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SI Units
SI is the abbreviation of Système International d' Unités. The SI units for mass, length, time and force are kilogram, meter, second and newton respectively. The unit of length is meter, m, defined as
1 650 763.73 wavelengths in vacuum of the radiation corresponding to the orange-red line of the spectrum of Krypton-86. The unit of time is second, s. The second is defined as the duration of
9 192 631 770 cycles of the radiation associated with a specified transition of the Cesium 133 atom. The unit of mass is kilogram, kg. It is equal to the mass of a particular cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures. The amount of substance can also be expressed in terms of the mole (mol). One kilomole of a substance is the amount of that substance in kilograms numerically equal to its molecular weight. The number of kilomoles of a substance, n , is obtained by dividing the mass (m) in kilograms by the moleculare weight (M), in kg/ kmol.
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The unit for temperature is Kelvin, K . One K is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. Quite often the Celsius, oC , is used to express the temperature of a substance.
The SI unit of force, called the newton, N is a secondary unit. The, N , is the force required to accelerate a mass of 1 kilogram at the rate of 1 meter per (second)2 .
| 1 N = (1kg) (1m/s2 )= 1kg m/s2 |
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The smaller or bigger quantities are expressed using the following prefixes
Factor |
Prefix |
Symbol |
Factor |
Prefix |
Symbol |
1012 |
tera |
T |
10-2 |
centi |
c |
109 |
giga |
G |
10-3 |
milli |
m |
106 |
mega |
M |
10-6 |
micro |
μ |
103 |
kilo |
k |
10-9 |
nano |
n |
102 |
hecto |
h |
10-12 |
pico |
p |
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