Problem with units

  1. I'm having great difficulties with CGS-units. In an experiment involving NMR I have to calculate the proton-proton distance in CaSO4.2H2O.

    I have done the measurements and they are, I think, pretty good however I have some problems with the calculations because of the units.

    The formula I have to use is the following:

    a = (3/2)(u*u0)/r^3

    where u = 2.79*5.05*10^-27 J/T = 2.79*5.05*10^-24 erg/Gauss and u0 is the Permeability of vacuum. a is something I measured and has a value of 4.5 Gauss.

    I know that r should be about 1.5 Angstrom (spelling?). My instructor told me to use CGS-units (thus putting u0=1) but does that mean that the rest of the formula stays the same? Or are there other changes like an additional 4pi or something. If I just put u0=1 I get r = 1.67*10^-8 (but I don't know the units, I believe the CGS-unit for distance is cm in which case r=1.67*10^-10 m = 1.67 A which is pretty close to 1.5 A).
     
  2. jcsd
  3. vanesch

    vanesch 6,236
    Staff Emeritus
    Science Advisor
    Gold Member

    Why don't you do the calculations first in SI units, and convert in the end in CGS ? I also get confused often between CGS, Gaussian, Natural,... etc units concerning EM quantitities because indeed, some include a 4 pi, others don't etc...

    So I then do everything in SI units, and fill in the 4 pi in the other units in order to obtain the same result :blushing:
     
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