Basic physical quantities in the si system. System for measuring si - history, purpose, role in physics

, amount of substance and the power of light... The units of measurement for them are the basic SI units - meter, kilogram, second, ampere, kelvin, mole and candela respectively .

A full official description of the basic SI units, as well as the SI as a whole, together with its interpretation, is contained in the current version of the SI Brochure (fr. and presented on the BIPM website.

The rest of the SI units are derivatives and are formed from the basic ones using equations connecting with each other physical quantities The international system quantities.

The base unit can also be used for a derived quantity of the same dimension. For example, the amount of precipitation is determined as the quotient of dividing the volume by the area and in SI is expressed in meters. In this case, the meter is used as a coherent derived unit.

The names and designations of basic units, as well as all other SI units, are written in small letters (for example, meter and its designation m). There is an exception to this rule: the designations of units named by the surnames of scientists are written with a capital letter (for example, ampere denoted by the symbol A).

Basic units

The table shows all the main SI units together with their definitions, designations, physical quantities to which they refer, as well as with a brief justification of their origin.

Improving the system of units

XXI General Conference on Weights and Measures (1999) recommended in the XXI century "National laboratories to continue research to link the mass to fundamental or mass constants to determine the mass of the kilogram." Most of the expectations were associated with the Planck constant and the Avogadro number.

V explanatory note addressed to the CIPM in October 2009, the President of the CIPM Advisory Council on Units listed the uncertainties of the physical fundamental constants using the current definitions and what those uncertainties would become when using the new proposed definitions of units. He recommended that the CIPM accept the proposed changes in the “definition kilograms, ampere, kelvin and praying so that they are expressed in terms of the values ​​of the fundamental constants h , e , k, and N A ».

XXIV General Conference on Weights and Measures

At the XXIV General Conference on Weights and Measures, on October 17-21, 2011, a Resolution was adopted, according to which it is proposed in a future revision of the International System of Units to redefine the basic units so that they are based not on human-made artifacts (standards), but on fundamental physical constants or properties of atoms, the numerical values ​​of which are fixed and assumed to be exact by definition.

Kilogram, ampere, kelvin, mole

In accordance with the decisions of the XXIV GCMW, the most important changes should affect the four basic SI units: kilogram, ampere, kelvin and mol. The new definitions of these units will be based on fixed numerical values ​​of the following fundamental physical constants: Planck's constant, elementary electric charge, Boltzmann's constant and Avogadro's number, respectively. All of these quantities will be assigned precise values ​​based on the most accurate measurements recommended by the Committee on Data for Science and Technology (CODATA).

The Resolution contains the following provisions for these units:

• The kilogram will remain the unit of mass; but its value will be set by fixing the numerical value of Planck's constant equal to exactly 6.626 06X⋅10 −34, when it is expressed in the SI unit m 2 · kg · s −1, which is equivalent to J · s.
• The ampere will remain the unit of electric current; but its value will be set by fixing the numerical value of the elementary electric charge equal to exactly 1.602 17X⋅10 −19, when it is expressed in SI unit s · A, which is equivalent to Cl.
• Kelvin will remain the unit of thermodynamic temperature; but its value will be set by fixing the numerical value of the Boltzmann constant equal to exactly 1.380 6X⋅10 −23 when it is expressed in the SI unit m −2 · kg · s −2 · K −1, which is equivalent to J · K −1.
• The mole will remain the unit of the amount of matter; but its value will be set by fixing the numerical value of Avogadro's constant equal to exactly 6.022 14X⋅10 23 mol −1 when it is expressed in the SI unit mol −1.

Meter, second, candela

The definitions of meter and second are already currently associated with exact values constants such as the speed of light and the magnitude of the splitting of the ground state of the cesium atom, respectively. The existing definition of candela, although not tied to any fundamental constant, nevertheless, can also be considered as related to the exact value of the invariant of nature. Based on the foregoing, it is not intended to change essentially the definitions of meter, second and candela. However, in order to maintain the unity of style, it is planned to adopt new, completely equivalent to the existing ones, formulations of definitions in the following form:

• The meter, symbol m, is the unit of length; its value is set by fixing the numerical value of the speed of light in a vacuum equal to exactly 299 792 458, when it is expressed in the SI unit m · s −1.
• The second, symbol c, is the unit of time; its value is established by fixing the numerical value of the frequency of hyperfine splitting of the ground state of the cesium-133 atom at a temperature of 0 K equal to exactly 9 192 631 770, when it is expressed in SI unit s −1, which is equivalent to Hz.
• Candela, the cd symbol, is the unit of luminous intensity in a given direction; its value is set by fixing the numerical value of the luminous efficiency of monochromatic radiation with a frequency of 540 × 10 12 Hz equal to exactly 683, when it is expressed in the SI unit m −2 kg −1 s 3 cd sr or cd sr W −1, which is equivalent to lm · W −1.

