Let us consider some of the phenomena that experimentally confirm the main provisions and conclusions of the molecular kinetic theory.

1. Brownian motion. The Scottish botanist R. Brown (1773-1858), observing a suspension of pollen in water under a microscope, found that the pollen particles moved briskly and randomly, then rotating, then moving from place to place, like dust particles in a sunbeam. Subsequently, it turned out that such a complex zigzag motion is characteristic of any particles of small size (1 μm) suspended in a gas or liquid. The intensity of this movement, called Brownian, increases with increasing temperature of the medium, with decreasing viscosity and particle size (regardless of their chemical nature). The reason for the Brownian motion remained unclear for a long time. Only 80 years after the discovery of this effect, he was given an explanation: the Brownian motion of suspended particles is caused by the impact of molecules of the medium in which the particles are suspended. Since the molecules move chaotically, the Brownian particles receive shocks from different directions, and therefore they move in such a bizarre shape. Thus, the Brownian motion is a confirmation of the conclusions of the molecular kinetic theory about the chaotic thermal motion of atoms and molecules.

2. Stern's experience. The first experimental determination of the velocities of molecules was carried out by the German physicist O. Stern (1888-1970). His experiments also made it possible to estimate the velocity distribution of molecules. The Stern setup is shown in Fig. 70. A platinum wire coated with a layer of silver is stretched along the axis of the inner cylinder with a slot, which is heated by a current when the air is pumped out. Silver evaporates when heated. Silver atoms, escaping through the slit, hit the inner surface of the second cylinder, giving an image of the slit O.

If the device is brought into rotation around the common axis of the cylinders, then the silver atoms will not settle against the gap, but will shift from the point O for some distance s. The slit image is blurred. By examining the thickness of the deposited layer, it is possible to estimate the velocity distribution of molecules, which corresponds to the Maxwellian distribution.

Knowing the radii of the cylinders, their angular velocity of rotation, as well as measuring s, it is possible to calculate the speed of motion of silver atoms at a given temperature of the wire. The results of the experiment showed that the average velocity of silver atoms is close to that which follows from the Maxwellian velocity distribution of molecules.

3. Experience Lammert. This experiment makes it possible to more accurately determine the velocity distribution law for molecules. The diagram of the vacuum installation is shown in Fig. 71. The molecular beam formed by the source, passing through the slit, enters the receiver. Two discs with slots fixed on a common axis are placed between the source and the receiver. When the disks are stationary, the molecules reach the receiver, passing through the slots in both

disks. If the axis is brought into rotation, then the receiver is reached only by those molecules that have passed the slot in the first disk and which spend a time equal to or multiple of the disk rotation time to travel between the disks. Other molecules are retained by the second disc. By changing the angular velocity of rotation of the disks and measuring the number of molecules entering the receiver, it is possible to reveal the law of distribution of the velocities of molecules. This experiment also confirmed the validity of the Maxwellian velocity distribution of molecules.

4. Experimental determination of the Avogadro constant. Taking advantage of the idea of ​​the distribution of molecules by height (see formula (45.4)), the French scientist J. Perrin (1870-1942) experimentally determined the Avogadro constant. Examining Brownian motion under a microscope, he made sure that Brownian particles are distributed along the height like gas molecules in a gravitational field. Applying the Boltzmann distribution to them, we can write

where m- particle mass, m 1 is the mass of the liquid displaced by it: m = 4/3 r 3 , m 1 = 4/3 r 3  1 (r - particle radius,  - particle density,  1 - liquid density).

If n 1 and n 2 are particle concentrations at levels h 1 and h 2, a k = R / N A , then

Meaning N a, obtained from the works of J. Perrin, corresponded to the values ​​obtained in other experiments, which confirms the applicability of distribution (45.4) to Brownian particles.

