Two systems of international units. In the international system of units, seven basic units and units derived from basic units according to the principle of consistency are collectively called SI units. For example, the derivative units of the international system of units include those units given special names by the international metrology conference, such as Newton, Watt, Volt, Lumen, etc. It also includes those units without special names, such as meters per second, joules per kelvin, radian per second, etc.
SI prefix When a unit is prefixed with SI, it constitutes a new whole. So when there is an index, it refers to the whole, not just the units without prefix. For example, when expressed as cm3, it refers to cubic centimeters; When expressed as μ s- 1, it means every microsecond; When expressed as mm2/s, it refers to square millimeters per second. Under no circumstances can you use the prefix SI alone, for example, you can't use k instead of Kg or kω, or 103.
SI unit of dimensionless quantity. The dimension of a considerable number of physical quantities is "1". For example: refractive index n, dynamic friction coefficient μ, linear strain ε, relative atomic mass ar, proton number Z, power Lp, plane angle φ. The SI unit of all these quantities is the ratio of two identical SI units. For example, the SI unit of refractive index is the ratio of SI units of two velocities, that is, Ms-1(Ms-1) =1; The SI unit of dynamic friction coefficient is the ratio of the SI units of two forces, that is, N/N= 1. The unit of multiples and fractions is not composed of the SI prefix, but the power of 10, such as 106, 103, 10-6, 10-3, etc. You can also use the mathematical symbol% instead of 10-2, but you can also use micrograms per gram. The definition of the basic unit begins with 1889. In the past century, due to the development of science and technology, their definitions are constantly changing. The following is a brief description of their definition and evolution. Physical quantity name physical quantity symbol unit name unit symbol unit definition length l m m 1 m Light in vacuum 1/ Travel mass in the time interval of 299792458 seconds m kg kg 1 kg is18×14074481.
Duration 9 19263 1770 cycle current ι ampere) A is 1/ 273. 16 The amount of substance n(ν) mol 1 mol is the amount of substance in a system, and the basic contents of this system are
The number of atoms of carbon-12 is equal. Luminous intensity I(Iv) Candeira cd 1 is the luminous intensity of a light source in a given direction. The light source emits monochromatic radiation with the frequency of 540× 10 12 Hz, and the radiation intensity in this direction is 1/683 watts per ball note: 1. In people's life and trade, quality may be.
2. Unit name and unit symbol, with Chinese symbol before and international symbol after. Example: "Ampere" can be abbreviated as "An" and can also be used as a Chinese symbol. The words in brackets are synonyms of the former. Example: "kilogram" can also be called "kilogram".
3.Kg (kg) Original name: G (grave). ① unit of length-meter (m). 1889 1 the length of the international rice prototype (platinum-iridium meter ruler) approved by the international metrology conference is 1 meter. 1927 The definition of rice was strictly defined in the Seventh Metrology Congress as follows: the distance between the axes of two middle scribes on the scale of platinum-iridium meter kept by the International Bureau of Metrology at 0℃ (platinum-iridium meter scale is a ruler with an approximate H-shaped cross section, and three lines perpendicular to the scale are engraved on the two ends of its middle transverse rib, and the middle scribes refer to the middle scribes of every three lines). The ruler is stored at 1 standard atmospheric pressure and placed on two cylinders with a diameter of at least 1cm, symmetrically placed on the same horizontal plane with a distance of 571mm.
The uncertainty of the above definition of rice is about 1× 10-7. Due to the development of science and technology, it can no longer meet the needs of precision measurement such as metrology. In 1950s, with the development of isotope spectrum light source, krypton -86 isotope spectrum line with extremely narrow width was discovered. With the success of interference technology, people finally found a natural reference that is not easy to destroy, that is, the natural reference with the wavelength of light wave as the length unit.
Therefore, the definition of rice in 1 1 960 was changed to: "The length of rice is equal to 1650763.73 times of the radiation wavelength of krypton -86 atoms passing between 2p 10 and 5d5 levels in vacuum." The limit uncertainty of krypton -86 length datum is 4× 10-9. After the definition of rice is changed, the original international rice instruments are still kept in the International Bureau of Metrology according to the original conditions.
