Editorial Information
| Quantity | Unit | |
|---|---|---|
| Name | Symbol | |
| Time | second | s |
| Length | metre | m |
| Mass | kilogram | kg |
| Electric current | ampere | A |
| Thermodynamic temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Definitions of SI base units:
Unit of time
The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency Δ ν Cs , the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s –1 .
Unit of length
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m/s, where the second is defined in terms of Δ ν Cs .
Unit of mass
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6,626 070 15 × 10 –34 when expressed in the unit J s, which is equal to kg m 2 s –1 , where the metre and the second are defined in terms of c and Δ ν Cs .
Unit of electric current
The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1,602 176 634 × 10 –19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of Δ ν Cs .
Unit of thermodynamic temperature
The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1,380 649 × 10 –23 when expressed in the unit J K –1 , which is equal to kg m 2 s –2 K –1 , where the kilogram, metre and second are defined in terms of h , c and Δ ν Cs .
Unit of amount of substance
The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6,022 140 76 × 10 23 elementary entities. This number is the fixed numerical value of the Avogadro constant, N A, when expressed in the unit mol –1 and is called the Avogadro number.
The amount of substance, symbol n , of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
Unit of luminous intensity
The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10 12 Hz, K cd , to be 683 when expressed in the unit lm W –1 , which is equal to cd sr W –1 , or cd sr kg –1 m –2 s 3 , where the kilogram, metre and second are defined in terms of h , c and Δ ν Cs .
| Quantity | Unit | |
|---|---|---|
| Name | Symbol | |
| Celsius temperature | degree Celsius | °C |
Celsius temperature t is defined as the difference t = T – T 0 between the two thermodynamic temperatures T and T 0 where T 0 = 273,15 K. An interval or difference of temperature may be expressed either in kelvins or in degrees Celsius. The unit ‘ degree Celsius ’ is equal to the unit ‘ kelvin ’ .] ]