Molecular kinetic theory is a branch of physics that studies the properties of various states of matter, based on the concept of the existence of molecules and atoms as the smallest particles of matter. The ICT is based on three main principles: 1. All substances are made up of tiny particles: molecules, atoms or ions. 2. These particles are in continuous chaotic motion, the speed of which determines the temperature of the substance. 3. There are forces of attraction and repulsion between the particles, the nature of which depends on the distance between them. The main provisions of the ICT are confirmed by many experimental facts. The existence of molecules, atoms and ions has been proven experimentally, the molecules have been sufficiently studied and even photographed using electron microscopes. The ability of gases to expand and occupy indefinitely the whole the volume provided to them is explained by the continuous chaotic movement of molecules. Elasticity gases, solids and liquids, the ability of liquids

wetting some solids, the processes of coloring, gluing, maintaining shape with solids, and much more speak about the existence of forces of attraction and repulsion between molecules. The phenomenon of diffusion - the ability of molecules of one substance to penetrate into the gaps between the molecules of another - also confirms the main provisions of the MCT. The phenomenon of diffusion explains, for example, the spread of odors, mixing dissimilar liquids, the process of dissolving solids in liquids, welding metals by melting them or by pressure. A confirmation of the continuous chaotic movement of molecules is also the Brownian movement - the continuous chaotic movement of microscopic particles insoluble in a liquid.

The movement of Brownian particles is explained by the chaotic movement of liquid particles, which collide with microscopic particles and set them in motion. It was experimentally proved that the speed of Brownian particles depends on the temperature of the liquid. The theory of Brownian motion was developed by A. Einstein. The laws of motion of particles are of a statistical, probabilistic nature. There is only one known way to reduce the intensity of Brownian motion - to decrease the temperature. The existence of Brownian motion convincingly confirms the motion of molecules.

Any substance consists of particles, therefore amount of substance it is considered to be proportional to the number of particles, i.e. structural elements contained in the body, v.

The unit of the amount of a substance is mole.Moth is the amount of a substance containing as many structural elements of any substance as there are atoms in 12 g of carbon C 12. The ratio of the number of molecules of a substance to the amount of a substance is called Avogadro's constant:

n a = N / v. na = 6.02 10 23 mol -1.

Avogadro's constant shows how many atoms and molecules are contained in one mole of a substance. Molar mass is called a value equal to the ratio of the mass of a substance to the amount of a substance:

Molar mass is expressed in kg / mol. Knowing the molar mass, you can calculate the mass of one molecule:

m 0 = m / N = m / vN A = M / N A

The average mass of molecules is usually determined chemical methods, Avogadro's constant is determined with high accuracy by several physical methods. The masses of molecules and atoms are determined with a high degree of accuracy using a mass spectrograph. The masses of molecules are very small. For example, the mass of a water molecule: t = 29.9 10 -27 kg.

Molar mass is related to the relative molecular mass of Mr. Relative molar mass is a value equal to the ratio of the mass of a molecule of this substance to 1/12 of the mass of a carbon atom C 12. If known chemical formula substance, then using the periodic table can be determined by its relative mass, which, when expressed in kilograms, indicates the magnitude of the molar mass of this substance.

2) Oscillatory motion of molecules in nature and technology. Harmonic vibrations... Amplitude, period, frequency and phase of oscillations. Determine empirically the frequency of the proposed oscillatory system.

Mechanical vibrations are called the movements of bodies, repeating exactly or approximately the same at regular intervals. The forces acting between bodies inside the considered system of bodies are called internal forces. The forces acting on the bodies of the system from the side of other bodies are called external forces. Free vibrations vibrations that have arisen under the influence of internal forces are called, for example, a pendulum on a string. Oscillations under the action of external forces - forced oscillations, for example, a piston in an engine. Common signs all modes of vibration is the repetition of the process of movement after a certain interval of time. The vibrations described by the equation are called harmonic. In particular, vibrations arising in a system with one restoring force proportional to deformation are harmonic. The minimum interval at which the body's movement is repeated is called the oscillation period T. Physical quantity, the reciprocal of the oscillation period and characterizing the number of oscillations per unit of time, is called the frequency. Frequency is measured in hertz, 1 Hz = 1 s -1. The concept of cyclic frequency is also used, which determines the number of oscillations in 2p seconds. The modulus of maximum displacement from the equilibrium position is called the amplitude. The value under the cosine sign is the oscillation phase, j 0 is the initial oscillation phase. The derivatives also change harmoniously, and the total mechanical energy with arbitrary deviation NS(angle, coordinate, etc.) equals where A and V- constants determined by system parameters. Differentiating this expression and taking into account the absence of external forces, it is possible to write down what, where.

