International System of Units

The system of standard units of measurement commonly used in the world

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International System of Units (French: Syst è me International d'Unit é s Symbol: SI), derived from Metric or metric system , old name“ Universal Metric System ”Is the most commonly used standard in the world Weights and measures Unit system, using decimal system Carry system. yes eighteenth century The efforts of late scientists were first made French Revolution In 1799, it was used by France as a unit of measurement. The International System of Units is Metric The unit system developed on the basis of International Conference on Metrology It is approved and recommended to be adopted by all countries. Its international abbreviation is SI.

Chinese name: International System of Units
Foreign name: Système International d'Unités

Abbreviation: SI
Unit classification: Basic unit ， Export Units ， Auxiliary unit
Prepared by: International Conference on Metrology

catalog

▪ Length unit - meter (m)
▪ Mass unit - kg
▪ Time unit - second (s)
▪ Current intensity unit - ampere (A)
▪ Thermodynamic temperature unit Kelvin (K)

▪ Unit of measure of substance - mole
▪ Luminous intensity unit - candela (cd)
▪ Auxiliary unit
▪ Export Units
▪ SI prefix
five status

six physical quantity
▪ Unit system
▪ Centimeter gram second system (CGS system)
▪ Unit table

brief introduction

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The international system of units is a consistent system of units adopted and recommended by the International Conference on Metrology (CGPM). In the International System of Units, units are divided into three categories: basic units, derived units and auxiliary units. The seven strictly defined basic units are: length （ rice ）、 quality （ Kg ）、 time （ second ）、 electric current （ ampere ）、 Thermodynamic temperature （ Kelvin ）、 Amount of substance （ mole ）And Luminous intensity （ Candela ）。 Basic units are dimensionally independent of each other, and many derived units are composed of basic units. There are only two auxiliary units, purely geometric units. Of course, the auxiliary company can also form an export company. Various physical quantities are related to each other through the equations describing natural laws and their definitions. For convenience, a group of mutually independent physical quantities are selected as basic quantities, and other quantities are expressed according to basic quantities and relevant equations, which are called derived quantities^[1] 。

history

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According to the resolution of the 9th International Conference on Metrology in 1948, the International Commission on Metrology (CIPM) was tasked to "study and formulate a set of rules for measurement units", trying to establish a scientific and practical system of measurement units. In 1954, the 10th International Conference on Metrology decided to adopt six quantities of length, mass, time, current, thermodynamic temperature and luminous intensity as the basic quantities of the practical measurement system. According to the resolution of the 11th International Conference on Metrology in 1960, this practical unit of measurement was called the International System of Units, and SI was used as the general abbreviation symbol of the International System of Units; Formulate the rules for the prefixes (called SI prefixes), SI derived units, SI auxiliary units and other provisions used to form multiple and fractional units, and form a set of rules for units of measurement. In 1971, the 14th International Conference on Metrology decided to add "quantity of matter" as the seventh basic quantity of the International System of Units on the basis of the first six quantities, and adopted their corresponding units as the basic units of the International System of Units^[2] 。

On November 16, 2018, the International Conference on Metrology adopted a resolution that the three basic units in the International System of Units（ Kg 、 Kelvin 、 ampere ）It will be defined by natural constants and will take effect on the International Metrology Day 2019 (May 20). So far, the seven basic units of the International System of Units will all be defined by basic physical constants, and the realization of the value of the quantity has entered the era of quantization.^[4] (Definition mole Of Avogadro constant It is not a natural constant, but a fixed value used to represent the number of particles for teaching reasons.)

In November 2022, four new prefixes will be added to the International System of Units, namely ronna, ronto, quetta and quecto.^[6] On May 20, 2023, the National Science and Technology Terminology Examination and Approval Committee and the State Administration of Market Supervision and Administration jointly released the Chinese name of the new prefix of the International System of Units at the main commemoration event of World Metrology Day in China. The Chinese names are: Capacity [That] soft [Support] Kun [It] Loss [Coto], respectively 10 twenty-seven 、10 -27 、10 thirty 、10 -30 。^[10]

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Constitution rules

International System of Units system of units of measurement The principle of decimal system Constitute its multiple and Fractional unit ； The units of multiples and fractions can only be formed by SI prefixes. The basic units and their definitions can only be determined by the International Conference on Metrology. The special names and symbols of SI derived units can only be selected by the International Conference on Metrology. According to the above rules, such as volume unit rise , mass unit ton Luminance Company Nite (nt, 1 nit=1 kan/m2) are not SI units.

Two types of SI units: in the International System of Units, the seven basic units and the units derived from the basic units according to the principle of consistency are collectively called SI units. For example, SI derived units include those units with special names given by the International Conference on Metrology, such as Newton 、 watt 、 Volts 、 Lumen Etc; It also includes those units without special names, such as Meters per second , Joule per Kelvin Radians per second Etc.

SI prefix: When the SI prefix is added to the unit, a new whole is formed. Therefore, when there is an index, it refers to the whole, not only to the unit with no prefix. For example, when expressed as cm ³, it means cubic centimeter; When expressed as μ s ⁻⁻, it refers to every microsecond; When expressed in mm ²/s, it refers to the square millimeter per second. The SI prefix cannot be used alone under any circumstances, for example, k cannot be used instead of kg or k Ω, or 10 ³.

