Kinetic energy is that energy, which is determined by the speed of various points belonging to this system. In this case, one should distinguish between the energy that characterizes the translational motion and the rotational motion. Moreover, the average kinetic energy is the average difference between the total energy of the entire system and its rest energy, that is, in essence, its value is the average value of potential energy.
Its physical value is determined by the formula 3/2 kT, in which are indicated: T - temperature, k - Boltzmann constant. This value can serve as a kind of comparison criterion (standard) for the energies contained in various types of thermal motion. For example, the average kinetic energy for gas molecules in the study of translational motion is 17 (- 10) nJ at a gas temperature of 500 C. As a rule, electrons have the greatest energy during translational motion, but the energy of neutral atoms and ions is much less.
This value, if we consider any solution, gas or liquid at a given temperature, has a constant value. This statement is also true for colloidal solutions.
The situation with solids is somewhat different. In these substances, the average kinetic energy of any particle is too small to overcome the forces of molecular attraction, and therefore it can only move around a point that conditionally fixes a certain equilibrium position of the particle over a long period of time. This property allows the solid substance to be sufficiently stable in shape and volume.
If we consider the conditions: translational motion and ideal gas, then here the average kinetic energy is not a quantity dependent on the molecular mass, and therefore it is defined as a value directly proportional to the absolute temperature.
We made all these judgments in order to show that they are valid for all types of aggregate states of matter - in any of them, temperature acts as the main characteristic that reflects the dynamics and intensity of the thermal motion of elements. And this is the essence of molecular kinetic theory and the content of the concept of thermal equilibrium.
As you know, if two physical bodies interact with each other, then a heat exchange process occurs between them. If the body is a closed system, that is, it does not interact with any bodies, then its heat transfer process will last as long as it takes to equalize the temperatures of this body and the environment. This state is called thermodynamic equilibrium. This conclusion has been repeatedly confirmed by the results of experiments. To determine the average kinetic energy, one should turn to the characteristics of the temperature of a given body and its heat transfer properties.
It is also important to consider that microprocesses inside bodies do not end even when the body enters thermodynamic equilibrium. In this state, the movement of molecules occurs inside the bodies, their velocities change, shock and collision. Therefore, only one of several of our statements is fulfilled - body volume, pressure (if we are talking about gas) can vary, but the temperature will still remain constant. This once again confirms the statement that the average kinetic energy of thermal motion in isolated systems is determined solely by the temperature indicator.
This pattern was established during the experiments of Charles in 1787. Carrying out experiments, he noticed that when bodies (gases) are heated by the same amount, their pressure changes in accordance with a directly proportional law. This observation made it possible to create many useful devices and things, in particular, a gas thermometer.