03.02.2015

Lesson 39 (Grade 10)

Theme. The main provisions of the MCT of the structure of matter and its experimental justification

1. Objectives of the course molecular physics and MCT; macro- and micro-objects

To begin with, let's recall all the previous sections of physics that we studied, and understand that all this time we have been considering the processes occurring with macroscopic bodies (or objects of the macrocosm). Now we will study their structure and the processes taking place inside them.

Definition. Macroscopic body- a body consisting of a large number particles. For example: a car, a person, a planet, a billiard ball ...

Microscopic body - a body consisting of one or more particles. For example: atom, molecule, electron ... (Fig. 1)

Rice. 1. Examples of micro and macro objects, respectively

Having thus determined the subject of study of the MKT course, we should now talk about the main goals that the MKT course sets for itself, namely:

1. Study of the processes occurring inside a macroscopic body (movement and interaction of particles)

2. Properties of bodies (density, mass, pressure (for gases) ...)

3. Study of thermal phenomena (heating-cooling, changes aggregate states body)

The study of these issues, which will take place throughout the entire topic, will begin now with the fact that we will formulate the so-called basic provisions of the ICT, that is, some statements, the truth of which has not been questioned for a long time, and, starting from which, the entire further course will be built ...

Let's analyze them one by one:

2. The first basic provision of the ICB; molecules, atoms

All substances are composed of a large number of particles - molecules and atoms.

Definition. Atom- the smallest particle of a chemical element. The sizes of atoms (their diameter) are of the order of cm. It should be noted that different types atoms, unlike molecules, are relatively few. All their varieties that are known to man today are collected in the so-called periodic table (see Fig. 2)

Rice. 2. Periodic table chemical elements(in fact, varieties of atoms) D.I. Mendeleev

Molecule- a structural unit of matter, consisting of atoms. Unlike atoms, they are larger and heavier than the latter, and most importantly, they have a huge variety.

A substance whose molecules consist of one atom are called atomic, from more - molecular... For example: oxygen, water, table salt () - molecular; helium silver (He, Ag) - atomic.

Moreover, it should be understood that the properties of macroscopic bodies will depend not only on quantitative characteristics their microscopic composition, but also of quality.

If in the structure of atoms the substance has a certain geometry ( crystal lattice), or, conversely, does not, then these bodies will be inherent various properties... For example, amorphous bodies do not have a strict melting point. The most famous examples are amorphous graphite and crystalline diamond. Both substances are made up of carbon atoms.

Rice. 3. Graphite and diamond, respectively

Thus, "how many, in what mutual arrangement and what atoms and molecules does the substance consist of?" - the first question, the answer to which will bring us closer to understanding the properties of bodies.

3. The second main provision of the ICB

All particles are in continuous thermal chaotic motion.

Just as in the examples considered above, it is important to understand not only the quantitative aspects of this movement, but also the qualitative ones for various substances.

Molecules and atoms of solids undergo only small vibrations about their constant position; liquid - also vibrate, but due to the large size of the intermolecular space, they sometimes change places with each other; the gas particles, in turn, practically without colliding, freely move in space.

4. The third main provision of the ICB

The particles interact with each other.

This interaction is electromagnetic in nature (interaction of nuclei and electrons of an atom) and acts in both directions (both attraction and repulsion).

Here: d- distance between particles; a- particle size (diameter).

For the first time the concept of "atom" was introduced by the ancient Greek philosopher and natural scientist Democritus (Fig. 4). In a later period, the Russian scientist Lomonosov actively asked about the structure of the microworld (Fig. 5).