Dimensionless quantity SI units of. There are quite a number of physical quantities dimension It is "1". For example: refractive index n ， Dynamic friction coefficient μ ， Linear strain ε ， Relative atomic mass A r ， Proton number Z , power level L p ， Plane angle φ 。 The SI units of all such quantities are the ratio of two identical SI units. For example, the SI unit of refractive index is the ratio of the SI units of two velocities, that is, m · s ⁻ ¹/(m · s ⁻ ¹)=1; The SI unit of dynamic friction coefficient is the ratio of the SI units of two forces, that is, N/N=1. Its multiples and fractional units are not formed with SI prefixes but with powers of 10, such as 10, 10 ³, 10 ⁻, 10 ⁻, etc. The mathematical symbol% can also be used to replace 10 ⁻ ², but units such as microgram per gram (μ g/g), milliliter per cubic meter (mL/m ³) can also be used to replace 10 ⁻, but such abbreviations as ppm should not be used^[2] 。

Usage rules

one
The name of the unit and its abbreviation have been clearly defined. Abbreviation can be used as its full name without confusion. It is customary to continue to use the units that only use abbreviation, for example, in some decimal multiple units, if only“ Milliampere ”Instead of milliamperes. However, the use of "milliampere" is not excluded.
two
The name of the combination unit is consistent with the order in which its symbols are written. In symbol multiplication sign No corresponding name division sign The corresponding name is "every". No matter how many units are in the denominator, "every" only appears once in the place of the division sign. For example, the symbol of acceleration SI unit is m/s ², and its name is "meters per second to the second power" instead of "meters per second per second"; The common unit symbol of electric energy kW · h is“ Kilowatt hour ”Instead of "kilowatt hours".
three
Power The order of the unit name in the form of is that the index name precedes the unit name, and the corresponding index name is formed by adding the word "power" to the number. For example, the name of the unit symbol of the moment of inertia of the section m ∨ is "square meter" instead of "square meter".
four
The index is a unit of - 1, or a unit with a numerator of 1. Its name begins with the word "every". For example, the SI unit of the coefficient of linear expansion is ℃ ⁻⁻⁻⁻ or K ⁻⁻⁻, and its name is "per ℃" or "per Kelvin" instead of "minus first power Celsius" or "minus first power Kelvin".
five
If the second and third powers of length refer to area and volume, the corresponding indices are named "square" and "cube" and are placed before the name of the length unit. For example, the SI unit symbol m ³ of volume is called "cubic meter", not "cubic meter" or "cubic meter", and the common unit symbol km ² of area is called "square kilometer", not "square kilometer" or "square kilometer".
six
The multiples and fractional units selected should generally be in the range of 0.1~1000. For example, 1.2 × 10 ∨ N can be written as 12kN; 0.00394m can be written as 3.94mm; 11401Pa can be written as 11.401kPa; 3.1 × 10 ⁻ s can be written as 31ns. Units used in some occasions are not subject to the above restrictions. For example: Mechanical drawing In millimetres; Land area unit Square kilometers ； Unit used for cross-sectional area of conductor square millimetre Etc. In the value table of the same quantity and in the narrative article, for the convenience of comparison, the same unit can also be used without considering whether the value is in the range of 0.1~1000.
seven
Prefixes: 100, 10, fen, and li (h, da, d, c) are generally only used for certain lengths, areas, volumes, and other occasions that have long been used to. For example, it can be used for decibel dB, etc.
eight
In some units other than the International System of Units, prefixes can be used to form multiples or fractional units. stay Legal measurement unit Medium, only tons rise 、 Electron volt Decibel Turks These units are sometimes prefixed.
nine
In the legal measurement unit, non decimal system Units and Celsius temperature units are not used as prefixes.
ten
Do not overlap prefixes. For example, "pico farad" μ μ F shall not be used, but "pico farad" or "pico farad" pF shall be used instead; "Nanometer" or "nanometer" nm should not be used instead of "nanometer" m μ m. However, for example, "3000 kW" can be used because it is a colloquial description of "3000 kW", of which only the second "1000" is the prefix.
eleven
Sometimes it is confusing to use a fraction word as the Chinese name of the prefix. For example, 1kg and 1000g are both "one kilogram" in oral narration, which cannot be distinguished. If it is necessary to strictly distinguish, 1000g can be read as "one hundred and zero grams" or "one thousand grams".
twelve
100 million (10% eight ）The use of numerals such as,,, (10 ∨) is unrestricted. They can also form multiple units with units, but they are not prefixes. For example, the unit of transportation volume is "ten thousand ton km", and the symbol can be 10 ≮ t · km or ten thousand t · km.
thirteen
When the combination unit of multiplication form forms its multiples and fractional units by adding a prefix, the prefix is generally added to the first unit. For example: moment The SI unit of is N · m, and its multiples and fractional units can be MN · m, kN · m, mN · m, μ N · m, etc., instead of prefixing m.
fourteen
The combination unit of the division form, when adding the prefix to form a multiple and fractional unit, the prefix is generally added to the first unit of the numerator. For example: heat capacity The SI unit of is J/K, and its multiple unit can be kJ/K instead of J/mk; The SI unit of momentum is kg · m/s, and its multiple unit can be Mg · m/s instead of kg · km/s.
fifteen
When the denominator of a combination unit is a unit of length, area or volume, the prefixes can also be used to form multiples and fractional units of the combination unit according to custom and convenience. For example, the SI unit of density is kg/m ³, and its multiple unit can be g/cm ³; The SI unit of charge volume density is C/m ³, and its multiples and fractional units can be MC/m ³, C/mm ³ or C/cm ³; The SI unit of electric field intensity is V/m, and its multiple unit can be kV/m or V/mm, etc.
sixteen
Generally, two units with prefixes are not used in combination units, nor are prefixes used in numerators and denominators at the same time. The prefixes in the SI unit kg of mass are not treated as prefixes here, but g is Fractional unit Inaction has no prefix. For example: Linear density The SI unit of is kg/m, and the fractional unit can be g/km.
seventeen
The exponent of a multiple or fractional unit in the form of a power, belonging to a multiple or fractional unit including the prefix. For example, 1cm ²=1 × (10 ⁻ ² m) ²=1 × 10 ⁻ ∨ m ², and 1cm ² ≠ 10 ⁻ ² m ². Another example: 1 μ s ⁻⁻⁻⁶⁶⁶⁶⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻=1 × (10.
eighteen
In physics equation If all the quantities are expressed in SI units equation The form of is not different from that of the physical equation. This can avoid mistakes and unnecessary coefficient Enter the calculation equation. Therefore, it is suggested that all values should be expressed in SI units in calculation, while Prefix With the corresponding 10 Power Instead. For example, the relationship between the speed v, time t and the distance s of a uniformly moving object is: v=s/t. Let an object pass through a distance of 9 km in 1.5 min, and calculate the speed. Here, both kilometers and minutes are Legal measurement unit But they are not SI units. Their corresponding SI units are seconds and meters. If these three quantities are expressed in SI units, the calculation formula will be completely consistent with the above relationship without bringing other coefficients. s=9km=9×10³m，t=1.5min=1.5×60s=90s。 The SI unit of v is m/s, so: v=s/t=9 × 10 ³ m/90s=100 m/s.
nineteen
When the abbreviation of the SI prefix Chinese name is placed before the abbreviation of the unit name to form a Chinese symbol, care should be taken to avoid confusion, and parentheses should be used if necessary. For example: represents the rotation frequency measurement It shall not be written as 3000 seconds. For example, "three per thousand seconds" should be written as "3 (thousand seconds) ⁻⁻", where "thousand" is the prefix; If it means "three thousand per second", it should be written as "three thousand (seconds) ⁻⁻⁻", where "thousand" is a numeral. The volume value shall not be written as 2 km ³. If it means "two cubic kilometers", it should be written as "2 (kilometers) ³". Here, "thousand" is the prefix; If it means "two thousand cubic meters", it should write "two thousand (meters) ³", where "thousand" is a numeral.

