Units may be defined as a part of a quantity by which comparison of the magnitude could be made.
There are two kinds of units:
- Fundamental units
- Derived units
The units for the fundamental for base quantities are called fundamental or base units. The units for all other physical quantities can be expressed as a combination of the base units. Such units obtained for the derived quantities are called derived units. A complete set of these units, both the base and derived units is known as the system of units.
Systems of units: Several systems of units have been in use to describe measurements. The common systems are the CGS systems(Centimetre, Gram, Second); the FPS system (Foot, Pound, Second) which is the British system; the MKS system (Meter, Kilogram and Second) and the now internationally accepted which is known as International Systems of Units(came from French name “Le Systeme International d’ Unites”). abbreviated SI.
SI Base Quantities And Units
|Base quantities||Units||Symbol is||Definitions|
|Length||meter||m||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. (1983)|
|Mass||kilogram||kg||The kilogram is equal to the mass of international prototype of the kilogram (a platinum-irridium alloy cylinder) at international bureau of weights and measures, at Severs, near Paris, France. (1889)|
|Time||second||s||The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state to the Cesium-133 atom. (1967)|
|Electric current||ampere||A||The ampere is that constant current, if maintained into straight parallel conductors of in finite length, of negligible circular cross-section, and placed 1 m apart in a vacuum, would produce between these conductors of force equal to 2×10-7 newton per metre of length.(1948)|
|Thermodynamic temperature||Kelvin||K||The kelvin, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. (1967)|
|Amount of substance||Mole||mol||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.(1971)|
|Luminous intensity||candela||cd||The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x 1012 Hertz and that has a radiant intensity in the direction of 1/683 watt per steradian. (1979)|