Re: Divergence and Dirac Delta Function

*To*: mathgroup at smc.vnet.net*Subject*: [mg9142] Re: [mg9127] Divergence and Dirac Delta Function*From*: Axel Schwalm <a3023575 at smail3.rrz.Uni-Koeln.DE>*Date*: Tue, 14 Oct 1997 10:01:56 -0400*Sender*: owner-wri-mathgroup at wolfram.com

>I am a physics student currently enrolled in an upper level >Electrodynamics course and, while playing around with Mathematica 3.0 >for Students, I discovered that if I tell Mathematica to compute the >divergence of: unit vector(r) / r^2 it will return the value of >zero. However, I know that, at the very least in Electrodynamics, the >correct value should be: 4*Pi*(DiracDelta Function). Is it possible to >get Mathematica to output this value without explictly telling the >program that this is the value? > Elvis Dieguez ****************************************** Dear Elvis, whether You get "1" or "4*Pi" as a result from considering vector matters, depends on the unit-system You make use of (SI- or Gaussian system, physically speaking). One simplifies calculations, the other one is easier to understand (and to make a lecture with it). Thus, Mathematicas result is as correct as Yours. In the long run, You will get used to switch between both description and discover, that "4*Pi" is an integrational constant, that comes all along in electrodynamics. To understand the different descriptions, look, for example, into the courses of Feynman and Landau-Lifschitz. Yours, Axel Schwalm University of Cologne