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SI Units By Dr. Ahmed Mostafa Assist. Prof. of anesthesia & I.C.U

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The Systéme International d’Unités (SI) was adopted in 1960 after a meeting of the Conférences Générales des Poids et Mesures (CGPM) – an international convention of 50 member states including all the major industrialized nations.

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◙ The system defines seven fundamental (Basic) units that are dimensionally independent. ◙ The fundamental units can be algebraically combined to create derived units. ◙ The SI also defines prefixes for multiples and submultiples of SI units to avoid very large or small numbers and defines the correct usage of SI notation. The prefix attaches directly to the name or symbol of a unit.

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Fundamental SI units

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-Time (second, s): The duration of a given number of oscillations of the caesium-133 atom -Length (metre, m): The length of the path travelled by light in vacuum during a certain fraction of a second.

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Fundamental SI units -Mass (kilogram, kg): The mass of the international prototype of the kilogram held in Śevres, France. -Electric current (ampere, A): The current in two parallel conductors of infinite length and placed 1 metre apart in vacuum, which would produce between them a force of 2 ×10 -7 N.m -1

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Fundamental SI units -Thermodynamic temperature (kelvin, K): The kelvin is the fraction 1/ of the thermodynamic temperature of the triple point of water. The triple point of water is the unique temperature and pressure in a special water cell at which all three phases of water coexist (about 0.01 K above water’s freezing point). This can be measured accurately to about 1 m K. From this point, gas and radiation thermometers can be calibrated.

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Fundamental SI units -Amount of substance (mole, mol): The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in kilogram of carbon 12. -Luminous intensity (candela, cd): Luminous intensity in a given direction of a source that emits monochromatic light at a specific frequency.

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Derived SI units Derived units are those which may be expressed in terms of multiplication or division of fundamental units. Certain derived units have been named and given special symbols and may be further combined to express other derived units.

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Named derived SI units

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Un-named derived SI units

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N.B: Some everyday units are recognized by the system although they themselves are not true SI units. Examples include the litre (10 -3 m 3 ), the minute (60 s), and the bar (10 5 Pa). One litre is the volume occupied by 1 kg of water but was redefined in the 1960s as being equal to 1000 cm 3.

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Decimal multiples and fractions

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Advantages of SI units 1.Standardization: the use of one standard unit for each physical quantity. 2.Decimalization: decimal multiples and submultiples of any unit can be derived by adding one standard prefix to the unit. 3.Simplification.

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Dr. Ahmed Mostafa Thank you

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