Function of the state of a thermodynamic system whose change in any differential reversible process is equal to the heat absorbed by the system from its surroundings divided by the absolute temperature of the system. Also known as thermal charge.

Related Terms


A process in which the temperature of a system is reduced without any heat being exchanged between the system and its surroundings.


  1. The theorem that all Carnot engines operating between two given temperatures have the same efficiency, and no cyclic heat engine operating between two given temperatures is more efficient than a Carnot engine. 2. The theorem that any system has two properties, the thermodynamic temperature T and the entropy S, such that the amount of heat exchanged in an infinitesimal reversible process is given by d


The principle that for any system executing a cyclical process, the integral over the cycle of the infinitesimal amount of heat transferred to the system divided by its temperature is equal to or less than zero. Also known as Clausius theorem; inequality of Clausius.


A change in state of aggregation of a system accompanied by a discontinuous change in enthalpy, entropy, and volume at a single temperature and pressure.


A parameter devised to measure the air's cooling effect upon a human body; it is determined by the amount of heat required by a device to maintain the device at a constant temperature (usually 34 C); the entire system should be made to correspond, as closely as possible, to the external heat exchange mechanism of the human body.


The heat required to cause a change in state of a substance from solid to liquid (latent heat of fusion) or from liquid to vapor (latent heat of vaporization). These phase changes occur without change of temperature at the melting point and boiling point, respectively.


The transfer of heat or mass by large-scale fluid movements. When the process occurs, due to density and temperature differences, it is termed natural convection. When the process occurs due to external devices (such as fans), it is termed forced convection.


All warm bodies emit light (electromagnetic radiation, mostly infrared). When this radiation is absorbed or emitted by a body, heat is transferred and termed 'heat transfer by radiation'. Such heat transfer requires a line of sight (view factor) and is proportional to the fourth power of the absolute temperature difference between bodies and the emissivity of the bodies.


The sum of sensible heat and latent heat between an arbitrary datum point and the temperature and state under consideration.


A thermo-dynamic process in which no heat is extracted from or added to the system of the process.

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