Rice. 4. Democritus Fig. 5. Lomonosov

5. Various options for justifying the provisions of the ICB

To begin with, let's recall the main provisions of the ICT, namely:

1. All bodies are composed of small particles - molecules and atoms,

2. These particles are in constant chaotic motion,

3. These particles interact continuously with each other.

So how do you get empirical confirmation of these claims? In fact, every person, without exception, is familiar with one of the methods. This is diffusion, or mixing, in simple terms.

Definition. Diffusion- the process of mutual penetration of molecules of one substance into the space between the molecules of another (Fig. 6).

Rice. 6. Process of diffusion in gases

Diffusion can occur both in gases (we can observe this process, feeling the spread of odors), in liquids (mixing colored water of different colors) and even in solids (if very smooth sheets of glass or metal are put on top of each other for a long time, it is impossible will distinguish where one leaf ends and another begins). Moreover, there is also mixed diffusion, that is, the penetration of gas molecules into solid and liquid bodies (otherwise the fish in water could not breathe), etc. (Fig. 7)

Rice. 7.various examples of diffusion

Indeed, if we assume that matter is a kind of continuous structure, it becomes completely incomprehensible how to explain all of the above phenomena.

However, the main argument in explaining the main provisions of the MCT is the Brownian motion.

6. Description of Brown's experiment

Definition. Brownian motion- continuous thermal chaotic movement of molecules of matter (Fig. 8).

This term came into use after, in 1827, Scottish botanist Robert Brown, mixing the pollen of a driftwood with water and examining a drop of the mixture under a microscope, observed the aforementioned movement.

Rice. 8. Trajectory of a particle in Brownian motion

7. Explanation of Brown's experiment

However, since Brown could see only pollen particles through a microscope, he misinterpreted his discovery (he thought that the pollen was alive). The Brownian motion can only be explained on the basis of molecular kinetic theory.

The reason for the Brownian motion of a particle is that the impacts of liquid molecules on a particle do not cancel each other out..

Figure 8.4 schematically shows the position of one Brownian particle and the molecules closest to it. When molecules move randomly, the impulses transmitted by them to a Brownian particle, for example, to the left and to the right, are not the same. Therefore, the resulting force of pressure of liquid molecules on a Brownian particle is nonzero. This force also causes a change in the motion of the particle.

Rice. 9. Brownian particle of pollen in water

The average pressure has a certain value in both gas and liquid. But there are always slight random deviations from this average. The smaller the surface area of ​​the body, the more noticeable the relative changes in the pressure force acting on this area. So, for example, if the area has a size of the order of several diameters of a molecule, then the pressure force acting on it changes abruptly from zero to a certain value when the molecule enters this area.
The construction of the theory of Brownian motion and its experimental confirmation by the French physicist J. Perrin finally completed the victory of the molecular-kinetic theory. Almost a century later, the German physicist Albert Einstein (1879-1955) realized that a large particle of pollen is simply pushed by much smaller water molecules, which themselves are already moving chaotically (Fig. 9).

Such observations can be carried out in many other ways: drop paint into water and look at the mixture under a microscope, observe a separate speck of dust moving in your apartment ...

8. Proof of key points

Thus, the presence of Brownian motion is fully confirmed by the introduced provisions of the MCT. The very fact of the movement of pollen confirms them. Since the pollen is moving, it means that forces are acting on it. The only one possible reason the emergence of these forces is the collision of any small bodies. Therefore, it is no longer possible to doubt the first two provisions. And since a particle of pollen changes its direction, it means that at different times the number of blows to pollen from a certain side is different, which means that there is no doubt that water molecules interact with each other.

Brownian motion is thermal motion, and it cannot stop. With increasing temperature, its intensity increases. Figure 8.3 shows a diagram of the motion of Brownian particles. The positions of the particles, marked with dots, are determined at regular intervals - 30 s. These points are connected by straight lines. In reality, the trajectory of the particles is much more complex.

