Today we will take a closer look at an important topic - we will define 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 study physics. Meanwhile, it was this science that once made it possible to discover America!
Let's start from afar. The ancient civilizations of the Mediterranean were, in a sense, lucky: they developed on the shores of a closed inland body of water. The Mediterranean Sea is called that way because it is surrounded on all sides by land. And the ancient travelers could travel 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 up descriptively rather than geographically. Thanks to these relatively short voyages, the Greeks, Phoenicians and Egyptians became very good at building ships. 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 enter the ocean. While sailing across the endless expanses between the continents, the sailors saw only water for many months, and they had to somehow find their way. The invention of accurate watches and a high-quality compass helped determine one’s coordinates.
Clock and compass
The invention of small hand-held chronometers greatly helped sailors. To determine exactly where they were, they needed to have a simple instrument that measured the height of the sun above the horizon, and to know when exactly noon was. And thanks to the compass, ship captains 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, unexplored lands. Strange plants grew on them and strange animals were found.
Plants and Physics
All naturalists of the civilized world rushed to study these new strange ecological systems. And of course, they sought to benefit from them.
Robert Brown was an English botanist. He traveled 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. One day, while observing the movement of pollen in plant sap, he noticed: small particles constantly make chaotic zigzag movements. This is the definition of Brownian motion of small elements in gases and liquids. Thanks to the discovery, the amazing botanist wrote his name in the history of physics!
Brown and Gooey
In European science it is customary to name an effect or phenomenon after the person who discovered it. But often this happens by accident. But the person who describes, discovers the importance of, or explores in more detail a physical law finds himself in the shadows. This happened with the Frenchman Louis Georges Gouy. It was he who gave the definition of Brownian motion (7th grade definitely doesn’t hear about it when studying this topic in physics).
Gouy's research and properties of Brownian motion
French experimenter Louis Georges Gouy observed the movement of different types of particles in several liquids, including solutions. The science of that time was already able to accurately determine the size of pieces of matter down to tenths of a micrometer. While exploring what Brownian motion is (it was Gouy who gave the definition of this phenomenon in physics), the scientist realized: the intensity of particle movement increases if they are placed in a less viscous medium. Being a broad-spectrum experimenter, he exposed the suspension to light and electromagnetic fields different power. The scientist found that these factors do not in any way affect the chaotic zigzag jumps of particles. Gouy unambiguously showed what Brownian motion proves: the thermal movement of molecules of a liquid or gas.
Team and mass
Now let’s describe in more detail the mechanism of zigzag jumps of small pieces of matter in a liquid.
Any substance consists of atoms or molecules. These elements of the world are very small; no optical microscope can see them. In liquid they oscillate and move all the time. When any visible particle enters a solution, its mass is thousands of times greater than one atom. Brownian motion liquid molecules occurs chaotically. But nevertheless, all atoms or molecules are a collective, they are connected to each other, like people who join hands. Therefore, it sometimes happens that the atoms of the liquid on one side of the particle move in such a way that they “press” on it, while a less dense environment is created on the other side of the particle. Therefore, the dust particle moves in the space of the solution. Elsewhere, the collective movement of fluid molecules randomly affects the other side of a more massive component. This is exactly how Brownian motion of particles occurs.
Time and Einstein
If a substance has a non-zero temperature, its atoms undergo 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. Anyone 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 the Nobel Prize, and special theory relativity. But few people know that Einstein developed a formula for Brownian motion.
Based on molecular kinetic theory, the scientist derived the diffusion coefficient of suspended particles in liquid. And this 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 the absolute temperature (expressed in Kelvin), N A is Avogadro’s constant (corresponds to one mole of a substance, or approximately 10 23 molecules), a is the approximate average radius of particles, ξ is the dynamic viscosity of a 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 influence of the environment on many particles. But even one foreign element in a liquid can give rise to some patterns and dependencies. For example, if you observe a Brownian particle for a long time, then all its movements can be recorded. And out of this chaos a harmonious system will emerge. The average movement of a Brownian particle along any one direction is proportional to time.
In experiments on a particle in a liquid, the following quantities were refined:
- Boltzmann's constant;
- Avogadro's number.
