Basic units of si. Si measurement system - history, purpose, role in physics Basic units of si measurement
In 1875, the International Bureau of Weights and Measures was founded by the Metric Conference; its goal was to create a unified measurement system that would be used throughout the world. It was decided to take as a basis the metric system, which appeared during the French Revolution and was based on the meter and kilogram. Later, the standards of the meter and kilogram were approved. Over time, the system of units of measurement has evolved and currently has seven basic units of measurement. In 1960, this system of units received the modern name International System of Units (SI System) (Systeme Internatinal d "Unites (SI)). The SI system is not static; it is developing in accordance with the requirements that are currently imposed on measurements in science and technology.
Basic units of measurement of the International System of Units
The definition of all auxiliary units in the SI system is based on seven basic units of measurement. The main physical quantities in the International System of Units (SI) are: length ($l$); mass ($m$); time ($t$); electric current ($I$); Kelvin temperature (thermodynamic temperature) ($T$); amount of substance ($\nu$); luminous intensity ($I_v$).
The basic units in the SI system are the units of the above-mentioned quantities:
\[\left=m;;\ \left=kg;;\ \left=s;\ \left=A;;\ \left=K;;\ \ \left[\nu \right]=mol;;\ \left=cd\ (candela).\]
Standards of basic units of measurement in SI
Let us present the definitions of the standards of basic units of measurement as done in the SI system.
Meter (m) is the length of the path that light travels in a vacuum in a time equal to $\frac(1)(299792458)$ s.
Standard mass for SI is a weight in the shape of a straight cylinder, the height and diameter of which is 39 mm, consisting of an alloy of platinum and iridium weighing 1 kg.
One second (s) called a time interval that is equal to 9192631779 periods of radiation, which corresponds to the transition between two hyperfine levels of the ground state of the cesium atom (133).
One ampere (A)- this is the current strength passing in two straight infinitely thin and long conductors located at a distance of 1 meter, located in a vacuum, generating the Ampere force (the force of interaction of conductors) equal to $2\cdot (10)^(-7)N$ for each meter of conductor .
One kelvin (K)- this is the thermodynamic temperature equal to $\frac(1)(273.16)$ part of the triple point temperature of water.
One mole (mole)- this is the amount of a substance that has the same number of atoms as there are in 0.012 kg of carbon (12).
One candela (cd) equal to the intensity of light emitted by a monochromatic source with a frequency of $540\cdot (10)^(12)$Hz with an energy force in the direction of radiation $\frac(1)(683)\frac(W)(avg).$
Science is developing, measuring technology is being improved, and definitions of units of measurement are being revised. The higher the measurement accuracy, the greater the requirements for determining units of measurement.
SI derived quantities
All other quantities are considered in the SI system as derivatives of the basic ones. The units of measurement of derived quantities are defined as the result of the product (taking into account the degree) of the basic ones. Let us give examples of derived quantities and their units in the SI system.
The SI system also has dimensionless quantities, for example, reflection coefficient or relative dielectric constant. These quantities have dimension one.
The SI system includes derived units with special names. These names are compact forms of representing combinations of basic quantities. Let us give examples of SI units that have their own names (Table 2).
Each SI quantity has only one unit, but the same unit can be used for different quantities. Joule is a unit of measurement for the amount of heat and work.
SI system, units of measurement multiples and submultiples
The International System of Units has a set of prefixes for units of measurement that are used if the numerical values of the quantities in question are significantly greater or less than the unit of the system that is used without the prefix. These prefixes are used with any units of measurement; in the SI system they are decimal.
Let us give examples of such prefixes (Table 3).
When writing, the prefix and the name of the unit are written together, so that the prefix and the unit of measurement form a single symbol.
Note that the unit of mass in the SI system (kilogram) has historically already had a prefix. Decimal multiples and submultiples of the kilogram are obtained by connecting the prefix to the gram.
Non-system units
The SI system is universal and convenient in international communication. Almost all units that are not included in the SI system can be defined using SI terms. The use of the SI system is preferred in science education. However, there are some quantities that are not included in the SI, but are widely used. Thus, units of time such as minute, hour, day are part of culture. Some units are used for historical reasons. When using units that do not belong to the SI system, it is necessary to indicate how they are converted to SI units. An example of units is given in Table 4.
