Brownian motion. Brownian motion - Knowledge Hypermarket Brownian motion galileo
Today we will take a closer look important topic- Let's define the Brownian motion of small pieces of matter in a liquid or gas.
Map and coordinates
Some schoolchildren, tormented by boring lessons, do not understand why they should study physics. Meanwhile, it was this science that once made it possible to discover America!
Let's start from afar. In a sense, the ancient civilizations of the Mediterranean were lucky: they developed on the shores of a closed inland reservoir. The Mediterranean Sea is called so because it is surrounded on all sides by land. And ancient travelers could advance quite far with their expedition without losing sight of the shores. The outlines of the land helped to navigate. And the first maps were drawn more descriptively than geographically. Thanks to these relatively short voyages, the Greeks, Phoenicians and Egyptians learned how to build ships well. And where the best equipment is, there is the desire to push the boundaries of your world.
Therefore, one fine day, the European powers decided to go out into the ocean. While sailing through the vast expanses between the continents, sailors saw only water for many months, and they had to somehow navigate. The invention of an accurate watch and a high-quality compass helped determine their coordinates.
Clock and compass
The invention of small hand-held chronometers helped navigators a lot. To determine exactly where they were, they needed to have a simple instrument that measured the height of the sun above the horizon, and know exactly when it was noon. And thanks to the compass, the captains of the ships knew where they were going. Both the clock and the properties of the magnetic needle were studied and created by physicists. Thanks to this, the whole world was opened to Europeans.
The new continents were terra incognita, uncharted lands. Strange plants grew on them and incomprehensible animals were found.
Plants and physics
All natural scientists of the civilized world rushed to study these new strange ecological systems. And of course, they wanted to take advantage of them.
Robert Brown was an English botanist. He made trips to Australia and Tasmania, collecting plant collections there. Already at home, in England, he worked hard on the description and classification of the brought material. And this scientist was very meticulous. Once, while observing the movement of pollen in plant sap, he noticed that small particles constantly make chaotic zigzag movements. This is the definition of the Brownian motion of small elements in gases and liquids. Thanks to the discovery, the amazing botanist wrote his name into the history of physics!
Brown and Gooey
In European science, it is customary to name an effect or phenomenon by the name of the one who discovered it. But often it happens by accident. But a person who describes, discovers the importance, or explores a physical law in more detail, finds himself in the shadows. So it happened with the Frenchman Louis Georges Gui. It was he who gave the definition of Brownian motion (Grade 7 definitely does not hear about him when he studies this topic in physics).
Gouy's research and properties of Brownian motion
The French experimenter Louis Georges Gouy observed the movement of various types of particles in several liquids, including solutions. The science of that time already knew how to accurately determine the size of pieces of matter up to tenths of a micrometer. Exploring what Brownian motion is (it was Gouy who gave the definition in physics to this phenomenon), the scientist realized that the intensity of the movement of particles increases if they are placed in a less viscous medium. Being a broad-spectrum experimenter, he exposed the suspension to the action of light and electromagnetic fields of various powers. The scientist found that these factors do not affect the chaotic zigzag jumps of particles. Gouy unequivocally showed what Brownian motion proves: the thermal movement of the molecules of a liquid or gas.
Collective and mass
And now we will describe in more detail the mechanism of zigzag jumps of small pieces of matter in a liquid.
Any substance is made up of atoms or molecules. These elements of the world are very small, not a single optical microscope is able to see them. In a liquid, they vibrate and move all the time. When any visible particle enters the solution, its mass is thousands of times greater than one atom. Brownian motion liquid molecules occurs randomly. But nevertheless, all atoms or molecules are a collective, they are connected to each other, like people who join hands. Therefore, sometimes it happens that the atoms of the liquid on one side of the particle move in such a way that they "press" on it, while on the other side of the particle a less dense medium is created. Therefore, the dust particle moves in the space of the solution. Elsewhere, the collective motion of fluid molecules randomly acts on the other side of the more massive component. This is exactly how the Brownian motion of particles takes place.
