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 kilo 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 (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)

The prefix:

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)
molar is added to an extensive quantity to mean divided by amount of substance (e.g. molar volume V_m = V/n).