Brownian motion can also be observed in gas. It is carried out by particles of dust or smoke suspended in the air. The German physicist R. Paul (1884-1976) colorfully describes the Brownian motion: “Few phenomena are capable of captivating the observer as much as the Brownian motion. Here the observer is allowed to look behind the curtains.

what happens in nature. A new world opens before him - the non-stop hustle and bustle of a huge number of particles. The smallest particles quickly fly into the field of view of the microscope, almost instantly changing the direction of movement. Larger particles move more slowly, but they also constantly change direction. Large particles are practically pushed together in place. Their protrusions clearly show the rotation of particles around their axis, which constantly changes direction in space. There is not a trace of system or order anywhere. The domination of blind chance - this is what a strong, overwhelming impression this picture makes on the observer. " Present concept Brownian motion used in more broad sense... For example, Brownian motion is the trembling of the arrows of sensitive measuring instruments, which occurs due to the thermal motion of atoms of the parts of the instruments and the environment.

Perrin's experiments. The idea behind Perrin's experiments is as follows.
It is known that the concentration of gas molecules in the atmosphere decreases with height. If there was no thermal motion, then all the molecules would fall to the Earth and the atmosphere would disappear. However, if there was no attraction to the Earth, then due to the thermal motion, the molecules would leave the Earth, since the gas is capable of unlimited expansion. As a result of the action of these opposite factors, a certain distribution of molecules along the height is established, as mentioned above, that is, the concentration of molecules decreases rather quickly with height. Moreover, what more mass molecules, the faster their concentration decreases with height.
Brownian particles participate in thermal motion. Since their interaction is negligible, the set of these particles in a gas or liquid can be considered as an ideal gas of very heavy molecules. Consequently, the concentration of Brownian particles in a gas or liquid in the Earth's gravity field should decrease according to the same law as the concentration of gas molecules. This law is well known.
Perrin, using a microscope of high magnification and shallow depth of field (shallow depth of field), observed Brownian particles in very thin layers of liquid. Counting the concentration of particles at different heights, he found that this concentration decreases with height according to the same law as the concentration of gas molecules. The difference is that, due to the large mass of Brownian particles, the decrease occurs very quickly.
Moreover, counting Brownian particles at different heights allowed Perrin to determine Avogadro's constant with a completely new method. The value of this constant coincided with the known one.
All these facts testify to the correctness of the theory of Brownian motion and, accordingly, to the fact that Brownian particles participate in the thermal motion of molecules.

Molecular kinetic theory is a branch of physics that studies the properties of various states of matter, based on the concept of the existence of molecules and atoms as the smallest particles of matter. The ICT is based on three main principles:

1. All substances are made up of tiny particles: molecules, atoms or ions.

2. These particles are in continuous chaotic motion, the speed of which determines the temperature of the substance.

3. There are forces of attraction and repulsion between the particles, the nature of which depends on the distance between them.

The main provisions of the ICT are confirmed by many experimental facts. The existence of molecules, atoms and ions has been proven experimentally, the molecules have been sufficiently studied and even photographed using electron microscopes. The ability of gases to expand indefinitely and occupy the entire volume provided to them is explained by the continuous chaotic movement of molecules. The elasticity of gases, solids and liquids, the ability of liquids to wet some solids, the processes of coloring, gluing, retention of shape by solids, and much more speak about the existence of forces of attraction and repulsion between molecules. The phenomenon of diffusion - the ability of molecules of one substance to penetrate into the gaps between the molecules of another - also confirms the main provisions of the MCT. The phenomenon of diffusion explains, for example, the spread of odors, mixing of dissimilar liquids, the process of dissolution of solids in liquids, welding of metals by melting them or by pressure. A confirmation of the continuous chaotic movement of molecules is also the Brownian movement - the continuous chaotic movement of microscopic particles insoluble in a liquid.

The movement of Brownian particles is explained by the chaotic movement of liquid particles, which collide with microscopic particles and set them in motion. It was experimentally proved that the speed of Brownian particles depends on the temperature of the liquid. The theory of Brownian motion was developed by A. Einstein. The laws of motion of particles are of a statistical, probabilistic nature. There is only one known way to reduce the intensity of Brownian motion - to decrease the temperature. The existence of Brownian motion convincingly confirms the motion of molecules.

Any substance consists of particles, therefore, the amount of substance v is considered to be proportional to the number of particles, i.e., structural elements contained in the body.

