Physics describes the laws 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.
Fundamental and Derived Quantities :
- The fundamental quantities should be independent of each Other.
- All other quantities may be expressed in terms of the fundamental quantities.
- fundamental quantities are also known as BASE QUANTITIES.
SI Units :
In 1971 CGPM held its meeting and decided a system of units which is known as the International system of Units. It is abbreviated as SI .
This system is widely used throughout the World.
Fundamental Quantities and their Units in SI :
Supplementary Units :
1. Plane Angle – Unit ‘Radian’
2. Solid Angle- Unit ‘Steradian’
Definitions of Base Units :
1.Meter:- It is the unit of length. The distance travelled by light in vacuum in 1/299,792,458 second is called 1m.
2. Kilogram:- The mass of a cylinder made of platinum-Iridium alloy kept at International Bureau of weights and measures is defined as 1KG.
3. Second:- Cesium-133 atom emits Electromagnetic radiation of several wavelength. 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.
4. Ampere:- Suppose two long straight wires with negligible cross-section are placed parallel to each other in vacuum at a separation of 1m 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*10^-7 N/m of the wires, the Current in any of the wires is called 1A. Here ,Newton(N) is the SI unit of force.
5. Kelvin:- The fraction 1/273.16 of the thermodynamic temperature of Triple point of water is Called 1K .
6. 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 0.012kg of Carbon is called a mole.
7. Candela:- The SI unit of luminous Intensity is 1cd which is the luminous intensity of a blackbody of surface area 1/600,000m^2 placed at the temperature of freezing platinum and at a pressure of 101,325N/m^2, in the direction perpendicular to its surface.
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DEEPENDRA SINGH GURJAR
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IIT JEE & NEET Physics 