Standing on the table on a hot kettle after a while cools down. The sun's rays in summer significantly heat the surface of the Earth. Metal handle of the pan, standing on a gas stove, very hot. All examples of heat transfer. The exchange of internal energy between bodies and the environment or between parts of the body without doing mechanical work is called heat transfer.

The exchange of energy by heat exchange caused by many acts of individual interactions between molecules, in other words, many microprocesso. For example, cooling of hot water in the air due to the energy exchange between molecules of water and air when they collide. The heated air and the cooling water is due to the fact, when most of these collisions the water molecules lose energy, as air molecules get her. However, individual collisions of air molecules can lose energy, and water molecules — buy, as some molecules of energy can significantly differ from its average value. Such cases will occur more often, the closer will be the temperature of water and air. And by equality of their temperatures, the cases of increasing and decreasing energy of air molecules in collisions with water molecules are equally probable and no energy transfer from water to air or Vice versa generally will not occur.

 Thus, during the heat exchange of the majority of molecular interactions contribute to the energy transfer from the body with higher temperature to body with lower temperature and leads to the equalization of the temperatures of these bodies.

Due to historical conditions, the change in internal energy of the body when the heat exchange is often called transferred or received by the amount of heat Q. If in the process of heat transfer the internal energy of a body is increased by ∆U, they say, that the body gets the amount of heat Q. If, however, this decreases the body's energy by ∆U, they say, the body gave the quantity of heat Q. Therefore, the Q-value is a numerical expression of energy, transmitted or received by the body in heat transfer. This means, the amount of heat Q must be measured in the units of, as the energy or work. The SI unit of quantity of heat is Joule (J.) (Before the unit of heat was calorie serving (Kal) and calories (kcal))

We emphasize, what amount of heat is a measure of the change in the internal energy of bodies in heat transfer and essentially depends on the kind of process. This means, what about the amount of heat it is possible to speak only in connection with any process. When the body is in any particular state, neither of which the amount of heat in the body of the question. In this case we can speak only about the internal energy of the body.