New look of SI

In 2019, the SI issue based on fundamental constants will enter into force, in which:

see also

Notes (edit)

1. The SI Brochure Description of SI on the website of the International Bureau of Weights and Measures (eng.)

General information

Prefixes can be used before unit names; they mean that one must be multiplied or divided by a certain integer, a power of 10. For example, the prefix "kilo" means multiplication by 1000 (kilometer = 1000 meters). SI prefixes are also called decimal prefixes.

International and Russian designations

Subsequently, basic units were introduced for physical quantities in the field of electricity and optics.

SI units

SI units are written with lowercase letter, after the designations of SI units, a dot is not put, in contrast to the usual abbreviations.

Basic units

Derived units

Derived units can be expressed in terms of basic ones using mathematical operations: multiplication and division. For convenience, some of the derived units have been assigned their own names; such units can also be used in mathematical expressions to form other derived units.

The mathematical expression for the derived unit of measurement follows from physical law, with the help of which this unit of measurement is determined or determination of the physical quantity for which it is entered. For example, speed is the distance that a body travels per unit of time; accordingly, the unit of measurement for speed is m / s (meter per second).

Often the same unit can be written in different ways, using a different set of basic and derived units (see, for example, the last column in the table ). However, in practice, established (or simply generally accepted) expressions are used that the best way reflect physical meaning magnitudes. For example, Nm should be used to record the torque value, and mN or J should not be used.

The Kelvin and Celsius scales are related as follows: ° C = K - 273.15

Non-SI units

Certain non-SI units, by decision of the General Conference on Weights and Measures, are “allowed for use in conjunction with the SI”.

Other units are not allowed.

However, in different areas sometimes other units are used.

• System units

System of units of physical quantities, a modern version of the metric system. SI is the most widely used system of units in the world, as in Everyday life and in science and technology. Currently, SI is accepted as the main system of units by most countries in the world and is almost always used in the field of technology, even in those countries in which traditional units are used in everyday life. In these few countries (for example, the United States), the definitions of traditional units have been changed so as to associate them with fixed coefficients with the corresponding SI units.

The SI was adopted by the XI General Conference on Weights and Measures in 1960, some subsequent conferences made a number of changes to the SI.

In 1971, the XIV General Conference on Weights and Measures amended the SI, adding, in particular, the unit of the amount of substance (mol).

In 1979, the XVI General Conference on Weights and Measures adopted a new definition of candela, which is in force today.

In 1983, the XVII General Conference on Weights and Measures adopted a new definition of the meter, which is in force today.

SI defines seven basic and derived units of physical quantities (hereinafter referred to as units), as well as a set of prefixes. Standard abbreviations for units and rules for writing derived units have been established.

The basic units are kilogram, meter, second, ampere, kelvin, mole, and candela. Within the SI, these units are considered to have independent dimensions, that is, none of the basic units can be obtained from others.

Derived units are derived from fundamental units using algebraic operations such as multiplication and division. Some of the derived units in SI have their own names, for example, the radian.

Prefixes can be used before unit names; they mean that one must be multiplied or divided by a certain integer, a power of 10. For example, the prefix "kilo" means multiplication by 1000 (kilometer = 1000 meters). SI prefixes are also called decimal prefixes.

Many non-SI units, such as, for example, ton, hour, liter and electron volt are not included in the SI, but they are "allowed to be used on a par with SI units."

Seven basic units and the dependence of their definitions

SI base units

Derived units with their own names

Units that are not part of the SI, but by decision of the General Conference on Weights and Measures "are allowed for use in conjunction with the SI".

Rules for writing unit notation

The designations of units are printed in roman type, a dot is not put after the designation as an abbreviation sign.