Because the laser with stable saturation absorption has high frequency stability and reproducibility, its wavelength is more reproducible than that of Krypton -86, and its accuracy may be further improved. Therefore, four stable laser wavelength values were recommended at the two meetings of the rice definition advisory committee 1973 and 1979, which were used in parallel with the krypton -86 wavelength with the same accuracy.
Since 1973, the frequency values of various spectral lines from infrared band to visible band have been measured accurately. According to the frequency and wavelength values V and λ of methane spectral line, the light speed value с=λv=299792458 m/s in vacuum is obtained. This value is very accurate, so people decide to take this light speed value as the definition value, and deduce the definition of length L (or wavelength) from time T (or frequency) through the formula l=сt (or λ=с/v). 1983 17 the international metrology conference formally adopted the following new definition: "the meter is the length of light propagating in a vacuum at an interval of 1/299792458 seconds."
Old definition: 1790 In May, a special committee composed of French scientists suggested that one fortieth of the total length of the earth meridian passing through Paris should be used as the unit of length-meter.
② Mass unit-kilogram (kg). 1889 1 the international metrology conference approved the international original kilogram device and announced that it will be used as the quality unit in the future. In order to avoid the ambiguity of the commonly used word "weight", 190 1 the third international metrology conference stipulates that the kilogram is the unit of mass (not weight), which is equal to the mass of the original international kilogram device. The original platinum-iridium kilogram device was kept in the International Bureau of Metrology according to the conditions stipulated in 1889 1 International Metrology Conference.
New definition: Ronald Fox, an emeritus professor at the Physics Branch of Georgia Institute of Technology, proposed that gram (one thousandth of a kilogram) should be strictly defined as the weight of18×14074481c-12 atom from now on. At least two other proposals to redefine kilograms are under discussion. They include: replacing Pt-Ir mixed cylinder with pure silicon atomium at 1; 2 Use the known "Watt Balance" device and use electromagnetic energy to define the kilogram.
Old definition: the mass of 1 liter pure water at 4℃ is 1Kg.
③ Time unit-second. Initially, the time unit "second" was defined as 1/86400 of the average solar day. The precise definition of "average solar day" is left to astronomers. However, measurements show that the average solar day cannot guarantee the necessary accuracy. In order to define the time unit more accurately, the 11International Metrology Conference of1960 approved the definition based on the tropical year stipulated by the International Astronomical Association: "1900 1/35438 with the tropical year as the second starts from October of Gregorian calendar time. However, the accuracy of this definition still can't meet the requirements of precise measurement at that time, so 13 International Metrology Conference in 1967 decided to modify the definition of second to 9 1926365438, which is the radiation period of two ultra-fine energy level transitions in cesium-133 atomic ground state.
The International Atomic Time is an international reference time scale defined according to Article 2 above and belongs to the International System of Units (SI).
④ Current intensity unit-ampere (A). 1893 quoted the so-called "international" electrical unit of current and resistance at the international electrotechnical conference held in Chicago. The definitions of "international" ampere and "international" ohm were approved at the London International Conference on 1908. Although during the Eighth International Metrology Congress in 1933, it was explicitly and unanimously required to replace these "international" units with so-called "absolute" units, it was not until the Ninth International Metrology Congress in 1948 that these "international" units were formally decided to be abolished and the following definitions of current intensity units were adopted:
A constant current passes through two parallel straight lines with infinite length and negligible circular cross-section, and the distance between the two straight lines in vacuum is 1 meter. If this constant current makes the force per meter between the two wires equal to 2× 10-7N (Newton), then the current intensity of this constant current is 1A (ampere).
⑤ Thermodynamic degree-day-Kelvin (k). 1954 The first 10 international metrology conference stipulated the definition of thermodynamic degree-day, and the triple point of water was selected as the basic fixed point, and its temperature was defined as 273. 16K. 1967 13 the name (symbol k) in Kelvin is adopted by the international metrology conference instead of "Kelvin", which is formally defined as:
Thermodynamic degree-day kelvin is 1/273.438+06 of the thermodynamic temperature of water triple point. At the same time, the conference also decided to use the unit Kelvin and its symbol K to represent the temperature range or temperature difference.
