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International System of Units

## Description

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).

Abbreviation: SI

## SI base quantities / SI base units

» time / [second]
» length / [meter]
» mass / [kilogram]
» electric current / [ampere]
» thermodynamic temperature / [kelvin]
» amount of substance / [mole]
» luminous intensity / [candela]
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