Number density

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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

dV is a volume element.

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

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