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A physical quantity is either a quantity within physics that can be measured (e.g. mass, volume, etc.), or a result of measurement, and is usually expressed as the product of a numerical value and a physical unit (whereby SI units are usually preferred).
Example
- P = 42.3 x 103 W = 42.3 kW
where
- P represents the physical quantity of power
- 42.3 x 103 is the numerical value
- k is the SI prefix kilo representing 103
- W is the symbol for the unit of power, the watt
- kW is the kilowatt (= 103W)
Subscripted variables
Usually, the symbols for physical quantities are chosen to be a single lower case or capital letter of the Latin or Greek alphabet. Often, the symbols are modified by subscripts or superscripts.
Examples
- Ep - potential energy
- cp - heat capacity at constant pressure
Extensive vs. Intensive
A quantity is called:
- extensive when its magnitude is additive for subsystems (e.g. volume, V, or mass, m)
- intensive when the magnitude is independent of the extent of the system (e.g. temperature, T, or pressure, p)
The following variables are neither extensive nor intensive:
Prefixes
Some extensive physical quantities may be prefixed to qualify the meaning:
- specific is added to refer to the quantity divided by its mass
- molar is added to refer to the quantity divided by amount of substance
Examples
- specific volume v = V/m
- molar volume Vm = V/n
See also
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