Understanding SI Units: Base, Supplementary, and Derived Units

Sunday, 21st November 2021 | SEO Content for Google

Welcome to a detailed exploration of the International System of Units (SI) and its various components. This comprehensive article will delve into the intricacies of base units, supplementary units, and derived units, providing clarity on the SI system as it stands and the proposed redefinition that has sparked global attention.

Introduction to the Base Units

The International System of Units (SI) is the foundation of modern measurement and is utilized globally. It consists of seven base units, defined by seven fundamental physical quantities. These base units are:

Meter (m): Unit of length, defined as the distance traveled by light in a vacuum in 1/299792458 of a second. Kilogram (kg): Unit of mass, originally defined by a physical prototype but now redefined in terms of fundamental constants. Second (s): Unit of time, defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. Ampère (A): Unit of electric current, defined by the force between two wires carrying a current of one ampere. Kelvin (K): Unit of thermodynamic temperature, defined by the Boltzmann constant and the thermodynamic temperature of the triple point of water. Mole (mol): Unit of amount of substance, defined by the number of carbon-12 atoms in 0.012 kilograms. Candela (cd): Unit of luminous intensity, defined by the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

The Proposed Redefinition of SI Units

On 16 November 2018, the Conférence générale des poids et mesures (CGPM) voted on proposals to redefine the SI base units based on physical constants. The implementation of these new definitions was set for May 20, 2019. This change primarily affects the definitions of the second, the kilogram, and the ampere.

Redefined Definitions

Second (s): The second is now defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. This definition is based on the BIPM (Bureau International des Poids et Mesures) and is consistent with the current definition.

Kilogram (kg): The kilogram, the last base unit to be defined physically, is now based on the Planck constant. The precise value of the Planck constant is now 6.62607015 × 10^-34 joule seconds. This definition removes the need for the International Prototype Kilogram (le grand K).

Ampere (A): The numerical value of the elementary charge is now 1.602176634 × 10^-19 coulombs. The ampere is defined as the constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one meter apart in vacuum, would produce between these conductors a force equal to 2 × 10^-7 newton per meter of length.

Supplementary Units

Supplementary units in the SI are two: the radian (rad) and the steradian (sr). These are dimensionless units used to measure plane and solid angles. The radian measures the plane angle at the center of a circle that intercepts an arc equal in length to the radius of the circle. The steradian measures the solid angle subtended at the center of a sphere by a portion of the surface of the sphere, with an area equal to the square of the radius of the sphere.

Derived Units

Derived units are formed by combining base units in various ways. These units have specific names and are used to measure quantities that can be expressed as products or quotients of the base units. Some examples include:

Meter per second (m/s): Unit of velocity, indicating the distance traveled per unit of time. Meter per second squared (m/s2): Unit of acceleration, indicating the change in velocity per unit of time. Newton (N): Unit of force, defined as one kg m/s2 by the relation 1 N 1 kg m/s2.

A table of all SI derived units and their definitions in terms of base units can be found in the BIPM’s brochure on the International System of Units (Le Système International d’Unites).

Conclusion

The International System of Units (SI) plays a crucial role in scientific and technical communications. With recent redefinitions, its base and derived units are now more precise and based on fundamental constants, ensuring universal consistency and accuracy in measurements.