Time and Einstein
If a substance has a non-zero temperature, its atoms perform thermal vibrations. Therefore, even in a very cold or supercooled liquid, Brownian motion exists. These chaotic jumps of small suspended particles never stop.
Albert Einstein is perhaps the most famous scientist of the twentieth century. Everyone who is at least somewhat interested in physics knows the formula E = mc 2 . Also, many can remember the photoelectric effect for which he was given Nobel Prize, and about special theory relativity. But few people know that Einstein developed the formula for Brownian motion.
Based on the molecular kinetic theory, the scientist derived the diffusion coefficient of suspended particles in a liquid. And it happened in 1905. The formula looks like this:
D = (R * T) / (6 * N A * a * π * ξ),
where D is the desired coefficient, R is the universal gas constant, T is absolute temperature(expressed in Kelvin), N A is Avogadro's constant (corresponding to one mole of a substance, or about 10 23 molecules), a is the approximate average radius of the particles, ξ is the dynamic viscosity of the liquid or solution.
And already in 1908, the French physicist Jean Perrin and his students experimentally proved the correctness of Einstein's calculations.
One particle in the warrior field
Above, we described the collective action of the medium on many particles. But even one foreign element in a liquid can give some regularities and dependencies. For example, if you observe a Brownian particle for a long time, then you can fix all its movements. And out of this chaos, a coherent system will emerge. The average advance of a Brownian particle along any one direction is proportional to time.
During experiments on a particle in a liquid, the following quantities were refined:
- Boltzmann's constant;
- Avogadro's number.
Apart from linear motion, chaotic rotation is also characteristic. And the average angular displacement is also proportional to the observation time.
Sizes and shapes
After such reasoning, a logical question may arise: why is this effect not observed for large bodies? Because when the length of an object immersed in a liquid is greater than a certain value, then all these random collective “shocks” of molecules turn into constant pressure, as they are averaged. And the general Archimedes is already acting on the body. Thus, a large piece of iron sinks, and metal dust floats in the water.
The particle size, on the example of which the fluctuation of liquid molecules is revealed, should not exceed 5 micrometers. As for objects with large sizes, this effect will not be noticeable here.
- Forces of interaction between molecules……………………4
- Why was Giordano Bruno burnt?...................... ................7
- Did Galileo Galilei renounce his scientific views? .......... .............................. .......9
- Bibliography……………………………......... ........................... ... .13
Brownian motion
Brownian motion, random movement of small particles suspended in a liquid or gas, occurring under the action of shocks from the molecules of the environment. Opened by Robert Brown in 1827. Visible only under a microscope, suspended particles move independently of each other and describe complex zigzag trajectories. Brownian motion does not weaken with time and does not depend on the chemical properties of the medium. The intensity of the Brownian motion increases with an increase in the temperature of the medium and with a decrease in its viscosity and particle size.
When observing Brownian motion, the position of a particle is fixed at regular intervals. Of course, between observations, the particle does not move in a straight line, but the connection of successive positions by straight lines gives a conditional picture of movement.
The theory of Brownian motion explains the random motion of a particle by the action of random forces from molecules and friction forces. The random nature of the force means that its action for the time interval t 1 is completely independent of the action for the interval t 2 if these intervals do not overlap. The force averaged over a sufficiently long time is zero, and the average displacement of a Brownian particle also turns out to be zero.
The theory of Brownian motion has played an important role in the foundation of statistical mechanics. In addition, she also has practical value. First of all, Brownian motion limits the accuracy of measuring instruments. For example, the limit of accuracy of readings of a mirror galvanometer is determined by the trembling of the mirror, like a Brownian particle bombarded by air molecules. The laws of Brownian motion determine the random movement of electrons, causing noises
in electrical circuits. Dielectric losses in dielectrics are explained by the random motions of dipole molecules that make up the dielectric. Random movements of ions in electrolyte solutions increase their electrical resistance.