In addition to 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 extent of an object immersed in a liquid is greater than a certain value, then all these random collective “pushes” 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 size of particles, as an example of which the fluctuation of liquid molecules is revealed, should not exceed 5 micrometers. As for large objects, this effect will not be noticeable.
- Interaction forces between molecules……………………4
- Why was Giordano Bruno burned?............................................7
- Did Galileo Galilei renounce his scientific views? .......... .............................. .......9
- Bibliography……………………………......... ........................... ... .13
Brownian motion
Brownian motion, the random movement of small particles suspended in a liquid or gas, occurring under the influence of shocks from the molecules of the environment. Opened by Robert Brown in 1827. Suspended particles, visible only under a microscope, move independently of each other and describe complex zigzag trajectories. Brownian motion does not weaken over time and does not depend on the chemical properties of the medium. The intensity of Brownian motion increases with increasing temperature of the medium and with a decrease in its viscosity and particle size.
When observing Brownian motion, the position of the particle is recorded at regular intervals. Of course, between observations the particle does not move rectilinearly, but connecting successive positions with straight lines gives a conventional picture of the movement.
The theory of Brownian motion explains the random movements of a particle by the action of random forces from molecules and friction forces. The random nature of the force means that its action during the time interval t 1 is completely independent of the action during the interval t 2 if these intervals do not overlap. The average force over a sufficiently long time is zero, and the average displacement of the Brownian particle also turns out to be zero.
The theory of Brownian motion played an important role in the foundation of statistical mechanics. In addition, she also has practical significance. First of all, Brownian motion limits the accuracy of measuring instruments. For example, the limit of accuracy of the readings of a mirror galvanometer is determined by the vibration 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 movements of the 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 taken into account for the first timeJ. D. van der Waals (1873
) to explain the properties of real gases and liquids.
Orientation forces act between polar molecules, that is, those withelectric dipole moments. The force of attraction between two polar molecules is greatest when their dipole moments are aligned along the same line. This force arises due to the fact that the distances between unlike charges are slightly smaller than between like ones. As a result, the attraction of the dipoles exceeds their repulsion. The interaction of dipoles depends on their mutual orientation, and therefore the forces of dipole interaction are called orientational. Chaotic thermal motion continuously changes the orientation of polar molecules, but, as calculations show, the average value of the force over all possible orientations has a certain value that is not equal to zero.
Inductive (or polarization) forces act between polar and nonpolar molecules. A polar molecule createselectric field, which polarizes a molecule with electric charges uniformly distributed throughout the volume. Positive charges are displaced in the direction of the electric field (that is, away from the positive pole), and negative charges are shifted against (toward the positive pole). As a result, a dipole moment is induced in a nonpolar molecule.
This energy is called induction, since it appears due to the polarization of molecules caused byelectrostatic induction. Inductive forces ( F ind ?r? 7) also act between polar molecules.
Acts between non-polar molecules dispersive intermolecular interaction. The nature of this interaction was fully clarified only after the creationquantum mechanics. In atoms and molecules electrons move around the nuclei in a complex way. On average over time, the dipole moments of nonpolar molecules turn out to be zero. But at every moment the electrons occupy some position. Therefore, the instantaneous value of the dipole moment (for example, for a hydrogen atom) is different from zero. An instantaneous dipole creates an electric field that polarizes neighboring molecules. The result is an interaction instantaneous dipoles. The energy of interaction between nonpolar molecules is the average result of the interaction of all possible instantaneous dipoles with the dipole moments that they induce in neighboring molecules due to induction.
Intermolecular interaction of this type called dispersive because light dispersion in a substance 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 exceed in magnitude both orientational and inductive ones. Only during the interaction of molecules with large dipole moments, for example water molecules, F or
> F disp(3 times for water molecules). When interacting with such polar molecules as CO, HI, HBr and others, dispersion forces are tens and hundreds of times greater than all others.
It is very significant that all three types of intermolecular interactions decrease in the same way with distance:
U = U or
+ U ind + U disp ?r
? 6
Repulsive forces act between molecules at very short 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 depend, to a greater extent than the attractive forces, on the individuality of the molecules
Why was Giordano Bruno burned?
