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Definitions of the Seven Basic S I Units

metre [m]
The metre is the basic unit of length. It is the distance light travels, in a vacuum, in
1/299792458th of a second.

kilogram [kg]
The kilogram is the basic unit of mass. It is the mass of an international prototype in
the form of a platinum-iridium cylinder kept at Sevres in France. It is now the only
basic unit still defined in terms of a material object, and also the only one with a
prefix[kilo] already in place.

second [s]
The second is the basic unit of time. It is the length of time taken for 9192631770
periods of vibration of the caesium-133 atom to occur.

ampere [A]
The ampere is the basic unit of electric current. It is that current which produces a
specified force between two parallel wires which are 1 metre apart in a vacuum.It is
named after the French physicist Andre Ampere (1775-1836).

kelvin [K]
The kelvin is the basic unit of temperature. It is 1/273.16th of the thermodynamic
temperature of the triple point of water. It is named after the Scottish mathematician
and physicist William Thomson 1st Lord Kelvin (1824-1907).

mole [mol]
The mole is the basic unit of substance. It is the amount of substance that contains as
many elementary units as there are atoms in 0.012 kg of carbon-12.

candela [cd]
The candela is the basic unit of luminous intensity. It is the intensity of a source of
light of a specified frequency, which gives a specified amount of power in a given
direction.




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