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;\n* 42.3 x 10³ being the numerical value which is split up into\n** 42.3 and \n** k, the SI prefix kilo representing 10³\n* 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:\n* E_p for potential energy (note: p is upright)\n* 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:\n* extensive when its magnitude is additive for subsystems (e.g. volume V or the mass m)\n* intensive where the magnitude is independent of the extent of the system (e.g. temperature T, pressure p) The prefix:\n* 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)\n* molar is added to an extensive quantity to mean divided by amount of substance (e.g. molar volume V_m = V/n). See also: \n* List of physical quantities\n* SI \n\n