查看完整版本 : 光速可以不靠測量計算出來嗎?

LT3648 2017-12-6 11:40 AM

光速可以不靠測量計算出來嗎?

電磁常數[url=https://zh.wikipedia.org/wiki/%E7%BB%8F%E5%85%B8%E7%94%B5%E7%A3%81%E5%AD%A6]馬克士威電磁理論[/url]將[i]c[/i]與[url=https://zh.wikipedia.org/wiki/%E7%9C%9F%E7%A9%BA%E7%94%B5%E5%AE%B9%E7%8E%87]真空電容率[/url][i]ε[/i]0和[url=https://zh.wikipedia.org/wiki/%E7%9C%9F%E7%A9%BA%E7%A3%81%E5%B0%8E%E7%8E%87]真空磁導率[/url][i]μ[/i]0聯繫起來:[i]c[/i]2 = 1/([i]ε[/i]0[i]μ[/i]0)。真空電容率可以通過測量已知大小電容器的[url=https://zh.wikipedia.org/wiki/%E9%9B%BB%E5%AE%B9]電容[/url]所得;由於[url=https://zh.wikipedia.org/wiki/%E5%AE%89%E5%9F%B9]安培[/url]單位的定義,真空磁導率的準確值被固定在4π×10−7 H·m−1。Rosa和Dorsey在1907年用這種方法得出的光速值為299,710±22 km/h。


請問[url=https://zh.wikipedia.org/wiki/%E7%9C%9F%E7%A9%BA%E7%94%B5%E5%AE%B9%E7%8E%87]真空電容率[/url][i]ε[/i]0和[url=https://zh.wikipedia.org/wiki/%E7%9C%9F%E7%A9%BA%E7%A3%81%E5%B0%8E%E7%8E%87]真空磁導率[/url][i]μ[/i]0是否靠實驗得出的,若是,以上求得之光速,嚴格上亦是靠測量。:smile_45:

LT3648 2017-12-7 05:29 PM

[quote]原帖由 [i]LT3648[/i] 於 2017-12-6 11:40 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=471978618&ptid=27104572][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
電磁常數馬克士威電磁理論將c與真空電容率ε0和真空磁導率μ0聯繫起來:c2 = 1/(ε0μ0)。真空電容率可以通過測量已知大小電容器的電容所得;由於安培單位的定義,真空磁導率的準確值被固定在4π×10−7 H·m&# ... [/quote]


請問[url=https://zh.wikipedia.org/wiki/%E7%9C%9F%E7%A9%BA%E7%94%B5%E5%AE%B9%E7%8E%87]真空電容率[/url][i]ε[/i]0和[url=https://zh.wikipedia.org/wiki/%E7%9C%9F%E7%A9%BA%E7%A3%81%E5%B0%8E%E7%8E%87]真空磁導率[/url][i]μ[/i]0是否靠實驗得出的,若是,以上求得之光速,嚴格上亦是靠測量。:smile_45:
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請問各位,以上兩個值,當時科學家是如何測量和計算得出的?相信不需用測量光速的距離要幾km遠吧?所以當時科學家得出ε0和μ0,間接計到光速,是非常了不起的。:loveliness:

[[i] 本帖最後由 LT3648 於 2017-12-7 10:39 PM 編輯 [/i]]

LT3648 2017-12-7 09:53 PM

吊!真空電容率ε0原來是用光速和此公式c2 = 1/(ε0μ0)計的::Q

Determination of a value for ε0[edit]
One now adds the requirement that one wants force to be measured in newtons, distance in metres, and charge to be measured in the engineers' practical unit, the coulomb, which is defined as the charge accumulated when a current of 1 ampere flows for one second. This shows that the parameter ε0 should be allocated the unit C2⋅N−1⋅m−2 (or equivalent units – in practice "farads per metre").

