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• Topic
• International System of Unit (SI)

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SI Definitions
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meter The meter is the length of the path
travelled by light in vacuum during a time
interval of 1/299 792 458 of a second.
Historical International Prototype Metre bar,
made of an alloy of platinum and iridium, that
was the standard from 1889 to 1960, then this
was replaced by the above- mentioned definition.
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SI Definitions
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kilogram The kilogram is the unit of mass it
is equal to the mass of the international
prototype of the kilogram. This is made of
platinum-iridium it is kept at the BIPM, in
France. Photograph _at_ BIPM Visit the page
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SI Definitions
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second The unit of time, it was defined
originally as the fraction 1/86 400 of the mean
solar day. In 1967 it was replaced by the
following definition The second is 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.
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SI Definitions
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ampere 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
meter of length.
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SI Definitions
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candela 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 (http//en.wikipedia.org/wiki/Steradian)
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SI Definitions
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kelvin The kelvin, unit of thermodynamic
temperature, is the fraction 1/273.16 of the
thermodynamic temperature of the triple point of
water. mole 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.
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Derived units
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Derived units are combinations of fundamental
units. Examples speed (v) distance/time units
m/s acceleration (a) velocity / time units
m/s/s m/s2 force (F) mass x acceleration
units kgm/s2 N (Newton) force x
distance units kgm2/s2 Nm J (Joule)
energy (E)
charge (Q) current x time units As C
(Coulomb)
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