Physical quantity - Definition and Overview

A physical quantity is either a quantity within Physics that can be measured (e.g. mass, volume, etc.), or it 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 of 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)

Subscripted variables

Usually, the symbols for 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)

Extensive vs Intensive

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)

Prefixes

Some extensive physical quantities may be prefixed to qualify the meaning:

• specific is added to refer to the quantity divided by its mass
• molar is added to refer to the quantity divided by amount of substance

Examples

• the specific volume v = V/m
• the molar volume V_m = V/n

See also

• List of physical quantities
• Physical unit
• SI