Brownian movement of people. Brownian motion (motion of molecules). Observation of Brownian motion

In the 6th grade, you got acquainted with diffusion - the mixing of gases, liquids and solids through direct contact. This phenomenon can be explained by the random movement of molecules. But the most obvious evidence of the movement of molecules can be obtained by observing under a microscope the smallest particles of any solid substance suspended in water. These particles make random motion, which is called Brownian.

Brownian motion is the thermal motion of particles suspended in a liquid (or gas).

Observation of Brownian motion. The English botanist Brown first observed this phenomenon in 1827, examining the spores of the club moss suspended in water under a microscope. Nowadays, particles of gummigut paint, which are insoluble in water, are usually used. These particles perform random motion. The most striking and unusual thing is that this movement never stops. We are accustomed to the fact that any moving body sooner or later stops. Brownian motion is thermal motion, and it cannot stop. As the temperature increases, its intensity increases. Figure 6 shows a diagram of the movement of Brownian particles. The positions of the particles marked with dots are determined at regular intervals of 30 s. These points are connected by straight lines. In reality, particle trajectories are much more complicated.

Brownian motion can be observed in a gas. It is carried out by particles of dust or smoke suspended in the air.

Currently, the concept of "Brownian motion" is used in a broader sense. So, for example, Brownian motion is the trembling of the arrows of sensitive measuring instruments. It occurs due to the thermal movement of atoms of instrument parts and the environment.

Explanation of Brownian motion. An explanation of Brownian motion can only be given on the basis of molecular kinetic theory. The reason for the Brownian motion of a particle is that the impacts of molecules on it do not cancel each other out. Figure 7 schematically shows the position of one Brownian particle and the molecules closest to it. In the chaotic motion of molecules, the impulses transmitted by them to a Brownian particle, for example, from the left and from the right, are not the same. Therefore, the resulting pressure force is different from zero, which causes a change in the motion of a Brownian particle.

The mean pressure has a certain value in both gas and liquid. But there are always minor random deviations from the mean. The smaller the body area, the greater the relative changes in the pressure force acting on this area. So, if the area has dimensions of the order of several diameters of the molecule, then the force acting on it changes abruptly from zero to a certain value when the molecule enters this area.

The molecular-kinetic theory of Brownian motion was created by A. Einstein in 1905. 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.

« Physics - Grade 10 "

Recall the diffusion phenomenon from the basic school physics course.
How can this phenomenon be explained?

Previously, you learned what diffusion, i.e., the penetration of molecules of one substance into the intermolecular space of another substance. This phenomenon is determined by the random movement of molecules. This can explain, for example, the fact that the volume of a mixture of water and alcohol is less than the volume of its components.

But the most obvious evidence of the movement of molecules can be obtained by observing under a microscope the smallest particles of any solid substance suspended in water. These particles move randomly, which is called Brownian.

Brownian motion- this is the thermal movement of particles suspended in a liquid (or gas).

Observation of Brownian motion.

The English botanist R. Brown (1773-1858) first observed this phenomenon in 1827, examining the moss spores suspended in water under a microscope.

Later, he considered other small particles, including particles of stone from the Egyptian pyramids. Now, to observe Brownian motion, particles of gummigut paint, which is insoluble in water, are used. These particles move randomly. The most striking and unusual thing for us is that this movement never stops. We are accustomed to the fact that any moving body sooner or later stops. Brown initially thought that the spores of the club moss showed signs of life.

Brownian motion is thermal motion, and it cannot stop. As the temperature increases, its intensity increases.

Figure 8.3 shows the trajectories of Brownian particles. The positions of the particles marked with dots are determined at regular intervals of 30 s. These points are connected by straight lines. In reality, the particle trajectory is much more complicated.

Explanation of Brownian motion.

Brownian motion can be explained only on the basis of molecular-kinetic theory.

“Few phenomena can captivate the observer as much as Brownian motion. Here the observer is allowed to look behind the scenes of what happens in nature. A new world opens before him - a non-stop hustle and bustle of a huge number of particles. The smallest particles fly quickly 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 practically jostle in place. Their protrusions clearly show the rotation of particles around their axis, which constantly changes direction in space. Nowhere is there a trace of system or order. The dominance of blind chance - that's what a strong, overwhelming impression this picture makes on the observer. R. Paul (1884-1976).

