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ERASED sentences explaining why the editor put less decimals on furmlas. unneeded and DISTRACTING — Preceding unsigned comment added by 77.210.125.31 ( talk) 17:32, 15 May 2012 (UTC)
How can a measured value be defined? Electric permittivity is measured using a capacitor circuit. It then so happens that the inverse of the product εμ is close to the square of the speed of light. But you cannot apply the defined speed of light in SI units order to determine the measured value of ε.
This all goes back to Weber and Kohlrausch in 1856. They did an experiment using a Leyden jar and obtained an electromagnetic/electrostatic ratio that was closely linked to the measured speed of light. The physical importance of it all lies in the convergence of two measured results. We cannot replace these experiments with definitions of c and μ.
The lead in this article is totally confused as it is attempting to explain what cannot be explained. It needs to be drastically re-written. David Tombe ( talk) 12:24, 13 August 2009 (UTC)
Steven, There was an experiment with an electric circuit involving a capacitor which was used to measure the electric permittivity of the vacuum, prior to the 1983 SI definition of the metre. The value obtained could be subsituted into the equation c^2 = 1/(εμ) to obtain a value that is very close to the speed of light.
We have got no automatic right to reverse the situation using the directly measured speed of light in order to obtain the value of ε through this equation. We have two independent measurements of two different quantities which appear to be linked through the equation c^2 = 1/(εμ). We cannot deny the significance of this important result in physics simply by invoking a new SI definition of the metre.
It is a total tautology, based on the benefit of hindsight, to suggest that we can obtain the value of ε by using the equation c^2 = 1/(εμ) and the post-1983 defined speed of light. David Tombe ( talk) 11:50, 14 August 2009 (UTC)
The defined exact value of the electric constant is discussed here. It is not seen as a problem. Like the other defined constants, it could be taken to be 1 in the proper set of units, but that's not the units we chose. Dicklyon ( talk) 15:14, 14 August 2009 (UTC)
Here's a 1993 book by a guy who seems to have not got the 1983 memo. Dicklyon ( talk) 15:22, 14 August 2009 (UTC)
Here is a sensible discussion by Halliday. As you see, the problem is not with this article. It's just the way it is; the electric constant is now a constant, not a measured value. Dicklyon ( talk) 15:22, 14 August 2009 (UTC)
Steven, You asked me for a source. Nelkon & Parker "Advanced Level Physics" (1979) describes the experiment that is used to determine the value of electric permittivity. It uses an electric circuit with a capacitor in it. Are you seriously trying to tell me that that experiment became null and void when the metre was re-defined in 1983?
The equation c^2 = 1/(εμ) came about in the first place as a consequence of the experimental determination of the electric permittivity (ε). That equation yields a number that is very close to the speed of light, and that fact is a matter of great interest to physicists. You cannot then work backwards using a defined speed of light in order to obtain a defined electric permittivity (ε). That is known as cooking the books with the benefit of hindsight.
You cannot wipe out history with a mere definition. The 1856 experiment with the Leyden jar was one of the most important experiments in the history of electromagnetism. David Tombe ( talk) 19:17, 14 August 2009 (UTC)
Steven, I'm fully aware of the current definitions in SI units. And I fully understand your point of view. Your point of view is that since both c and μ are defined, then ε automatically becomes defined through the equation c^2 = 1/(εμ). That is all pretty straightforward.
However, the point that everybody seems to be missing is that the equation c^2 = 1/(εμ) arises in the first place because of experimental measurements of ε. Hence it is a tautology to define ε using an equation which only exists because of experimental determinations of ε. And this tautology pulls the mat from underneath the famous work of Wilhelm Eduard Weber and Rudolf Kohlrausch in 1856 with the Leyden jar. It reduces the equation c^2 = 1/(εμ) to a meaningless conversion factor. This is a classic case of maths having gone off the rails and lost all connection with the physics that it was supposed to be describing. David Tombe ( talk) 11:49, 15 August 2009 (UTC)
Steven, I have a reputable source. I have an advanced level physics textbook which describes the experiment for measuring the electric permittivity. The experiment involves an electric capacitor circuit with a vibrating reed switch. It utilizes the equations Q = CV and C = εA/d. It doesn't make any difference what system of units is used to define the metre. The only thing of importance in the experiment is that we can actually measure A, V, d, and the time derivative of Q.
