The meter, symbol m, is the SI unit of the SI base quantity length l. It is defined by taking the fixed numerical value of the speed of light c in vacuum to be 299 792 458 when expressed in the unit m·s−1, where the second is defined in terms of the caesium frequency ΔνCs.
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 unit-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 unit-entity U 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 unit-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)). A unit-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 a unit-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 entity X, which is a constant for any defined entity X. QX = nX·zX·F
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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]
MitoPedia concepts: Ergodynamics