Kompressiblen

Kompressiblen is a term used in physics, particularly in German-language literature, to describe substances or media that exhibit changes in density when subjected to pressure. In English-language usage, the analogous concept is compressibility, with the reciprocal of compressibility known as the bulk modulus.

Definition and scope: A kompressibler Stoff or medium has a finite compressibility: κ = −(1/V)(∂V/∂P)_T. The smaller the κ, the less compressible the material. Two related quantities are isothermal compressibility κ_T and adiabatic compressibility κ_S, reflecting changes under constant temperature or constant entropy.

In gases, changes in density with pressure are significant; for ideal gas, p = ρ R T / M. Liquids have smaller κ, solids even smaller; under extreme pressures all states may increase in compressibility.

Dynamics: In fluid dynamics, compressible flow accounts for density variations in time and space; speed of sound c = sqrt( (∂p/∂ρ)_s ). The Mach number M = v/c determines whether compressibility effects are important; M << 1 allows incompressible approximations, M ~ 1 or greater requires full treatment and can generate shocks.

Applications include aerospace, acoustics, meteorology, geophysics and materials science. Examples: Air and steam are highly compressible; water is relatively incompressible under ordinary conditions; some metals show measurable compressibility at high pressures.

Etymology: from Latin compressus; in German, Kompressibilität is the property, and kompressiblen appears as the adjective or plural in context.