Overview#

International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the "metric" Unit of measurement system.

Standard prefixes for the SI units of measure#

Prefix name decahectokilomegagigaterapetaexazettayotta
Prefix symbol dahkMGTPEZY
Factor10010110210310610910121015101810211024

Submultiples#

Prefix namedecicentimillimicronanopicofemtoattozeptoyocto
Prefix symboldcmμnpfazy
Factor10010-110-210-310-610-910-1210-1510-1810-2110-24

SI base unit#

Unit name Unit symbol Dimension symbol Quantity name Typical symbols

NameSymbolMeasurePost-2019 formal definition[1]Historical origin / justificationDimensionsymbol
secondstime"The second, symbol s, is the SI unit of  time . It is defined by taking the fixed numerical value of the caesium frequency ∆νCs, the  unperturbed ground-state hyperfine transition frequency of the caesium 133 atom , to be 9192631770 when expressed in the unit Hz, which is equal to s−1."[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 . Historically this day was defined as the  mean solar day ; i.e., the average time between two successive occurrences of local apparent solar  noon .		T
metremlength"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 m s−1, where the second is defined in terms of  ∆''ν''%%sub Cs/% ."[1]1 / 10000000 of the distance from the  Earth 's equator to the North Pole measured on the  meridian arc through Paris .L
kilogramkgmass"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 6.62607015×10−34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and ∆νCs."[1]The mass of one  litre  of  water  at the temperature of melting ice. A litre is one thousandth of a cubic metre.M
ampereAelectric current"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 1.602176634×10−19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆νCs."[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 . 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  apart in vacuum, would produce between these conductors a force equal to 2×10−7  newtons  per metre of length." This had the effect of defining the  vacuum permeability  to beμ0 = 4π×10−7  H / m  or  N / A 2 or  T ⋅m/A or  Wb /(A⋅m) or  V ⋅ s /( A ⋅m)I
kelvinKthermodynamic temperature"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 1.380649×10−23 when expressed in the unit J K−1, which is equal to kg m2 s−2 K−1, where the kilogram, metre and second are defined in terms of h, c and ∆νCs."[1]The  Celsius scale : the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is  absolute zero ).Θ
molemolamount of substance"The mole, symbol mol, is the SI unit of  amount of substance . One mole contains exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the  Avogadro constant , NA, when expressed in the unit mol−1 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."[1]Atomic weight  or  molecular weight  divided by the  molar mass constant , 1 g/mol.N
candelacdluminous intensity"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 540×1012 Hz, Kcd, to be 683 when expressed in the unit  lm  W−1, which is equal to cd  sr  W−1, or cd sr kg−1 m−2 s3, where the kilogram, metre and second are defined in terms of h, c and ∆νCs."[1]The  candlepower , which is based on the light emitted from a burning candle of standard properties.J

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This particular version was published on 29-Nov-2024 16:16 by -jim.Top