# sources and sinks of vector field

Let the vector field $\vec{U}$ of $\mathbb{R}^{3}$ be interpreted, as in the remark of the parent entry (http://planetmath.org/Flux), as the velocity of a liquid.  Then the flux

 $\oint_{a}\vec{U}\cdot d\vec{a}$

of $\vec{U}$ through a closed surface $a$ expresses how much more liquid per time-unit it comes from inside of $a$ to outside than contrarily.  Since for a usual incompressible liquid, the outwards flow and the inwards flow are equal, we must think in the case that the flux differs from 0 either that the flowing liquid is suitably compressible or that there are inside the surface some sources creating liquid and sinks annihilating liquid.  Ordinarily, one uses the latter idea.  Both the sources and the sinks may be called sources, when the sinks are negative sources.  The flux of the vector $\vec{U}$ through $a$ is called the productivity or the strength of the sources inside $a$.

For example, the sources and sinks of an electric field ($\vec{E}$) are the locations containing positive and negative charges, respectively.  The gravitational field has only sinks, which are the locations containing .

The expression

 $\frac{1}{\Delta v}\oint_{\partial\Delta v}\vec{U}\cdot d\vec{a},$

where $\Delta v$ means a region in the vector field and also its volume, is the productivity of the sources in $\Delta v$ per a volume-unit.  When we let $\Delta v$ to shrink towards a point $P$ in it, to an infinitesimal  volume-element $dv$, we get the limiting value

 $\displaystyle\varrho\;:=\;\frac{1}{dv}\oint_{\partial dv}\vec{U}\cdot d\vec{a},$ (1)

called the source density in $P$.  Thus the productivity of the source in $P$ is $\varrho\,dv$.  If  $\varrho=0$, there is in $P$ neither a source, nor a sink.

The Gauss’s theorem

 $\int_{v}\nabla\cdot\vec{U}\,dv\;=\;\oint_{a}\vec{U}\cdot d\vec{a}$

applied to $dv$ says that

 $\displaystyle\nabla\cdot\vec{U}\;=\;\frac{1}{dv}\oint_{\partial dv}\vec{U}% \cdot d\vec{a}.$ (2)

Accordingly,

 $\displaystyle\varrho\;=\;\nabla\cdot\vec{U}$ (3)

and

 $\displaystyle\oint_{a}\vec{U}\cdot d\vec{a}\;=\;\int_{v}\varrho\,dv.$ (4)

This last can be read that  the flux of the vector through a closed surface equals to the total productivity of the sources inside the surface.  For example, if $\vec{U}$ is the electric flux density $\vec{D}$, (4) means that the electric flux through a closed surface equals to the total charge inside.

 Title sources and sinks of vector field Canonical name SourcesAndSinksOfVectorField Date of creation 2013-03-22 18:47:03 Last modified on 2013-03-22 18:47:03 Owner pahio (2872) Last modified by pahio (2872) Numerical id 12 Author pahio (2872) Entry type Definition Classification msc 26B15 Classification msc 26B12 Related topic Divergence Related topic SolenoidalField Related topic CirculationAndVorticity Defines source Defines sink Defines source of vector field Defines productivity Defines strength Defines source density