Interaction forces between molecules
Intermolecular interaction is the interaction between electrically neutral molecules or atoms . The forces of intermolecular interaction were first taken into accountJ. D. Van der Waals (1873
) to explain the properties of real gases and liquids.
Orientation forces act between polar molecules, that is, havingdipole electric moments. The attractive force between two polar molecules is maximum when their dipole moments are located along the same line. This force arises due to the fact that the distances between unlike charges are slightly less than between like charges. As a result, the attraction of dipoles exceeds their repulsion. The interaction of dipoles depends on their mutual orientation, and therefore the dipole interaction forces are called orientation. Chaotic thermal motion continuously changes the orientation of polar molecules, but, as the calculation shows, the average value of the force over all possible orientations has a certain value that is not equal to zero.
Inductive (or polarizing) forces act between polar and nonpolar molecules. The polar molecule createselectric field, which polarizes a molecule with electric charges uniformly distributed over the volume. Positive charges move in the direction of the electric field (that is, away from the positive pole), while negative charges move against (toward the positive pole). As a result, a dipole moment is induced in the nonpolar molecule.
This energy is called induction, since it appears due to the polarization of molecules caused byelectrostatic induction. Induction forces ( F ind ?r? 7) also act between polar molecules.
between non-polar molecules dispersion intermolecular interaction. The nature of this interaction was fully elucidated only after the creationquantum mechanics. In atoms and molecules electrons move in a complex way around the nuclei. On average over time, the dipole moments of nonpolar molecules turn out to be equal to zero. But at every moment the electrons occupy a certain position. Therefore, the instantaneous value of the dipole moment (for example, for a hydrogen atom) is nonzero. An instantaneous dipole creates an electric field that polarizes neighboring molecules. The result is an interaction instantaneous dipoles. The interaction energy between non-polar molecules is the average result of the interaction of all possible instantaneous dipoles with the dipole moments they induce in neighboring molecules due to induction.
Intermolecular interaction of this type is called dispersive because light dispersion in matter is determined by the same properties of molecules as this interaction. Dispersion forces act between all atoms and molecules, since the mechanism of their appearance does not depend on whether the molecules (atoms) have permanent dipole moments or not. Usually these forces are larger than both orientational and induction forces. Only when interacting molecules with large dipole moments, such as water molecules, F or
> F disp(3 times for water molecules). In the interaction of such polar molecules as CO, HI, HBr and others, dispersion forces are tens and hundreds of times greater than all the others.
It is very important that all three types of intermolecular interaction decrease in the same way with distance:
U = U or
+ U ind + U disp ?r
? 6
repulsive forces act between molecules at very small distances when filledelectron shellsatoms that make up molecules. Existing in quantum mechanics Pauli principle prohibits the penetration of filled electron shells into each other. The repulsive forces that arise in this case depend to a greater extent than the attractive forces on the individuality of the molecules
Why was Giordano Bruno burnt?
Bruno (Bruno) Giordano Philippe (1548, Nola, - 17.2.1600, Rome), Italian philosopher and poet, representative pantheism . Persecuted by churchmen for his views, he left Italy and lived in France, England, Germany. Upon his return to Italy (1592), he was accused of heresy and freethinking, and after eight years in prison, he was burned at the stake.
