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Section 5.4 Divergence

Subsection 5.4.1 Illustration

Consider \(\vec{F}(x,y,z)=\langle P(x,y,z),Q(x,y,z),R(x,y,z) \rangle.\) Note this has 3 inputs and three outputs. For convenience \(\frac{\partial\vec{F}_x}{\partial x} = \frac{\partial P(x,y,z)}{\partial x}.\)

Activity 74.

The purpose of this activity is to illustrate the concept of divergence in a vector field.
(a)
If people arrive at the gas station at a rate of 6 every 15 minutes and leave at a rate of 3 every 15 minutes what will be the result?
(b)
If people are arriving at lover’s leap at a rate of 1 person per hour and leaving at a rate of 1 per minute, what will be the result?
(c)
In terms of rates what information do \(\frac{\partial \vec{F}_x}{\partial x},\) \(\frac{\partial \vec{F}_y}{\partial y}\text{,}\) and \(\frac{\partial \vec{F}_z}{\partial z}\) provide?
(d)
As a result how can \(\frac{\partial \vec{F}_x}{\partial x}+\frac{\partial \vec{F}_y}{\partial y}+\frac{\partial \vec{F}_z}{\partial z}\) be described?

Subsection 5.4.2 Definition

Definition 5.4.1. Div.

For a vector field \(F\text{,}\) the divergence is
\begin{equation*} \mbox{div } \vec{F} = \frac{\partial \vec{F}_x}{\partial x}+\frac{\partial \vec{F}_y}{\partial y}+\frac{\partial \vec{F}_z}{\partial z}. \end{equation*}

Exercises 5.4.3 Exercises

Calculate the divergence as directed and compare the result to the vector field images.

1.

\(\vec{F}(x,y,z)=\langle -x^2,-y^2,-z^2 \rangle\)
(a)
\(\mbox{div}(\vec{F})(0,0,0)\)
(b)
\(\mbox{div}(\vec{F})(-1,-1,-1)\)
(c)
\(\mbox{div}(\vec{F})(1,1,1)\)

2.

\(\vec{G}(x,y,z)=\langle x^3,y^3,3 \rangle\)
(a)
\(\mbox{div}(\vec{G})(0,0,0)\)
(b)
\(\mbox{div}(\vec{G})(-1,-1,-1)\)
(c)
\(\mbox{div}(\vec{G})(1,1,1)\)

3.

\(\vec{H}(x,y,z)=\langle x,xy,0 \rangle\)
(a)
\(\mbox{div}(\vec{H})(0,0,0)\)
(b)
\(\mbox{div}(\vec{H})(-2,-1,-1)\)
(c)
\(\mbox{div}(\vec{H})(2,1,1)\)

4.

\(\vec{J}(x,y,z)=\langle z^3y,x^2z,xy \rangle\)
(a)
\(\mbox{div}(\vec{J})(0,0,0)\)
(b)
\(\mbox{div}(\vec{J})(-1,-1,-1)\)
(c)
\(\mbox{div}(\vec{J})(1,1,1)\)