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.

Figure 8.4 schematically shows the position of one Brownian particle and the molecules closest to it.

When molecules move randomly, the impulses they transmit to a Brownian particle, for example, from the left and from the right, are not the same. Therefore, the resulting pressure force of liquid molecules on a Brownian particle is nonzero. This force causes a change in the motion of the particle.

The molecular-kinetic theory of Brownian motion was created in 1905 by A. Einstein (1879-1955). 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. In 1926, J. Perrin received Nobel Prize for the study of the structure of matter.

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 were 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 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, i.e., the concentration of molecules decreases rather quickly with height. Moreover, the larger the mass of molecules, the faster their concentration decreases with height.

Brownian particles participate in thermal motion. Since their interaction is negligible, the totality 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 gravitational field must decrease according to the same law as the concentration of gas molecules. This law is known.

Perrin, using a microscope of high magnification and a small depth of field (small depth of field), observed Brownian particles in very thin layers of liquid. Calculating 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.

All these facts testify to the correctness of the theory of Brownian motion and to the fact that Brownian particles participate in the thermal motion of molecules.

Counting Brownian particles at different heights allowed Perrin to determine Avogadro's constant in a completely new way. The value of this constant coincided with the previously known one.

One of the most convincing proofs of the reality of the movement of molecules is the phenomenon of the so-called Brownian movement, discovered in 1827 by the English botanist Brown when studying the smallest spores suspended in water. He found, when examined under a microscope at high magnification, that these spores were in constant erratic movement, as if performing a wild fantastic dance.

Further experiments showed that these movements are not related to the biological origin of the particles or to any fluid movements. Similar movements are made by any small particles suspended in a liquid or gas. Such erratic movements are made, for example, by particles of smoke in still air. This random movement of particles suspended in a liquid or gas is called Brownian motion.

Special studies have shown that the nature of Brownian motion depends on the properties of the liquid or gas in which the particles are suspended, but does not depend on the properties of the substance of the particles themselves. The speed of movement of Brownian particles increases with increasing temperature and with decreasing particle size.

All these regularities can be easily explained if we accept that the movements of suspended particles arise as a result of the shocks they experience from the moving molecules of the liquid or gas in which they are located.

Of course, every Brownian particle is subjected to such impacts from all sides. With the complete disorder of molecular motions, it would seem that the number of impacts falling on a particle from any direction should be exactly equal to the number of impacts from the opposite direction,

so that all these shocks must cancel each other out completely and the particles must remain motionless.

This is exactly what happens if the particles are not too small. But when we are dealing with microscopic particles, see), the situation is different. Indeed, from the fact that molecular motions are chaotic, it only follows that, on the average, the number of impacts in different directions is the same. But in such statistical system, like liquid or gas, deviations from average values ​​are inevitable. Such deviations from the average values ​​of certain quantities that occur in a small volume or over short periods of time are called fluctuations. If a body of ordinary dimensions is in a liquid or gas, then the number of shocks that it experiences from the side of the molecules is so great that neither individual shocks nor the random predominance of shocks in one direction over shocks in other directions can be noticed. For small particles total number the shocks they experience are comparatively small, so that the predominance of the number of shocks from one direction or the other becomes noticeable, and it is precisely due to such fluctuations in the number of shocks that those characteristic, as it were, convulsive movements of suspended particles arise, which are called Brownian motion.

It is clear that the motions of Brownian particles are not molecular motions: we see not the result of the impact of one molecule, but the result of the predominance of the number of impacts in one direction over the number of impacts in the opposite direction. Brownian motion only very clearly reveals the very existence of random molecular motions.

Thus, Brownian motion is explained by the fact that due to the random difference in the number of impacts of molecules on a particle from different directions, a certain resultant force of a certain direction arises. Since fluctuations are usually short-term, after a short period of time the direction of the resultant will change, and with it the direction of particle movement will also change. Hence the observed randomness of Brownian motions, reflecting the randomness of molecular motion.