In Bruno's philosophy, ideas Neoplatonism (in particular, the idea of a single beginning and the world soul as the driving principle of the Universe, which led Bruno to hylozoism ) intersected with the strong influence of the views of ancient materialists, as well as the Pythagoreans. The design of Bruno's pantheistic natural philosophy, directed against scholastic Aristotelianism, was largely facilitated by Bruno's acquaintance with the philosophy of Nicholas of Cusa (from whom Bruno also learned the idea of "negative theology", based on the impossibility of a positive definition of God). Based on these sources, Bruno considered the goal of philosophy to be knowledge not of a supernatural god, but of nature, which is "God in things." Developing the heliocentric theory of N. Copernicus ,
which had a great influence on him, Bruno expressed ideas about the infinity of nature and an infinite number of worlds, asserted the physical homogeneity of the world (the doctrine of the 5 elements that make up all bodies - earth, water, fire, air and ether). The idea of a single infinite simple substance, from which many things arise, was associated by Bruno with the idea of internal kinship and the coincidence of opposites (“On Cause, Beginning and One”, 1584). In infinity, being identified, the straight line and the circle, the center and the periphery, the form and matter, etc. merge. The basic unit of being is monad , in whose activity corporal and spiritual, object and subject merge. The highest substance is the "monad of monads", or God; as a whole, it manifests itself in everything individual - "everything in everything." These ideas had a great influence on the development of the philosophy of modern times: the idea of a single substance in its relation to individual things was developed by Bruno Spinoza, the idea of a monad by G. Leibniz, the idea of the unity of being and the "coincidence of opposites" - in the dialectic of F. Schelling and G. Hegel. Thus, Bruno's philosophy was a transitional link from medieval philosophical systems to the philosophical concepts of modern times.
V. V. Sokolov.
In cosmology, Bruno expressed a number of conjectures that were ahead of his era and justified only by subsequent astronomical discoveries: about the existence of planets unknown in his time within our solar system, about the rotation of the Sun and stars around an axis (“On the Immeasurable and Incalculable”, 1591), about that in the Universe there are countless bodies similar to our Sun, etc. Bruno refuted medieval ideas about the opposition between Earth and sky and spoke out against anthropocentrism, speaking about the habitability of other worlds.
As a poet, Bruno belonged to the opponents of classicism. Actually, Bruno's work of art: the anti-clerical satirical poem "Noah's Ark", philosophical sonnets, the comedy "Candlestick" (1582, Russian translation 1940), in which Bruno breaks with the canons of "learned comedy" and creates a free dramatic form that allows a realistic depiction of life and customs Neapolitan street. In this comedy, Bruno ridicules pedantry and superstition, with caustic sarcasm he falls upon the stupid and hypocritical immorality that the Catholic reaction brought with it.
R. I. Khlodovsky
Did Galileo Galilei renounce his scientific views?
In 1609, on the basis of the information that reached him about the spotting scope invented in Holland, Galileo built his first telescope, giving approximately 3-fold magnification. The work of the telescope was demonstrated from the tower of St. Mark in Venice and made a huge impression. Soon Galileo built a telescope with a magnification of 32 times. The observations made with its help destroyed the "ideal spheres" of Aristotle and the dogma of the perfection of celestial bodies: the surface of the Moon turned out to be covered with mountains and pitted with craters, the stars lost their apparent size, and for the first time their colossal remoteness was comprehended. Jupiter discovered 4 satellites, a huge number of new stars became visible in the sky. Milky Way broke up into separate stars. Galileo described his observations in The Starry Messenger (1610-11), which made a stunning impression. At the same time, a fierce controversy began. Galileo was accused of the fact that everything he saw was an optical illusion, and they argued simply that his observations contradict Aristotle, and therefore are erroneous.
Astronomical discoveries served as a turning point in the life of Galileo: he freed himself from teaching and, at the invitation of Duke Cosimo II de Medici, moved to Florence. Here he becomes the court "philosopher" and the "first mathematician" of the university, without the obligation to lecture.
Continuing telescopic observations, Galileo discovered the phases of Venus, sunspots and the rotation of the Sun, studied the motion of Jupiter's satellites, and observed Saturn. In 1611, Galileo traveled to Rome, where he was given an enthusiastic reception at the papal court and where he struck up a friendship with Prince Cesi, the founder of the Accademia dei Lincei ("Academy of the Lynx-eyed"), of which he became a member. At the insistence of the duke, Galileo published his first anti-Aristotelian essay - "Discourse on bodies in water and those that move in it" (1612), where he applied the principle of equal moments to the derivation of equilibrium conditions in liquid bodies.
