|WikiProject Measurement||(Rated C-class, Mid-importance)|
|WikiProject Physics||(Rated C-class, Mid-importance)|
Finger error in a formula
Gaussian or CGS?
If, as the article states, the term "cgs units" is deprecated, why is it used several times, with no further qualification, in the text? For those who know, would it be safe to change existing unqualified references in this article from "cgs units" to "Gaussian units"?
This article does a pretty comprehensive job of describing how Gaussian units differ from SI ones, but offers no explanation for why factors of 4pi, epsilon_zero etc. differ between the two systems. There's some (not very clear) discussion at Centimetre_gram_second_system_of_units#Derivation_of_CGS_units_in_electromagnetism, but this article doesn't mention that and only links to it in passing in the lead. There needs to be a brief description of why these factors disappear, and a clear link to the relevant place for more information. I'd fix it myself, but frankly I don't follow the reasoning (despite having a degree in physics and working in a field where Gaussian units are still used), so I doubt I could write anything that was correct. Modest Genius talk 00:35, 27 December 2012 (UTC)
- About the 4pi: It's not clear to me exactly what question you're looking to answer:
- "Why is it that if Coulomb's law has an explicit 4pi then Gauss's law does not have an explicit 4pi, and vice-versa?" I think the article answers that well: "The quantity 4π appears because 4πr2 is the surface area of the sphere of radius r. For details, see the articles Relation between Gauss's law and Coulomb's law and Inverse-square law."
- "What are the advantages and disadvantages of rationalized versus non-rationalized units?" This is a subjective question. One person might think that something is an advantage, someone else with different preferences might think the same thing is a disadvantage. It's not impossible to discuss subjective questions in a wikipedia article, but it is difficult. You need to fairly summarize all the major points of view, and find references for them. The latter is very difficult; even if lots of physicists say to each other over coffee "Gaussian units make XYZ unnecessarily complicated", they rarely would write that into a publication that we can reference.
- "What are the historical circumstances that caused SI to be rationalized while Gaussian is not?" It looks like Reference 5 discusses the history and I'm sure you can find the information you're looking for there. If you want to add better historical information to the article, that would be great, and I'm happy to help as I can.
- About epsilon0: The main thing is that in Gaussian units,
- 1 statC = 1 g1/2 cm3/2 s−1
- while in SI, there is no analogous relation between Coulombs and the "mechanical" units (length, mass, time). This difference leads to a wide variety of formula differences, especially when epsilon0 and mu0 are involved.
- Again, maybe you're wondering what the advantages and disadvantages are to defining charge in terms of mechanical units. Or maybe you're wondering why, theoretically, people have a choice in the matter at all. Or maybe you're wondering why, historically, different systems developed in different ways. Which is it? --Steve (talk) 18:15, 27 December 2012 (UTC)
Contradiction with cgs article
I think there is a contradiction between the articles on cgs unit system http://en.wikipedia.org/wiki/Centimetre_gram_second_system_of_units#Electrostatic_units_.28ESU.29 and the one on the Gaussian unit system http://en.wikipedia.org/wiki/Gaussian_units.
In the Table Electromagnetic units in various CGS systems of http://en.wikipedia.org/wiki/Centimetre_gram_second_system_of_units#Electrostatic_units_.28ESU.29 the conversion of magnetic field units between SI unit system (i.e. Tesla) and ESU unit system is give as 10^4 statT/c. This conversion is quite consistent when one begins with the Lorentz force equation in SI units and converts it to ESU. In this way the additional (1/c) term in the denominator of the corresponding equation in ESU is automatically obtained. In the same table the conversion between Tesla and Gauss is 1 T = 10^4 G. In the light of the conversion between SI and USU this implies that whenever one replaces 1 T in the Lorentz equation in SI units by 10^4 G one obtains the corresponding equation in Gaussian units. On the other hand in the table Electromagnetic unit names of http://en.wikipedia.org/wiki/Gaussian_units the conversion between SI and Gaussian units is given as 1 T = 10^4 Gauss. Probably this does not mean that whenever one replaces 1 T in the Lorentz equation in SI units by 10^4 Gauss one obtains the corresponding equation in Gaussian units. I think this point requires clarification. — Preceding unsigned comment added by 220.127.116.11 (talk) 14:11, 2 April 2013 (UTC)
- ESU is not the same as Gaussian. But anyway, you're correct that you cannot replace "1 T" in an SI equation by "10^4 G" in a gaussian or ESU equation. There is already some text about this in the article: " The symbol "↔" was used instead of "=" as a reminder that the SI and Gaussian units are corresponding but not equal because they have incompatible dimensions. For example, according to the top row of the table, something with a charge of 1 C also has a charge of (10^−1 c) Fr, but it is usually incorrect to replace "1 C" with "(10−1 c) Fr" within an equation or formula...". There is similar text in the statcoulomb article. I have just now copied the exact same text and notation to the table in Cgs units#Electromagnetic units in various CGS systems. Does that help? --Steve (talk) 17:49, 2 April 2013 (UTC)