In addition to the thermodynamic temperature expressed in Kelvin (symbol T, see thermodynamic temperature scale), the temperature expressed in degrees Celsius (symbol T) defined by the formula t = t-t0 is also used. Where T0=273. 15K is the thermodynamic temperature of water freezing point, which is 0.0 1K (Kelvin) different from the thermodynamic temperature of water triple point. The unit of temperature is Celsius (symbol℃). Therefore, the unit "Celsius" is equal to the unit "Kelvin". The temperature interval or temperature difference is expressed in degrees Celsius.
According to the definition of moderate-day kelvin in thermodynamics, the absolute measurement of temperature must rely on thermodynamic thermometers, such as gas thermometers. Thermodynamic temperature scale is reasonable in theory, but it is very difficult to realize. Therefore, it is decided to adopt the practical temperature scale in the world, which cannot replace the thermodynamic temperature scale, but it is necessary to improve the accuracy as much as possible according to the measurement technology level at that time and approach the thermodynamic temperature scale. According to the requirement of practicality, it should be unified internationally.
1927 the seventh international metrology conference adopted the first international temperature scale. This international temperature scale was revised in 1948, and was named 1948 international practical temperature scale (code IPTS-48) by 1960 international metrology conference in1. Later, there was a revised version of IPTS-48 of 1960. The corrected fixed-point temperature value still keeps the value of 1948.
1968, the international metrology Committee adopted a new international practical temperature scale, which is consistent with the best thermodynamic results known at present. The code name of this thermometer is IPTS-68. It is based on the following two points: firstly, there are 1 1 reproducible fixed points, and the temperature value of the fixed points is determined by a gas thermometer in the range of 13.8 1K to 1337.58K; Secondly, standard instruments (13.8 1K to 903.89K are platinum resistance thermometers, 903.89K to 1337.58K are platinum-rhodium-platinum thermocouples, and spectral pyrometers and constants с 2 = 0.0 are used above1337.58k. ..
It is particularly important to note that the triple point of water is not the freezing point, but the freezing point is related to air pressure and solute (such as air) in water, and the triple point is only related to the nature of water itself. The calculated size of 1K is equal to 1℃, and the melting point of water is about 273.10/.325 Pa.
6. The unit of matter-mole. This unit is closely related to atomic weight. Initially, "atomic weight" was based on the atomic weight of chemical element O (oxygen) (defined as 16). But chemists set the value of natural oxygen element as O- 16, which is a mixture of O- 17 and O- 18. Physicists, on the other hand, set the value of oxygen-16 as 16, which is very inconsistent. 1959-1960, this inconsistent situation ended after the agreement was reached between the International Union of Pure and Applied Physics (IUPAP) and the International Union of Pure and Applied Chemistry (IUPAC). It is decided that the carbon isotope C- 12 is the standard and its atomic weight is 12. On this basis, the numerical value of "relative atomic mass" is given. The remaining problem is to define the unit of substance quantity by determining the mass corresponding to C- 12. According to international agreements, the C- 12 of the unit of "material quantity" should be 0.0 12Kg (kg). The unit of "the amount of matter" thus defined is named mole.
According to the suggestions of the International Union of Pure and Applied Physics, the International Union of Pure and Applied Chemistry and the International Organization for Standardization, the International Metrology Committee formulated the definition of Moore in 1967, which was approved in 1969 and finally adopted by the International Metrology Conference in 197 14. Definition: Mole is the quantity of a system substance, and the basic unit should be specified when using Mole, which can be atoms, molecules, ions, electrons and other particles; Or a specific combination of these particles. At the same time, this definition of mole strictly defines the properties of molar quantity.
According to scientific determination, the number of C atoms contained in 12g C- 12 is about 6.0220943x1023. Represented by the symbol NA, it is called avogadro constant.
Definition: The amount of any substance containing Avon Gadereau constant (about 6.022× 1023) of structural particles is 1mol.