We will now supplement the above qualitative explanation of Brownian motion with a quantitative consideration of this phenomenon. Its quantitative theory was first given by Einstein and, independently, by Smoluchowski (1905). We present here a simpler derivation of the main relation of this theory than those of these authors.

Due to the incomplete compensation of molecular impacts, as we have seen, a certain resultant force acts on a Brownian particle, under the action of which the particle moves. In addition to this force, the particle is affected by the friction force caused by the viscosity of the medium and directed against the force

For simplicity, we assume that the particle has the shape of a sphere of radius a. Then the friction force can be expressed by the Stokes formula:

where is the coefficient of internal friction of the liquid (or gas), the velocity of the particle. The equation of particle motion (Newton's second law) therefore has the form:

Here is the mass of the particle, its radius vector with respect to an arbitrary coordinate system, the velocity of the particle, and the resultant of the forces caused by the impacts of the molecules.

Consider the projection of the radius vector on one of the coordinate axes, for example, on the axis For this component, equation (7.1) will be rewritten in the form:

where is the component of the resulting force along the axis

Our task is to find the displacement x of a Brownian particle, which it receives under the influence of molecular impacts. Each of the particles is constantly subjected to collisions with molecules, after which it changes the direction of its movement. Various particles receive displacements that differ both in magnitude and in direction. The probable value of the sum of the displacements of all particles is equal to zero, since the displacements can have both positive and positive displacements with equal probability. negative sign. The average value of the particle displacement projection x will therefore be zero. However, the average value of the square of the displacement will not be equal to zero, i.e., the quantity x, since it does not change its sign when the sign of x changes. Let us therefore transform equation (7.2) so that it contains the quantity To do this, we multiply both sides of this equation by

We use the obvious identities:

Substituting these expressions into (7.3), we obtain:

This equality is valid for any particle and therefore it is also valid for the average values ​​of the quantities included in it,

if the averaging is carried out over a sufficiently large number of particles. Therefore, you can write:

where is the average value of the square of the particle's displacement, the average value of the square of its velocity. As for the average value of the quantity included in the equality, it is equal to zero, since for a large number of particles both positive and negative values. Equation (7.2) therefore takes the form:

The value in this equation is the average value of the square of the velocity projections on the axis. Since the motions of the particles are completely chaotic, the average values ​​of the squares of the velocity projections along all three coordinate axes must be equal to each other, i.e.

It is also obvious that the sum of these quantities should be equal to the average value of the square of the particle velocity

Consequently,

Thus, the expression of interest to us, which is included in (7.4), is equal to:

The value is the average kinetic energy Brownian particle. Colliding with molecules of a liquid or gas, Brownian particles exchange energy with them and are in thermal equilibrium with the medium in which they move. Therefore, the average kinetic energy of the translational motion of a Brownian particle must be equal to the average kinetic energy of the molecules

liquid (or gas), which, as we know, is

and therefore

The fact that the average kinetic energy of a Brownian particle is equal (as for a gas molecule!) is of fundamental importance. Indeed, the basic equation (3.1) we derived earlier is valid for any particles that do not interact with each other and perform chaotic motions. Whether it will be molecules invisible to the eye or much larger Brownian particles containing billions of molecules is immaterial. From the molecular-kinetic point of view, a Brownian particle can be treated as a giant molecule. Therefore, the expression for the average kinetic energy of such a particle must be the same as for the molecule. The velocities of Brownian particles, of course, are incomparably less, corresponding to their greater mass.

Let us now return to equation (7.4) and, taking into account (7.5), we rewrite it

This equation is easily integrated. Denoting we get:

and after separating the variables, our equation is transformed into:

Integrating the left side of this equation in the range from 0 to and the right side from to we get:

The value, as can be easily seen, is negligible under normal experimental conditions. Indeed, the size of Brownian particles does not exceed cm, the viscosity of a liquid is usually close to the viscosity of water, i.e. approximately equal to Therefore, if the time interval between successive observations of a Brownian particle exceeds which, of course, always takes place, then

For finite time intervals and corresponding displacements, equation (7.6) can be rewritten as:

The average value of the squared displacement of a Brownian particle over a time interval along the X axis, or any other axis, is proportional to this time interval.