Surely you are not seriously trying to tell me that this experiment became defunct in 1983 following the re-definition of the metre in SI units? The reputable source is "Nelkon & Parker" 'Advanced Level Physics. It is the 1979 version. I will be interested to find out if this experiment has been dropped from the most up to date version. If it has been dropped, I will accept that your reversion has merit under wikipedia's rules. But I will never accept that this experiment has been nullified by a mere re-definition of the metre. David Tombe ( talk) 01:00, 16 August 2009 (UTC)
Steven, A 1993 reference was supplied above [1]. It uses SI units and it points out that electric permittivity is an experimentally measured quantity.
Also, you are now contradicting yourself. One moment you are saying that it is impossible to measure the permittivity of the vacuum within the current definition of SI units, and the next moment you are saying that we can still measure it, but that the interpretation of the measured quantity has changed from what it used to be.
We are measuring the quantity ε as per the equation C = εA/d. There hasn't been a physical interpretation of this quantity since the time of Maxwell, so I can't see how any interpretation could have changed as a consequence of the re-definition of the metre in 1983. In the experiment in question, d will be substantially the same whether based on the pre-1983 definition of the metre, or the post-1983 definition of the metre. So I can't see that there is any argument at all to say that ε is not a measured quantity. It can only become a defined quantity if we work backwards through an equation that only came about in the first place because of the measured value. David Tombe ( talk) 14:39, 16 August 2009 (UTC)
Steven, The source is quite clear that it is talking about the absolute permittivity of free space, and it gives the measured value. The formula in question is C = εA/d, and the vibrating reed switch/capacitor experiment can be used in conjunction with that formula to measure the permittivity of any material, including the vacuum. Nothing can possibly have changed in relation to this experiment as result of the 1983 re-definition of the metre. If the space between the capacitor plates, d, was 1cm before 1983, it will likely have remained at 1cm after 1983. It's a simple matter of knowing the values of A, d, Q/t and V and we will obtain an experimental value for ε.
The precision of this experiment is irrelevant. It was never very precise. But nevertheless, you keep overlooking the fact that the equation c^2 =1/(εμ) came about because of experimental measurements. We should not therefore be using that equation in reverse to determine ε, even though that has been common practice, even before the 1983 re-definition of the metre, due to the fact that the experimental method was difficult. David Tombe ( talk) 17:13, 16 August 2009 (UTC)
Dick, The problem is that the equation in question comes from the experimental determination of ε. Steven has been trying to tell me that since 1983, the experiment no longer means what it meant before, and that we can only determine ε theoretically from the equation that was first based on the experiment.
This is an extended tautology of the already existing tautology that lies in the poost-1983 speed of light. David Tombe ( talk) 17:47, 16 August 2009 (UTC)
Dick, The linkage of the equation c^2 = 1/(εμ) to the speed of light has always been purely experimental. The theoretical equation itself was Newton's equation for the speed of sound [equation (132) in Maxwell's 1861 paper] but the numbers, and hence the linkage with the speed of light, began with Wilhelm Eduard Weber and Rudolf Kohlrausch in 1856. If we do away with the experiments that produced that linkage to the speed of light, then we do away with the equation altogether in relation to the speed of light. We cannot retain the equation with its connection to the speed of light and use it in reverse to define ε. That truly is cooking the books. The introduction to this article is sheer propaganda, and it is a new physics which was unknown even in recent times. It doesn't appear in my textbooks. I'm not going to discuss the matter anymore on this page. I'm going to take the matter to a wider arena because this article is the nonsensical conclusion of what was only the tip of the iceberg at the speed of light page. David Tombe ( talk) 21:47, 16 August 2009 (UTC)
Timothy, I've looked into magnetic permeability already and I know that it is a defined unit. I know the story. It was Gregorio de Giorgi of Rome's idea, and he promoted it at the sixth international electrical congress in St. Louis, Missouri in 1904. I don't have a problem with it because it doesn't lead to any tautologies. So long as permittivity remains a measurable quantity then nothing is destroyed as regards the 1856 experiment of Weber and Kohlrausch, which is of course about a numerical ratio.
