Heat is a physical phenomenon that is produced as a result of a mechanical work or a chemical reaction. The concept of temperature is derived from the general concept of heat and cold. Temperature is the average of kinetic energy. Proportional, differences in temperature maintain heat transfer. It is one of the examples, that the change in size or volume of a system is related to changes in temperature; The most obvious example is the expansion of hot air; When the air heats up, its viscosity increases and its density decreases relative to the surrounding air, the same behavior occurs in all liquids and gases, the natural heat transfer in the houses, oceans and air systems increases; Solids also have thermal expansion properties; Railroads and bridges, for example, have expansion joints to allow them to expand freely and adjust to changes in temperature. What is the main cause of thermal expansion? As mentioned earlier, an increase in temperature means an increase in the kinetic energy of individual atoms. These right potentials are non-convergent; Because the potential energy increases sharply when the molecules are close to each other compared to when they are far away from each other; The moving distance is greater when the neighbors are further away from each other than when they are moving towards each other. The result is that as the kinetic energy (increase in temperature) increases, the average distance between the molecules is greater, the substance becomes compact, and for most substances this is a great approximation in normal conditions, that there is no better direction (solid gases). is tropic) meaning that an increase in temperature leads to an increase in self-size by a certain amount in each dimension; If the solid is free, compressed or compact, its ratio remains constant, only its total energy changes. This is Library research that has been used from internal and external sources.