SI Base Units

The base unit defines the base quantity. For example, length is a physical quantity and the meter is a unit of length that defines the predetermined length. When we say 10 meters, we mean that the length we are referring is 10 times the predetermined length called meter.

Table 1: SI base units

 

Definition of SI base units

• Meter: Unit of length

                Symbol: m

                Purpose: To measure length

                Definition: The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

• Kilogram: Unit of mass

                Symbol: kg

                Purpose: To measure the quantity of matter contained in a body or substance

                Definition: It is equal to the mass of the international prototype of the kilogram.

• Second: Unit of time

                Symbol: t

                Purpose: To measure time

                Definition: The second is 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.

• Ampere: Unit of electric current

                Symbol: A

                Purpose: To measure electric current

                Definition: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross- section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per meter of length.

• Kelvin: Unit of thermodynamic temperature

                Symbol: K

                Purpose: To measure temperature

                Definition: The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

• Mole: Unit of amount of substance

                Symbol: mol

                Purpose:  To specify amounts of chemical elements or compounds.

                Definition: The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol.” When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

• Candela: Unit of luminous intensity

                Symbol: cd

                Purpose: To measure the luminous intensity of a heat or light source

                Definition: The candela is 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.

 

Reference: http://www.bipm.org/en/CGPM/db/17/1/