The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10−34 when expressed in the unit J s, which is equal to kg m2 s−1, where the meter and the second are defined in terms of c and ΔνCs.
Reference: Bureau International des Poids et Mesures (2019) The International System of Units (SI). 9th edition:117-216 ISBN 978-92-822-2272-0. - »Open Access pdf«
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 *,€ Qe = 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; Qe.
- § 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. Qe = 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 elementary 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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