Definitions of Base Units

Hey There! We have studied that there are 7 base units in physics. Today we are going to learn their definitions. So, Let’s start….      

Any standard unit should have the following two properties:      

1. 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.
2. Availability: The standard unit should be easily made available for comparing with other quantities.

  Also Read – Unit System & Measurement      

The procedures to define a standard value as a unit are quite often not very simple and use modern equipment.

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 traveled 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 the International Bureau of Weights and Measures is defined as 1 kg.      

Second

Cesium-133 atom emits electromagnetic radiation of several wavelengths. Particular radiation is selected which corresponds to the transition between the two hyperfine levels of the ground state of Cs-133. 4

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 the 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 meter 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 273.16 of the thermodynamic temperature of triple point of water is called 1 K.      

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 10²³ with an uncertainty of about 0’000031 x 10²³.      

Candela

The SI unit of luminous intensity is 1 cd which is the luminous intensity of a blackbody of surface area 1/600 000 m²placed at the temperature of freezing platinum and at a pressure of 101,325 N/m², in the direction perpendicular to its surface.