Physical size is a physical property of a material object, process, physical phenomenon, characterized quantitatively.
Physical quantity value expressed by one or more numbers characterizing this physical quantity, indicating the unit of measurement.
The size of a physical quantity are the values of numbers appearing in the value of a physical quantity.
Units of measurement of physical quantities.
Unit of measurement of physical quantity is a quantity of fixed size that is assigned a numerical value equal to one. It is used for the quantitative expression of physical quantities homogeneous with it. A system of units of physical quantities is a set of basic and derived units based on a certain system of quantities.
Only a few systems of units have become widespread. In most cases, many countries use the metric system.
Basic units.
Measure a physical quantity - means to compare it with another similar physical quantity taken as a unit.
The length of an object is compared with a unit of length, the mass of a body with a unit of weight, etc. But if one researcher measures the length in fathoms and another in feet, it will be difficult for them to compare the two values. Therefore, all physical quantities throughout the world are usually measured in the same units. In 1963, the International System of Units SI (System international - SI) was adopted.
For each physical quantity in the system of units there must be a corresponding unit of measurement. Standard units is its physical implementation.
The length standard is meter- the distance between two strokes applied on a specially shaped rod made of an alloy of platinum and iridium.
Standard time serves as the duration of any regularly repeating process, for which the movement of the Earth around the Sun is chosen: the Earth makes one revolution per year. But the unit of time is taken not to be a year, but give me a sec.
For a unit speed take the speed of such uniform rectilinear motion at which the body moves 1 m in 1 s.
A separate unit of measurement is used for area, volume, length, etc. Each unit is determined when choosing a particular standard. But the system of units is much more convenient if only a few units are selected as the main ones, and the rest are determined through the main ones. For example, if the unit of length is a meter, then the unit of area will be a square meter, volume will be a cubic meter, speed will be a meter per second, etc.
Basic units The physical quantities in the International System of Units (SI) are: meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), candela (cd) and mole (mol).
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Basic SI units |
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Magnitude |
Unit |
Designation |
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Name |
Russian |
international |
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Electric current strength |
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Thermodynamic temperature |
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The power of light |
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Quantity of substance |
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There are also derived SI units that have their own names:
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Derived SI units with their own names |
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Unit |
Derived unit expression |
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Magnitude |
Name |
Designation |
Through other SI units |
Through SI major and supplementary units |
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Pressure |
m -1 ChkgChs -2 |
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Energy, work, amount of heat |
m 2 ChkgChs -2 |
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Power, energy flow |
m 2 ChkgChs -3 |
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Amount of electricity, electric charge |
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Electrical voltage, electrical potential |
m 2 ChkgChs -3 ChA -1 |
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Electrical capacity |
m -2 Chkg -1 Ch 4 Ch 2 |
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Electrical resistance |
m 2 ChkgChs -3 ChA -2 |
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Electrical conductivity |
m -2 Chkg -1 Ch 3 Ch 2 |
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Magnetic induction flux |
m 2 ChkgChs -2 ChA -1 |
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Magnetic induction |
kgHs -2 HA -1 |
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Inductance |
m 2 ChkgChs -2 ChA -2 |
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Light flow |
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Illumination |
m 2 ChkdChsr |
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Radioactive source activity |
becquerel |
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Absorbed radiation dose |
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ANDmeasurements. To obtain an accurate, objective and easily reproducible description of a physical quantity, measurements are used. Without measurements, a physical quantity cannot be characterized quantitatively. Definitions such as “low” or “high” pressure, “low” or “high” temperature reflect only subjective opinions and do not contain comparisons with reference values. When measuring a physical quantity, a certain numerical value is assigned to it.
Measurements are carried out using measuring instruments. There are quite a large number of measuring instruments and devices, from the simplest to the most complex. For example, length is measured with a ruler or tape measure, temperature with a thermometer, width with calipers.
Measuring instruments are classified: by the method of presenting information (displaying or recording), by the method of measurement (direct action and comparison), by the form of presentation of readings (analog and digital), etc.
The following parameters are typical for measuring instruments:
Measuring range- the range of values of the measured quantity for which the device is designed during its normal operation (with a given measurement accuracy).
Sensitivity threshold- the minimum (threshold) value of the measured value, distinguished by the device.
Sensitivity- connects the value of the measured parameter and the corresponding change in the instrument readings.
