International System of Units

Transcript

International System of Units The internationally accepted system of measurement is the Système International d’Unités, abbreviated as SI. Earlier, different systems of units were used: CGS system: Length — centimetre Mass — gram Time — second FPS system: Length — foot Mass — pound Time — second MKS system: Length — metre Mass — kilogram Time — second Using different systems could create confusion. SI provides a common measurement system. THE SEVEN SI BASE UNITS Length — metre (m) Mass — kilogram (kg) Time — second (s) Electric current — ampere (A) Thermodynamic temperature — kelvin (K) Amount of substance — mole (mol) Luminous intensity — candela (cd) DEFINITIONS Metre: One metre is the distance travelled by light in vacuum in 1/299,792,458 of a second. Second: One second corresponds to 9,192,631,770 periods of the radiation associated with the specified transition between two hyperfine levels of the ground state of a caesium-133 atom. Kilogram: The kilogram is defined by fixing Planck’s constant at 6.62607015 × 10⁻³⁴ J s, equivalent to kg m² s⁻¹. Ampere: The ampere is defined by fixing the elementary charge at 1.602176634 × 10⁻¹⁹ C, where C = A s. Kelvin: The kelvin is defined by fixing Boltzmann’s constant at 1.380649 × 10⁻²³ J K⁻¹. Mole: One mole contains exactly 6.02214076 × 10²³ specified elementary entities. Candela: The candela is defined using monochromatic radiation of frequency 540 × 10¹² Hz, with its luminous efficacy fixed at 683 lm W⁻¹. ANGLE UNITS Plane angle is measured in radians (rad). Solid angle is measured in steradians (sr). Both are dimensionless derived quantities.