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Basic unit

Basic unit The definition of Science and technology Their definitions are also changing definition and evolution The situation.

Physical quantity name	Physical quantity symbol	Physical quantity unit	Name of unit	Symbol of unit	Unit definition
time	t	1s	second	s	One second is the time of 9192631770 cycles of radiation corresponding to the transition between two hyperfine levels of cesium-133 atom in the ground state
length	L	1m	rice	m	1 meter is light stay vacuum Travel in (299792458) ⁻⁻ s in
quality	m	1kg	Kg	kg	1kg is Planck constant Is the mass of 6.62607015 × 10 ⁻ ³ ≮ J · s (6.62607015 × 10 ⁻ ³ ≮ kg · m ² · s ⁻ ¹)
electric current	Ι	1A	ampere	A	1 ampere is (1.602176634) × 10 nineteen The current corresponding to the individual charge, that is, 1 ampere is the current passing through 1 coulomb charge within 1 s at a certain point
Thermodynamic temperature	T	1K	Kelvin	K	1 Kelvin is the thermodynamic temperature when the Boltzmann constant is 1.380649 × 10J · K ⁻ (1.380649 × 10 ⁻ ² kg · m ² · s ⁻ ² · K ⁻)
Amount of substance	n （ ν ）	1mol	mole	mol	1 mole is the amount of substance in a system that accurately contains 6.02214076 × 10 ² ³ atoms or molecules and other basic units
Luminous intensity	I （ Iv ）	1cd	Candela	cd	1 candela is a light source emitted in a given direction frequency Is the monochromatic radiation of 540 × 10 ^ 12s ⁻⁻, and the radiation intensity in this direction is the luminous intensity when (683) ⁻ kg · m ² · s ⁻ ³ · sr ⁻^{[11 ]}
Note: 1 In people's life and trade, mass may be mistaken for weight. In fact, the unit of weight is 1N, while the unit of mass is 1kg. 2. The name of the unit and the symbol of the unit are two columns, the Chinese symbol of the unit suffix is in the front, and the international symbol of the unit suffix is in the rear. Example: "Ampere" can be used as the Chinese symbol of "A". The former name of 3. kg (kilogram): G (Grave).

Length unit - meter (m)

Platinum iridium meter gauge

Approved by the 1st International Conference on Metrology in 1889 International rice prototype (platinum iridium meter gauge) is 1 meter long. In 1927, the 7th Metrology Conference made the following strict provisions on the definition of meter: International Bureau of Metrology The distance between the axes of the two middle scribed lines engraved on the preserved platinum iridium meter ruler at 0 ℃ (the platinum iridium meter ruler is a piece of cross section approximately H Three lines perpendicular to the longitudinal direction of the ruler are carved on the two ends of the middle transverse rib of the ruler, and the middle score refers to the middle score of every three lines). This ruler is stored in 1 standard Atmospheric pressure And put it on two pieces with a diameter of at least 1cm symmetrically placed on the same horizontal plane and 571mm apart cylinder On.

The above definition of meter has an uncertainty of about 1 × 10 ⁻⁷. Due to the development of science and technology, it can not meet the requirements of metrology and other precision measurement Needs. In the 1950s, with isotope spectrum light source And found krypton 86 isotope spectrum line with very narrow width interference With the success of technology, people have finally found an indestructible Natural benchmark , which is to take the wavelength of light wave as Length unit The natural benchmark of.

Therefore, the definition of meter at the 11th International Conference on Metrology in 1960 was changed as follows: "The length of meter is equal to 2p of krypton 86 atom ten And 5d five The radiation of transitions between energy levels is 1650763.73 times the wavelength in vacuum. " The limit uncertainty of krypton 86 length standard is ± 4 × 10 ⁻ ⁹. After the definition of meter is changed, International rice prototype It shall still be kept in the International Bureau of Metrology under the original conditions.

Due to the saturated absorption stability of the laser Frequency stability and Reproducibility Compared with the wavelength of krypton 86, their wavelengths are easier to reproduce and their accuracy may be further improved. Therefore, at the two meetings of the Advisory Committee on the Definition of Meters in 1973 and 1979, the wavelength values of four kinds of stable lasers were recommended successively, which are used in parallel with the wavelength of krypton 86, and have the same Accuracy 。

Since 1973 precision measurement From infrared band until visible light Various bands Spectral line The frequency value of. According to the frequency and wavelength value v and λ , got vacuum In light speed Value с= λv =299792458 m/s. This value is very accurate, so people decided to take this value of light speed as the definition value, while the length l (or wavelength) is defined by time t (or frequency) by formula l =с t (or λ =с/ v ）Export.

