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Table 2.1 SI Base Quantities and Units*, , metre The metre, symbol m, is the SI unit of length. It is defined by taking the, fixed numerical value of the speed of light in vacuum c to be 299792458, when expressed in the unit ms“, where the second is defined in terms of, the caesium frequency Aycs., , Mass Kilogram kg 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 6.62607015x10 when, expressed in the unit J s, which is equal to kg m?s*, where the metre and, the second are defined in terms of c and Aves., , Time second s The second, symbol s, is the SI unit of time. It is defined by taking the fixed, numerical value of the caesium frequency Aves, the unperturbed groundstate hyperfine transition frequency of the caesium-133 atom, to be, 9192631770 when expressed in the unit Hz, which is equal to s+., , Electric ampere A The ampere, symbol A, is the SI unit of electric current. It is defined by, taking the fixed numerical value of the elementary charge e to be, 1.602176634x10"” when expressed in the unit C, which is equal to A s,, where the second is defined in terms of Avcs., , , , , , , , , Thermo kelvin K The kelvin, symbol K, is the SI unit of thermodynamic temperature., , dynamic It is defined by taking the fixed numerical value of the Boltzmann constant, , Temperature k to be 1.380649x10* when expressed in the unit J K"!, which is equal to, kg m’s*k", where the kilogram, metre and second are defined in terms of, h, cand Aves., , Amount of mole mol The mole, symbol mol, is the SI unit of amount of substance. One mole, , substance contains exactly 6.02214076x10° elementary entities. This number is the, , fixed numerical value of the Avogadro constant, N,, when expressed in the, unit mol" and is called the Avogadro number. The amount of substance,, symbol n, of a system is a measure of the number of specified elementary, entities. An elementary entity may be an atom, a molecule, an ion, an electron,, any other particle or specified group of particles., , Luminous candela cd The candela, symbol cd, is the SI unit of luminous intensity in given direction., , intensity It is defined by taking the fixed numerical value of the luminous efficacy of, monochromatic radiation of frequency 540x10'? Hz, K,,, to be 683 when expressed, in the unit Im W", which is equal to cd sr W", or cd sr kg-'m”s*, where the, kilogram, metre and second are defined in terms of h, cand Aves.