# Vector calculus fundamental theorem corollaries

1. Sep 27, 2009

### Adam Lewis

1. The problem statement, all variables and given/known data
Prove

$$\int_{V}\nabla\ T d\tau\ = \oint_{S}Td\vec{a}$$

2. Relevant equations

Divergence theorem:
$$\int_{V}(\nabla\bullet\vec{A})d\tau\ = \oint_{S}\vec{A}\bullet\ d\vec{a}$$

3. The attempt at a solution
By using the divergence theorem with the product rule for divergences and setting $$\vec{A}\ = T\vec{c}$$ where c is a constant vector, I've got it down to

$$\int_{V}\vec{c}\bullet\nabla\ Td \tau\ = \oint_{S}T\vec{c}\bullet\ d\vec{a}$$

Which is exactly what we're looking for except for that annoying c. You can work out the dot products in the integrals and cancel off the components of c, but this kills the vector nature of the expression. We need the gradient *vector* and the surface element *vector* to be in there. How do I get rid of c without turning everything scalar? Alternatively, how do I restore the expressions to vector-hood after getting rid of c?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 28, 2009

### gabbagabbahey

First, since $T$ is a scalar function, $T\vec{c} \cdot d\vec{a}=\vec{c} \cdot Td\vec{a}$, from there, just use the fact that if $\vec{c}$ is a constant vector it is uniform over space, and hence it is treated as a constant for the spacial integrations:

$$\int_{V}\vec{c}\cdot\vec{v}d \tau=\vec{c}\cdot\int_{V}\vec{v}d \tau$$

(for any vector $\vec{v}$ ) and

$$\oint_{S}\vec{c}\cdot\ Td\vec{a}=\vec{c}\cdot\oint_{S} Td\vec{a}$$

If you haven't already proven these assertions in any of your calculus courses, it is a fairly straight forward process of writing the vectors in Cartesian coordinates (Since Cartesian unit vector are position independent, they can be pulled out of the integrals) and breaking up the integral into three pieces, and pulling out the constants $c_x$, $c_y$ and $c_z$.

3. Sep 30, 2009

### Adam Lewis

Hi again,
Thanks a bundle. I guess the dot when c is pulled out represents scalar multiplication? The integral without c is of course a scalar, which was confusing me.
Thanks!

4. Sep 30, 2009

### gabbagabbahey

No, the 'dot' is still a vector dot product, the integral of a vector, is a vector.

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