Designations are placed behind the numerical values ​​of the quantities separated by a space; a line break is not allowed. Exceptions are designations in the form of a sign above the line, without a space in front of them. Examples: 10 m / s, 15 °.

If the numeric value is a fraction with a slash, it is enclosed in parentheses, for example: (1/60) s −1.

When specifying the values ​​of quantities with maximum deviations, they are enclosed in brackets or the designation of the unit for numerical value values ​​and beyond its maximum deviation: (100.0 ± 0.1) kg, 50 g ± 1 g.

The designations of the units included in the product are separated by dots on the center line (N · m, Pa · s); it is not allowed to use the symbol "×" for this purpose. In typewritten texts, it is allowed not to raise the point or to separate the symbols with spaces, if this cannot cause misunderstandings.

You can use a horizontal bar or a forward slash (only one) as a division mark in designations. When using a slash, if the denominator contains a product of units, it is enclosed in parentheses. Correct: W / (mK), wrong: W / m / K, W / mK.

It is allowed to use the designations of units in the form of a product of the designations of units raised to a power (positive and negative): W · m −2 · K −1, A · m². When using negative exponents, it is not allowed to use a horizontal or forward slash (division sign).

It is allowed to use combinations of special characters with letter designations, for example: ° / s (degree per second).

It is not allowed to combine designations and full names of units. Wrong: km / h, correct: km / h.

Unit designations derived from surnames are written with capital letter, including those with SI prefixes, for example: ampere - A, megapascal - MPa, kilonewton - kN, gigahertz - GHz.

• 1 General
• 2 History
• 3 SI units
  • 3.1 Basic units
  • 3.2 Derived units
• 4 Non-SI units
• Prefixes

General information

The SI system was adopted by the XI General Conference on Weights and Measures; some subsequent conferences made a number of changes to the SI.

The SI system defines seven major and derivatives units of measure as well as a set. Standard abbreviations for units of measure and rules for writing derived units have been established.

In Russia, GOST 8.417-2002 is in force, which prescribes the mandatory use of SI. It lists the units of measurement, their Russian and international names and the rules for their application have been established. According to these rules, only international symbols may be used in international documents and on instrument scales. In internal documents and publications, you can use either international or Russian designations (but not both at the same time).

Basic units: kilogram, meter, second, ampere, kelvin, mole and candela. Within the SI, these units are considered to have independent dimensions, that is, none of the basic units can be derived from others.

Derived units are derived from basic ones using algebraic operations such as multiplication and division. Some of the derived units in the SI system have their own names.

Prefixes can be used before the names of units of measurement; they mean that the unit of measurement must be multiplied or divided by a certain integer, a power of 10. For example, the prefix "kilo" means multiplication by 1000 (kilometer = 1000 meters). SI prefixes are also called decimal prefixes.

History

The SI system is based on the metric system of measures, which was created by French scientists and was first widely introduced after the Great French revolution... Before the introduction of the metric system, units of measurement were chosen randomly and independently of each other. Therefore, the conversion from one unit of measurement to another was difficult. In addition, different units of measurement were used in different places, sometimes with the same names. The metric system was supposed to become a convenient and unified system of measures and weights.

In 1799, two standards were approved - for the unit of measurement of length (meter) and for the unit of measurement of weight (kilogram).

In 1874, the CGS system was introduced, based on three units of measurement - centimeter, gram and second. Decimal prefixes from micro to mega were also introduced.

In 1889, the 1st General Conference on Weights and Measures adopted a system of measures similar to the GHS, but based on the meter, kilogram and second, since these units were recognized as more convenient for practical use.

Subsequently, basic units were introduced for measuring physical quantities in the field of electricity and optics.

In 1960, the XI General Conference on Weights and Measures adopted a standard that was first called the "International System of Units (SI)".

In 1971, the IV General Conference on Weights and Measures amended the SI, adding, in particular, a unit for measuring the amount of a substance (mol).

Currently, the SI is accepted as a legal system of units of measurement by most countries in the world and is almost always used in the field of science (even in those countries that have not adopted the SI).

SI units

After the designations of units of the SI System and their derivatives, a dot is not put, in contrast to the usual abbreviations.