Accuracy- the ability of the device to indicate the true value of the measured indicator.
Stability- the ability of the device to maintain a given measurement accuracy for a certain time after calibration.
How was the meter determined?In the 17th century, with the development of science in Europe, calls began to be increasingly heard to introduce a universal measure or Catholic meter. It would be a decimal measure based on a natural phenomenon, and independent of the decrees of the person in power. Such a measure would replace the many different systems of measures that existed at that time.
The British philosopher John Wilkins proposed taking the length of a pendulum as a unit of length, the half-period of which would be equal to one second. However, depending on the location of measurements, the value was different. French astronomer Jean Richet established this fact during his trip to South America (1671 - 1673).
In 1790, Minister Talleyrand proposed measuring the standard length by placing a pendulum at a strictly established latitude between Bordeaux and Grenoble - 45° north latitude. As a result, on May 8, 1790, the French National Assembly decided that the meter is the length of a pendulum with a half-period of oscillation at a latitude of 45° equal to 1 s. According to today's SI, that meter would be equal to 0.994 m. This definition, however, did not suit the scientific community.
On March 30, 1791, the French Academy of Sciences accepted a proposal to establish a standard meter as part of the Paris meridian. The new unit was to be one ten-millionth of the distance from the equator to the North Pole, that is, one ten-millionth of a quarter of the circumference of the Earth, measured along the Paris meridian. This became known as the “Genuine and Definitive Meter.”
On April 7, 1795, the National Convention adopted a law introducing the metric system in France and instructed commissioners, which included S. O. Coulon, J. L. Lagrange, P.-S. Laplace and other scientists experimentally determined units of length and mass.
In the period from 1792 to 1797, by decision of the revolutionary Convention, the French scientists Delambre (1749-1822) and Mechain (1744-1804) measured the arc of the Paris meridian with a length of 9 ° 40 "from Dunkirk to Barcelona in 6 years , laying a chain of 115 triangles across all of France and part of Spain.
Subsequently, however, it turned out that due to incorrect consideration of the Earth’s polar compression, the standard turned out to be 0.2 mm shorter. Thus, the meridian length of 40,000 km is only approximate. The first prototype of a brass meter standard, however, was made in 1795. It should be noted that the unit of mass (the kilogram, the definition of which was based on the mass of one cubic decimeter of water), was also tied to the definition of the meter.
History of the formation of the SI system
On June 22, 1799, two platinum standards were made in France - a standard meter and a standard kilogram. This date can rightly be considered the beginning of the development of the current SI system.
In 1832, Gauss created the so-called absolute system of units, taking as the main three units: the unit of time - the second, the unit of length - the millimeter, and the unit of mass - the gram, because using these very units the scientist was able to measure the absolute value of the Earth's magnetic field (this system received the name GHS Gauss).
In the 1860s, under the influence of Maxwell and Thomson, the requirement was formulated that basic and derived units must be consistent with each other. As a result, the GHS system was introduced in 1874, while prefixes were also allocated to designate submultiples and multiples of units from micro to mega.
In 1875, representatives of 17 states, including Russia, the USA, France, Germany, Italy, signed the Metric Convention, according to which the International Bureau of Measures, the International Committee of Measures were established and the regular convening of the General Conference on Weights and Measures (GCPM) began to operate. . At the same time, work began on the development of an international standard of the kilogram and a standard of the meter.
In 1889, at the first CGPM conference, the MKS system was adopted, based on the meter, kilogram and second, similar to the GHS, but the MKS units were seen as more acceptable due to the convenience of practical use. Units for optics and electricity will be introduced later.
In 1948, by order of the French government and the International Union of Theoretical and Applied Physics, the Ninth General Conference on Weights and Measures instructed the International Committee on Weights and Measures to propose, in order to unify the system of units of measurement, its ideas for creating a unified system of units of measurement, which could be accepted by all member states of the Meter Convention.
As a result, in 1954, at the tenth CGPM, the following six units were proposed and adopted: meter, kilogram, second, ampere, Kelvin and candela. In 1956, the system received the name “Système International d’Unités” - the international system of units. In 1960, a standard was adopted, which for the first time was called the “International System of Units”, and the abbreviation “SI” was assigned. The basic units remain the same six units: meter, kilogram, second, ampere, Kelvin and candela. (The Russian abbreviation “SI” can be deciphered as “International System”).
