### number density

(number of objects per volume)

**Number density** is a term for the number of objects
per unit volume.

*N*
*n* = —
*V*

for number density *n*, number of objects, *N* and volume *V*.
The term is often used for atoms or molecules, e.g., in a gas,
in contrast to other measures such as mass density.

Number density can also be used for larger objects, e.g., stars.

A volume may have "partial" number densities,
i.e., specific to one type of the objects in the volume.
An alternative measure for some purposes is **partial pressure**,
a calculation of the pressure contribution of one type of object,
e.g., calculated by the ideal gas law.
An example is **electron pressure**,
which the Saha equation can be adapted to use.

(*chemistry,measure*)
http://en.wikipedia.org/wiki/Number_density

**Referenced by:**

absorption coefficient

Boltzmann equation

column density

cooling function

luminosity function (LF)

Saha equation

Strömgren sphere

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