The use of eV-s is fairly common in physics (especially when you deal with elementary particles). As long as you know that it is nothing but J-s [MKS] or erg-s [CGS] multiplied by the appropriate number, you don't have to worry about any special "system of units". The answer (which includes the number as well as the units) should be the same in all units.
(The Particle Data Group, the standard reference book for all particle physicists, in fact gives h/(2 pi) in the units of both, J-s as well as MeV-s:
http://pdg.lbl.gov/2007/reviews/contents_sports.html#constantsetc . Go to "Physical Constants")
Oh, I know. I have no objection to anyone using eV. In fact I couldn't care less if anyone uses slugs and standard banana lengths!
The problem arises if you use the units inconsistently. My criticism is aimed at post #4 in this thread where Schneibster does two calculations to derive h and hbar in eV seconds. Having done so he then claims that it is obvious from a dimensional analysis (not given there) that the standard SI formula (with all inputs in SI units) will give an answer in eV. That is simply not true (as he himself confirms later in another post in which his dimensional analysis gives the answer in joules). Setting aside the issue of whether the answer is in joules or eV, the question arises, why did he bother to calculate h and hbar in eV seconds? The equation he stated is correct for SI which doesn't use eV seconds. eV seconds are not an SI unit. And he never bothered to give a form of the equation for which h in eV seconds
would be valid - or explain which system of units would be required for the
other variables if the
same equation was used. The entire eV seconds calculation he gives above is meaningless and pointless in the context in which it is given.
That is my criticism. His calculation is misdirection, a sleight of hand. It looks impressive if you don't bother to actually check it - if you do, it doesn't make sense. Pointing that out does not imply that I have any problem with eV or eV seconds. The problem I have with it is inappropriate, inconsistent and meaningless use out of context.
Going back to the factor of e^4 vs e^3 discussed earlier in the thread: the expressions that have e^3 in them are simply wrong, and one should not use them even if they are in the textbook. One should, rather, point out to the instructor (if any) that they are wrong and use the correct ones for solving problems.
-Dorman
It depends on what you are trying to do. I agree they are strictly wrong, however they do give the numerically right answer in eV. The proper SI equations are dimensionally correct but do not give the answer in eV. I suppose I should have pointed out that the correction factor for joules to eV is just 1.6 x 10
-19 as opposed to "e" which is 1.6 x 10
-19 coulombs and on that basis you are correct, I apologise for any lack of clarity on my part.
All of this confusion arises from the introduction of various unit systems which have nothing to do with the original question. It was obvious that the question was based on cgs and I answered that correctly the first time. Now it's perfectly reasonable to add the SI formula and explain that this gives an answer in joules which needs conversion. Everything else just confuses the issue.
