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Re: Faraday, a unit of charge



Regarding Ludwik's puzzlement:

The 94868 C, used in the footnote of my yesterday's
message was based on the k=9*10^9 factor (Coulomb's
law). It would be 94803 if the exact value of k was used
instead. This is totally irrelevant in the context of my
message (devoted to the 1/4*Pi factor) but I am puzzled.

As I mentioned in my previous post the number should be
sqrt(10000000)/299792458 C = (1/94802.699262) C not 94868 C nor 94803 C.

I suspect your puzzlement might possibly be caused by a mere numerical
near coincidence.

According to Handbook of Chemistry and Physics the
new Faraday (unit of charge since 1960) is 96516 C

by physical scale or 96489 C by chemical scale. The
second number is nearly the same as the product of
the e (charge of one electron in C) and N (the Avogadro
number). Is this because one mole in physics is still not
exactly the same as one mole in chemistry?

The definition of a mole is the same for physics and chemistry. A
Faraday of charge is *defined* as the amount of charge in a mole of
elementary (electron or proton) charges. It has *nothing* to do with
your homemade unit above as the amount of electric static charge that
exerts a 1 N force on another similar static charge separated from it
by 1 m. What makes you think these two things ought to be related
somehow? The concept of a mole has to do with the number of C-12 atoms
in their ground state that happen to have a mass of exactly 0.012 kg.
It has nothing to do with the idea of a measured amount of electrical
force of 1 N at a distance separation of 1 m.

Maybe your above puzzlement concerns *why* two such unrelated quantities
would coincidentally come out numerically so close (after a reciprocal is
taken)?

The Handbook (69th edition) was printed in 1988. Is
it not true that by that time physicists and chemists
already used C-12 as the common base for the atomic
mass unit (thus eliminating the old "O-16 versus
O-natural -mixture" discrepancy)?

Yes. As of 1988 the mole (and thus the unified mass unit) had
already been redefined decades earlier in terms of C-12.

David Bowman
David_Bowman@georgetowncollege.edu