SI Units

For any measurement of a quantity such as distance, time, mass, force, etc. it is important to specify the units of the measurement. A distance of 3 is meaningless unless we indicate whether we mean 3 metres, 3 kilometres or 3 Megaparsecs.

The most widely used set of units is known variously as the Systéme Internationale, International System of Units or SI Units. These units are a modernised version of the metric system, and make use of 7 base units:

Measurement Name Symbol
Length Metre m
Mass Kilogram kg
Time Second s
Current Ampere A
Thermodynamic Temperature Kelvin K
Quantity of substance Mole mol
Luminous Intensity Candela cd

The SI units are complemented by two angular units:

Measurement Name Symbol
Plane angle radian rad
Solid angle steradian sr

It should be noted that two of the SI units are somewhat redundant. The mole is a measure of the number of atoms or molecules, so it is not really a unit. Candelas can be specified in terms of m, kg and s (1 Cd = 1/683 W sr-1= 1/683 kg m2 s-3 sr-1 at 5.40 × 1014 Hertz).

Commonly derived units include:

Measurement Name Symbol Derived Unit
Force Newton N m kg s-2
Pressure Pascal Pa N m-2
Energy Joule J N m
Power Watt W J s-1
Electric charge Coloumb C A s

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