Old definition: In May 1790, a special committee composed of French scientists proposed to pass the Earth in Paris meridian One forty millionth of the total length is taken as the unit of length - meter.

In October 1983, the 17th International Conference on Metrology formally adopted the following new definitions: "1 meter is the travel of light in vacuum within (299792458) ⁻ s" 。

Mass unit - kg

international prototype kilogram

In 1889, the 1st International Conference on Metrology approved international prototype kilogram , and announced that this prototype will be used as the mass unit in the future.

In order to avoid ambiguity in the meaning of the word "weight" in general use, the Third International Conference on Metrology in 1901 stipulated that:

The kilogram is the unit of mass (not weight), which is equal to the mass of the international kilogram prototype. This platinum iridium kilogram prototype is kept at the International Bureau of Metrology in accordance with the conditions stipulated by the First International Conference on Metrology in 1889.

New definition: Georgia Institute of Technology Ronald Fox, an emeritus professor at the School of Physics, proposed that henceforth grams (one thousandth of a kilogram) would be strictly defined as 18 × 14074481 C-12 atom Weight of. At least two other proposals to redefine the kilogram are under discussion. They include: 1 ° replaced by pure silicon atomic sphere Platinum and iridium blend cylinder ； 2 ° Use a known "watt balance" device and use Electromagnetic energy Define the kilogram.

Old definition: the mass of 1L pure water at 4 ℃ is 1kg.

On November 16, 2018, the International Conference on Metrology adopted a resolution that 1 kg was defined as“ corresponding Planck constant^[3] Is the mass of 6.62607015 × 10 ⁻ ³ ≮ J · s (6.62607015 × 10 ⁻ ³ ≮ kg · m ² · s ⁻ ¹) ”。

Time unit - second (s)

first, Time unit "Second" is defined as Average solar day 1/86400. The precise definition of "average solar day" is left to astronomers. But the measurement shows that the average solar day cannot guarantee the necessary Accuracy 。 In order to define the time unit more accurately, the 11th International Congress of Metrology in 1960 approved the Regression year The definition based on: "The second is 1/31556925.9747 of the regression year calculated from 12:00 in the calendar book on January 0, 1900." However, this definition accuracy Still can't meet the requirements at that time precise Metrology Therefore, the 13th International Metrology Conference in 1967 decided to change the definition of second as follows according to the level of atomic energy level transition measurement technology at that time: "One second is the time of 9192631770 cycles of radiation corresponding to the transition between two hyperfine levels of cesium-133 atom in the ground state ”。

International Atomic Time According to the above definition of seconds reference Time scale , which belongs to the International System of Units (SI).

Current intensity unit - ampere (A)

Current and resistance The so-called "international" electrical unit was held in Chicago in 1893 International Electrical Congress The referenced on the. The definitions of "international" ampere and "international" ohm were approved by the London International Conference of Representatives in 1908.

Although during the 8th International Conference on Metrology in 1933, it was clearly and unanimously required to use the so-called "absolute" units to replace these "international" units, it was not until the 9th International Conference on Metrology in 1948 that the "international" units were officially abolished and the following definitions of current intensity units were adopted:

A constant current passes through two infinite straight parallel wires with negligible circular cross-section 1 m apart in vacuum. If the constant current makes the force generated per meter length between the two wires equal to 2 × 10 ⁻⁷⁷⁷⁷ N (Newton), the current intensity of this constant current is 1A (ampere).

On November 16, 2018, the International Metrology Conference adopted a resolution“ One ampere passes (1.602176634) × 10 ^ 19 in one second Electronic charge Corresponding current ”^[3] 。

Thermodynamic temperature unit Kelvin (K)

Three phase point of water

The 10th International Conference on Metrology in 1954 stipulated that Thermodynamic temperature Definition of unit, which selects the Three phase point Is the basic fixed point, and its temperature is defined as 273.16K. In 1967, the 13th International Conference on Metrology adopted the Kelvin name (symbol K) instead of "Kelvin degree" (symbol K), whose formal definition is:

Thermodynamic temperature unit Kelvin Is 1/273.16 of the thermodynamic temperature of the triple point of water. At the same time, the General Assembly also decided to use the unit Kelvin and its symbol K to indicate the temperature interval or Temperature difference 。

In addition to the thermodynamic temperature in Kelvin (symbol T, see Thermodynamic temperature scale ）In addition, you can also use the formula

t = T － T zero

Defined Celsius temperature (symbol t )。 Where T zero =273.15K is water freezing point It is 0.01K (Kelvin) different from the thermodynamic temperature of the triple point of water. The unit of temperature is Celsius (symbol ℃). Therefore, the unit of "Celsius" is equal to the unit of "Kelvin". The Celsius temperature interval or difference is expressed in degrees Celsius.

According to the definition of Kelvin, the unit of thermodynamic temperature, the absolute measurement of temperature must be carried out with the help of thermodynamic thermometers, such as Gas thermometer 。

Theoretically, the thermodynamic temperature scale is reasonable, but its implementation is very difficult. Therefore, the international community decided to adopt the practical temperature scale, which can not replace the thermodynamic temperature scale, but improve the accuracy as much as possible and approach the thermodynamic temperature scale according to the level of measurement technology at that time. According to the requirements of practicality, it should also be unified internationally.

In 1927, the 7th International Conference on Metrology adopted the first International temperature scale 。 This international temperature scale was revised in 1948, and was named the 1948 International Practical Temperature Scale (code: IPTS-48) by the 11th International Conference on Metrology in 1960. Then came the 1960 revision of IPTS-48. The fixed point temperature value of the revised version remains the same as that of 1948.