⑦ Luminous intensity unit-Candeira (cd). In 1948, the unit of luminous intensity based on flame or incandescent filament used in various countries was changed to "new candlelight". This decision was made by CIE and IMC before 1937. According to the power granted by 1933, the eighth international metrology conference, the international metrology commission promulgated it at 1946. 1948 The 9th International Metrology Congress approved this decision of the International Metrology Committee and agreed to give this luminous intensity unit a new international name "Candeira" (symbol cd). 1967 13 The metrology meeting formally adopted the following revised definitions:
1cd (Candeira) is the luminous intensity of the blackbody surface in the vertical direction at the platinum solidification temperature under the pressure of 10 1325N/m2.
The above definition has been used until 1979. In practice, it is found that there are great differences when laboratories in different countries reproduce cd (Candeira) in original bold. During this period, the radiation measurement technology developed rapidly, and its accuracy was comparable to that of photometric measurement. Through radiation measurement, cd (Candeira) can be directly reproduced. In view of this situation, 1977 International Metrology Committee defined the ratio of luminescence measurement to radiation measurement, and stipulated that the spectral light efficiency of monochromatic radiation with a frequency of 540× 10 12Hz (Hz) was 683lm/W (lumens per watt). This value is accurate enough for bright visual light; For dark vision light, there is only about 3% variation.
At the 1979 16 metrology conference held in 10, it was formally decided to abolish the definition of 1967 and make the following new definition of cd (Candeira):
1cd (Candeira) is the luminous intensity of the light source in a given direction. The light source emits monochromatic radiation with the frequency of 540× 10 12Hz (Hz), and the radiation intensity in this direction is 1/683 W/sr (watts/sphericity).
The radiation wavelength defined by 540× 10 12Hz is about 555nm, which is the most sensitive wavelength for human eyes. SI derived units are derived from SI basic units or auxiliary units according to the definition, and there are many. There are 19 SI derivative units with special names. There are 17 named after outstanding scientists, such as Newton, Pascal and Joule, to commemorate their contributions in this field. They have their own special names and symbols, and these special names and symbols can be used to form other export units, which is simpler than expressing them in basic units. At the same time, for the convenience of representation, these derived units can also be combined with other units to represent other more complex derived units.
The following are the definitions of some export units with special names.
Hertz (frequency unit)-The frequency of periodic phenomena with a period of 1s (seconds) is 1Hz (hertz), that is, 1hz = 1s- 1. Newton (unit of force)-the force that causes the mass of 1Kg (kg) to produce an acceleration of 1m/s2 (m/s.sup.2), that is,1n =1kg m/s2. Pascal (pressure unit)-the pressure per square meter 1N (Newton) force, i.e. 1Pa= 1N/m2. Joule (unit of energy or work)-1n (Newton) The work done when the point of force moves in the direction of force 1m (m), that is,1j =1n m watt (unit of power)-/kloc. That is, 1W =1j/s 1A s volts (unit of potential difference and electromotive force)-if the power consumed between two points is1a (ampere) in a conductor with constant current. That is, 1v = 1w/a ... farad (capacitance unit)-when the capacitor is charged with 1C (coulomb), there is a potential difference of 1V (volt) between the two plates, so the capacitance of this capacitor is1f. If a constant current of 1A (ampere) is generated in the conductor and there is no other electromotive force in the conductor, the resistance between these two points is1Ω (ohm), that is,1Ω =1v. Siemens (conductance unit)-the negative power of ω (ohm), that is, 1s = 1ω- 1. Henry (inductance unit)-Let the current flowing through the closed loop change evenly at the speed of 1A/s (ampere per second). If an electromotive force of 1V (volts) is generated in the loop, the inductance of this loop is 1H (Henry), that is,1h =1v s.
