What is this: thermal motion? With what notions is it related?

The events of the physical world are inextricably linked with changes in temperature. With her every person gets acquainted in early childhood, when he understands that the ice is cold, and boiling water burns. At the same time, it is understood that the processes of temperature change do not occur instantaneously. Only then in school the student learns that this is due to the thermal movement. And the processes associated with the temperature, identified a whole section of physics.

What is the temperature?

This scientific concept is introduced to replace commonplace terms. In everyday life, words such as hot, cold or warm constantly appear. They all talk about the degree of warmth of the body. This is how it is defined in physics, only with the addition that it is a scalar quantity. After all, the temperature has no direction, but only a numerical value.

In the international system of units (SI), the temperature is measured in degrees Celsius (° C). But in many formulas describing thermal phenomena, it is required to translate it into Kelvin (K). For this, there is a simple formula: T = t + 273. In it, T is the temperature in Kelvin, and t is in Celsius. The Kelvin scale is associated with the concept of absolute zero temperature.

There are several more temperature scales. In Europe and America, for example, in the course of Fahrenheit (F). Therefore, they must be able to write down in Celsius. To do this, it is necessary to subtract 32 from the readings in F, then divide it by 1.8.

Home experiment

In his explanation it is required to know such concepts as temperature, thermal motion. Yes, and to perform this experience is easy.

It will take three tanks for him. They should be large enough to fit easily in their hands. Fill them with water of different temperatures. In the first, it must be very cold. In the second - warmed up. In the third, pour hot water, one in which the hand will be possible to hold.

Now the experience itself. Lower your left hand into a container of cold water, the right one with the hottest. Wait a couple of minutes. Remove them and immediately immerse them in a vessel with warm water.

The result will be unexpected. The left hand will feel that the water is warm, the right will have a feeling of cold water. This is due to the fact that in the beginning thermal equilibrium is established with those liquids into which the hands are immersed initially. And then this balance is sharply violated.

The main provisions of the molecular-kinetic theory

It describes all the heat phenomena. And these statements are quite simple. Therefore, it is necessary to know these provisions in a conversation about the thermal motion.

First: substances are formed by minute particles, located at some distance from each other. And these particles can be both molecules and atoms. And the distance between them is many times larger than the particle size.

Second: in all substances, the thermal motion of molecules is observed, which never stops. The particles move randomly (chaotically).

Third: the particles interact with each other. This action is due to the forces of attraction and repulsion. Their value depends on the distance between the particles.

Confirmation of the first MKT provision

Proof of the fact that the bodies consist of particles, between which there are gaps, is their thermal expansion. Thus, when the body is heated, its size increases. This is due to the removal of particles from each other.

Another confirmation of this is diffusion. That is, the penetration of molecules of one substance between the particles of another. And this movement turns out to be mutual. Diffusion proceeds the faster, the further apart molecules are located. Therefore, in gases, mutual penetration will occur much faster than in liquids. And in solids, diffusion requires years.

By the way, the last process explains the thermal motion. After all, the mutual penetration of substances into each other occurs without any interference from the outside. But it can be accelerated if the body is heated.

Confirmation of the second MKT provision

A vivid proof of the existence of a thermal motion is the Brownian motion of particles. It is considered for suspended particles, that is, for those that are substantially larger than molecules of matter. These particles can be dust particles or grains. And put them in water or gas.

The reason for the random motion of the suspended particle is that molecules are acting on all sides. Their action is random. The magnitude of the impacts at each time is different. Therefore, the resultant force is directed one way and the other.

If we talk about the rate of thermal motion of molecules, then for it there is a special name - the mean quadratic. It can be calculated by the formula:

V = √ [(3kT) / m ₀ ].

In it, T is the temperature in Kelvin, m ₀ is the mass of one molecule, k is the Boltzmann constant (k = 1.38 * 10 ^-23 J / K).

Confirmation of the third provision of ICB

The particles attract and repel. In the explanation of many processes associated with thermal motion, this knowledge is important.

After all, the forces of interaction depend on the aggregate state of matter. So, for gases there are practically none, since the particles are removed so much that their action is not manifested. In liquids and solids they are perceptible and ensure the preservation of the volume of matter. In the latter they guarantee also the maintenance of the form.

The proof of the existence of the forces of attraction and repulsion is the appearance of elastic forces in the deformation of bodies. Thus, when the elongation increases, the forces of attraction between molecules increase, and in the case of compression, the forces of attraction are repulsed. But in both cases they return the body to its original form.

Average energy of thermal motion

It can be written from the basic MKT equation :

(PV) / N = (2E) / 3.

In this formula, p is the pressure, V is the volume, N is the number of molecules, and E is the average kinetic energy.

On the other hand, this equation can be written as:

(PV) / N = kT.

If you combine them, you get the following equality:

(2E) / 3 = kT.

From this follows a formula for the average kinetic energy of molecules:

E = (3kT) / 2.

Hence it is clear that the energy is proportional to the temperature of the substance. That is, when the latter increases, the particles move faster. This is the essence of the thermal motion that exists, as long as there is a temperature different from absolute zero.