In 1968, the International Metrology Commission adopted a new International practical temperature scale It is consistent with the best known thermodynamic results. The code of this temperature scale is IPTS － 68. It is based on the following two points: first, there are 11 reproducible fixed points, and it is specified to use gas thermometer to measure the temperature value of fixed points within the range of 13.81K to 1337.58K; Secondly, standard instruments (13.81K to 903.89K are Platinum resistance thermometer 903.89K to 1337.58K are platinum rhodium platinum thermocouples, and spectral pyrometers and constants are used above 1337.58K с two =0.014338m · K), indexing according to the specified fixed point (see temperature measurement ）。

Special attention should be paid to the fact that the triple point of water is not the freezing point solute The triple point is only related to the nature of water itself. The calculated 1K is equal to 1 ℃, and the melting point of water at 101.325kPa is about 273.15K.

On November 16, 2018, the International Metrology Conference adopted a resolution“ 1 Kelvin is corresponding Boltzmann constant^[3] The thermodynamic temperature at 1.380649 × 10 ⁻ ³ J · K ⁻ (1.380649 × 10 ⁻ ³ kg · m ² · s ⁻ ² · K ⁻) ”。

Unit of measure of substance - mole

This unit is the same as Atomic weight There is close relationship. Initially, the "atomic weight" was based on the atomic weight of the chemical element O (oxygen) (specified as 16). But chemists set the value of the mixture of O (oxygen) isotopes O-16, O-17 and O-18, that is, the natural oxygen element, at 16. Physicists set the value of one isotope of oxygen, namely, oxygen-16, at 16, which is quite inconsistent.

From 1959 to 1960, International Union of Pure and Applied Physics （ IUPAP ）And International Union of Pure and Applied Chemistry （ IUPAC ）After reaching a consensus agreement, this inconsistency ended. Decide to switch to carbon isotope C-12 is used as the standard, its atomic weight is set as 12, and based on this, the value of "relative atomic mass" is given. The remaining problem is to define the unit of quantity of substance by determining the corresponding mass of C-12. According to international agreements, one“ Amount of substance ”The unit C-12 shall have 0.012kg (kg). The unit of "quantity of substance" defined in this way is named mole (symbol mol).

International Commission on Metrology According to the International Union of Pure and Applied Physics, the International Union of Pure and Applied Chemistry and International Organization for Standardization The definition of mole was formulated in 1967 and approved in 1969. It was finally adopted by the 14th International Conference on Metrology in 1971. The definition is as follows:

Mole is the amount of material in a system, and the number of basic units contained in the system is equal to the number of atoms in 0.012kg C-12.

When using mole Basic unit It should be specified that it can be atom 、 molecule , ions, electrons and other particles; Or a specific combination of these particles. This definition of mole also strictly defines the nature of the quantity in moles.

According to scientific determination, the atomic number of C contained in 12g C-12 is about 6.0220943 × 10 ² ³. With symbol N A Express Avogadro constant 。

Definition: The amount of a substance containing Avogadro constant structural particles (about 6.022 × 10 ² ³) is 1mol (mole).

On November 16, 2018, the International Metrology Conference adopted a resolution“ 1 mole is the amount of substances in a system that accurately contains 6.02214076 × 10 ² ³ atoms or molecules and other basic units ”。 At the same time Avogadro constant^[3] It is 6.02214076 × 10 ² ³ mol ⁻.

Luminous intensity unit - candela (cd)

What countries use flame or Incandescent lamp Based on wire datum Luminous intensity Unit, changed to“ New candlelight ”。 This decision is International Commission on Illumination (CIE) and International Commission on Metrology It was made before 1937. Promulgated by the International Commission on Metrology at its 1946 meeting in accordance with the powers granted by the 8th International Conference on Metrology in 1933. The 9th session in 1948 International Conference on Metrology Approved the decision of the International Metrology Commission and agreed to give the luminous intensity unit a new international name "candela" (symbol cd). The 13th Metrology Conference in 1967 formally adopted the following revised definitions:

1 cd (candela) is 101325 N/m ² (Newton per square meter) pressure Lower, at platinum Solidification temperature Of Blackbody The luminous intensity of 1/60000 m2 (square meter) surface in the vertical direction.

The above definition was used until 1979. In use, it is found that laboratories in various countries use boldface material object When the original device replicates cd (candela), there is a big difference between them. in the meantime, Radiometry With the rapid development of technology, its accuracy has been comparable to that of luminosity Compared with measurement, cd (candela) can be reproduced directly by radiation measurement. In view of this situation, in 1977, the International Metrology Commission made it clear that luminescence Measurement and radiation The ratio between measurements, the specified frequency is 540 × 10 ^ 12Hz（ hertz ）Of Monochromatic radiation Of spectrum The light efficiency is 683lm/W (lumens per watt). This value is accurate enough for photopic light; For dark visual light, there is only about 3% change.

At the 16th Metrology Conference held in October 1979, it was formally decided to abolish the 1967 definition and make the following new definition of cd (candela): "1 candela is a light source emitted in a given direction frequency Is the monochromatic radiation of 540 × 10 ^ 12s ⁻⁻, and the radiation intensity in this direction is the luminous intensity when (683) ⁻ kg · m ² · s ⁻ ³ · sr ⁻ ”。

The 540 × 10 ^ 12Hz (Hz) radiation wavelength in the definition is about 555nm, which is the most sensitive feeling of human eyes wavelength 。

Auxiliary unit

There are two international units Auxiliary unit (incorporated into the export unit), i.e radian and Sphericity 。

Export Units

SI derived units are derived from SI base units or Auxiliary unit Press Definitional expression There are many exported products. There are 20 SI export units with special names. 18 are named after outstanding scientists, such as Newton 、 Pascal 、 joule To commemorate their contributions in the field of this discipline. They have their own special names and symbols. These special names and symbols can be used to form other export units, which is simpler than basic units. At the same time, for convenience Export Units You can also combine with other units to represent other, more complex, derived units.

