A physical quantity is either a quantity within physics that can be measured (e.g. mass, volume, etc.), or it is the result of measurement and 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 letter of the Latin or Greek alphabet, printed in italic. Both lower and capital letters are used. Often, the symbols are modified by subscripts or superscripts. If these sub- or superscripts are themselves symbols for physical quantities or numbers, they are printed in italic. Other sub- and superscripts are printed upright (roman).
Examples
- Ep for potential energy (note: p is upright)
- cp for heat capacity at constant pressure (note: p represents the physical quantity of pressure and is therefore printed italic)
Extensive vs Intensive
A quantity is called:
- extensive when its magnitude is additive for subsystems (e.g. volume V or the mass m)
- intensive where the magnitude is independent of the extent of the system (e.g. temperature T, pressure p)
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
- the specific volume v = V/m
- the molar volume Vm = V/n
See also