请输入您要查询的字词:

 

单词 SourcesAndSinksOfVectorField
释义

sources and sinks of vector field


Let the vector field U of 3 be interpreted, as in the remark of the parent entry (http://planetmath.org/Flux), as the velocity of a liquid.  Then the flux

aU𝑑a

of 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 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 (E) are the locations containing positive and negative charges, respectively.  The gravitational field has only sinks, which are the locations containing .

The expression

1ΔvΔvU𝑑a,

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

ϱ:=1dvdvU𝑑a,(1)

called the source density in P.  Thus the productivity of the source in P is ϱdv.  If ϱ=0, there is in P neither a source, nor a sink.

The Gauss’s theorem

vU𝑑v=aU𝑑a

applied to dv says that

U=1dvdvU𝑑a.(2)

Accordingly,

ϱ=U(3)

and

aU𝑑a=vϱ𝑑v.(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 U is the electric flux densityD, (4) means that the electric flux through a closed surface equals to the total charge inside.

随便看

 

数学辞典收录了18232条数学词条,基本涵盖了常用数学知识及数学英语单词词组的翻译及用法,是数学学习的有利工具。

 

Copyright © 2000-2023 Newdu.com.com All Rights Reserved
更新时间:2025/5/5 2:22:05