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Physical constants and their evaluation


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

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