Physical quantity

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).

Example:

P = 42.3 x 10³ W = 42.3 kW

With

P being the physical quantity for power;
42.3 x 10³ being the numerical value which is split up into
- 42.3 and
- k, the SI prefix representing 10³
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).

Examples:

E_p for potential energy (note: p is upright)
c_p 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 = V/m). Similarly, the expression molar before an extensive quantity means divided by amount of substance (e.g. molar volume V_m = V/n).