The following is the definition of some export units with special names.

hertz （ frequency The frequency of a periodic phenomenon with a period of 1s (seconds) is 1Hz (Hz), that is, 1Hz=1s ⁻⁻.

Newton （ power Unit) - the force that causes the mass of 1kg (kg) to produce an acceleration of 1m/s ² (meters per second to the second power), that is, 1N=1kg · m/s ².

Pascal （ pressure Unit) - pressure of 1N (Newton) force per m2 (square meter) area, i.e. 1Pa=1N/m2.

joule （ can or Work Unit) - the action point of 1 N (Newton) force moves 1 m (m) in the direction of the force Work , i.e. 1J=1N · m.

watt （ power Unit) - power giving 1J (joule) energy in 1s (second), i.e. 1W=1J/s.

Coulomb （ Electricity Unit) - 1A (ampere) current delivered within 1s (second) Electricity That is, 1C=1A · s.

Volts （ Potential difference and emf Unit) - When flowing 1A (ampere) constant current wireway If the power consumed between the two points is 1W (watt), then Potential difference 1V（ Volts ）, i.e. 1V=1W/A.

Farad （ capacitance Unit) -- For capacitor When charging 1C (coulomb), there is a potential difference of 1V (volt) between the two plates capacitance 1F（ Farad ）, i.e. 1F=1C/V.

Ohms （ resistance Unit) - at conductor Add constant potential difference of 1V (volt) between two points, if 1A is generated in the conductor（ ampere ）And there is no other emf , the resistance between these two points is 1 Ω（ Ohms ）That is, 1 Ω=1V/A.

Siemens? （ conductance Unit) - the negative power of Ω (ohm), that is, 1S=1 Ω ⁻ ¹.

Henry （ inductance Unit) - Let flow through one Closed loop At a current of 1 A/s (amperes per second) rate Uniform change. If 1V (volt) electromotive force is generated in the circuit inductance Is 1H (Henry), i.e. 1H=1V · s/A.

weber （ Magnetic flux Unit) - Let only One turn Of Ring Rd In Magnetic flux The magnetic flux in the loop is 1 if the electromotive force of 1V (volt) is generated in the loop（ weber ）, i.e. 1Wb=1Vs.

Tesla （ Magnetic induction or Magnetic flux density Unit) - The magnetic flux per m ² (square meter) is 1Wb (Weber) Magnetic induction That is, 1 T=1 Wb/m ².

Lumen （ Luminous flux Unit) - luminous flux emitted from a uniform point light source with luminous intensity of 1 cd (candela) to sr (unit solid angle within spherical degree), that is, 1 lm=1 cd · sr.

Lux （ Illuminance Unit: 1 lm (lumen) per m ² (square meter) Luminous flux Of Illuminance That is, 1 lx=1lm/m ².

Becquerel （ Activity Unit) - 1 spontaneous occurrence within 1s (second) nuclear transmutation or transition , 1Bq（ Becquerel ）That is, 1Bq=1s ⁻ ¹.

Gray (Unit of absorbed dose of ionizing radiation energy) - the unit that grants 1kg (kg) of irradiated material with 1J (joule) energy absorbed dose , i.e. 1Gy=1J/kg^[1] 。

Hiwart （ dose Equivalent) - 1 J (joule) per kg (kg) Dose equivalent , i.e. 1Sv=1J/kg.

radian (rad) and Sphericity (sr) (purely geometric units), incorporated into the derived units. It is defined as follows:

radian (rad) - one circular Between two inner radii Plane angle 。 The two radii are intercepted on the circumference arc length Equal to the radius.

Sphericity (sr) - one Solid angle , its vertex is at the center of the sphere, and the area it intercepts on the sphere is equal to the radius of the sphere Side length Of square Area of^[1] 。

SI prefix

International System of Units Prefixes
multiplication factor	Chinese name	Symbol	english
ten thirty	Kun [It]	Q	quetta
ten twenty-seven	Capacity [That]	R	ronna
ten twenty-four	Yao [It]	Y	yotta
ten twenty-one	Ze [It]	Z	zetta
ten eighteen	Ai [Cosa]	E	exa
ten fifteen	beat [It]	P	peta
ten twelve	too [Pull]	T	tera
ten nine	luck [Coffee]	G	giga
ten six	Mega	M	mega
ten three	thousand	k	kilo
ten two	hundred	h	hecto
ten one	ten	da	deca
ten -1	branch	d	deci
ten -2	Per cent	c	centi
ten -3	Millionth	m	milli
ten -6	tiny	μ	micro
ten -9	Na [No]	n	nano
ten -12	skin [May]	p	pico
ten -15	fly [Mother care]	f	femto
ten -18	A [support]	a	atto
ten -21	Oblique [Putuo]	z	zepto
ten -24	Unitary [Coto]	y	yocto
ten -27	soft [support]	r	ronto
ten -30	Loss [Coto]	q	quecto
reference material:^[6-10]

