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SI Units

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International System of Base Units (The seven base units):

Time

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 caesium-133 atom.

Mass

The kilogram is defined as a mass equal to that of the International Prototype Kilogram - a platinum-iridium cylinder which in 1889  replaced the previous platinum protype matching the mass of 1 cubic decimetre of pure water at the temperature of its maximum density (4.0 C/39.2 F)).

Length

The metre was 1960 redefined as 1,650,763.73 wavelengths of the reddish-orange light emitted by the isotope krypton-86. The metre was again redefined in 1983 as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.

Temperature

Since 1960 the temperature scale is based on the triple point of water. The triple point of a substance is the temperature and pressure at which its solid, liquid, and gas forms are in equilibrium. The temperature of 273.16 K (kelvins) was assigned to this point. The freezing point of water was designated as 273.15 K, equalling exactly 0 on the Celsius temperature scale. The Celsius scale has been redefined in terms of Kelvin temperatures, so that now absolute zero, 0 K, is at -273.15 C. As a consequence the freezing and boiling points of water are at present determined as 0 C and 99.974 C respectively.

Ampere

The ampere was defined as the constant electric current that, flowing in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2 10-7 newton per metre of length.

Mole

In 1971 the mole was defined as the amount of substance of a system that contains as many elementary entities—which may be molecules, atoms, ions, and so on—as there are atoms in 0.012 kilogram of carbon-12. This number, known as Avogadro’s Number, is about 6.022 1023.

Light

In 1979 the candela was redefined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 1012 hertz (cycles per second) and that has a radiant intensity of (1/638) watt per steradian.

International System of Supplementary Units (Two supplementary units):

Radian

The radian is the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius.

Steradian

The steradian is the solid angle that, having its vertex at the centre of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere. The radian and the steradian are classed as derived units.

Derived Units (Nine common units derived from SI base units)

Area square

meter

m2

Volume

cubic meter

m3

Velocity

meter per second

m/s

Acceleration

meter per second squared

m/s2

Density

kilogram per cubic meter

kg/m3

Current density

ampere per square meter

A/m2

Magnetic field strength

ampere per meter

A/m

Specific volume

cubic meter per kilogram

m3/kg

Luminance

candela per square meter

cd/m2

Derived Units assigned special names (Units derived from SI base and supplementary units):

Frequency

hertz

Hz

1/s

Force

newton

N

kgm/s2

Pressure/stress

pascal

Pa

N/m2

Energy/work/quantity of heat

joule

J

Nm

Power

watt

W

J/s

Quantity of electricity

coulomb

C

As

Electric potential

volt

V

W/A

Capacitance

farad

F

G/V

Electric resistance

ohm

W

V/A

Conductance

siemens

S

A/V

Magnetic flux

weber

Wb

Vs

Magnetic flux density

tesla

T

Wb/m2

Inductance

henry

H

Wb/A

Luminous flux

lumen

lm

cdsr

Illuminance

lux

lx

lm/m2

Activity of radionuclides

becquerel

Bq

1/s

Absorbed dose

gray

Gy

J/kg

Dose equivalent

sievert

Sv

J/kg

Prefix

Factor Name Symbol
1024 yotta Y
1021 zetta Z
1018 exa E
1015 peta P
1012 tera T
109 giga G
106 mega M
103 kilo k
102 hecto h
10 deca da
10-1 deci d
10-2 centi c
10-3 milli m
10-6 micro
10-9 nano n
10-12 pico p
10-15 femto f
10-18 atto a
10-21 zepto z
10-24 yocto y

 

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