Heat is energy transferred from one system to another by thermal interaction. In contrast to work, heat is always accompanied by a transfer of entropy. Heat flow is characteristic of macroscopic objects and systems, but its origin and properties can be understood in terms of their microscopic constituents.

Heat flow from a high to a low temperature body occurs spontaneously. This flow of energy can be harnessed and converted into useful work by means of a heat engine. The second law of thermodynamics prohibits heat flow from a low to a high temperature body, but with the aid of a heat pump external work can be used to transport energy from low to the high temperature.

The SI unit of heat is the Joule. Heat can be measured with a calorimeter, or determined indirectly by calculations based on other quantities, relying for instance on the first law of thermodynamics.

Heat transfer can occur in a variety of ways: by conduction, radiation, convection, net mass transfer, friction or viscosity, and by chemical dissipation.

In physics, especially in calorimetry, and in meteorology, the concepts of latent heat and of sensible heat are used. Latent heat is associated with phase changes, while sensible heat is associated with temperature change.

Because it is by definition a transfer of energy, heat is always associated with a process of some kind, and "heat" is used interchangeably with "heat flow" and "heat transfer". In common usage, the noun heat has a broader meaning, and can refer to temperature or to the sensation felt when touching or being close to a high-temperature object.

Source: http://en.wikipedia.org/w/index.php?title=Heat&oldid=493156204

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