Physical quantity

From Exampleproblems

A physical quantity is either a quantity within physics that can be measured (e.g. mass, volume, etc.), or a result of measurement, and is usually expressed as the product of a numerical value and a physical unit (whereby SI units are usually preferred).

Contents 1 Example 2 Subscripted variables 2.1 Examples 3 Extensive vs. Intensive 4 Prefixes 4.1 Examples 5 See also

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 lower case or capital letter of the Latin or Greek alphabet. Often, the symbols are modified by subscripts or superscripts.

Examples

• E_p - potential energy
• c_p - heat capacity at constant pressure

Extensive vs. Intensive

A quantity is called:

• extensive when its magnitude is additive for subsystems (e.g. volume, V, or mass, m)
• intensive when the magnitude is independent of the extent of the system (e.g. temperature, T, or pressure, p)

The following variables are neither extensive nor intensive:

• Angular momentum
• Area
• Force
• Length
• Time

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

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

See also

• International standard ISO 31: Quantities and units
• List of physical quantities
• Physical unit
• SIcs:Veličina

fr:Quantité physique ko:물리량 hu:Fizikai_mennyiség ja:物理量 pl:Wielkość fizyczna sl:fizikalna količina sv:Storhet fi:Suure vi:Đại lượng vật lý zh:物理量