UNITS
Physics describes the law of nature. This description is quantitative and involves measurement and comparison of physical quantities. To measure a physical quantity we need some standard unit of that quantity.
Who Decides the Units ?
How is a standard unit chosen for a physical quantity ? The first thing is that it should have international acceptance. Otherwise, everyone will choose his or her own unit for the quantity and it will be difficult to communicate freely among the persons distributed over the world. A body named Conference Generale des poids et Mesures or CGPM also known as General Conference on Weight and Measures English has been given the authority to decide the units by international agreement. It holds its meetings and any changes in standard units are communicated through the publications of the conference.
Definitions Of Base Units
Any standard unit should have the following two properties :
(a) Invariability : The standard unit must be invariable. Thus, defining distance between the tip of the middle finger and the elbow as a unit of length is not invariable.
(b) Availability : The standard unit should be easily made available for comparing with other quantities.
The procedures to define a standard value as a unit are quite often not very simple and use modern equipments. Thus, a complete understanding of these procedures cannot be given in the first chapter. We briefly mention the definitions of the base units which may serve as a reference if needed.
Metre
It is the unit of length. the distance travelled by light in vacuum in 1/299,792,458 second is called 1 m.
Kilogram
The mass of a cylinder made of platinum-iridium alloy kept at International Bureau of Weights and Measures is defined as 1 kg.
Second
Cesium-133 atom emits electromagnetic radiation of several wavelengths. A particular radiation is selected which corresponds to the transition between the two hyperfine levels of the ground state of Cs-133. Each radiation has a time period of repetition of certain characteristics. The time duration in 9,192,631,770 time periods of the selected transition is defined as 1 s.
Ampere
Suppose two long straight wires with negligible cross-section are placed parallel to each other in vacuum at a separation of 1 m and electric currents are established in the two in same direction. The wires attract each other. If equal currents are maintained in the two wires so that the force between them is 2 ｘ 10^-7 newton per metre of the wires, the current in any of the wires is called 1 A. Here, newton is the SI unit of force.
Kelvin
The fraction 1/273.16 of the thermodynamic temperature of triple point of water is called 1K.
Mole
The amount of a substance that contains as many elementary entities (molecules or atoms if the substance is monatomic) as there are number of atoms in 0.012 kg of carbon-12 is called a mole. This number (number of atoms in 0.012 kg of carbon-12) is called Avogadro constant and its best value available is 6.022045 ｘ 10 ^23 with an uncertainty of about 0.000031 ｘ 10^23 .
Candela
The SI unit of luminous intensity is 1 cd which is the luminous intensity of a blackbody of surface area 1/600,000 m^2 placed at the temperature of freezing platinum and at a pressure of 101,325 N/m^2 , in the direction perpendicular to its surface.
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