01 Jan 2018

SI Units

The SI system ( Systeme International d'Unite) consists of 7 basic units.

This rationalised system of units was adopted internationally in 1960.

Unit SI name SI symbol
length metre m
mass kilogramme kg
time second s
electric current Ampere A
temperature difference Kelvin K
luminous intensity candela cd
quantity of substance mole mol

From these basic units useful derived units  have been defined.

Unit SI name SI symbol Relationship SI basic units
frequency hertz Hz   s-1
speed, velocity       m / s
plane angle radian rad   m x m-1 = 1
angular velocity     rad / s s-1
acceleration       m / s2
angular acceleration     rad / s2 s-2
area       m2
volume       m3
mass density       kg / m3
specific volume       m3 / kg
force newton N   kg x m x s-2
work, energy, heat joule J N x m m2 x kg x s-2
pressure, stress pascal Pa N / m2 m-1 x kg x s-2
power watt W J / s m2 x kg x s-3
electric potential difference, EMF volt V W / A m2 x kg x s-3 x A-1
electric resistance ohm Ω V / A m2 x kg x s-3 x A-2
electric conductance siemens S A / V m-2 x kg-1 x s3 x A2
current density       A / m2
electric charge coulomb C   s x A
capacitance farad F C / V m-2 x kg-1 x s4 x A2
magnetic flux weber Wb V x s m2 x kg x s-2 x A-1
magnetic flux density tesla T Wb / m2 kg x s-2 x A-1
magnetic field strength       A / m
inductance henry H Wb/ A m2 x kg x s-2 x A-2
Celsius temperature degree Celsius °C   K

Prefixes can be added to indicate multiples of the basic SI unit.

Multiplication factor Prefix SI symbol
1012 tera T
109 giga G
106 mega M
103 kilo k
102 hecto h
101 deca da
10-1 deci d
10-2 centi c
10-3 milli m
10-6 micro µ
10-9 nano n
10-12 pico p

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