Base unit

Base unit is a fundamental measurement in a particular system of units to which all other units can be related. The International System of Units (SI) defines seven base units that are used to describe everything from the amount of substance to the intensity of light.

Definition

A base unit in a system of measurement is a defined unit that cannot be expressed as a combination of other units. These units are used as the building blocks for other units within the system. For example, in the SI system, the base unit of length is the metre.

International System of Units

The International System of Units (SI) is the most widely used system of measurement. It consists of seven base units: the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity.

Metre

The metre is the base unit of length in the SI system. It is defined as the distance travelled by light in a vacuum in 1/299,792,458 of a second.

Kilogram

The kilogram is the base unit of mass in the SI system. It is defined by the Planck constant, which is exactly 6.62607015×10−34 joule-seconds.

Second

The second is the base unit of time in the SI system. It is 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 caesium-133 atom.

Ampere

The ampere is the base unit of electric current in the SI system. It is defined by the elementary charge, which is exactly 1.602176634×10−19 coulombs.

Kelvin

The kelvin is the base unit of thermodynamic temperature in the SI system. It is defined by the Boltzmann constant, which is exactly 1.380649×10−23 joule per kelvin.

Mole

The mole is the base unit of amount of substance in the SI system. It is defined by the Avogadro constant, which is exactly 6.02214076×10^23 entities per mole.

Candela

The candela is the base unit of luminous intensity in the SI system. It is defined by the luminous efficacy of monochromatic radiation of frequency 540×10^12 hertz, which is exactly 683 lumens per watt.

See also

• Derived unit
• Physical constant
• Dimensional analysis