However, in 1613 Galileo's letter to Abbot Castelli became known, in which he defended the views of Copernicus. The letter served as a pretext for a direct denunciation of Galileo to the Inquisition. In 1616, the Jesuit congregation declared the teachings of Copernicus heretical, the book of Copernicus was included in the list of banned books. The name of Galileo was not named in the decree, but he was privately ordered to refuse to defend this doctrine. Galileo formally obeyed the decree. For several years he was forced to remain silent about the Copernican system or to speak about it in hints. Galileo travels to Rome in 1616. In the papal palace, theologians, the so-called "preparers of cases for the Inquisition" gather to discuss and test the Copernican doctrine, and then issue an edict forbidding the preaching of Copernican views. This was the first official ban. But Galileo did not abandon his views. Just got more careful. Deprived of the right to preach the teachings of Copernicus, he directed his criticism against Aristotle. Galileo's only major work during this period was The Assayer, a polemical treatise on the three comets that appeared in 1618. In terms of literary form, wit, and refinement of style, this is one of Galileo's most remarkable works.
Convinced of the validity of the Copernican system, Galileo set to work on a large astronomical treatise “Dialogue of two major systems world - Ptolemaic and Copernican" (1632). In this work, the advantages of the Copernican doctrine are so convincingly proved, and the pope, bred under the guise of a simple-minded loser Simplicio, a supporter of the Aristotelian concept, looks like such a fool that the thunder was not slow to strike. Dad was offended. Galileo's enemies took advantage of this and summoned him to court. The spirit of the seventy-year-old Galileo was broken. The aged scientist was forced to public repentance, and in the last years of his life he spent under house arrest and the supervision of the Inquisition. In 1635 he renounced "his heretical doctrine." The scientist Galileo was not a hero. He pleaded defeated. But in the history of science, he remained a great scientist, and the trial of Galileo, even in the words of the adherents of the Catholic religion, “was the most fatal mistake ever made by church authorities regarding science.”
In 1623 Galileo's friend Cardinal Maffeo Barberini ascended the papacy under the name of Urban VIII. For Galileo, this event seemed tantamount to liberation from the bonds of the interdict (decree). In 1630, he arrived in Rome with the finished manuscript of the Dialogue on the Ebb and Flow (the first title of the Dialogue on the Two Chief Systems of the World), in which the systems of Copernicus and Ptolemy are presented in the conversations of three interlocutors: Sagredo, Salviati and Simplicio.
etc.................
« 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. Before him opens new world- 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 the Nobel Prize for his 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 large mass Brownian particles decrease 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.
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 with 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 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 molecular diameters, 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
The Scottish botanist Robert Brown, during his lifetime, as the best connoisseur of plants, received the title of "prince of botanists." He made many wonderful discoveries. In 1805, after a four-year expedition to Australia, he brought to England about 4,000 species of Australian plants unknown to scientists and spent many years studying them. Described plants brought from Indonesia and Central Africa. Studied plant physiology, first described the nucleus in detail plant cell. But the name of the scientist is now widely known not because of these works.
In 1827, Brown conducted research on plant pollen. He, in particular, was interested in how pollen is involved in the process of fertilization. Once, under a microscope, he examined elongated cytoplasmic grains suspended in water isolated from the pollen cells of the North American plant Clarkia pulchella (pretty clarkia). Suddenly, Brown saw that the smallest hard grains, which could hardly be seen in a drop of water, were constantly trembling and moving from place to place. He found that these movements, in his words, "are not associated either with flows in the liquid or with its gradual evaporation, but are inherent in the particles themselves."
Brown's observation was confirmed by other scientists. The smallest particles behaved as if they were alive, and the “dance” of the particles accelerated with increasing temperature and decreasing particle size and clearly slowed down when water was replaced by a more viscous medium. This amazing phenomenon never stopped: it could be observed for an arbitrarily long time. At first, Brown even thought that living creatures really got into the field of the microscope, especially since pollen is the male sex cells of plants, but particles from dead plants, even from those dried a hundred years earlier in herbariums, also led. Then Brown wondered if these were the "elementary molecules of living beings", which the famous French naturalist Georges Buffon (1707-1788), the author of the 36-volume Natural History, spoke about. This assumption fell away when Brown began to explore apparently inanimate objects; at first these were very small particles of coal, as well as soot and dust from the London air, then finely ground inorganic substances: glass, many different minerals. “Active molecules” were everywhere: “In every mineral,” Brown wrote, “that I managed to grind into dust to such an extent that it could be suspended in water for some time, I found, in greater or lesser quantities, these molecules.
