perunitarea

Per unit area expresses a quantity distributed over a two-dimensional region by normalizing to its area. In science and engineering, this is commonly called areal density, surface density, or areal concentration. Presenting a quantity as a density allows comparison across regions of different size and clarifies how much is present per unit area.

Mathematically, if a quantity Q is defined over a planar region of area A, its areal density is sigma = Q/A. For a position-dependent quantity f(x,y), the total over region R is ∫∫_R f dA and the average density is (1/Area(R)) ∫∫_R f dA. In discrete form, sigma = (sum_i q_i)/A.

Common units include kg/m^2 for mass per area, C/m^2 for charge per area, J/m^2 for energy per area, and individuals/m^2 for population density. Radiative quantities use irradiance, measured in W/m^2, as energy flux per area.

Applications span physics, engineering, ecology, and materials science. Areal density describes thin films and coatings, surface charge on conductors, rainfall or light flux onto surfaces, and spatial distribution of organisms.

Terminology varies; the phrase is usually written as "per unit area" in prose, while per-area metrics are often called areal or surface density. The concept is a specific form of density, contrasted with quantities defined per length or per volume.