Definitions of the SI Base Units(physical quantities,units and measurement)

Definitions of the SI Base Units

Length: metre (m)
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Mass: kilogram (kg)
The kilogram is equal to the mass of the international prototype of the kilogram: a piece of platinum-iridium alloy kept at the International Bureau of Weights and Measures, Sévres, France.

Time: second (s)
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.

Electric current: ampere (A)
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 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per metre of length.

Thermodynamic temperature: kelvin (K)
The kelvin is 1/273.16 of the thermodynamic temperature of the triple point of water.

The unit kelvin and its symbol K should be used to express both thermodynamic temperature and an interval or a difference of temperature.
In addition to the thermodynamic temperature (symbol T) there is also the Celsius (symbol t) defined by the equation t=T-T0 where T0=273.15 K. Celsius temperature is expressed in degree Celsius (symbol C). The unit 'degree Celsius' is equal to the unit 'kelvin', and a temperature interval or a difference of temperature may also be expressed in degrees Celsius.

by:Amelina bte Ahsemat