Bruno Giordano Filippe (1548, Nola, - 17.2.1600, Rome), Italian philosopher and poet, representative pantheism . Persecuted by clergy for his views, he left Italy and lived in France, England, and Germany. Upon returning to Italy (1592), he was accused of heresy and freethinking and, after eight years in prison, was burned at the stake.
In Bruno's philosophy ideas Neoplatonism (especially the ideas about a single beginning and the world soul as the driving principle of the Universe, which led Bruno to hylozoism ) crossed with the strong influence of the views of ancient materialists, as well as the Pythagoreans. The formation 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 N. Copernicus ,
which had a huge influence on him, Bruno expressed ideas about the infinity of nature and the 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). Bruno associated the idea of a single infinite simple substance from which many things arise with the idea of internal kinship and coincidence of opposites (“On Cause, Beginning and One,” 1584). In infinity, being identified, the straight line and the circle, the center and the periphery, form and matter, etc. merge. The basic unit of existence is monad , in the activity of which the bodily and spiritual, object and subject merge. The supreme 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 modern philosophy: 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 existence and the “coincidence of opposites” - in the dialectics 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 guesses 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 Innumerable,” 1591), about the that in the Universe there are countless bodies similar to our Sun, etc. Bruno refuted the medieval ideas about the opposition between the Earth and the sky and spoke against anthropocentrism, speaking about the habitability of other worlds.
As a poet, Bruno belonged to the opponents of classicism. Bruno's own work of art: the anti-clerical satirical poem "Noah's Ark", philosophical sonnets, the comedy "The 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, and with caustic sarcasm attacks the stupid and hypocritical immorality that the Catholic reaction brought with it.
R. I. Khlodovsky
Did Galileo Galilei renounce his scientific views?
In 1609, based on the information that reached him about the telescope invented in Holland, Galileo built his first telescope, giving approximately 3x magnification. The operation of the telescope was demonstrated from the tower of St. The stamp was in Venice and made a huge impression. Galileo soon built a telescope with a magnification of 32 times. Observations made with its help destroyed Aristotle’s “ideal spheres” 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 their colossal distance was understood for the first time. Jupiter discovered 4 satellites, and a huge number of new stars became visible in the sky. Milky Way broke up into separate stars. Galileo described his observations in the work “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; they also argued simply that his observations contradicted Aristotle, and therefore were erroneous.
Astronomical discoveries served as a turning point in Galileo’s life: he was freed from teaching and, at the invitation of Duke Cosimo II de’ Medici, moved to Florence. Here he becomes the court "philosopher" and "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 movement of the satellites of Jupiter, observed Saturn. In 1611, Galileo traveled to Rome, where he received an enthusiastic reception at the papal court and where he struck up a friendship with Prince Cesi, founder of the Accademia dei Lincei (“Lynx-Eyed Academy”), of which he became a member. At the insistence of the Duke, Galileo published his first anti-Aristotelian work, “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, a letter from Galileo to Abbot Castelli became known, in which he defended the views of Copernicus. The letter served as a reason for direct denunciation of Galileo to the Inquisition. In 1616, the Jesuit congregation declared the teachings of Copernicus heretical, and Copernicus's book was included in the list of prohibited books. Galileo was not named in the decree, but he was privately ordered to renounce his defense of this doctrine. Galileo formally submitted to the decree. For several years he was forced to remain silent about the Copernican system or speak about it in hints. Galileo travels to Rome in 1616. Theologians, the so-called “case preparers for the Inquisition,” gather in the papal palace to discuss and test the Copernican doctrine, and then issue an edict prohibiting the preaching of Copernicus’s views. This was the first official ban. But Galileo did not give up his views. I just became 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 sophistication of style, this is one of Galileo's most remarkable works
Convinced of the validity of the Copernican system, Galileo began to work on a large astronomical treatise, “Dialogue on the two most important systems of the world - Ptolemaic and Copernican” (1632). This work so convincingly proves the advantages of the Copernican teaching, and the pope, portrayed under the guise of the simple-minded loser Simplicio, a supporter of the Aristotelian concept, looks like such a fool that thunder was not slow to strike. Dad was offended. Galileo's enemies took advantage of this and he was summoned to court. The spirit of seventy-year-old Galileo was broken. The elderly scientist was forced to publicly repent, and in the last years of his life he spent under house arrest and surveillance of the Inquisition. In 1635 he renounced “his heretical teaching.” The scientist Galileo was not a hero. He admitted defeat. But in the history of science he remained a great scientist, and the trial of Galileo, even in the words of adherents of the Catholic religion, “was the most fatal mistake that church authorities have ever made regarding science.”
