Heissystem

Heissystem is a thermodynamic term used primarily in German-language literature to denote a system whose temperature exceeds that of its surroundings or of a reference reservoir. The defining characteristic is a net flow of heat from the hot system to the cooler environment when the two are brought into contact, until thermal equilibrium is reached. The concept is relative; a system can be considered hot only with respect to a chosen surroundings or reservoir, and its temperature is described by state variables such as internal energy, pressure, and volume.

In the framework of the first law of thermodynamics, the change in the system’s internal energy dU is equal to the heat added δQ minus the work done δW. For a hot system releasing heat to the surroundings, δQ is negative for the system, and the process can be accompanied by work if the system expands or contracts. The second law states that the total entropy of system plus surroundings does not decrease; hot-to-cold heat transfer increases the entropy of the universe, and reversible cases occur only in idealized limits.

Examples include a cup of hot coffee in a room, a heated metal rod cooling to room temperature, or a high-temperature gas in contact with a cooler reservoir. In engineering, distinguishing hot and cold systems helps analyze heat transfer, thermal equilibration, and the operation of heat engines and refrigerators. The term is mainly used descriptively and is understood relative to a defined ambient or reference state.