# 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？

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]

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

LT3648 2017-12-7 09:53 PM

Determination of a value for ε0
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

LT3648 2017-12-7 10:14 PM

Systems of units and historical origin of value of μ0
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]