Heat

HEAT. Direct sensation allows us to distinguish cold bodies from warm ones. The state of a body can be determined more precisely by using the changes that substances undergo under the action of heat: one can use the expansion of bodies depending on the action of heat, the change in their color, the excitation of an electric current, or a change in the state of matter. Thus, it is possible to quantitatively determine the thermal state of a body, its temperature (see Thermometer, Body temperature). The study of thermal changes in a body, combined with the measurement of temperature, allows for the determination of the amount of heat passing from one body to another. The branch of the study of heat that examines quantitative relationships during the heating of a body is called calorimetry (see). The heating of bodies can lead to a change in the state of substances, whereby during gradual heating, a solid substance first turns into a liquid, and the liquid turns into a gaseous state. These changes, associated with the absorption of a certain amount of heat, are not accompanied by a change in the temperature of the body near the melting, boiling, and evaporation points; the heat spent on the process of melting and evaporation (referred to a mass of 1 g) is called the latent heat of fusion and vaporization. The study of melting in solutions and alloys depending on temperature has found wide application in chemistry and constitutes a large branch of physical-chemical analysis. Heat, according to modern theoretical views, is a form of energy and therefore obeys the fundamental law of physics—the law of conservation of energy. The theory of thermal phenomena can be constructed on the basis of the law of conservation of energy and certain other principles (the second law of thermodynamics, Nernst's theorem, see Thermodynamics). We can also consider the theory of thermal phenomena by specializing our ideas about the nature of thermal energy and attributing certain motions to the atoms and molecules of a body (see Gases, Kinetic theory). Heat can be transferred from body to body by the transmission of molecular motions (by thermal conductivity). In bodies with mobile molecules (in liquids and gases), heat can be transferred together with the molecules themselves (convection). Finally, heat can be transferred in the form of radiant heat, and the theoretical study of radiation phenomena leads to the recognition of the divisibility of energy into elements of finite size, called quanta or photons (see Quantum theory).