Weber (magnetic flux unit)-Let the magnetic flux in the loop with only one turn be reduced to zero evenly within 1s (seconds). If an electromotive force of 1V (volts) is generated in the loop, the magnetic flux in the loop is 1 (Weber), that is,1WB =1vs. Tesla (magnetic induction intensity or magnetic flux density unit)-magnetic induction intensity with magnetic flux of 1Wb (Weber) per square meter (square meter), i.e. 1 T= 1 Wb/m2. Lumen (luminous flux unit)-luminous flux emitted by a uniform point light source with luminous intensity of 1cd (Candeira) to sr (unit solid angle within sphericity), that is, 1LM = 1cd Sr. Lux (illumination unit)-per square meter 1LM (lumen Bekkerel (radioactivity unit)-1 spontaneous nuclear transformation or transition occurs within 1s (seconds), i.e. 1Bq (Bekkerel), i.e. 1bq = 1s- 1. Gray (specific energy unit)-1kg (kg) The absorbed dose of the irradiated substance is 1J (joule), that is, 1Gy= 1J/Kg.
Sewart (dose equivalent)-the dose equivalent of 1J (joule) per kilogram (Kg), that is, 1Sv= 1J/Kg.
Radius (rad) and sphericity (sr) (pure geometric units) have been incorporated into derived units. Its definition is as follows: radian (rad)-the plane angle between two radii in a circle. The arc length of these two radii on the circumference is equal to the radius. Sphericity (Sr)—— A solid angle whose vertex is located in the center of a sphere, and the area it cuts on the sphere is equal to the area of a square with the radius of the sphere as its side. Physics is an experimental science, and its theory is based on experimental observation. Experimental observation is inseparable from the measurement of physical quantities. In order to quantitatively express the size of the observed quantity, for the same physical quantity (such as length), it is necessary to select a specific quantity as the unit (such as 1 meter). Any other quantity in this class can be expressed by the product of this unit and a number, which is the value of the physical quantity with the above specific quantity as the unit.
Physics has established many systems of units in history. 197 1 year later, the international system of units based on seven basic quantities was established.
Various physical quantities are interrelated through the equations describing the laws of nature and the definitions of new physical quantities. For convenience, a group of independent physical quantities are usually selected as the basic physical quantities, and other quantities are expressed according to the basic quantities and related equations, which are called derived quantities. The earliest branch of physics that people studied was mechanics. In the field of mechanics, the unit system with length, mass and time as the basic physical quantities was first established, which was called 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 in Paris in 17 by 1875. Metric agreed length is in meters, mass in kilograms (kilograms) and time in seconds. This system of units is called the meter-kilogram-second system. With the development of electromagnetism, thermodynamics, light radiation and microphysics, the basic physical quantities have gradually expanded from three to seven. The system of units developed on the basis of the meter-kilogram-second system was established, which was confirmed by the International Metrology Conference in1/and was called the International System of Units (SI). The principle of the international system of units is scientific and mostly practical, involving all professional fields. Popularizing the international unit system can eliminate the confusion caused by various unit systems and the coexistence of units, save a lot of manpower and material resources, and promote the further development of national economy and international exchanges. At present, most industrialized countries are actively promoting the international system of units, and countries that used the English system have also decided to abandon the English system and adopt or prepare to adopt the international system of units. Because in physics, especially in theoretical physics, the centimeter-gram-second unit and its developed electromagnetic unit are sometimes needed, the centimeter-gram-second unit system is still used as a reserved unit system. The International Metrology Committee believes that when using the cm-g-s system, it is generally best not to use it with the international system. In particle physics, a special unit system-natural unit system is still widely used. In the natural unit system, the basic physical constants H (Planck's constant divided by 2π) and с (the speed of light) are taken as 1. So the basic physical quantity can be simplified, so that energy can be selected as the only basic physical quantity. In other physics disciplines closely related to particle physics, the natural unit system is sometimes adopted.
Cm g s system (CGS system). In many books and papers on physics, especially theoretical physics, the centimeter-gram-second system (CGS system) is still widely used. This system of units chooses centimeters, grams and seconds as the basic units. One of the conveniences of the cm-g-s system is that 1 cubic centimeter of water has a mass of about 1 gram at its maximum density. This unit system was established at 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, derived units. But when it comes to electromagnetic phenomena, the establishment of export units is not unique. In other words, there are two different ways. The starting point of one method is that the force between two magnetic poles is inversely proportional to the square of distance, and the starting point of the other method is that the force between two charges is inversely proportional to the square of distance. In 185 1, W. Weber obtained two consistent "absolute" unit systems along these two paths. Electrostatic interaction based on charge is called absolute electrostatic unit (CGSE), and magnetic interaction is called absolute electromagnetic unit (CGSM).
