Talk:Atomic units

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1 Atomic units related to magnetic quantities
2 The derived au table needs some attention
3 Comparison with Planck units
4 Boltzmann's constant

[edit] Atomic units related to magnetic quantities

In this article, there is a sentence:

" In atomic units, the Bohr magneton

μ B = 1 / 2

, ..."

However, as described in Bohr magneton page, $μ B$ is expressed in S.I. units as

$\mu_B = {{e \hbar} \over {2 m_e}}$

and in Gaussian centimeter-gram-second units as

$\mu_B = {{e \hbar} \over {2 m_e c}}$ .

If $e$ , $\hbar$ , and $m e$ are set equal to unity in each of these expressions, the Bohr magneton is

μ B = 1 / 2

in, S.I.-a.u., say, and

μ B = 1 / (2 c)

in, Gaussian-a.u., say, and they have different values.

By virtue of the equality

4πε 0 = 1

electric quantities have the same values in both S.I.-a.u. and Gauss-a.u.. For the magnetic quantities like $μ B$ , however, this does not seem to be the case, and there seems to be a freedom or ambiguity which of S.I. or Gauss to be chosen.

Now, my question is, when a.u. is referred to, does it imply what I wrote as S.I.-a.u., as can be read from the part of article cited above? Is there a consensus, or a rule?

NorioTakemoto 15:14, 15 February 2006 (UTC)

A nice little problem in undergraduate physics! Your (and now my) perplexity purely concerns the cgs formula for μ_B, and I gather that cgs units are only of historical interest nowadays. That pesky little c... tsk-tsk. I will think about this.202.36.179.65 19:09, 9 April 2006 (UTC)

[edit] The derived au table needs some attention

i might get to it myself at a later time. it would be good if au, Planck, Stoney, etc and all "natural units" were tied together into articles of consistent format. r b-j 02:36, 16 May 2006 (UTC)

I spent some time cleaning up both tables in the article. It didn't make sense to have Boltzmann and gravitational constants in the table. They were defined very differently from the other dimensional scales. Wigie 14:18, 18 May 2006 (UTC)

[edit] Comparison with Planck units

I made some changes here, but I'm still not happy with it. It seems that this section doesn't make its point very concisely, and in the process "hides" some interesting au values like the speed of light and the Bohr magneton. Wigie 14:23, 18 May 2006 (UTC)

[edit] Boltzmann's constant

The article says:

Finally, au normalize a unit of atomic energy to 1, while Planck units normalize to 1 Boltzmann's constant k, which relates energy and temperature.

However, if I understand the table of derived units correctly, atomic units also normalize Boltzmann's constant to 1. Am I missing something? Henning Makholm 15:01, 6 July 2006 (UTC)

Hi, same problem for me. Can someone give a reference to an external document where it is shown how the atomic unit for temperature is derived? As far as I can see, one has to define (not derive) $\frac{E_h}{k_B} = 1 a.u.$ . I'm not convinced that this is a commonly used definition. Tovrstra 14:48, 19 March 2007 (UTC)