Title prefix	Symbol Prefix	chinese Prefix	english	science Counting method	Etymological interpretation	type	Name example (taking meter as an example)	Example of symbol (taking meter as an example)
quetta	Q	Kun [It]
ronna	R	Capacity [That]
yotta	Y	Yao [It]	Septillion	1×10²⁴	It means "the penultimate symbol" (Y) and "eight" (otta)	Integer unit	yottametre	Ym
zetta	Z	Ze [It]	Sextillion	1×10²¹	It means "the last symbol" (Z)	Integer unit	zettametre	Zm
exa	E	Ai [Cosa]	Quintillion	1×10¹⁸	It means "six"	Integer unit	exametre	Em
peta	P	beat [It]	Quadrillion	1×10¹⁵	It means "penta", which is copied from the Greek writing and becomes a metric unit	Integer unit	petametre	Pm
tera	T	too [Pull]	Trillion	1×10¹²	It means "four"	Integer unit	terametre	Tm
giga	G	luck [Coffee]	Billion	1×10⁹	It means "one billion"	Integer unit	gigametre	Gm
mega	M	Mega	Million	1×10⁶	It means "million"	Integer unit	megametre	Mm
kilo	k	thousand	Thousand	1×10³	It means "thousand"	Integer unit	kilometre	km
hecto	h	hundred	Hundred	1×10²	It is also written as hecta, which is usually used as the prefix of compound words of metric units, meaning "hundred"	Integer unit	hectometre	hm
deca	da	ten	Ten	1×10	It is also written deka, which is usually used as the prefix of compound words of metric units, meaning "ten"	Integer unit	decametre	dam
			One	one			metre	m
deci	d	branch	Tenth	1×10⁻¹	It means "one tenth"	Decimal unit	decimetre	dm
centi	c	Per cent	Hundredth	1×10⁻²	It means "per cent, one percent"	Decimal unit	centimetre	cm
milli	m	Millionth	Thousandth	1×10⁻³	It means "one millionth, one thousandth"	Decimal unit	millimetre	mm
myrio	mo		Ten-thousandth	1×10⁻⁴		Decimal unit	myriometre	mom
micro	μ	tiny	Millionth	1×10⁻⁶	It means "tiny"	Decimal unit	micrometre	μm
nano	n	Na [No]	Billionth	1×10⁻⁹	It means "dwarf". This unit is often used in electronics or other scientific fields	Decimal unit	nanometre	nm
pico	p	skin [May]	Trillionth	1×10⁻¹²	It means "small"	Decimal unit	picometre	pm
femto	f	fly [Mother care]	Quadrillionth	1×10⁻¹⁵	It means "15"	Decimal unit	femtometre	fm
atto	a	A [support]	Quintillionth	1×10⁻¹⁸	It means "eighteen"	Decimal unit	attometre	am
zepto	z	Oblique [Putuo]	Sextillionth	1×10⁻²¹	It means "seven" (1000 ⁻⁷⁷⁷⁻⁷⁷⁻⁷⁷⁻⁷⁷⁻⁷⁷⁻⁷	Decimal unit	zeptometre	zm
yocto	y	Unitary [Coto]	Septillionth	1×10⁻²⁴	It means "eight" (1000 ⁻⁸⁸)	Decimal unit	yoctometre	ym
ronto	r	soft [support]
quecto	q	Loss [Coto]
Due to historical problems, the SI prefix "k" has been included in the SI unit "1kg" of mass, so the SI prefix of mass is added before "g". If "1mg" is used, "1 μ kg" shall not be used.

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The International System of Units is Metrology The foundation and core of the research. Especially the seven basic units Recurrence , Save and Value transmission It is the most fundamental research topic of metrology.

physical quantity

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Unit system

Physics is an experimental science whose theory is based on experimental observation. Experimental observation cannot be separated from the measurement of physical quantities, in order to ration For the size of apparent measurement value, for the same type of physical quantity (such as length), a specific quantity needs to be selected as the unit (such as 1 meter). Any other quantity in this type can be expressed by the product of this unit and a number, which is called the physical quantity The value in the above specified quantity.

In history, physics has established many systems of units. After 1971, the international system of units based on seven basic quantities was established^[5] 。

Various physical quantities are described natural law Of equation And the definition of new physical quantities. For convenience, a group of mutually independent physical quantities are usually selected as Basic physical quantity Other quantities are expressed according to basic quantities and relevant equations, which are called derived quantities.

The earliest branch of physics was mechanics. In the field of mechanics, the unit system with length, mass and time as basic physical quantities was first established, which is known as the centimeter gram second (CGS) system.

In order to meet the needs of international trade, industry and scientific and technological exchanges, the metric convention was formulated by 17 foreign ministers in Paris in 1875. The metric convention stipulates that the length is in meters and the mass is in kilograms（ kg . ）Is the unit, and the time is in seconds. This system of units is called the meter kilogram second system.

along with electromagnetics 、 thermodynamics , optical radiation and Microphysics With the development of, the basic physical quantities have gradually expanded from 3 to 7. The unit system developed on the basis of the meter kilogram second system was established. It was awarded the 11th session in 1960 International Conference on Metrology The confirmation of is called the International System of Units (SI).

International System of Units The principle of composition is relatively scientific, most units are very practical, and involve all professional fields. The universal promotion of the International System of Units can eliminate the confusion caused by the coexistence of multiple systems and units, and save a lot of manpower and material resources Which is conducive to the further development of the national economy and international exchanges.

Most of today's industrial developed countries actively promote the International System of Units, which was originally adopted Imperial % of the countries also decided to abandon the British system and adopt or prepare to adopt the International System of Units.

Because in physics, especially Theoretical physics The centimeter gram second system unit and its development electromagnetism Therefore, the centimeter gram second unit system is still used as a reserved unit system. International Commission on Metrology It is believed that when the centimeter gram second system is used, it is generally better not to use it together with the international system units.

stay Particle physics A special system of units, namely Natural system of units 。 In the natural system of units Basic physical constants h （ Planck constant Divide by 2 π ）And с（ light speed ）All are taken as 1. Therefore, the basic physical quantity can be reduced, so that energy can be selected as the basic physical quantity. In other physics disciplines closely related to particle physics, sometimes the natural unit system is also used^[1] 。

Centimeter gram second system (CGS system)

In many books and papers on physics, especially in theoretical physics, the centimeter gram second system (CGS system) is still widely used. This system uses centimeter, gram and second as its basic units. One convenience of the centimeter gram second system is that 1 cubic centimeter of water has a mass of approximately 1 gram at its maximum density. This system of units was established under the initiative of the Standards Committee of the British Association for the Advancement of Science. After the three basic units are determined, all other units can be determined according to the requirements of consistency, that is, the derived unit. But when it comes to Electromagnetic phenomenon There are two different ways to establish the derived unit. The starting point of one approach is two magnetic pole The force between two charges is inversely proportional to the square of the distance Force Inversely proportional to the square of the distance. W. weber In 1851, following these two approaches, two consistent "absolute" unit systems were obtained. Charge dependent static electricity The interaction is called the absolute electrostatic system unit (CGSE), and the magnetic interaction is called the absolute electromagnetic system unit (CGSM).

