sides and on the bottom.
Solution
Let’s start this off with a sketch of the surface.
The region E for the triple integral is then the region enclosed by these surfaces. Note that cylindrical coordinates would be a perfect coordinate system for this region. If we do that here are the limits for the ranges.
We’ll also need the divergence of the vector field so let’s get that.
The integral is then,