Formula (7.7) makes it possible to calculate the average value of the square of displacements, and the average is taken over all the particles participating in the phenomenon. But this formula is also valid for the average value of the square of many successive displacements of a single particle in equal time intervals. From an experimental point of view, it is more convenient to observe the displacements of a single particle. Such observations were made by Perrin in 1909.

Perrin observed the movement of particles through a microscope, the eyepiece of which was equipped with a grid of mutually perpendicular lines that served as a coordinate system. Using the grid, Perrin marked on it the successive positions of one particle he had chosen at certain intervals of time (for example, 30 s). Connecting then the points marking the positions of the particle on the grid, he obtained a picture similar to that shown in Fig. 7. This figure shows both the displacements of the particle and their projections on the axis

It should be borne in mind that the motion of a particle is much more complicated than can be judged from Fig. 7, since the positions are marked here at not too short time intervals (of the order of 30 s). If these gaps are reduced, then it turns out that each straight line segment in the figure unfolds into the same complex zigzag trajectory as the entire figure. 7.

Since the constant can be determined from the state equation.

Perrin's experiments had great importance for the final substantiation of the molecular-kinetic theory.

Brownian motion- in natural science, the random movement of microscopic, visible particles of a solid substance suspended in a liquid (or gas), caused by the thermal movement of particles of a liquid (or gas).

Brownian motion occurs due to the fact that all liquids and gases consist of atoms or molecules - the smallest particles that are in constant chaotic thermal motion, and therefore continuously push the Brownian particle from different sides. It was found that large particles with dimensions of more than 5 microns practically do not participate in Brownian motion, smaller particles (less than 3 microns) move forward along very complex trajectories or rotate. When a large body is immersed in the medium, the shocks that occur in large numbers are averaged and form a constant pressure. If a large body is surrounded by a medium on all sides, then the pressure is practically balanced, only the lifting force of Archimedes remains - such a body smoothly floats up or sinks. If the body is small, like a Brownian particle, then pressure fluctuations become noticeable, which create a noticeable randomly changing force, leading to oscillations of the particle. Brownian particles usually do not sink or float, but are suspended in a medium.

The basic physical principle underlying Brownian motion is that the average kinetic energy of the movement of the molecules of a liquid (or gas) is equal to the average kinetic energy of any particle suspended in this medium. Therefore, the average kinetic energy< E> translational motion of a Brownian particle is equal to:

< E> =m<v 2 >/ 2 = 3kT/2,

where m is the mass of the Brownian particle, v- her speed k is the Boltzmann constant, T- temperature. We can see from this formula that the average kinetic energy of a Brownian particle, and hence the intensity of its motion, increases with increasing temperature.

The Brownian particle will move along a zigzag path, moving away gradually from the starting point. Calculations show that the value of the mean square of the displacement of a Brownian particle r 2 =x 2 +y 2 +z 2 is described by the formula:

< r 2 > = 6kTBt

where B- particle mobility, which is inversely proportional to the viscosity of the medium and particle size. This formula, called Einstein's formula, was experimentally confirmed with all possible care by the French physicist Jean Perrin (1870-1942). Based on the measurement of the parameters of the motion of a Brownian particle, Perrin obtained the values ​​of the Boltzmann constant and the Avogadro number, which are in good agreement with the values ​​obtained by other methods within the limits of measurement errors.

15. The first law of thermodynamics. Work, heat, internal energy.

Formulation: the amount of heat received by the system goes to changing its internal energy and doing work against external forces.

The first law (first law) of thermodynamics can be formulated as follows: "The change in the total energy of the system in quasi-static process is equal to the amount of heat Q reported to the system, in total with the change in energy associated with the amount of substance N at the chemical potential, and the work A "performed on the system by external forces and fields, minus the work A performed by the system itself against external forces":.