In order to establish the numerical value of ε0, one makes use of the fact that if one uses the rationalized forms of Coulomb's law and Ampère's force law (and other ideas) to develop Maxwell's equations, then the relationship stated above is found to exist between ε0, μ0 and c0. In principle, one has a choice of deciding whether to make the coulomb or the ampere the fundamental unit of electricity and magnetism. The decision was taken internationally to use the ampere. [color=#ff0000]This means that the value of ε0 is determined by the values of c0 and μ0[/color], as stated above. For a brief explanation of how the value of μ0 is decided, see the article about μ0.

[[i] 本帖最後由 LT3648 於 2017-12-7 10:23 PM 編輯 [/i]]

fb2d905c69ae04 2017-12-7 10:08 PM

梗係啦,光速不變,現代物理反過來以光速定義長度。

LT3648 2017-12-7 10:14 PM

吊!真空磁導率μ0原來是督數的,不是量度出來的::Q
又係靠ε0和光速fit入此公式而合理化的:c2 = 1/(ε0μ0)。

Systems of units and historical origin of value of μ0[edit]
In principle, there are several equation systems that could be used to set up a system of electrical quantities and units.[11] Since the late 19th century, the fundamental definitions of current units have been related to the definitions of mass, length and time units, using Ampère's force law. However, the precise way in which this has "officially" been done has changed many times, as measurement techniques and thinking on the topic developed. The overall history of the unit of electric current, and of the related question of how to define a set of equations for describing electromagnetic phenomena, is very complicated. Briefly, the basic reason why μ0 has the value it does is as follows.

Ampère's force law describes the experimentally-derived fact that, for two thin, straight, stationary, parallel wires, a distance r apart, in each of which a current I flows, the force per unit length, Fm, that one wire exerts upon the other in the vacuum of free space would be given by


Writing the constant of proportionality as km gives


The form of km needs to be chosen in order to set up a system of equations, and a value then needs to be allocated in order to define the unit of current.

In the old "electromagnetic (emu)" system of equations defined in the late 1800s, km was chosen to be a pure number, 2, distance was measured in centimetres, force was measured in the cgs unit dyne, and the currents defined by this equation were measured in the "electromagnetic unit (emu) of current" (also called the "abampere"). A practical unit to be used by electricians and engineers, the ampere, was then defined as equal to one tenth of the electromagnetic unit of current.

In another system, the "rationalized metre–kilogram–second (rmks) system" (or alternatively the "metre–kilogram–second–ampere (mksa) system"), km is written as μ0/2π, where μ0is a measurement-system constant called the "magnetic constant".[12] The value of μ0 was chosen such that the rmks unit of current is equal in size to the ampere in the emu system: μ0 is defined to be 4π × 10−7 H/m.[5]

Historically, several different systems (including the two described above) were in use simultaneously. In particular, physicists and engineers used different systems, and physicists used three different systems for different parts of physics theory and a fourth different system (the engineers' system) for laboratory experiments. In 1948, international decisions were made by standards organizations to adopt the rmks system, and its related set of electrical quantities and units, as the single main international system for describing electromagnetic phenomena in the International System of Units.

Ampère's law as stated above describes a physical property of the world. However, the choices about the form of km and [color=#ff0000]the value of μ0 are totally human decisions, taken by international bodies composed of representatives of the national standards organizations of all participating countries. The parameter μ0 is a measurement-system constant, not a physical constant that can be measured. It does not, in any meaningful sense, describe a physical property of the vacuum.[[/color]13] This is why the relevant Standards Organizations prefer the name "magnetic constant", rather than any name that carries the hidden and misleading implication that μ0 describes some physical property.[citation needed]

[[i] 本帖最後由 LT3648 於 2017-12-7 10:41 PM 編輯 [/i]]

LT3648 2017-12-7 10:20 PM

[quote]原帖由 [i]fb2d905c69ae04[/i] 於 2017-12-7 10:08 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=472061713&ptid=27104572][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
梗係啦,光速不變,現代物理反過來以光速定義長度。 [/quote]

吊!又話Rosa和Dorsey在1907年用這種這條式: c2 = 1/(ε0μ0)得出的光速值為299,710±22 km/h,原來是搵笨柒的,你明白嗎?;P
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