Number density

Number density is the number of specified particles or objects contained in a unit volume. Its value depends on which entities are counted and the volume used.

Volume: The amount of three-dimensional space occupied by a region or object. Number density divides a count by the volume containing the counted entities.
Ideal gas law: The equation relating pressure, volume, temperature, and amount of an ideal gas. For a gas, pressure can be written as number density times Boltzmann's constant and temperature.
Phase-space distribution: A function describing how a system's particles are distributed over position and momentum. Integrating over momentum yields a spatial particle density in kinetic descriptions.
Mass concentration: The mass of a substance per unit volume of a mixture or solution. It measures mass per volume, not the number of specified entities per volume.
Counting measure: A measure that assigns each object in a set a total count. The numerator must specify which entities count as one object.
Plasma frequency: The natural oscillation frequency of charged particles in a plasma. Its value depends on the electron number density.
Boltzmann equation: An equation describing the evolution of a particle distribution through motion and collisions. Its distribution function encodes local particle number density.
Molarity: The amount of solute in moles per unit volume of solution. It uses moles as the counted quantity and is commonly used for solutions.
Dimensional analysis: The use of physical dimensions to check equations and characterize quantities. It identifies number density as an inverse-volume quantity.
Debye length: The characteristic distance over which mobile charges screen an electric field in a plasma or electrolyte. The number density of charged particles helps set the screening distance.

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