LDAPWiki: International System of Units

International System of Units

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		deca	hecto	kilo	mega	giga	tera	peta	exa	zetta	yotta
Prefix symbol		da	h	k	M	G	T	P	E	Z	Y
Factor	100	101	102	103	106	109	1012	1015	1018	1021	1024

Submultiples#

Prefix name	deci	centi	milli	micro	nano	pico	femto	atto	zepto	yocto
Prefix symbol	d	c	m	μ	n	p	f	a	z	y
Factor	100	10-1	10-2	10-3	10-6	10-9	10-12	10-15	10-18	10-21	10-24

SI base unit#

Name	Symbol	Measure	Post-2019 formal definition[1]	Historical origin / justification	Dimensionsymbol
second	s	time	"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
metre	m	length	"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
kilogram	kg	mass	"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
ampere	A	electric 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
kelvin	K	thermodynamic 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
mole	mol	amount 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
candela	cd	luminous 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

More Information#

There might be more information for this subject on one of the following:

[#1] - SI_base_unit - based on information obtained 2022-05-27

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