Base quantity

Base Quantity is a term used in the field of physics and measurement to denote a set of independent dimensions or quantities in terms of which all the other dimensions or quantities may be expressed. The seven base quantities are length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.

Definition

A base quantity is a physical quantity in a subset of a given system of quantities that is chosen by convention, where no quantity in the set can be expressed in terms of the others. The International System of Units (SI) defines seven base quantities. These quantities are defined in terms of a set of seven base units.

Base Quantities in the International System of Units

The International System of Units (SI) defines seven base quantities:

• Length: The metre (m) is the base unit of length in SI. It is defined by taking the fixed numerical value of the speed of light in vacuum to be 299,792,458 when expressed in the unit m·s⁻¹.
• Mass: The kilogram (kg) is the base unit of mass in SI. It is defined by taking the fixed numerical value of the Planck constant to be 6.62607015×10⁻³⁴ when expressed in the unit J·s.
• Time: The second (s) is the base unit of time in SI. It is defined by taking the fixed numerical value of the caesium frequency, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9,192,631,770 when expressed in the unit Hz.
• Electric current: The ampere (A) is the base unit of electric current in SI. It is defined by taking the fixed numerical value of the elementary charge to be 1.602176634×10⁻¹⁹ when expressed in the unit C.
• Thermodynamic temperature: The kelvin (K) is the base unit of thermodynamic temperature in SI. It is defined by taking the fixed numerical value of the Boltzmann constant to be 1.380649×10⁻²³ when expressed in the unit J·K⁻¹.
• Amount of substance: The mole (mol) is the base unit of amount of substance in SI. It is defined by taking the fixed numerical value of the Avogadro constant to be 6.02214076×10²³ when expressed in the unit mol⁻¹.
• Luminous intensity: The candela (cd) is the base unit of luminous intensity in SI. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10¹² Hz to be 683 when expressed in the unit lm·W⁻¹.

See also

• Derived quantity
• International System of Units
• Physical quantity