Definition of the hottest Si basic unit

Abstract: the 9th International Conference on Metrology held in 1948 decided to require the International Committee on metrology to create a simple and scientific practical system of units that can be used by all Member States of the metric convention organization. In 1954, the 10th International Conference on Metrology decided to adopt meter (m), kilogram (kg), second (s), ampere (a), Kelvin (k) and candela (CD) as the basic units

keyword: unit of measurement absolute zero Avogadro constant

length: meter (m)

meter is the length of light travel in vacuum within a time interval of 1/s

old definition:

1 In May 1790, a special committee composed of French scientists suggested that the length unit of the earth's meridian passing through Paris should be 1/40 million of the total length - meters

2 In 1960, the 11th International metrology conference: "the length of meter is equal to the radiation of the transition between 2p10 and 5d1 energy levels of krypton-86 atom. 7 wavelength in vacuum. Product use: the tensile testing machine of steel bar is three times that of the special testing equipment for food packaging"

mass: kilogram (kg)

kilogram is defined as the mass of the international kilogram prototype

old definition: the mass of 1 liter of pure water at 4 degrees Celsius is 1 kilogram

time: second (s)

at the 13th International Conference on weights and measures in 1967, a resolution was adopted to adopt the following definition to replace the astronomical definition of second: one second is the duration of 9192631770 weeks of transition radiation between two hyperfine energy levels of cesium-133 atom ground state

international atomic time is an international reference time scale based on the definition of seconds above, belonging to the international system of units (SI)

current: Ampere [Pei] (s.l.10% equity a)

ampere is a constant current. If it is maintained in two infinite parallel straight wires 1 meter apart in vacuum, and the circular section can be ignored, the force generated between the two wires is equal to 2 per meter of length × Newton. The definition was approved at the 9th International Conference on Metrology in 1948, and the ampere was officially adopted as one of the basic units of the international system of units at the 11th International Conference on Metrology in 1960. Ampere is named in memory of French physicist a.-m. ampere

thermodynamic temperature: Kelvin (k)

Kelvin English is Kelvin for short, international code K, the unit of thermodynamic temperature. Kelvin is one of the seven basic units in the international system of units (SI). Taking absolute zero (0k) as the minimum temperature, it stipulates that the temperature of the three-phase point of water is 273.16k, and 1K is equal to 1/273.16 of the temperature of the three-phase point of water. It should be noted that the three-phase point of water is not the freezing point, which is related to the air pressure and the solute in the water (such as air). The three-phase point is only related to the nature of the water itself, which is a constant. The size of 1K calculated from this is roughly equal to 1 ° C, and the melting point of water at 101325pa is about 273.15k, so the conversion between the Celsius temperature scale and the international temperature scale is about tc=tk-273.15. Kelvin was named in memory of Lord Kelvin, a British physicist

luminous intensity: Kan [della] (CD)

Kan from nothing to have della is the luminous intensity of a light source in a given direction, and the light source emits a frequency of 540 × Monochromatic radiation of 1012 Hz, and the radiation intensity in this direction is 1/683 watt per sphere degree.

540 in the definition × The wavelength of 1012 Hz radiation is about 555nm, which is the most sensitive wavelength for human eyes to feel.

the amount of substance: mol

indicates the physical quantity of the number of particles that make up the substance (the amount of substance is a special term, which can not be separated and omitted)

the mole is the unit of the amount of physical quantity substance

according to scientific measurement, the number of C atoms contained in 12g 12C is about 6 × 10^23 is represented by the symbol Na, which is called Avogadro constant

Avogadro constant (NA) has a similar value of 6.02 sold for nearly 1million yuan × 10^23

definition: all structural particles containing Avogadro constant (about 6.02 × 10^23), the amount of which is 1 mole. (end)