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[{$pagename}] is a coherent system of units of measurement starting with seven [SI Base Units]
[{$pagename}] magnitudes of all SI units have been defined by declaring that seven [SI Defining Constants] have certain exact numerical values when expressed in terms of their SI units.
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!! SI base unit
Unit name Unit symbol Dimension symbol Quantity name Typical symbols
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||Name||Symbol||Measure||Post-2019 formal definition%%sup [[1]/%||Historical origin / justification||[Dimensionsymbol|https://en.wikipedia.org/wiki/Dimensional_analysis]
|[second|https://en.wikipedia.org/wiki/Second]|s|[time|https://en.wikipedia.org/wiki/Time]|"The second, symbol s, is the SI unit of  [time|https://en.wikipedia.org/wiki/Time] . It is defined by taking the fixed numerical value of the caesium frequency ∆''ν''%%sub Cs/%, the  [unperturbed ground-state hyperfine transition frequency of the caesium 133 atom|https://en.wikipedia.org/wiki/Caesium_standard] , to be 9192631770 when expressed in the unit Hz, which is equal to s%%sup −1/%."%%sup [[1]/%|The day is divided in 24 hours, each hour divided in 60 minutes, each minute divided in 60 seconds. \\ A second is 1 / (24 × 60 × 60) of the  [day|https://en.wikipedia.org/wiki/Day] . Historically this day was defined as the  [mean solar day|https://en.wikipedia.org/wiki/Mean_solar_day] ; i.e., the average time between two successive occurrences of local apparent solar  [noon|https://en.wikipedia.org/wiki/Noon] .|T
|[metre|https://en.wikipedia.org/wiki/Metre]|m|[length|https://en.wikipedia.org/wiki/Length]|"The metre, symbol m, is the SI unit of  [length|https://en.wikipedia.org/wiki/Length] . It is defined by taking the fixed numerical value of the  [speed of light in vacuum|https://en.wikipedia.org/wiki/Speed_of_light_in_vacuum]  ''c'' to be 299792458 when expressed in the unit m s%%sup −1/%, where the second is defined in terms of  [∆''ν''%%sub Cs/%|https://en.wikipedia.org/wiki/Caesium_standard] ."%%sup [[1]/%|1 / 10000000 of the distance from the  [Earth|https://en.wikipedia.org/wiki/Earth] 's equator to the North Pole measured on the  [meridian arc through Paris|https://en.wikipedia.org/wiki/Paris_meridian] .|L
|[kilogram|https://en.wikipedia.org/wiki/Kilogram]|kg|[mass|https://en.wikipedia.org/wiki/Mass]|"The kilogram, symbol kg, is the SI unit of  [mass|https://en.wikipedia.org/wiki/Mass] . It is defined by taking the fixed numerical value of the  [Planck constant|https://en.wikipedia.org/wiki/Planck_constant]  ''h'' to be 6.62607015×10%%sup −34/% when expressed in the unit J s, which is equal to kg m%%sup 2/% s%%sup −1/%, where the metre and the second are defined in terms of ''c'' and ∆''ν''%%sub Cs/%."%%sup [[1]/%|The mass of one  [litre|https://en.wikipedia.org/wiki/Litre]  of  [water|https://en.wikipedia.org/wiki/Water]  at the temperature of melting ice. A litre is one thousandth of a cubic metre.|M
|[ampere|https://en.wikipedia.org/wiki/Ampere]|A|[electric current|https://en.wikipedia.org/wiki/Electric_current]|"The ampere, symbol A, is the SI unit of  [electric current|https://en.wikipedia.org/wiki/Electric_current] . It is defined by taking the fixed numerical value of the  [elementary charge|https://en.wikipedia.org/wiki/Elementary_charge]  ''e'' to be 1.602176634×10%%sup −19/% when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆''ν''%%sub Cs/%."