Number density

Number density in physics is a subtle generalization of the concept of density and refers to the number of objects (particles, sound waves, galaxies, etc.) per volume.

Number density can be written as

$n = \frac{N}{V}$

where

n is the number density,

N is the total number of entities

V is the volume.

Conversely, the total number of entities can be written as

$\iiint_V n\;dV = N$

where

d V

is a volume element.

The number density of solutes in a solvent is called concentration.