The unit of magnetic field intensity specified in CGSM unit is called Oster, the unit of magnetic induction intensity is called Gauss, and 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 units are formed (see the electromagnetic quantity unit system).
When limited to the units of mechanical and electrical quantities, the international system of units includes current as the basic unit, that is, * * * has four basic units. The centimeter-gram-second system has only three basic units, and the current is the derived unit.
The international system of units adopts some so-called "practical units" (including some derivative units) recommended by the British Association for the Advancement of Science. For example, the unit of resistance is ohm and the unit of electromotive force is volt, which are equal to 109 and 108 times of the corresponding CGSM units, respectively. The proposal of the British Association for the Advancement of Science was adopted at the 1 International Electricity Conference held in Paris in 1. The congress also introduced a practical unit of current, ampere, which is equal to one tenth of the corresponding CGSM unit. Later, the practical unit of charge coulomb and the practical unit of capacitance farad were introduced.
People have formulated ohm physical standard (mercury column), volt physical standard (Weston battery) and ampere physical standard (silver electrolytic meter) for practical units, all of which are sub-standards. 1893 Chicago International Electric Power Conference gave the "legal" definitions of ohm, volt and ampere according to these physical standards. The International Conference on Electricity held in London from 65438 to 0908 decided to adopt a set of so-called "International System of Electrical Units", which is based on physical standards such as ohm and ampere in metrology.
1948 the ninth international metrology conference officially adopted the unit system of meters, kilograms, seconds and amperes, which is the basis of the current international system of units. Physical quantity name unit name derivation unit expression symbol derivation unit definition area A (S) m2 volume v m3 speed v m/s acceleration one meter per second m/s2 angular velocity ω radian per second rad/s frequency f (v) Hertz Hz 1 Hz =1s-1period is1 Frequency density of periodic phenomenon of seconds ρ kg Force f Newton per cubic meter n 1 nSeconds 2 Acceleration Moment M M Moment p kg m/s Pressure p Pa (Pascal) Pa 1 Pa= 1 N/m2 Pressure Work1Newton Energy (Energy) W(A J = 1 n m1j/s1s gives the charge amount of1focal energy q library (charge amount
1V = 1N m/c If the power consumed between two points in a conductor with a constant current of 1A is 1W, the potential difference between two points is 1V capacitance method (Farah) f1f =/kl. If there is a potential difference of 1 V between the two plates, the capacitance of the capacitor is 1 normal resistance Rω (ohm) ω 1ω= 1V/A, and a constant potential difference of 1V is added between two points of the conductor. If 1A is generated in the conductor, Then the resistance between two points is 1 ρ ρ (ohm) mωm magnetic induction intensity b Te (Tesla) T 1 T= 1 Wb/ magnetic flux is 1 Wei Φ Wei (Weber) WB 1 WB =/kloc-0 per square meter. The constant inductance l (Henry) H 1 H= 1 Wb/A makes the current flowing through the closed loop change uniformly at the rate of 1 A/s, Then the inductance of the loop is 1 constant conductance (Siemens) s 1s = 1ω- 1 ohm negative primary luminous flux (lumen) lm1lm =1cdx) LX/kloc-0. Kloc-0/ Bekkerel bq1bq =1s-1kloc-0/spontaneous nuclear transformation or transition absorbed dose 1 gy = 1 gy. Kg 1 kg irradiated materials were awarded the absorbed dose temperature of 1 coke. The degree of heat and cold of an object is T degrees Celsius (Fahrenheit)℃ (Fahrenheit). C specific heat capacity of coke per kilogram centigrade J/(kg * C). The calorific value of Q coke burned per kilogram of J/kg fuel is exothermic. Note: 1. Names and symbols in brackets are previous names and symbols.
2. Words in brackets can be omitted without causing confusion or misunderstanding. Remove the word in brackets, which is the abbreviation of its name.