For an elementary amount of heat, elementary work, and a small increment (total differential) of internal energy, the first law of thermodynamics has the form:

The division of work into two parts, one of which describes the work done on the system, and the second - the work done by the system itself, emphasizes that these works can be done by forces of a different nature due to different sources of forces.

Internal energybody is the total energy of this body minus the kinetic energy of the body as a whole and the potential energy of the body in the external field of forces. The internal energy is a single-valued function of the state of the system. This means that whenever a system finds itself in a given state, its internal energy assumes the value inherent in this state, regardless of the system's history. Consequently, the change in internal energy during the transition from one state to another will always be equal to the difference between its values ​​in the final and initial states, regardless of the path along which the transition was made.

The internal energy of a body cannot be measured directly. You can only determine the change in internal energy: where is the heat supplied to the body, measured in joules, is the work done by the body against external forces, measured in joules

The internal energy of an ideal gas depends only on its temperature and does not depend on the volume. The molecular-kinetic theory leads to the following expression for the internal energy of one mole of an ideal monatomic gas (helium, neon, etc.), whose molecules perform only translational motion:

Since the potential energy of the interaction of molecules depends on the distance between them, in the general case, the internal energy U of the body depends, along with the temperature T, also on the volume V: U = U (T, V).

The internal energy of the body can change if acting on it external forces do work (positive or negative). For example, if a gas is compressed in a cylinder under a piston, then external forces do some positive work A on the gas. At the same time, the pressure forces acting on the piston from the gas do work A = -A". If the gas volume has changed by a small amount ΔV, then the gas does work pSΔx = pΔV, where p is the gas pressure, S is the area of ​​the piston, Δx is its displacement (Fig. 3.8.1). When expanding, the work done by the gas is positive, while when compressed, it is negative. In the general case, during the transition from some initial state (1) to the final state (2), the work of the gas is expressed by the formula:

or in the limit as ΔV i → 0:

The work is numerically equal to the area under the process graph on the diagram (p, V). The amount of work depends on how the transition from the initial state to the final state was made. On fig. 3.8.2 shows three different processes that change the gas from state (1) to state (2). In all three cases, the gas does different work.

The processes depicted in fig. 3.8.2 can also be carried out in the opposite direction; then job A will simply reverse sign. Processes of this kind, which can be carried out in both directions, are called reversible. Unlike a gas, liquids and solids change their volume little, so that in many cases the work done during expansion or contraction can be neglected. However, the internal energy of liquid and solid bodies can also change as a result of work. During mechanical processing of parts (for example, when drilling), they heat up. This means that their internal energy changes. Another example is Joule's experiment (1843) on determining the mechanical equivalent of heat When a spinner immersed in a liquid rotates, external forces do positive work (A "\u003e 0); in this case, the liquid heats up due to the presence of internal friction forces, i.e. i.e. its internal energy increases.In these two examples, the processes cannot be carried out in the opposite direction.Such processes are called irreversible.

What is Brownian motion?

Now you will get acquainted with the most obvious proof of the thermal motion of molecules (the second main position of the molecular kinetic theory). Be sure to try to look through a microscope and see how the so-called Brownian particles move.

Previously, you learned what diffusion, i.e., the mixing of gases, liquids and solids in their direct contact. This phenomenon can be explained by the random movement of molecules and the penetration of molecules of one substance into the space between the molecules of another substance. This can explain, for example, the fact that the volume of a mixture of water and alcohol is less than the volume of its components. But the most obvious evidence of the movement of molecules can be obtained by observing under a microscope the smallest particles of any solid substance suspended in water. These particles move randomly, which is called Brownian.

This is the thermal movement of particles suspended in a liquid (or gas).

Observation of Brownian motion

The English botanist R. Brown (1773-1858) first observed this phenomenon in 1827, examining the moss spores suspended in water under a microscope. Later, he considered other small particles, including particles of stone from the Egyptian pyramids. Now, to observe Brownian motion, particles of gummigut paint, which is insoluble in water, are used. These particles move randomly. The most striking and unusual thing for us is that this movement never stops. We are accustomed to the fact that any moving body sooner or later stops. Brown initially thought that the spores of the club moss showed signs of life.