In 1963, in the USSR, according to GOST 9867-61 “International System of Units”, SI was adopted as preferable for areas of the national economy, in science and technology, as well as for teaching in educational institutions.
In 1968, at the thirteenth CGPM, the unit “degree Kelvin” was replaced by “kelvin”, and the designation “K” was also adopted. In addition, a new definition of a second was adopted: a second is a time interval equal to 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground quantum state of the cesium-133 atom. In 1997, a clarification will be adopted, according to which this time interval refers to the cesium-133 atom at rest at 0 K.
In 1971, at the 14th CGPM, another basic unit “mole” was added - a unit of quantity of a substance. A mole is the amount of substance in a system containing the same number of structural elements as there are atoms in carbon-12 weighing 0.012 kg. When using a mole, the structural elements must be specified and can be atoms, molecules, ions, electrons and other particles or specified groups of particles.
In 1979, the 16th CGPM adopted a new definition for the candela. Candela is the luminous intensity in a given direction of a source emitting monochromatic radiation with a frequency of 540·1012 Hz, the luminous energy intensity of which in this direction is 1/683 W/sr (watt per steradian).
In 1983, a new definition of the meter was given at the 17th CGPM. A meter is the distance traveled by light in a vacuum in (1/299,792,458) seconds.
In 2009, the Government of the Russian Federation approved the “Regulations on units of quantities allowed for use in the Russian Federation,” and in 2015, changes were made to it to eliminate the “validity period” of some non-system units.
Purpose of the SI system and its role in physics
Today, the international system of physical quantities SI is accepted throughout the world, and is used more than other systems both in science and technology, and in the everyday life of people - it is a modern version of the metric system.
Most countries use SI units in technology, even if in everyday life they use units traditional for these territories. In the USA, for example, customary units are defined in terms of SI units using fixed coefficients.
| Magnitude | Designation | ||
| Russian name | Russian | international | |
| Flat angle | radian | glad | rad |
| Solid angle | steradian | Wed | sr |
| Celsius temperature | degrees Celsius | o C | o C |
| Frequency | hertz | Hz | Hz |
| Force | newton | N | N |
| Energy | joule | J | J |
| Power | watt | W | W |
| Pressure | pascal | Pa | Pa |
| Light flow | lumen | lm | lm |
| Illumination | luxury | OK | lx |
| Electric charge | pendant | Cl | C |
| Potential difference | volt | IN | V |
| Resistance | ohm | Ohm | Ω |
| Electrical capacity | farad | F | F |
| Magnetic flux | weber | Wb | Wb |
| Magnetic induction | tesla | Tl | T |
| Inductance | Henry | Gn | H |
| Electrical conductivity | Siemens | Cm | S |
| Radioactive source activity | becquerel | Bk | Bq |
| Absorbed dose of ionizing radiation | gray | Gr | Gy |
| Effective dose of ionizing radiation | sievert | Sv | Sv |
| Catalyst activity | rolled | cat | kat |
A comprehensive detailed description of the SI system in official form is presented in the “SI Brochure” published since 1970 and in its supplement; these documents are published on the official website of the International Bureau of Weights and Measures. Since 1985, these documents have been issued in English and French, and are always translated into a number of languages, although the official language of the document is French.
The precise official definition of the SI system is formulated as follows: “The International System of Units (SI) is a system of units based on the International System of Units, together with the names and symbols, as well as a set of prefixes and their names and symbols, together with the rules for their application, adopted by the General Conference according to weights and measures (CGPM)".
The SI system defines seven basic units of physical quantities and their derivatives, as well as their prefixes. Standard abbreviations for unit designations and rules for writing derivatives are regulated. There are, as before, seven basic units: kilogram, meter, second, ampere, kelvin, mole, candela. Basic units have independent dimensions and cannot be derived from other units.
As for derived units, they can be obtained on the basis of the basic ones by performing mathematical operations such as division or multiplication. Some of the derived units, such as “radian”, “lumen”, “coulomb”, have their own names.
Before the name of the unit, you can use a prefix, such as a millimeter - a thousandth of a meter, and a kilometer - a thousand meters. The prefix means that one must be divided or multiplied by a whole number that is a specific power of ten.
