isotermaalne

Isotermaalne is an adjective used to describe phenomena that occur at a constant temperature. The term is employed in thermodynamics and related disciplines to denote states, processes, or surfaces where the temperature does not change during the observation or operation.

In thermodynamics, an isothermal process is one in which the temperature of the system remains fixed (dT = 0). For an ideal gas, this implies that the internal energy U depends only on temperature, so ΔU = 0 and the heat added to the system equals the work done by it (Q = W). On a pressure–volume diagram, an isothermal process for an ideal gas follows the hyperbola PV = constant.

Isothermal conditions are used in both theoretical models and practical applications where temperature control is feasible. Examples include isothermal expansion or compression of gases in engines or reactors kept at a constant temperature, and the use of isotherms—lines of equal temperature—in charts and maps. In geology and geophysics, isothermals (isotherms) denote lines or surfaces of equal temperature within a material or region; in meteorology, isothermal layers describe zones of roughly uniform air temperature.

Isothermal processes are contrasted with adiabatic (no heat transfer), isobaric (constant pressure), and isochoric (constant volume) processes. Real systems may deviate from perfect isothermality due to heat transfer limitations and material properties, but the isothermal idealization remains a common simplifying assumption in analysis and design.

See also: isotherm, isothermal process, temperature distribution.