What is unit
The collection of quantities like mass, length, volume, temperature, pressure etc. The measurement of any physical quantity involves comparison of its magnitude or size with a standard of the same kind. The standards are decided arbitrarily with some reasoning behind it. The arbitrarily decided or fixed standards are called as Units.
Characteristics of good unit:
A good unit must have following characteristics:
1) The magnitude of a unit should not be dependent on place and time. i.e. it should be invariable.
2) It should be easily reproducible.
3) It should be accepted universally.
Comman Systems of Units:
1) The centimeter- game second (CGS) system-
The system is based upon basic units, centimeter as the unit of length, gram as the unit of mass and the second as the unit of time.
2) The Foot-pound-second (FPS) system-
This system is based upon three basic units, foot, as the unit of length, pound as the unit of mass and the second as the unit of time.
3) The metre-kilogram-second (MKS) system or metric system-
This system is based upon three basic units, metre as the unit of length, kilogram as the unit of mass and the second as the unit of time.
The SI system:
With developments in science and it's communication, the need for a common system of measurement was felt worldwide. In order to make measurements more scientific and accurate, in the year 1791 AD, French academy of Science devised metric system of measurement. It was based on decimal system.
Fundamental units:
Later it was noticed that these three fundamental units can not define all the physical quantities like temperature, intensity, amount of substance etc. Therefore in addition to the units of mass, length and time, even the units of temperature, intensity, amount of substance, electric current etc. are also considered as Fundamental units.
All other units can be derived from these basic units, hence are known as Derived units. The derived units include units of area, volume, density, velocity, acceleration, force, pressure etc.
International system of units (SI Units) :
In October 1960,The general conference of Weights and measures (Conference General des poids at measures), adopted an International System of units. The units according to this system are called as SI Units (System International d' Unites).
Definitions of the seven basic SI units:
1) Metre (m) :
It is the distance traveled by light through vaccum in 1/299792458th of a second.
2) Kilogram (kg) :
It is the mass of an international prototype in the form of a platinum-iridium cylindrical bar kept in France. It is the only basic unit defined in terms of a material object and also the only one with a prefix (kilo).
3) Second (s) :
It is the length of time taken for 9192631770 periods of vibrations of caesium-133 atom to occur.
4) Ampere (A) :
It is the current that produces a specified force between two parallel wires, that are one meter apart in vaccum.
5) Kelvin (K) :
It is the 1/ 273 15th of the thermodynamic temperature of the triple point of water.
6) Mole (mol) :
It is the amount of a substance that contains as many fundamental particles as the number of atoms in 0.012 kg of carbon-12.
7) Candela (cd) :
It is the intensity of a source of light of a specific frequency, that gives a specific amount of power in a given direction.
Physical name of symbol quantity SI unit SI unit
1. Length Meter m
2. Mass kilogram kg
3.Time Second s
4.Temperature Kelvin K
5. Electric current Ampere A
6. Intensity Candela CD
7. Amount of Mole mol substance
Derived units:
The units of all physical quantities can be derived from the seven basic units. Hence these units are called as derived units.
The SI prefixes:
Units of Measurement - Definition, Different Systems - Chemosmart
By -
October 20, 2021