For about 30 years, Brown's discovery did not attract the interest of physicists. The new phenomenon was not given of great importance, believing that it is due to the trembling of the drug, or analogous to the movement of dust particles, which is observed in the atmosphere when a ray of light falls on them, and which, as was known, is caused by the movement of air. But if the motions of Brownian particles were caused by some flows in the liquid, then such neighboring particles would move in concert, which contradicts the observational data.
The explanation of Brownian motion (as this phenomenon was called) by the motion of invisible molecules was given only in the last quarter of the 19th century, but was not immediately accepted by all scientists. In 1863, Ludwig Christian Wiener (1826-1896), a teacher of descriptive geometry from Karlsruhe (Germany), suggested that the phenomenon is associated with the oscillatory movements of invisible atoms. It is important that Wiener saw an opportunity to penetrate the secrets of the structure of matter with the help of this phenomenon. He first tried to measure the speed of movement of Brownian particles and its dependence on their size. But Wiener's conclusions were complicated by the introduction of the concept of "atoms of the ether" in addition to the atoms of matter. In 1876, William Ramsay, and in 1877 the Belgian Jesuit priests Carbonel, Delso and Tirion, and finally, in 1888, Hui, clearly showed the thermal nature of Brownian motion [5].
“With a large area,” wrote Delso and Carbonel, “the impacts of molecules that cause pressure do not cause any shaking of the suspended body, because they together create uniform pressure on the body in all directions. But if the area is not sufficient to compensate for the unevenness, it is necessary to take into account the inequality of pressures and their continuous change from point to point. Law big numbers does not now reduce the effect of collisions to an average uniform pressure, their resultant will no longer be equal to zero, but will continuously change its direction and its magnitude.
If this explanation is accepted, then the phenomenon of thermal motion of liquids, postulated by the kinetic theory, can be said to be proven ad oculos (visibly). Just as it is possible, without distinguishing the waves in the distance from the sea, this will explain the rocking of the boat on the horizon by waves, in the same way, without seeing the movement of molecules, one can judge it by the movement of particles suspended in the liquid.
This explanation of Brownian motion is not only important as a confirmation of the kinetic theory, it also has important theoretical consequences. According to the law of conservation of energy, a change in the speed of a suspended particle must be accompanied by a change in temperature in the immediate vicinity of this particle: this temperature increases if the speed of the particle decreases, and decreases if the speed of the particle increases. Thus, the thermal equilibrium of a liquid is a statistical equilibrium.
An even more significant observation was made in 1888 by Huy: Brownian motion, strictly speaking, does not obey the second law of thermodynamics. Indeed, when a suspended particle rises spontaneously in a liquid, part of the heat of its environment spontaneously transforms into mechanical work, which is forbidden by the second law of thermodynamics. Observations, however, have shown that the particle rises less frequently, the heavier the particle. For particles of matter of ordinary sizes, this probability of such an uplift is practically zero.
Thus the second law of thermodynamics becomes a law of probability rather than a law of necessity. Previously, no experience has supported this statistical interpretation. It was enough to deny the existence of molecules, as was done, for example, by the school of energetics, which flourished under the leadership of Mach and Ostwald, for the second law of thermodynamics to become the law of necessity. But after the discovery of Brownian motion, a strict interpretation of the second law became already impossible: there was a real experience that showed that the second law of thermodynamics is constantly violated in nature, that a perpetual motion machine of the second kind is not only not excluded, but is constantly being realized right before our eyes.
Therefore, at the end of the last century, the study of Brownian motion acquired great theoretical significance and attracted the attention of many theoretical physicists, and in particular Einstein.
