are defined in terms of c and V Cs . Unit of time second The second symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency V Cs , the unperturbed ground-state hyperfine transition frequency of the caesium- atom, to be 9192631770 when expressed in the unit Hz, which is equal to s – . Unit of electric current ampere 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 .602176634× – when expressed in the unit C, which is equal to As, where the second is defined in terms of V Cs . Unit of thermodynamic temperature kelvin The kelvin, symbol k, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be .380649× – when expressed in the unit JK – , which is equal to kgm s – k – where the kilogram, metre and second are defined in terms of h , c and V Cs . Unit of amount of substance mole The mole, symbol mol, is the SI unit of amount of substance.
One mole contains exactly .02214076× elementary entities. This number is the fixed numerical value of the Avogadro constant, N A , 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.
Unit of luminous Intensity Candela The candela, symbol cd is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency × Hz, K cd , to be when expressed in the unit lm·W – , which is equal to cd·sr·W