Just like the fact that you don't measure mass in eV. You measure it in eV/c**2. Of course in your system of units you may have c dimensionless and equal to one but that is not SI anymore. That new system does not distinguish momentum-energy-mass or distance-time. SI distinguishes "quantity of matter" from "number of molecules"
On 15 January 2012 22:00, [email protected] <[email protected]> wrote: > I have two problems with this. First of all mol is not a number as > dozen. It's a measure of quantity of matter. The idea was created > before the atomistic theory of matter and the only important property > is additivity. You use the Avogadro's constant (again not a pure > number) to connect the two. So to use mols you don't need to know that > matter is made of discreet particles. > > Following the definitions of mol and Avogadro's constant and atomic > mass unit according to SI we must have the following identities: > > g/amu = avogadro_constant*mol = avogadro_number > > At the moment the thing called avogadro in physics.units is the > avogadro's constant. I suggest that we also add avogadros_constant = > avogadro and avogadro_number = avogadro*mol. But IMO it's important to > keep avogadro as it is because it's the standard defined in SI. > > Avogadro's constant is the number of atoms per mol. It's "number per > mol" so it should be with the dimensions of mol**-1. > > One mol is the quantity of matter in 12 grams of C12. [1] It does not > need atomistic theory to be defined. Just additivity of the quantity > of matter. > > atomic mass unit is 1/12 of the mass of one atom of C12 [2]. It's a > mass so it must be in kg. > > The fact that the [mass of an atom measured in atomic mass units] is > the same _number_ as the [molar mass of the element measured in > grams/mol] is a nice consequence of those definitions not an intrinsic > property. It would be nice to have a shorthand for that property but > correctness of the basic constructs is more important. > > Also wolframalpha agrees with what is currently in the pull request: > http://www.wolframalpha.com/input/?i=atomic+mass+unit > http://www.wolframalpha.com/input/?i=avogardo%27s+constant > [1] http://en.wikipedia.org/wiki/Mole_%28unit%29 > [2] http://en.wikipedia.org/wiki/Atomic_mass_unit > > On 15 January 2012 21:13, Chris Smith <[email protected]> wrote: >> cross posting again: >> >> OK, then what we should have is amu = amus = g/mol so when you divide >> mass by amu you get mols. If you want the number you multiply by >> avogadro('s constant) to get 6e23 (for 1 mol): >> >>>>> from sympy.physics import units as u >>>>> amu=u.g/u.mol >>>>> u.g/amu >> mol >>>>> _*u.avogadro >> 602214179000000000000000 >> >> I think this is all consistent: a mol (like dozen) is a number; >> mass/mass gives a number; when you use SI mass and amu you will get >> the number in terms of mol; if you want to see the actual number (like >> converting dozen to 12) you multiply by avogadro('s constant). >> >> -- >> You received this message because you are subscribed to the Google Groups >> "sympy" group. >> To post to this group, send email to [email protected]. >> To unsubscribe from this group, send email to >> [email protected]. >> For more options, visit this group at >> http://groups.google.com/group/sympy?hl=en. >> -- You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/sympy?hl=en.