Getting back to the main point, yes, Maxwell used the theoretical form of the equation c^2 = 1/(εμ). In fact in his 1861 paper, it appears at equation (132). It is in fact Newton's equation for the speed of sound. But the linkage to the number that closely relates to the speed of light is exclusively a consequence of the 1856 experiment. Maxwell never produced that number from his theory. He travelled down from Galloway to London in order to look up the results of Weber and Kohlrausch's experiment. That equation, when it involves the speed of light, is not a product of Maxwell's work alone. It is a combined product of Maxwell's theoretical work and the experimental work of Weber and Kohlrausch.
This matter is now being discussed at the wiki-physics project page. I no longer wish to discuss it on this page because the matter has a wider significance for physics in general beyond this particular article. David Tombe ( talk) 11:12, 17 August 2009 (UTC)
Would it be possible (without opening up a whole can of worms...) to put a sentence or two in the lede to explain how epsilon_0 is related to the physical quantities such as force and charge. Something along the lines of:
(with appropriate footnotes to make it clear that this relationship defines q, rather than provides a basis to measure , is only exact in free space, etc.)
As it stands, the article (and especially the intro) covers the metrology aspects, but doesn't explain why we bother defining it at all! Djr32 ( talk) 13:18, 10 October 2009 (UTC)
This was discussed at some length above, but that was back in 2007. I'll open the question again: can we move the article to electric constant, the name preferred by CODATA, the BIPM and NIST?
Let's take a look at the normal definition of permittivity:
where D is the electric displacement field and E is the electric field. You cannot use that definition to define a "vacuum permittivity" because there cannot be an electric displacement field in a vacuum (at least, not in classical terms). I guess that's why the name became "electric constant" in the first place, although I haven't got any reference to back up that hunch. The change is relatively recent – the 1986 CODATA set of values was still using "permittivity of vacuum" while the 1998 set uses "electric constant". Physchim62 (talk) 01:13, 1 April 2010 (UTC)
I agree. I've had 3 emags textbooks over the past 4 semesters, and all of them said "Electric constant". And while I know Google is not an oracle, "electric constant" is recognized by Google Calculator while "Vacuum Permittivity" and "Permittivity of free space" are not. Dmesg ( talk) 15:01, 1 April 2010 (UTC)
From the article: In the vacuum of classical electromagnetism, the polarization P = 0, so εr = 1 and ε = ε0.
Therefore D = ε0 . E. But what can be displaced in a vacuum??? — Preceding unsigned comment added by Koitus~nlwiki ( talk • contribs) 15:08, 20 February 2020 (UTC)
The Coulomb constant is already discussed fully at Vacuum permittivity; it does not warrant own page as this is just duplication of content. — Quondum 17:40, 8 March 2023 (UTC)
Due to importance of the SIThe SI's main goals are exactness, reproducibility and what is easy to measure, to the point of arriving to define the ampere instead of the coulomb as base unit (which would be a total idiocy if it weren't for the fact that an ampere is easier to build in a laboratory than a coulomb); this is simply how metrology works. But physics is something else, and having as main page a derived (rationalized) constant just because we deal with capacitors is for electricians, not physicists (and would have little encyclopedic value). At the moment the SI considers the Coulomb constant as derived from the vacuum permittivity, but (1) a stronger argument would be necessary to bring physics under the SI' exclusive jurisdiction (2) the SI changes position every 10-15 years and currently there is a lot of debate about the opportunity of choosing as base what is clearly derived (3) the Coulomb constant has quite a big historical significance. And even if all these three points are addressed, the Coulomb constant might still deserve its own page. -- Grufo ( talk) 17:29, 12 March 2023 (UTC)
Merged per consensus here. — Quondum 23:16, 9 May 2023 (UTC)
What is the "(14)" in 8.8541878188(14)×10−12 ? Times 14? Reference 14? 14 sig figs? Bystrc ( talk) 15:07, 10 June 2024 (UTC)