词条 | Base unit (measurement) |
释义 |
A base unit is one that has been explicitly so designated; a secondary unit for the same quantity is a derived unit. For example, when used with the International System of Units, the gram is a derived unit, not a base unit. In the language of measurement, quantities are quantifiable aspects of the world, such as time, distance, velocity, mass, temperature, energy, and weight, and units are used to describe their magnitude or quantity. Many of these quantities are related to each other by various physical laws, and as a result the units of a quantities can be generally be expressed as a product of powers of other units; for example, momentum is mass multiplied by velocity, while velocity is measured in distance divided by time. These relationships are discussed in dimensional analysis. Those that can be expressed in this fashion in terms of the base units are called {{Interlanguage link multi|Derived unit (measurement)|fr|3=Unité dérivée|lt=derived units}}. International System of Units{{main|SI base units}}In the International System of Units, there are seven base units: kilogram, metre, candela, second, ampere, kelvin, and mole. Natural units{{main|Natural units}}There are other relationships between physical quantities that can be expressed by means of fundamental constants, and to some extent it is an arbitrary decision whether to retain the fundamental constant as a quantity with dimensions or simply to define it as unity or a fixed dimensionless number, and reduce the number of explicit fundamental constants by one. For instance, time and distance are related to each other by the speed of light, c, which is a fundamental constant. It is possible to use this relationship to eliminate either the unit of time or that of distance. Similar considerations apply to the Planck constant, h, which relates energy (with dimension expressible in terms of mass, length and time) to frequency (with dimension expressible in terms of time). In theoretical physics it is customary to use such units (natural units) in which {{nowrap|1=c = 1}} and {{nowrap|1=ħ = 1}}. A similar choice can be applied to the vacuum permittivity or permittivity of free space, ε0.
A widely used choice, in particular for theoretical physics, is given by the system of Planck units, which are defined by setting {{nowrap|1=ħ = c = G = kB = ke = 1}}. That leaves every physical quantity expressed as a dimensionless number, so it is not surprising that there are also physicists who have cast doubt on the very existence of incompatible fundamental quantities.[1][2][3] See also{{Col-begin}}{{Col-break}}
References1. ^M. J. Duff, L. B. Okun and G. Veneziano, Trialogue on the number of fundamental constants, JHEP 0203, 023 (2002) [https://arxiv.org/abs/physics/0110060 preprint] [https://arxiv.org/pdf/physics/0110060v3.pdf pdf]. Mărimi fizice fundamentale2. ^{{cite book|last=Jackson|first=John David|title=Classical Electrodynamics|year=1998|publisher=John Wiley and Sons|pages=775|url=http://homepages.spa.umn.edu/~kd/Ast8001/em_units.pdf|accessdate=13 January 2014|chapter=Appendix on Units and Dimensions|quote=The arbitrariness in the number of fundamental units and in the dimensions of any physical quantity in terms of those units has been emphasized by Abraham, Plank, Bridgman, Birge, and others.}} 3. ^{{cite journal|last=Birge|first=Raymond T.|title=On the establishment of fundamental and derived units, with special reference to electric units. Part I.|journal=American Journal of Physics|year=1935|volume=3|pages=102–109|url=http://www.brynmawr.edu/physics/DJCross/docs/files/birge2.pdf|accessdate=13 January 2014|quote=Because, however, of the arbitrary character of dimensions, as presented so ably by Bridgman, the choice and number of fundamental units are arbitrary.|bibcode = 1935AmJPh...3..102B |doi = 10.1119/1.1992945 }} 2 : Measurement|Dimensional analysis |
随便看 |
|
开放百科全书收录14589846条英语、德语、日语等多语种百科知识,基本涵盖了大多数领域的百科知识,是一部内容自由、开放的电子版国际百科全书。