Basic units

Derived units

Derived units can be expressed in terms of basic ones using mathematical operations of multiplication and division. For convenience, some of the derived units have been assigned their own names; such units can also be used in mathematical expressions to form other derived units.

The mathematical expression for the derived unit of measurement follows from the physical law by which this unit of measurement is determined or the definition of the physical quantity for which it is entered. For example, speed is the distance that a body travels per unit of time. Accordingly, the unit of measure for speed is m / s (meter per second).

Often, the same unit of measurement can be written in different ways, using a different set of basic and derived units (see, for example, the last column in the table ). However, in practice, established (or simply generally accepted) expressions are used that best reflect the physical meaning of the measured quantity. For example, N × m should be used to record the moment of force, and m × N or J should not be used.

Non-SI units

Some units of measurement that are not included in the SI system, according to the decision of the General Conference on Weights and Measures, are "allowed for use in conjunction with SI".

The SI brochure has been published since 1970, since 1985 it has been published in French and English, has also been translated into a number of other languages, but the official text is only in French.

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General information

The strict definition of SI is formulated as follows:

The International System of Units (SI) is a system of units based on the International System of Units, together with names and symbols, as well as a set of prefixes and their names and symbols, together with rules for their use, adopted by the General Conference on Weights and Measures (CGPM).

Prefixes can be used before unit names. They mean that the unit must be multiplied or divided by a certain integer, a power of 10. For example, the prefix "kilo" means multiplication by 1000 (kilometer = 1000 meters). SI prefixes are also called decimal prefixes.

Names and designations of units

According to international documents (SI Brochure, ISO 80000, International Metrological Dictionary), SI units have names and designations. Unit names can be written and pronounced differently in different languages, for example: fr. kilogramme, eng. kilogram, port. quilograma, wall. cilogram, bulg. kilogram, Greek. χιλιόγραμμο , whale. 千克, jap. キ ロ グ ラ ム. The table gives the French and English names indicated in international documents. The designations of units, according to the SI Brochure, are not abbreviations, but mathematical objects(French entités mathématiques, English mathematical entities). They are included in the international scientific symbols ISO 80000 and do not depend on the language, for example: kg. In international designations of units, letters of the Latin alphabet are used, in some cases, Greek letters or special characters.

However, in the post-Soviet space (CIS, CIS-2, Georgia) and Mongolia, where an alphabet based on the Cyrillic alphabet is adopted, along with international designations (and in fact, instead of them), designations based on national names are used: “kilogram” - kg, arm ... կիլոգրամ -կգ, cargo. კილოგრამი - კგ, azerb. kiloqram - kq. Since 1978, Russian designations for units are subject to the same spelling rules as international ones (see below).

History

In 1874, the CGS system was introduced, based on three units - centimeter, gram and second - and decimal prefixes from micro to mega.

In 1875, representatives of seventeen states (Russia, Germany, USA, France, Italy, etc.) signed the Metric Convention, in accordance with which the International Committee of Weights and Measures (fr. Comité International des Poids et Mesures, CIPM) and the International Bureau of Weights and Measures (fr. Bureau International des Poids et Mesures, BIPM), and also provides for the regular convocation of General Conferences on Weights and Measures (GCMW) (fr. Conférence Générale des Poids et Mesures, CGPM). Work began on the development of international standards for meter and kilogram.

Subsequently, the basic units for physical quantities in the field of electricity and optics were introduced.

In 1956, the International Committee for Weights and Measures recommended that the system of units, based on the basic units adopted by the X CGPM, be given the name "Système International d'Unités".

In 1960, the XI CGPM adopted the standard, which was first called the "International System of Units", and established the international abbreviation for this system "SI". The main units in it are meter, kilogram, second, ampere, Kelvin and candela.

XIII GKMV (1967-1968) adopted a new definition of the unit of thermodynamic temperature, gave it the name "kelvin" and the designation "K" (earlier the unit was called "degree Kelvin", and its designation was "° K").

XIII CGPM (1967-1968) adopted a new definition of a second.

In 1971, the XIV GKMV introduced changes to the SI, adding, in particular, to the number of basic units the unit of the amount of substance (mol).

In 1979, the XVI CGPM adopted a new definition of candela.

In 1983, the XVII GKMV gave a new definition of the meter.

