physical quantity

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