The unit of the amount of a substance is the mole. A mole is the amount of a substance containing as many structural elements of any substance as there are atoms in 12 g of carbon C12. The ratio of the number of molecules of a substance to the amount of a substance is called Avogadro's constant:

Avogadro's constant shows how many atoms and molecules are contained in one mole of a substance. Molar mass is the mass of one mole of a substance, equal to the ratio of the mass of a substance to the amount of a substance:

Molar mass is expressed in kg / mol. Knowing the molar mass, you can calculate the mass of one molecule:

Molar mass is related to the relative molecular mass of Mg. Relative molecular weight is a value equal to the ratio of the mass of a molecule of a given substance to 1/12 of the mass of a carbon atom C12. If the chemical formula of a substance is known, then using the periodic table, its relative mass can be determined, which, when expressed in kilograms, shows the value molar mass of this substance.

The molecular-kinetic theory of the structure of matter is based on three statements:

• the substance is composed of particles;
• particles move randomly;
• particles interact with each other.

Each statement is rigorously proven through experiments.

Volume V of the oil layer is equal to the product its surface area S per layer thickness d, i.e. V = S * d / Therefore, the size of the olive oil molecule is:

The diameter of a water molecule is approximately 3 10 cm ... Assuming that each water molecule with a dense packing of molecules occupies a volume of approximately 3*10 8 cm 3 , the number of molecules in a drop can be found by dividing the drop volume 1 cm 3 per volume per molecule:

MOLECULE MASS. QUANTITY OF SUBSTANCE.

The masses of atoms and molecules differ significantly. What values ​​are convenient to characterize them? How to determine the number of atoms in any macroscopic body?

A new quantity appears - the amount of substance.

The mass of a water molecule. The masses of individual molecules and atoms are very small. For example, in 1 g water contains 3.7 * 10 22 molecules. Consequently, the mass of one water molecule (H 2 O) is equal to:

Since the masses of molecules are very small, it is convenient to use in calculations not absolute values ​​of masses, but relative ones. By international agreement, the masses of all atoms and molecules are compared to the mass of a carbon atom (the so-called carbon scale of atomic masses).

The relative molecular (or atomic) mass of the substance M r. called the ratio of the mass of a molecule (or atom) m 0 of a given substance to the mass of a carbon atom m o:

Amount of substance most naturally, it would be measured by the number of molecules or atoms in the body. But the number of molecules in any macroscopic body is so great that the calculations use not the absolute number of molecules, but the relative one.

In the International System of Units, the amount of a substance is expressed in moles.

One mole is the amount of a substance that contains as many molecules or atoms as there are atoms in carbon weighing 0.012 kg.

This means that 1 mole of any substance contains the same number of atoms or molecules. This number of atoms denotes N A and is called the constant Avogadro in honor of the Italian scientist (19th century).

N A - Avogadro's constant.

To determine the Avogadro constant, you need to find the mass of one carbon atom. A rough estimate of the mass can be made as was done above for the mass of a water molecule (the most accurate methods are based on the deflection of ion beams by an electromagnetic field).

Explanation of Brownian motion.

The Brownian motion can only be explained on the basis of molecular kinetic theory. The reason for the Brownian motion of a particle is that the impacts of liquid molecules on the particle do not cancel each other out. When molecules move randomly, the impulses transferred by them to a Brownian particle, for example, to the left and to the right, are not the same, therefore the resulting force of pressure of liquid molecules on a Brownian particle is nonzero, which causes a change in its motion.

Gases can be easily compressed, thus reducing the average distance

between molecules, but the molecules do not squeeze each other. The volume of the vessel is tens of thousands of times greater than the volume of those in

dumb molecules. Gases are easily compressed in this case, the average distance between molecules decreases, but the molecules do not squeeze each other.

Molecules move through space at tremendous speeds - hundreds of meters per second. When they collide, they bounce off each other in different sides like billiard balls. Weak forces of attraction of gas molecules are not able to keep them close to each other. That's why gases can expand indefinitely. They do not retain their shape or volume. Numerous impacts of molecules on the walls of the vessel create gas pressure.