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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

where

P represents the physical quantity for 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)

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:

• 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 (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 Vm = V/n).