SI units

The names of SI units are written with a lowercase letter, after the designations of SI units, a dot is not put, unlike ordinary abbreviations.

Basic units

Derived units

Derived units can be expressed in terms of basic ones using mathematical operations - multiplication and division. For convenience, some of the derived units have their own names; such units can also be used in mathematical expressions to form other derived units.

The mathematical expression for the derived unit of measurement follows from the physical law by which this unit of measurement is determined, or from the definition of the physical quantity for which it is entered. For example, speed is the distance that a body travels per unit of time; accordingly, the unit of measurement for speed is m / s (meter per second).

Often, the same unit can be written in different ways, using a different set of basic and derived units (see the last column of the table). However, in practice, established (or simply generally accepted) expressions are used that best reflect the physical meaning of the quantity. For example, Nm should be used to record the torque value, and mN or J should not be used.

The names of some derived units that have the same expression in terms of base units may be different. For example, the unit of measurement "second to minus the first power" (1 / s) is called hertz (Hz) when it is used to measure frequency and is called Becquerel (Bq) when it is used to measure the activity of radionuclides.

Overriding base units

At the XXIV CGPM on October 17-21, 2011, a resolution was unanimously adopted, in which, in particular, it was proposed in the future revision of the International System of Units to redefine the four basic SI units: kilogram, ampere, kelvin and mol. It is assumed that the new definitions will be based on fixed numerical values ​​of the Planck constant, elementary electric charge, Boltzmann constant and Avogadro's constant, respectively. All these quantities will be assigned accurate values ​​based on the most reliable measurement results recommended by the Committee on Data for Science and Technology (CODATA). By fixation (or fixation) is meant "the acceptance of some precise numerical value of a quantity by definition." The resolution contains the following provisions for these units:

• The kilogram will remain a unit of mass, but its value will be set by fixing the numerical value of Planck's constant equal to exactly 6.626 06X⋅10 −34, when it is expressed in the SI unit m 2 · kg · s −1, which is equivalent to J · s.
• The ampere will remain the unit of the electric current, but its value will be set by fixing the numerical value of the elementary electric charge equal to exactly 1.602 17X⋅10 −19, when it is expressed in SI unit s · A, which is equivalent to Cl.
• Kelvin will remain the unit of thermodynamic temperature, but its value will be set by fixing the numerical value of the Boltzmann constant equal to exactly 1.380 6X⋅10 −23 when it is expressed in SI unit m −2 kg s −2 K −1, which is equivalent to JK −1.
• The mole will remain the unit of the amount of substance, but its value will be set by fixing the numerical value of Avogadro's constant equal to exactly 6.022 14X⋅10 23 when it is expressed in the SI unit mol −1.

The resolution does not intend to change the essence of the definitions of meter, second and candela, however, in order to maintain the unity of style, it is planned to adopt new, completely equivalent to the existing ones, definitions in the following form:

• The meter, symbol m, is a unit of length; its value is set by fixing the numerical value of the speed of light in a vacuum equal to exactly 299 792 458, when it is expressed in the SI unit m · s −1.
• The second, symbol s, is a unit of time; its value is established by fixing the numerical value of the frequency of hyperfine splitting of the ground state of the cesium-133 atom at a temperature of 0 K equal to exactly 9 192 631 770, when it is expressed in SI unit s −1, which is equivalent to Hz.
• Candela, the designation cd, is the unit of luminous intensity in a given direction; its value is set by fixing the numerical value of the luminous efficiency of monochromatic radiation with a frequency of 540 × 10 12 Hz equal to exactly 683, when it is expressed in the SI unit m −2 kg −1 s 3 cd sr or cd sr W −1, which is equivalent to lm · W −1.

As a result of the implementation of the intentions formulated in the resolution, the SI in its new form will become a system of units in which:

XXV GCMW, held in 2014, decided to continue work on the preparation of a new revision of SI and planned to complete this work by 2018 in order to replace the existing SI with an updated version on XXVI GCMV in the same year.

Non-SI units

Some units that are not included in the SI, according to the decision of the SCMM, are "allowed for use in conjunction with the SI".

In addition, the Regulation on the units of quantities allowed for use in Russian Federation, allows the use of the following non-systemic units: carat, hail (gon), light year, parsec, foot, inch, kilogram-force per square centimeter,