# Homework Help: Problem with units

1. Oct 14, 2005

### Henk

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. Oct 15, 2005

### vanesch

Staff Emeritus
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