Isothermic

Isothermic is an adjective used to describe a process or condition that occurs at a constant temperature. In thermodynamics, isothermal processes are idealizations in which the system's temperature remains fixed while it exchanges heat with its surroundings. The term isothermic is often used interchangeably with isothermal, though some texts distinguish isothermal as the standard term.

For an ideal gas, an isothermal process implies that the internal energy depends only on temperature; since temperature is constant, U is constant. Consequently, all heat added to the system goes into doing work, and Q = W. The state variables satisfy PV = nRT with T constant, so pressure is inversely proportional to volume.

In real substances, internal energy can depend on both temperature and volume, so strictly isothermal processes require active heat transfer to offset any changes in internal energy and keep temperature constant. Isothermal conditions can occur during a phase change, where temperature remains at the phase-transition value while latent heat is absorbed or released, and pressure adjusts to maintain equilibrium along the phase boundary.

Isothermal processes are common in theoretical analyses of thermodynamic cycles and in contexts where temperature is regulated, such as chemical reactions conducted at a fixed temperature or systems connected to a heat reservoir. They are often contrasted with adiabatic (no heat transfer) and isobaric (constant pressure) processes.

Related concepts include isotherms, lines on a map representing constant temperature, which are used in meteorology and geology to visualize spatial temperature variations.