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A thermal transfer, more commonly called heat, is one of the modes of heat energy exchange between two systems. This is a fundamental notion of thermodynamics. Unlike work, heat is a disorderly microscopic energy transfer. There are three types of heat transfer, which can coexist:

  • conduction, due to the gradual diffusion of thermal agitation in the material;
  • convection, thermal transfer that accompanies the macroscopic movements of matter;
  • radiation, which corresponds to the propagation of photons.

The quantity of heat Q, is the quantity of energy exchanged by these three types of transfers, it is expressed in joule (J). By convention, Q > 0 if the system receives energy. Thermodynamics is based on the concept of heat in the formulation of the first and second principles of thermodynamics.

Sun and Earth (The Sun and Earth form an ongoing example of a heating process. Some of the Sun’s thermal radiation strikes and heats the Earth. Compared to the Sun, Earth has a much lower temperature and so sends far less thermal radiation back to the Sun. The heat of this process can be quantified by the net amount, and direction (Sun to Earth), of energy it transferred in a given period of time. Image credit: NASA Ames/JPL-Caltech/T. Pyle)

The meaning of the word heat in everyday language often causes ambiguities and confusion, especially with temperature. While it is true that spontaneous heat transfer occurs from the higher temperature regions to the lower temperature regions, it is nevertheless possible to carry out a heat transfer from a cold body to the hot body, using of a thermal machine like a refrigerator. Moreover, during a change of state, a pure body does not change temperature while it exchanges energy in the form of heat.

The simplest example of a situation involving a heat transfer is that of two bodies in contact having different temperatures. The hottest body yields energy to the coldest body by conduction; its temperature decreases, the disorder, the thermal agitation, decreases. In return, the temperature of the cold body increases, the thermal agitation increases within it.

History and evolution of terminology

Heat, in everyday language, is often confused with the notion of temperature. Although very different from a scientific point of view, the two notions are still related to each other and the history of the genesis of thermodynamics has sometimes led to this confusion. Expressions such as “hot water” could lead to the mistaken belief that heat is a property of the system when it is a transfer of energy. Also, it is incorrect to say “the water loses heat” when it cools. The expression “heat transfer” is a pleonasm that is very widespread.

Until the eighteenth century, scientists thought that heat consisted of a fluid that had been called phlogiston (theory of phlogiston).

In the nineteenth century, heat was assimilated to a fluid: caloric. The progress and success of calorimetry imposed this theory until the middle of the nineteenth century. This concept is, for example, taken up by Sadi Carnot: a heat engine can only work if the heat flows from a body whose temperature is higher to a body whose temperature is lower; reasoning corresponding to an analogy with a hydraulic machine that derives its energy from the passage of water from a high altitude reservoir to a lower altitude reservoir.

It is only with the advent of statistical thermodynamics that heat will be defined as a transfer of the thermal agitation of particles at the microscopic level. A system whose particles are statistically more agitated will have an equilibrium temperature, defined at the macroscopic scale, higher. Temperature is therefore a macroscopic quantity which is the statistical reflection of kinetic energies of particles at the microscopic scale. During random shocks, the most agitated particles transmit their kinetic energies to the least agitated particles. The balance of these transfers of microscopic kinetic energies corresponds to the heat exchanged between systems consisting of particles whose average thermal agitation is different.

Temperature is an intensive state function used to describe the equilibrium state of a system whereas heat is a transfer of thermal agitation comparable to a quantity of energy, associated with the evolution of a system between two distinct or identical states if the transformation is cyclic.

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