Specified by CGSM unit magnetic field intensity Is called Oster , specified Magnetic induction The unit is called Gaussian , the unit of magnetic induction flux is called maxwell 。 If all electrical quantities are in CGSE units and magnetic quantities are in CGSM units, the so-called absolute Gaussian system units are formed (see System of units for electromagnetic quantities ）。

When limited to the units of mechanical and electrical quantities, the International System of Units includes the current as the basic unit, that is, there are four basic units. In the centimeter gram second system, there are only three basic units, and the current is taken as the derived unit.

The International System of Units adopted some so-called "practical units" (including some derived units) recommended by the British Association for the Advancement of Science that year. for example resistance Unit: Ohms ， emf Unit: Volts , they are equal to 10 and 10 times of the corresponding CGSM system units respectively. The proposal of the British Association for the Advancement of Science was awarded the first prize in Paris in 1881 International Electrical Congress Approved. The conference also introduced the practical unit of current ampere , which is equal to one tenth of the corresponding CGSM system unit. Later, a practical unit of charge was introduced Coulomb and capacitance Utility unit Farad 。

The ohmic physical benchmark has been established for practical units（ mercury ）, volt physical reference (Weston battery) and ampere physical reference (silver electrolytic coulometer) Secondary datum use. 1893 Chicago According to these physical standards, the International Conference on Electricity has given "statutory" definitions to ohm, volt and ampere. The International Electrical Conference held in London in 1908 decided to Metrology A complete set of so-called "International Electrical System of Units" based on the physical standards of ohm and ampere is adopted in the.

The 9th session in 1948 International Conference on Metrology The metric kilogram second ampere system was formally adopted, which is the basis of the International System of Units^[1] 。

Unit table

Physical quantity name	Symbol	Unit name	Unit symbol	Export unit representation	Export unit definitions
the measure of area	A ( S )	square meter	m²
volume	V	cubic metre	m³
speed	v	Meters per second	m/s
acceleration	a	Meters per second square	m/s²
angular velocity	ω	Radians per second	rad/s
frequency	f ( v )	Hertz	Hz	1 Hz=1s⁻¹	Frequency of periodic phenomena with a period of 1 second
density	ρ	Kg/m3	kg/m³
power	F	N	N	1 N=1kg·m/s²	The force that causes 1 kg mass to produce an acceleration of 1 m/s ²
moment	M	N m	N·m
momentum	p	Kilograms meters per second	kg·m/s
pressure	p	Pascal	Pa	1 Pa=1 N/m²	Pressure of 1N per square meter
Work 、 can （ energy ）	W（A）	Joule	J	1 J=1 N·m	The work done by the action point of 1 N force moving 1 m in the direction of force
Work 、 can （ energy ）	E	Joule	J	1 J=1 N·m
power	P	Watts	W	1 W=1 J/s	Give power of 1 focal energy within 1 second
charge （ Charge quantity ）	Q	Coulomb	C	1 C=1 A·s	The amount of power delivered by a current of 1 ampere in 1 second
electric field intensity	E	Volts per meter	V/m
potential 、 Voltage 、 Potential difference	U ( V )	Volts	V	1 V=1 W/A 1 V=1 N·m/C	If the power consumed between two points in the wire flowing through a constant current of 1 A is 1 W, the potential difference between the two points is 1 V
capacitance	C	Farad	F	1 F=1 C/V	When the capacitor is charged with 1 bank of electricity, there is a potential difference of 1 volt between the two plates, then the capacitance of the capacitor is 1 method
resistance	R	Ohms	Ω	1 Ω=1 V/A	Add a constant potential difference of 1 V between two points of the conductor. If a constant current of 1 A is generated in the conductor and there is no other electromotive force in the conductor, the resistance between the two points is 1 Ω
resistivity	ρ	Ohm meter	Ω·m
Magnetic induction	B	TESLA	T	1 T=1 Wb/m²	The magnetic flux density with a magnetic flux of 1 Wei per square meter
flux （ Magnetic flux ）	Φ	Weber	Wb	1 Wb=1 V·s	Let the magnetic flux in the loop with only 1 turn uniformly reduce to zero within 1 second. If the electromotive force of 1 volt is generated in the loop, then the magnetic flux in the loop is 1 volt
inductance	L	Henry	H	1 H= 1 Wb/A	Let the current flowing through a closed loop change evenly at the rate of 1 A/s, then the inductance of the loop is 1 Ω
conductance	G	Siemens	S	1 S= 1Ω⁻¹	Ohms to the negative power
Luminous flux	Φ（Φr）	Flow (lumens)	lm	1 lm=1 cd·sr	Luminous flux emitted from a uniform point light source with luminous intensity of 1 ridge to unit solid angle (within sphericity)
Illuminance		Le (lux)	lx	1 lx=1 lm/m²	Illuminance with 1 streamer flux per square meter
Activity		Becquerel	Bq	1 Bq=1 s⁻¹	One spontaneous nuclear transition or transition occurs within one second
absorbed dose	D	Gore	Gy	1 Gy=1 J/kg	Give 1 kg of irradiated material an absorbed dose of 1 J energy
temperature	t	Celsius (Fahrenheit)	℃(℉)		Cold and hot degree of objects
Specific heat capacity	c	Coke/kg ℃	J/(kg*℃)		Heat absorption and release capacity of objects
calorific value	q	Coke per kilogram	J/kg		Heat release capacity of fuel combustion
Note: 1 The names and symbols in parentheses are synonyms of the previous names and symbols. 2. Words in parentheses can be omitted without causing confusion and misunderstanding. If the words in the brackets are removed, it is the abbreviation of its name.

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