The events of the physical world are inextricably linked with temperature changes. Every person gets acquainted with it in early childhood, when he realizes that the ice is cold, and he burns boiling water. At the same time, understanding comes that the processes of temperature change do not occur instantly. Later, at school, the student learns that this is due to thermal movement. And the processes associated with temperature, allocated a whole section of physics.
What is temperature?
This scientific concept is introduced to replace ordinary terms. In everyday life, words like hot, cold, or warm constantly appear. All of them talk about the degree of heating 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), temperature is measured in degrees Celsius (º). But in many formulas describing thermal phenomena, it is required to translate it into Kelvin (K). There is a simple formula for this: T = t + 273. In it, T is the temperature in Kelvins, and t in Celsius. The concept of absolute zero temperature is connected with the Kelvin scale.
There are several more temperature scales. In Europe and America, for example, in the course of Fahrenheit (F). Therefore, they must be able to record in Celsius. To do this, it is supposed to subtract 32 from the testimony in F., then divide it by 1.8.
Home experiment
In his explanation, it is required to know such concepts as temperature, thermal motion. And to carry out this experience is simple.
It will take three containers for it. They should be large enough so that they can easily fit the hands. Fill them with water of different temperatures. In the first, it should be very cold. In the second - heated. Pour hot water into the third one, in which it will be possible to hold the hand.
Now the experience itself. Lower the left hand in a container with cold water, the right - with the hottest. Wait a couple of minutes. Take them out and immediately immerse in a vessel with warm water.
The result will be unexpected. The left hand will feel that the water is warm, the right hand will have a feeling of cold water. This is due to the fact that at first thermal equilibrium is established with those fluids in which the hands are immersed initially. And then this balance is sharply disturbed.
The main provisions of the molecular kinetic theory
It describes all thermal phenomena. But these statements are quite simple. Therefore, in a conversation about thermal motion, these positions must be known.
First: substances are formed by the smallest particles located at some distance from each other. Moreover, these particles can be both molecules and atoms. And the distance between them is many times larger than the size of the particles.
Second: in all substances there is a thermal motion of the molecules, which never stops. Particles in this case move randomly (randomly).
Third: 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 provision of the ICB
The proof that the bodies are composed of particles between which there are gaps is their thermal expansion. So, when the body is heated, its size increases. This happens due to the removal of particles from each other.
Another confirmation of the above is diffusion. That is, the penetration of the molecules of one substance between the particles of another. Moreover, this movement is mutual. Diffusion proceeds the faster, the farther apart the molecules are. Therefore, mutual penetration in gases will occur much faster than in liquids. And in solids, diffusion takes years.
By the way, the latter process also explains the thermal motion. Indeed, the mutual penetration of substances into each other occurs without any intervention from the outside. But it can be accelerated by heating the body.
Confirmation of the second provision of the ICB
Bright evidence that there is thermal motion is Brownian motion of particles. It is considered for suspended particles, that is, for those that are substantially larger than the molecules of the substance. These particles can be dust particles or grains. And putting them relies on water or gas.
The reason for the random motion of a suspended particle is that molecules act on it from all sides. Their action is random. The magnitude of the effects at each point in time is different. Therefore, the resulting force is directed either one way or the other.
If we talk about the rate of thermal motion of molecules, then there is a special name for it - the quadratic mean. 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 the ICB
Particles are attracted and repelled. In the explanation of many processes associated with thermal motion, this knowledge is important.
After all, the forces of interaction depend on the state of aggregation of the substance. So, gases practically do not have them, since the particles are removed so much that their effect is not manifested. In liquids and solids, they are palpable and ensure the preservation of the volume of the substance. In the latter, they also guarantee the maintenance of form.
The proof of the existence of attractive and repulsive forces is the appearance of elastic forces during the deformation of bodies. So, with elongation, the forces of attraction between the molecules increase, and with compression, repulsions. But in both cases, they return the body to its original form.
Average thermal energy
It can be written from the basic equation of MKT :
(pV) / N = (2E) / 3.
In this formula, p is pressure, V is volume, N is the number of molecules, E is the average kinetic energy.
On the other hand, this equation can be written as follows:
(pV) / N = kT.
If you combine them, you get the following equality:
(2E) / 3 = kT.
From it follows such a formula for the average kinetic energy of molecules:
E = (3kT) / 2.
This shows that energy is proportional to the temperature of the substance. That is, with an increase in the latter, particles move faster. This is the essence of thermal motion, which exists as long as there is a temperature other than absolute zero.