A physical quantity 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).
- P = 42.3 x 103 W = 42.3 kW
- P being the physical quantity for power;
- 42.3 x 103 being the numerical value which is split up into
- W being the symbol for the unit of power, the watt.
Usually, the symbols of 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).
- 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)
A quantity is called extensive
when its magnitude is additive for subsystems as there are the volume V
or the mass m
. In cases where the magnitude is independent of the extent of the system (e.g. temperature T
, pressure p
) the quantity is called intensive
. The word specific
is added to an extensive quantity in order to refer to the quantity divided by its mass (e.g. the specific volume v
). Similarly, the expression molar
before an extensive quantity means divided by amount of substance (e.g. molar volume Vm