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MathGroup Archive 1999

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Physical constants and their evaluation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18107] Physical constants and their evaluation
  • From: John JOWETT <John.Jowett at cern.ch>
  • Date: Thu, 17 Jun 1999 12:26:41 -0400
  • Delivery-date: Thu Jun 17 16:21:22 1999
  • Organization: CERN
  • Sender: owner-wri-mathgroup at wolfram.com

Hello,
      I am wondering about the best policy for evaluating physical
formulas containing fundamental constants and the like.  I have often
used rules as in

Needs["Miscellaneous`PhysicalConstants`"]

   evaluateConstants={me->ElectronMass,
                      hbar->PlanckConstantReduced,
                      c->SpeedOfLight} 

that can be applied when desired to a symbolic expression such as 

   (1.234 me hbar/c) /. evaluateConstants

to get numerical values with units.  Up until then, the expressions
contain the constants symbolically

Another approach would be to define 

     N[me] = ElectronMass;
     N[hbar] = PlanckConstantReduced;
     N[c] = SpeedOfLight;

and then just use N[...] when the time comes to "plug in numbers". 

Is there any reason to prefer one method over the other?  I'd like to do
the  right thing as I plan to build up fairly elaborate packages on this
basis.

Thanks for any suggestions,
John Jowett

-- 
----------------------------------------------------------------------
http://wwwslap.cern.ch/~jowett/
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