Average kinetic energy
The kinetic energy is theenergy, which is determined by the speed of motion of various points belonging to this system. In this case, it is necessary to distinguish between the energy that characterizes the translational motion and the rotational motion. In this case, the average kinetic energy is the average difference between the total energy of the whole system and its rest energy, that is, in essence, its magnitude is the average value of the potential energy.
Its physical value is determined by the formula 3/ 2 kT, which denotes: T is the temperature, k is the Boltzmann constant. This value can serve as a kind of criterion for comparison (a standard) for energies enclosed in different 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 in translational motion, and the energy of neutral atoms and ions is much smaller.
This value, if we consider any solution, gas or liquid, which is at a given temperature, has a constant value. This statement is also true for colloidal solutions.
Somewhat different is the case with solidsubstances. 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 make a motion around a certain point that conditionally fixes a certain equilibrium position of the particle over a long period of time. This property and allows the solid substance to be sufficiently stable in form and volume.
If we consider the conditions: translational motion and ideal gas, then the average kinetic energy is not a quantity that is dependent on the molecular mass, and therefore is defined as a value directly proportional to the value of the absolute temperature.
All these judgments we have made with theshow that they are valid for all types of aggregate states of matter - in any of them the temperature acts as the main characteristic reflecting the dynamics and intensity of the thermal motion of the elements. And this is the essence of the molecular-kinetic theory and the content of the concept of thermal equilibrium.
As is known, if two physical bodies comein interaction with each other, then a heat exchange process arises between them. If the body is a closed system, that is, it does not interact with any bodies, then its heat exchange process will last as long as it takes to equalize the temperatures of this body and the environment. Such a state is called thermodynamic equilibrium. This conclusion was repeatedly confirmed by the results of experiments. To determine the average kinetic energy, one should refer to the characteristics of the temperature of a given body and its heat-exchange properties.
It is also important to take into account that the microprocesses insidebodies do not end even when the body enters a thermodynamic equilibrium. In this state, the molecules move, change in their velocities, impacts and collisions inside the bodies. Therefore, only one of several of our statements is satisfied: the volume of the body, the pressure (if it is a gas) can differ, but the temperature will still remain constant. This again confirms the assertion that the average kinetic energy of thermal motion in isolated systems is determined exclusively by the temperature index.
This pattern was established in the course of experiments by J. Charles in 1787. While conducting experiments, he noticed that when the 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.
