Re: Divergence and Dirac Delta Function

*To*: mathgroup at smc.vnet.net*Subject*: [mg9154] Re: [mg9142] Divergence and Dirac Delta Function*From*: Peter Jay Salzman <psalzman at landau.ucdavis.edu>*Date*: Thu, 16 Oct 1997 03:37:55 -0400*Sender*: owner-wri-mathgroup at wolfram.com

> >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 > 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). is this correct? i was under the impression that the 4 pi came from doing the surface integral around a sphere centred at the origin. the radial dependence drops out (1/r^2 cancels with the r^2 in the differential area element) and you're left with 4 pi. i think this result is invariant under change of unit system. or am i remembering incorrectly? peter -- -=><=-+-+-=><=-+-+-=><=-+-+-=><=-+-+-=><=-+-+-=><=-+-+-=><=-+-+-=><=-+-+-=><=- Do you hate spam? Join the Coalition Against Unsolicited Commercial Email (CAUCE) at http://www.cauce.org. Actions speak louder than words. Join Today! I BOYCOTT ANY COMPANY THAT USES MASS ADVERTISING ON THE INTERNET