國際單位制 (SI)

國際單位制是使用最廣泛的測量系統，是公制系統的現代形式。其核心是建立在七個基本單位之上。該系統規定了單位符號和名稱的二十個詞頭，以表示每個單位的倍數和分數。

國際單位制於 1960 年向公眾推出，這是 1948 年開始的早期研究的成果。SI 基於米-千克-秒單位制 (MKS)。SI 的創建是為了隨著時間的推移而發展。其目的是在國際層面上創建單位和詞頭，並修改單位定義，因為我們日常使用的測量工具的技術不斷變得更先進、更精確。這個不斷發展的系統的一個很好的例子是 2011 年和 2014 年舉行的第 24 屆和第 25 屆國際計量大會 (CGPM)，會上提出了一項修改「千克」定義的提案。提出這一點是因為有些人開始認為千克是自然的恆量，而不是質量的測量。

正如前一節所述，世界各地許多領域的各個行業之間缺乏令人難以置信的協調。為了創造統一性，SI 應運而生。由 1875 年《米制公約》設立的 CGPM 承擔了匯集眾多國際組織，就擬議新系統的定義和標準達成一致的使命。他們還聚集在一起，就這個新系統如何在世界各地呈現和接受的規則達成一致。

理論上，SI 可以用於任何類型的物理測量。儘管如此，不能忽視的事實是，在可預見的未來，非 SI 測量單位仍將出現在技術、科學和商業文獻中。此外，某些測量單位在歷史和某些文化中根深蒂固，它們將在很長一段時間內繼續使用。CIPM 已將這些單位分類並將其納入 SI 手冊。

以下是國際單位制。

Base Units

Amount Unit name Unit symbol Definition
Length Meter m The distance traveled by light in a vacuum in 1/299792458 second.
Weight Kilogram kg This is the unit for weight. The mass of the international prototype kilogram.
Time Second s The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
Current Ampere A The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2×10-7 newtons per meter of length.
Thermodynamic temperature Kelvin K 1/273.16 of the thermodynamic temperature of the triple point of water.
Substance amount Mole mol The amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. (Limited to objects with clarified composition.) Elementary entities are subatomic particles that compose matter and energy.
Luminosity Candela cd The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Supplementary Units

Amount Unit name Unit symbol Definition
Plane angle Radian rad Radian describes the plane angle subtended by an arc of a circle with the same length as the radius of that circle corresponds to an angle of 1 radian.
Solid angle Steradian sr A steradian is a solid angle at the center of a sphere subtending a section on the surface equal in area to the square of the radius of the sphere.

Derived Units

Derived units are a combination of base units and supplementary units and the mathematical symbols of multiplication and division.

Amount Unit name Unit symbol
Area Square meter m2
Volume Cubic meter m3
Speed Meter per second m/s
Acceleration Meter per second squared m/s2
Wavenumber Reciprocal meter m-1
Density Kilogram per cubic meter kg/m3
Current density Ampere per square meter A/m2
Magnetic field strength Ampere per meter A/m
Concentration (of amount of substance) Mole per cubic meter mol/m3
Specific volume Cubic meter per kilogram m3/kg
Luminance Candela per square meter cd/m2

Some derived units are given unique names.

Amount Unit name Unit symbol Composition
Frequency Hertz Hz 1Hz=1s-1
Force Newton N 1N=1kg･m/s2
Pressure, stress Pascal Pa 1Pa=1N/m2
Energy, work, amount of heat Joule J 1J=1N･m
Power, radiant flux Watt W W=1J/s
Electric charge, amount of electricity Coulomb C 1C=1A･s
Electric potential/electric potential difference, voltage, electromotive force Volt V 1V=1J/C
Resistance (electrical) Ohm Ω 1Ω=1V/A
Conductance (electrical) Siemens S 1S=1Ω-1
Magnetic Weber Wb 1Wb=1V･s
Magnetic flux density, magnetic induction Tesla T 1T=1Wb/m2
Inductance Henry H 1H=1Wb/A
Celsius temperature Degree Celsius ℃ 1t=T-To
Luminous flux Lumen lm 1lm=1cd･sr
Illuminance Lux lx 1lx=1lm/m2

Reference Information

SI unit prefixes indicating integer powers of ten

Factor Prefix Symbol Factor Prefix Symbol
1018 exa E 10-1 deci d
1015 peta P 10-2 centi c
1012 tera T 10-3 milli m
109 giga G 10-6 micro µ
106 mega M 10-9 nano n
103 kilo k 10-12 pico p
102 hecto h 10-15 femto f
10 deka da 10-18 atto a

Non-SI units

Amount Unit name Unit symbol Definition
Time Minute min 1min=60s
Hour h 1h=60min
Day d 1d=24h
Plane angle Degree ° 1°= (π/180) rad
Minute ′ 1′= (1/60) °
Second ″ 1″= (1/60) ′
Volume Liter l, L 1l=1dm3
Weight Metric ton t 1t=103kg

 

 

Reference: VITREK LLC