%%sup [[1]/%|The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of  [silver nitrate|https://en.wikipedia.org/wiki/Silver_nitrate] . Compared to the SI ampere, the difference is 0.015%. However, the most recent pre-2019 definition was: "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one  [metre|https://en.wikipedia.org/wiki/Metre]  apart in vacuum, would produce between these conductors a force equal to 2×10%%sup −7/%  [newtons|https://en.wikipedia.org/wiki/Newton_(unit)]  per metre of length." This had the effect of defining the  [vacuum permeability|https://en.wikipedia.org/wiki/Vacuum_permeability]  to be''μ''%%sub 0/% = 4''π''×10%%sup −7/%  [H|https://en.wikipedia.org/wiki/Henry_(unit)] / [m|https://en.wikipedia.org/wiki/Metre]  or  [N|https://en.wikipedia.org/wiki/Newton_(unit)] / [A|https://en.wikipedia.org/wiki/Ampere] %%sup 2/% or  [T|https://en.wikipedia.org/wiki/Tesla_(unit)] ⋅m/A or  [Wb|https://en.wikipedia.org/wiki/Weber_(unit)] /(A⋅m) or  [V|https://en.wikipedia.org/wiki/Volt] ⋅ [s|https://en.wikipedia.org/wiki/Second] /( [A|https://en.wikipedia.org/wiki/Ampere] ⋅m)|I
|[kelvin|https://en.wikipedia.org/wiki/Kelvin]|K|[thermodynamic temperature|https://en.wikipedia.org/wiki/Thermodynamic_temperature]|"The kelvin, symbol K, is the SI unit of  [thermodynamic temperature|https://en.wikipedia.org/wiki/Thermodynamic_temperature] . It is defined by taking the fixed numerical value of the  [Boltzmann constant|https://en.wikipedia.org/wiki/Boltzmann_constant]  ''k'' to be 1.380649×10%%sup −23/% when expressed in the unit J K%%sup −1/%, which is equal to kg m%%sup 2/% s%%sup −2/% K%%sup −1/%, where the kilogram, metre and second are defined in terms of ''h'', ''c'' and ∆''ν''%%sub Cs/%."%%sup [[1]/%|The  [Celsius scale|https://en.wikipedia.org/wiki/Celsius_scale] : the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is  [absolute zero|https://en.wikipedia.org/wiki/Absolute_zero] ).|Θ
|[mole|https://en.wikipedia.org/wiki/Mole_(unit)]|mol|[amount of substance|https://en.wikipedia.org/wiki/Amount_of_substance]|"The mole, symbol mol, is the SI unit of  [amount of substance|https://en.wikipedia.org/wiki/Amount_of_substance] . One mole contains exactly 6.022 140 76 × 10%%sup 23/% elementary entities. This number is the fixed numerical value of the  [Avogadro constant|https://en.wikipedia.org/wiki/Avogadro_constant] , ''N''%%sub A/%, when expressed in the unit mol%%sup −1/% and is called the  [Avogadro number|https://en.wikipedia.org/wiki/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."%%sup [[1]/%|[Atomic weight|https://en.wikipedia.org/wiki/Atomic_weight]  or  [molecular weight|https://en.wikipedia.org/wiki/Molecular_weight]  divided by the  [molar mass constant|https://en.wikipedia.org/wiki/Molar_mass_constant] , 1 g/mol.|N
|[candela|https://en.wikipedia.org/wiki/Candela]|cd|[luminous intensity|https://en.wikipedia.org/wiki/Luminous_intensity]|"The candela, symbol cd, is the SI unit of  [luminous intensity|https://en.wikipedia.org/wiki/Luminous_intensity]  in a given direction. It is defined by taking the fixed numerical value of the  [luminous efficacy|https://en.wikipedia.org/wiki/Luminous_efficacy]  of monochromatic radiation of frequency 540×10%%sup 12/% Hz, ''K''%%sub cd/%, to be 683 when expressed in the unit  [lm|https://en.wikipedia.org/wiki/Lumen_(unit)]  W%%sup −1/%, which is equal to cd  [sr|https://en.wikipedia.org/wiki/Steradian]  W%%sup −1/%, or cd sr kg%%sup −1/% m%%sup −2/% s%%sup 3/%, where the kilogram, metre and second are defined in terms of ''h'', ''c'' and ∆''ν''%%sub Cs/%."%%sup [[1]/%|The  [candlepower|https://en.wikipedia.org/wiki/Candlepower] , which is based on the light emitted from a burning candle of standard properties.|J \\
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This page (revision-8) was last changed on 29-Nov-2024 16:16 by -jimTop