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The number density (symbol: n or ρ N) is an intensive quantity used to describe the degree of concentration of countable objects (particles, molecules, phonons, cells, galaxies, etc.) in physical space: three-dimensional volumetric number density, two-dimensional areal number density, or one-dimensional linear number density.
The density of air or atmospheric density, denoted ρ, [1] is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. Air density, like air pressure, decreases with increasing altitude.
The number density of air is the number of molecules or atoms per unit volume: n air = A a v n V , {\displaystyle n_{\text{air}}={\frac {A_{av}n}{V}},} and when plugged into the real gas law, the number density of air is found by using pressure, temperature and the real gas constant:
The density of air at sea level is about 1.2 kg/m 3 (1.2 g/L, 0.0012 g/cm 3). Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of the ideal gas law). Atmospheric density decreases as the altitude increases.
at each geopotential altitude, where g is the standard acceleration of gravity, and R specific is the specific gas constant for dry air (287.0528J⋅kg −1 ⋅K −1). The solution is given by the barometric formula. Air density must be calculated in order to solve for the pressure, and is used in calculating dynamic pressure for moving vehicles.
Dimensionless numbers (or characteristic numbers) have an important role in analyzing the behavior of fluids and their flow as well as in other transport phenomena. [1] They include the Reynolds and the Mach numbers, which describe as ratios the relative magnitude of fluid and physical system characteristics, such as density, viscosity, speed of sound, and flow speed.
As in the previous section, the number density is defined as the number of molecules per (extensive) volume, or = /. The collision cross section per volume or collision cross section density is n σ {\displaystyle n\sigma } , and it is related to the mean free path ℓ {\displaystyle \ell } by ℓ = 1 n σ 2 {\displaystyle \ell ={\frac {1}{n ...
is the specific gas constant [L 2 T −2 θ −1] (287.05 J/(kg K) for air), is the density [M 1 L −3]. If the temperature is increased, but the volume kept constant, then the Knudsen number (and the mean free path) doesn't change (for an ideal gas). In this case, the density stays the same.