Average kinetic energy

Kinetic energy is the energy that is determined by the speed of motion of the 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 - temperature, k - Boltzmann constant. This value can serve as a kind of criterion for comparison (a standard) for energies contained 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 quantity, 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.

Something is different with solids. 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 and allows the solid substance to be sufficiently stable in form and volume.

If we consider conditions: translational motion and ideal gas, then the average kinetic energy is not a quantity 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 given to show 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 come into 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-transfer properties.

It is also important to consider that microprocesses within bodies do not end even when the body enters 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 true: 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 experiments of 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 instruments and things, in particular a gas thermometer.