Events of the physical world are inextricably linked with changes in temperature. Every person gets acquainted with it in early childhood, when he realizes that ice is cold, and boiling water burns. At the same time, the understanding comes that the processes of temperature change do not occur instantly. Later, at school, the student learns that this is connected with thermal motion. And a whole section of physics is dedicated to processes related to temperature.
What is temperature?
This is a scientific concept introduced to replace everyday terms. In everyday life, words such as hot, cold or warm constantly appear. All of them speak 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, temperature has no direction, but only a numerical value.
In the International System of Units (SI), temperature is measured in degrees Celsius (ºС). But in many formulas describing thermal phenomena, it is required to convert it to Kelvin (K). ForThere is a simple formula for this: T \u003d t + 273. In it, T is the temperature in Kelvin, and t is in Celsius. The concept of absolute zero temperature is associated with the Kelvin scale.
There are several other temperature scales. In Europe and America, for example, Fahrenheit (F) is used. Therefore, they must be able to write in Celsius. To do this, subtract 32 from the readings in F, then divide it by 1, 8.
Home experiment
In his explanation, you need to know such concepts as temperature, thermal motion. And it's easy to complete this experience.
It will take three containers. They should be large enough so that the hands can easily fit in them. Fill them with water of different temperatures. In the first, it must be very cold. In the second - heated. Pour hot water into the third, one in which it will be possible to hold a hand.
Now the experience itself. Dip your left hand in a container of cold water, right - with the hottest. Wait a couple of minutes. Take them out and immediately immerse them in a vessel of warm water.
The result will be unexpected. The left hand will feel that the water is warm, while the right hand will feel cold water. This is due to the fact that thermal equilibrium is first established with those liquids in which the hands are initially immersed. And then this balance is sharply disturbed.
Main tenets of molecular kinetic theory
It describes all thermal phenomena. And these statements are quite simple. Therefore, in a conversation about thermal motion, these provisions should be knownrequired.
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 greater than the size of the particles.
Second: in all substances there is a thermal movement of molecules, which never stops. The particles move randomly (chaotically).
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
Proof that bodies are composed of particles with gaps between them 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 what has been said is diffusion. That is, the penetration of molecules of one substance between the particles of another. Moreover, this movement is mutual. Diffusion proceeds the faster, the farther apart the molecules are located. Therefore, in gases, mutual penetration will occur much faster than in liquids. And in solids, diffusion takes years.
By the way, the last process also explains thermal motion. After all, the mutual penetration of substances into each other occurs without any interference from the outside. But it can be accelerated by heating the body.
Confirmation of the second position of the MKT
Bright proof that there isthermal motion is the Brownian motion of particles. It is considered for suspended particles, that is, for those that are significantly larger than the molecules of a substance. These particles can be dust particles or grains. And they are supposed to be placed in water or gas.
The reason for the random movement of a suspended particle is that molecules act on it from all sides. Their action is erratic. The magnitude of the impacts at each point in time is different. Therefore, the resulting force is directed either in one direction or the other.
If we talk about the speed of the thermal motion of molecules, then there is a special name for it - root mean square. It can be calculated using the formula:
v=√[(3kT)/m0].
In it, T is the temperature in Kelvin, m0 is the mass of one molecule, k is the Boltzmann constant (k=1, 3810-23 J/K).
Confirmation of the third provision of the ICB
Particles attract and repel. In explaining many processes associated with thermal motion, this knowledge turns out to be important.
After all, the forces of interaction depend on the aggregate state of matter. So, gases practically do not have them, since the particles are removed so far that their effect is not manifested. In liquids and solids, they are perceptible and ensure the conservation of the volume of the substance. In the latter, they also guarantee the maintenance of shape.
Evidence of the existence of forces of attraction and repulsion is the appearance of elastic forces during the deformation of bodies. So, with elongation, the forces of attraction between molecules increase, and withcompression - repulsion. But in both cases, they return the body to its original shape.
Average energy of thermal motion
It can be written from the basic MKT equation:
(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 the following formula for the average kinetic energy of molecules:
E=(3kT)/2.
From here 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 thermal motion, which exists as long as there is a temperature other than absolute zero.
