Number density ( $n$ )

The number density is the number of particles per unit volume. For a homogeneous species with

g

internal states and phase-space distribution

f (p)

in natural units,

n = g \int \frac{d ^{3} p}{( 2 π ) ^{3}} f (p) .

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Bacterial density ( $B (x, t)$ )

Bacterial density is the number of bacteria per unit length, area or volume in a spatial model. It is a field variable, distinct from the organisms themselves.

Column density ( $N = \int n d ℓ$ )

Column density is the number density integrated along a line of sight. Multiplication by an absorption cross-section gives an optical depth. A small column can result from a small local density, a small path length, or a small fraction of atoms in the absorbing state, so density inference requires these quantities to be constrained.

Electron number density ( $n_{e}$ )

Electron number density is the number of free electrons per unit volume. In fully ionized stellar matter,

n_{e} = ρ / (μ_{e} m_{u})

, with mass density

ρ

and mean molecular weight per electron

μ_{e}

.

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Number density is a physical quantity that describes the concentration of a number of particles, such as atoms, molecules, or other entities, in a given volume of space. It is typically represented by the symbol \( n \) and is measured in units of particles per unit volume, such as particles per cubic meter (m⁻³) or particles per cubic centimeter (cm⁻³).

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