The International System of Units (SI) is the modern form of the metric system of units for use in all aspects of life, including international trade, manufacturing, security, health and safety, protection of the environment, and in the basic science that underpins all of these. The system of quantities underlying the SI and the equations relating them are based on the present description of nature and are familiar to all scientists, technologists and engineers.
The definition of the SI units is established in terms of a set of seven defining constants. The complete system of units can be derived from the fixed values of these defining constants, expressed in the units of the SI. These seven defining constants are the most fundamental feature of the definition of the entire system of units. These particular constants were chosen after having been identified as being the best choice, taking into account the previous definition of the SI, which was based on seven base units, and progress in science (p. 125).
SI base quantities / SI base units
Quantity Symbol for quantity Q Symbol for dimension Name of SI unit Symbol for SI unit uQ [*] length l L meter m mass m M kilogram kg time t T second s electric current I I ampere A thermodynamic temperature T Θ kelvin K amount of substance *,§ nX = NX·NA-1 N mole mol count *,$ NX X elementary unit x elementary entity *,$ UX U elementary unit x charge *,€ QX = NX·zX·e I·T coulomb C = A·s luminous intensity Iv J candela cd
- [*] »SI base units, except for the canonical 'elementary unit' [x]. The following footnotes are canonical comments.
- * For the quantities n, N, U, and Q, the entity-type X of the elementary entity UX has to be specified in the text and indicated by a subscript: nO2; Nce; QX.
- § Amount nX is an elementary quantity, converting the elementary unit [x] into moles [mol] using the Avogadro constant, NA.
- $ Count NX equals the number of elementary entities UX. In the SI, the quantity 'count' is explicitly considered as an exception: "Each of the seven base quantities used in the SI is regarded as having its own dimension. .. All other quantities, with the exception of counts, are derived quantities" (Bureau International des Poids et Mesures 2019 The International System of Units (SI)). An elementary entity UX is not a count (UX is not a number of UX). NX has the dimension X of a count and UX has the dimension U of an elementary entity, and both quantities have the same unit, the 'elementary unit' [x].
- € Charge is a derived SI quantity. Charge is an elementary quantity, converting the elementary unit [x] into coulombs [C] using the elementary charge, e, or converting moles [mol] into coulombs [C] using the Faraday constant, F. zX is the charge number of elementary entity UX, which is a constant for any defined elementary entity UX. QX = nX·zX·F
|Bureau International des Poids et Mesures 2019 The International System of Units (SI)||Bureau International des Poids et Mesures (2019) The International System of Units (SI). 9th edition:117-216 ISBN 978-92-822-2272-0.||2019|
|Cohen 2008 IUPAC Green Book||Cohen ER, Cvitas T, Frey JG, Holmström B, Kuchitsu K, Marquardt R, Mills I, Pavese F, Quack M, Stohner J, Strauss HL, Takami M, Thor HL (2008) Quantities, Units and Symbols in Physical Chemistry. IUPAC Green Book 3rd Edition, 2nd Printing, IUPAC & RSC Publishing, Cambridge.||2008|
|MitoFit 2020.4.v0||Gnaiger Erich (2020) A X-mass Carol. The Elementary of Body Mass Excess and mitObesity. MitoFit Preprint Arch 2020.4.v0.|
|BEC 2020.1 doi10.26124bec2020-0001.v1||Gnaiger Erich et al ― MitoEAGLE Task Group (2020) Mitochondrial physiology. Bioenerg Commun 2020.1. doi:10.26124/bec:2020-0001.v1.||2020|
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- Entity, count, and number, and SI base quantities / SI base units
Quantity name Symbol Unit name Symbol Comment unit-entity UX elementary unit [x] UX, UB; [x] not in SI count NX elementary unit [x] NX, NB; [x] not in SI number N - dimensionless = NX·UX-1 amount of substance nB mole [mol] nX, nB electric current I ampere [A] A = C·s-1 time t second [s] length l meter [m] SI: metre mass m kilogram [kg] thermodynamic temperature T kelvin [K] luminous intensity IV candela [cd]
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