adiabatic

Adiabatic refers to processes or conditions in which no heat is exchanged with the environment (Q = 0). In thermodynamics, an adiabatic process is idealized as insulated so that any change in internal energy is due to work done on or by the system. If the process is reversible, it is also isentropic, meaning entropy remains constant. For an ideal gas, a reversible adiabatic process satisfies PV^γ = constant, where γ = Cp/Cv. Other relations include TV^{γ-1} = constant and TP^{1−γ} = constant; these enable connecting pressure, volume, and temperature during the process. In contrast, an adiabatic process can be irreversible, in which case entropy changes and the PV^γ relation need not hold exactly, though Q remains zero.

In meteorology and atmospheric science, adiabatic processes describe air parcel movements without heat exchange with surroundings, leading to adiabatic cooling as it rises and warming as it descends. The dry adiabatic lapse rate is about 9.8°C per kilometer; moist lifting releases latent heat, reducing the lapse rate.

In quantum mechanics, the adiabatic theorem states that a system with a slowly varying Hamiltonian will remain in its instantaneous eigenstate if energy gaps are nonzero, up to a phase factor. This underpins adiabatic processes in quantum computation and control.

The term derives from Greek adiasmos, meaning 'not passing through', and has analogous use in fields such as optics and acoustics where rapid exchanges are minimized.