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

Brownian motion can also be observed in a gas. It is carried out by particles of dust or smoke suspended in the air.

The German physicist R. Pohl (1884-1976) colorfully describes the Brownian motion: “Few phenomena can captivate the observer as much as the Brownian motion. Here the observer is allowed to look behind the scenes of what happens in nature. A new world opens before him - a non-stop hustle and bustle of a huge number of particles. The smallest particles fly quickly 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 practically jostle in place. Their protrusions clearly show the rotation of particles around their axis, which constantly changes direction in space. Nowhere is there a trace of system or order. The dominance of blind chance - that's what a strong, overwhelming impression this picture makes on the observer.

At present, the concept Brownian motion used in a broader sense. For example, Brownian motion is the trembling of the arrows of sensitive measuring instruments, which occurs due to the thermal movement of the atoms of the instrument parts and the environment.

Explanation of Brownian motion

Brownian motion can be explained only 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.. Figure 8.4 schematically shows the position of one Brownian particle and the molecules closest to it. When molecules move randomly, the impulses they transmit to a Brownian particle, for example, from the left and from the right, are not the same. Therefore, the resulting pressure force of liquid molecules on a Brownian particle is nonzero. This force causes a change in the motion of the particle.

The mean 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 the molecule, then the pressure force acting on it changes abruptly from zero to a certain value when the molecule enters this area.

The molecular-kinetic theory of Brownian motion was created in 1905 by A. Einstein (1879-1955).

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.

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 were 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 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, i.e., the concentration of molecules decreases rather quickly with height. Moreover, the greater the mass of molecules, the faster their concentration decreases with height.

Brownian particles participate in thermal motion. Since their interaction is negligible, the aggregate 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 gravitational field must decrease according to the same law as the concentration of gas molecules. This law is known.

Perrin, using a microscope of high magnification and a small depth of field (small depth of field), observed Brownian particles in very thin layers of liquid. Calculating 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 in a completely new way. 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.

You have clearly seen the existence of thermal motion; We saw the chaotic movement going on. Molecules move even more randomly than Brownian particles.

The essence of the phenomenon

Now let's try to understand the essence of the phenomenon of Brownian motion. And it happens because all absolutely liquids and gases consist of atoms or molecules. But we also know that these smallest particles, being in continuous chaotic motion, constantly push the Brownian particle from different sides.

But here's what's interesting, scientists have proven that particles of larger sizes that exceed 5 microns remain motionless and almost do not participate in Brownian motion, which cannot be said about smaller particles. Particles with a size of less than 3 microns are able to move forward, making rotations or writing out complex trajectories.

When immersed in the environment of a large body, the tremors occurring in a huge number seem to come out average level and maintain constant pressure. In this case, the theory of Archimedes comes into play, since a large body surrounded by a medium on all sides balances the pressure and the remaining lifting force allows this body to float or sink.

But if the body has dimensions such as a Brownian particle, that is, completely imperceptible, then pressure deviations become noticeable, which contribute to the creation of a random force that leads to oscillations of these particles. It can be concluded that Brownian particles in the medium are in suspension, in contrast to large particles that sink or float.

Significance of Brownian motion

Let's try to figure out if Brownian motion in the natural environment has any meaning:

First, Brownian motion plays a significant role in plant nutrition from the soil;
Secondly, in human and animal organisms, the absorption of nutrients occurs through the walls of the digestive organs due to Brownian motion;
Thirdly, in the implementation of skin respiration;
And lastly, Brownian motion matters in the spread of harmful substances in the air and in water.

Homework

Read the questions carefully and give written answers to them:

1. Remember what is called diffusion?
2. What is the relationship between diffusion and thermal motion of molecules?
3. Define Brownian motion.
4. What do you think, is Brownian motion thermal, and justify your answer?
5. Will the nature of Brownian motion change when heated? If it changes, then how?
6. What instrument is used in the study of Brownian motion?
7. Does the pattern of Brownian motion change with increasing temperature, and how exactly?
8. Will there be any change in Brownian motion if the aqueous emulsion is replaced with glycerol?

G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics Grade 10