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SI Base Units
SI base unit
The
SI
system of units defines seven SI base units:
fundamental
physical units defined by an
operational definition.
All other physical units can be derived from these base
units: these are known as
SI derived units. Derivation is by
dimensional analysis. Use
SI prefixes to abbreviate long numbers.
Unit:
metre (m)
One metre is defined as the
distance
light travels in a
vacuum in 1/299792458 second. This standard was adopted in 1983, when the
speed of light in vacuum was defined to be precisely 299792458 m/s.
Unit:
kilogram (kg)
One kilogram is defined to be the mass of a specific cylinder
of platinum-iridium alloy, kept at the
International Bureau of Weights and Measures (near Paris). There is an
ongoing effort to introduce a definition by way of fundamental or atomic
constants.
Unit:
second (s)
One second is defined as the time required for 9192631770
cycles of the transition between the two
hyperfine levels of the ground state of
caesium 133. This definition was adopted in 1967.
Unit:
ampere (A)
The ampere is that constant current which, if maintained in
two straight parallel conductors of infinite length, of negligible circular
cross-section, and placed 1 meter apart in vacuum, would produce between these
conductors a force equal to 2 x 10-7 newton per metre of length.
Unit:
Kelvin (K)
The Kelvin, unit of thermodynamic temperature, is the
fraction 1/273.16 of the thermodynamic temperature of the
triple point of water. Named after
Lord Kelvin.
Unit:
mole (mol)
-
The mole is the amount of substance of a system which
contains as many elementary entities as there are atoms in 0.012 kilogram of
carbon 12; its symbol is "mol".
-
When the mole is used, the elementary entities must be
specified and may be atoms, molecules, ions, electrons, other particles, or
specified groups of such particles.
Unit:
candela (cd)
The candela is the luminous intensity, in a given direction,
of a source that emits monochromatic radiation of frequency 540 x 1012
hertz and that has a radiant intensity in that direction of 1/683 watt per
steradian.
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