What does the internal energy change mean?

For several centuries, physicists assumed that the temperature is determined by the presence in the gases of an invisible and weightless substance of heat. Many theories have been put forward , explaining its movement within matter and between different objects. Only M.V. Lomonosov was able to explain the real nature of matter, creating a molecular-kinetic theory of gases. In his arguments and calculations, he managed to prove that there is no heat in nature. The temperature depends on the speed of the chaotic motion of the molecules. He introduced the concept of internal energy, and also explained how it changes in the real process.

What arguments led MV. Lomonosov for the proof of the molecular-kinetic theory of gases

Having stated for the first time the assumption that there is no heat in nature, he met a powerful resistance from venerable scientists of that period. They all recognized the presence of heat, and the beginning researcher - no. Then at one of the meetings with German and British physicists the following was said: "Dear teachers. Where did the heat in the body of the cow come from? She ate cold grass, and then her body warmed from the fact that inside her internal energy changed. Where did it originate? And the origin of heat in the body is explained by the fact that the grass has chemical energy, which the animal's body converted into this heat. Hence, we observe the phenomenon of energy transfer from one state to another. " He was listened to, asked dozens of questions. As a result of the discussion, the law of energy change was also formulated (it is also called the law of conservation of energy), which was recognized by all present. Later, a small collection of hypotheses was published, which was the first publication where the molecular-kinetic theory of gases was recognized.

What gave the researchers the theory of MV. Lomonosov

Today it seems that in thermodynamics everything is logical. But it should be remembered that more than 250 years passed from the first assumptions to the present day. The French researcher J. Charles discovered the law of proportionality of pressure growth with increasing gas temperature. He then explained the change in the internal energy of the gas when heated. He derived his formula. His research in 20 years, continued Gay-Lussac, who investigated the heating of gas at constant pressure. He watched as the position of the piston located inside the glass cylinder changes with its heating and cooling. Here he came very close to discovering the notion of a gas constant. He did not use the research that Robert Boyle did 140 years earlier . Only the work of Mariotte, carried out later and formulated in the Boyle-Mariott Act, helped Benois Paul Emile Clapeyron to formulate the first concept of the equation of state of an ideal gas.

After 40 years D.I. Mendeleev supplemented the equation of state with the results of his studies. Now the law of Kliperon-Mendeleev is the basis for thermodynamics of the whole world. He mathematically determines the change in internal energy from the temperature of the gas. The discovery of basic laws was also confirmed by practice. Thermal machines were created that operate on the thermodynamic cycles of Otto, Diesel, Trinkler and other scientists.

It was only at the beginning of the 20th century that the experience of predecessors was generalized and textbooks on molecular physics and technical thermodynamics were written, which harmoniously and logically set out the whole set of discoveries.

A few words about the law of ideal gas

PV = mRT

Today, when deriving any dependencies, the equation of state of an ideal gas is used. No one is embarrassed by the parameters that enter into it, which have quite definite concepts. The conclusions from the basic gas law give another important formula characterizing the change in internal energy:

DU = cvDT,

Here dU is the differential of the change in the internal energy, and cv is the heat capacity of the gas at constant volume. As a result of the discussion on the nature of the gas constant R, it is established that it characterizes the operation of the gas at constant pressure.