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You CAN Ace Calculus

17calculus > vector fields > divergence theorem

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free ideas to save on books - bags - supplies ATTENTION INSTRUCTORS: The new 2018 version of 17calculus will include changes to the practice problem numbering system. If you would like advance information to help you prepare for spring semester, send us an email at 2018info at 17calculus.com.

The Divergence Theorem

The Divergence Theorem is the three-dimensional version of the flux form of Green's Theorem and it relates the flow (or flux) through the boundary of a closed surface S to the divergence of the vector field through the volume Q. The surface S encloses the volume Q.

Divergence Theorem

If $$\vec{N}$$ is the unit normal vector directed outward from a solid region Q, then

$$\displaystyle{ \iint\limits_S { \vec{F} \cdot \vec{N} ~dS = \iiint\limits_Q { ( \nabla \vec{F} ) ~ dV } } }$$

Divergence Theorem in Detail

When you studied surface integrals, you learned how to calculate the flux integral $$\displaystyle{\iint_S{\vec{F} \cdot \vec{N} ~ dS} }$$ which is basically the flow through a surface S.
When you have a solid region bounded by a closed surface S, there is an easier way to calculate this integral, using the Divergence Theorem.

Basically what this Divergence Theorem says is that the flow or flux of the vector field $$\vec{F}$$ through the closed surface S, which is given by the integral $$\displaystyle{\iint_S{\vec{F} \cdot \vec{N} ~ dS} }$$ is equal to the divergence of $$\vec{F}$$ through the volume Q. This is given by the integral $$\displaystyle{ \iiint\limits_Q { ( \nabla \vec{F} ) ~ dV } }$$.

The required relationship between the surface S in the first integral and the volume Q in the second integral is that the volume Q must be bounded by the surface S.

Here is a great video explaining the Divergence Theorem.

 Dr Chris Tisdell - Divergence theorem of Gauss [12min-20secs]

Usually one of the integrals is easier to set up and evaluate than the other. Most of the time the easier integral is the volume integral since surface integrals are more difficult to set up in general. But you will get a feel for this as you get more experience.

Before we go on, let's look at an example.

 Divergence Theorem Example
 Example 1 - Verify the Divergence Theorem for $$\vec{F}(x,y,z)=xy\hat{i}+z\hat{j}+(x+y)\hat{k}$$ where the surface consists of the bounding planes $$y=4$$, $$z=4-x$$ and the coordinate planes.

Divergence Theorem in Depth

Here are several full length video lectures that explain the divergence theorem in more depth. These are important to watch in order to understand what is really going on with this theorem and how to use it properly.

 Evans Lawrence - Lecture 34 - Divergence Theorem [22min-22secs]
 MIT OCW - Lec 28-29 | MIT 18.02 Multivariable Calculus, Fall 2007 [2 videos 99min-27secs total]

Divergence Theorem Proof

Although not completely necessary to the use of the divergence theorem, going through the proof will help you understand in more detail how this proof works.

 Khan Academy - Divergence Theorem Proof [5 videos]
 Okay, you are now ready for some practice problems. The next logical step is the Stokes' Theorem, if you haven't already learned it. Stokes' Theorem →

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Practice Problems

Instructions - - Unless otherwise instructed, solve these problems using The Divergence Theorem.
- If you are asked to calculate the surface integral, you will usually evaluate the volume integral.
- Similarly, if you are asked to calculate the volume integral, you will usually evaluate the surface integral.

 Level A - Basic

Practice A01

Determine the flux of the vector field $$\vec{F}=\langle x^2,y^2,z^3\rangle$$ across the surface bounded by the planes $$x=0$$, $$x=2$$, $$y=0$$, $$y=2$$, $$z=0$$, $$z=4$$.

solution

Practice A02

Determine the flux of the vector field $$\vec{F}=\langle x^2/2,yz,-xz\rangle$$ across the surface $$z=\sqrt{4-x^2-y^2}$$, $$z=0$$.

solution

Practice A03

Compute the flux of $$\vec{F}=\langle x^4y, -2x^3y^2,z^2\rangle$$ through the surface of the solid bounded by $$z=0$$, $$z=h$$ and $$x^2+y^2=R^2$$.

solution

Practice A04

Calculate the surface integral for $$\vec{F}=\langle 4x^3z,4y^3z,3z^4\rangle$$ where the surface of the solid is bounded by the hemispheres $$z=\sqrt{4-x^2-y^2}$$, $$z=\sqrt{1-x^2-y^2}$$ and the plane $$z=0$$.

solution

Practice A05

Calculate the surface integral for $$\vec{F}=\langle 3xy^2,xe^z,z^3\rangle$$ where the solid is bounded by the cylinder $$y^2+z^2=25$$ and the planes $$x=-4$$ and $$x=3$$.

Calculate the surface integral for $$\vec{F}=\langle e^x\sin y, e^x\cos y, yz^2\rangle$$ where the surface is the box bounded by the planes $$x=4$$, $$y=5$$, $$z=1$$ and the coordinate planes.