Kompressible

Kompressible, in physics, describes a property of a substance whose density changes with pressure. A compressible fluid or solid will alter its volume in response to applied forces, whereas incompressible models assume constant density. The English term is compressible; in several languages, including German and Dutch, cognate forms such as kompressibel are used.

Key quantities: isothermal compressibility β_T = - (1/V) (∂V/∂P)_T and adiabatic compressibility β_S = - (1/V) (∂V/∂P)_S. The bulk modulus K_T = - V (∂P/∂V)_T equals 1/β_T for a given temperature. For adiabatic changes, K_S = - V (∂P/∂V)_S and β_S = 1/K_S. The speed of sound satisfies c^2 = (∂P/∂ρ)_s = K_S/ρ = 1/(ρ β_S).

Practical implications: gases are highly compressible compared with liquids, which are moderately compressible, and solids, which are very incompressible. Compressibility influences acoustic propagation, shock formation, and fluid-structure interactions. In geophysics, rock and magma compressibility affect wave speeds; in engineering, bulk modulus is a primary measure of material stiffness.

Measurement and notes: compressibility is often inferred from ultrasound experiments or from P–V isotherms. In anisotropic materials, compressibility can vary with direction. The term "kompressible" reflects the same concept in languages that spell with k; the English standard spelling remains compressible.