In 1623, Galileo's friend Cardinal Maffeo Barberini ascended the papal throne under the name of Urban VIII. For Galileo, this event seemed tantamount to liberation from the bonds of interdict (decree). In 1630, he arrived in Rome with the finished manuscript of the “Dialogue on the Ebb and Flow of the Tides” (the first title of the “Dialogue on the Two Major Systems of the World”), in which the systems of Copernicus and Ptolemy are presented in conversations between three interlocutors: Sagredo, Salviati and Simplicio.
etc.................
Brown's discovery.
The Scottish botanist Robert Brown (sometimes his last name is transcribed as Brown) during his lifetime, as the best plant expert, received the title “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 the physiology of plants, described the nucleus in detail for the first time plant cell. St. Petersburg Academy Sciences made him an honorary member. But the name of the scientist is now widely known not because of these works.
In 1827 Brown conducted research on plant pollen. He was particularly interested in how pollen participates in the process of fertilization. Once, under a microscope, he examined elongated cytoplasmic grains suspended in water from pollen cells of the North American plant Clarkia pulchella. Suddenly Brown saw that the smallest solid grains, which could barely 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 replacing water with a more viscous medium. This amazing phenomenon never stopped: it could be observed for as long as desired. At first, Brown even thought that living beings actually fell into the field of the microscope, especially since pollen is the male reproductive cells of plants, but there were also particles from dead plants, even from those dried a hundred years earlier in herbariums. Then Brown wondered if these were the “elementary molecules of living beings” that the famous French naturalist Georges Buffon (1707–1788), author of the 36-volume Natural History, spoke about. This assumption fell away when Brown began to examine apparently inanimate objects; at first it was 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,” wrote Brown, “which I have succeeded in pulverizing to such an extent that it can be suspended in water for some time, I have found, in greater or lesser quantities, these molecules."
It must be said that Brown did not have any of the latest microscopes. In his article, he specifically emphasizes that he had ordinary biconvex lenses, which he used for several years. And he goes on to say: “Throughout the entire study I continued to use the same lenses with which I began the work, in order to give more credibility to my statements and to make them as accessible as possible to ordinary observations.”
Now, to repeat Brown's observation, it is enough to have a not very strong microscope and use it to examine the smoke in a blackened box, illuminated through a side hole with a beam of intense light. In a gas, the phenomenon manifests itself much more clearly than in a liquid: small pieces of ash or soot (depending on the source of the smoke) are visible, scattering light, and continuously jumping back and forth.
As often happens in science, many years later historians discovered that back in 1670, the inventor of the microscope, the Dutchman Antonie Leeuwenhoek, apparently observed a similar phenomenon, but the rarity and imperfection of microscopes, the embryonic state of molecular science at that time did not attract attention to Leeuwenhoek’s observation, therefore the discovery is rightly attributed to Brown, who was the first to study and describe it in detail.
Brownian motion and atomic-molecular theory.
The phenomenon observed by Brown quickly became widely known. He himself showed his experiments to numerous colleagues (Brown lists two dozen names). But explain it mysterious phenomenon, which was called the “Brownian movement,” was not possible either by Brown himself or by many other scientists for many years. The movements of the particles were completely random: sketches of their positions made at different points in time (for example, every minute) did not at first glance make it possible to find any pattern in these movements.
An explanation of Brownian motion (as this phenomenon was called) by the movement of invisible molecules was given only in the last quarter of the 19th century, but was not immediately accepted by all scientists. In 1863, a teacher of descriptive geometry from Karlsruhe (Germany), Ludwig Christian Wiener (1826–1896), suggested that the phenomenon was associated with the oscillatory movements of invisible atoms. This was the first, although very far from modern, explanation of Brownian motion by the properties of the atoms and molecules themselves. It is important that Wiener saw the opportunity to use this phenomenon to penetrate the secrets of the structure of matter. He was the first to try to measure the speed of movement of Brownian particles and its dependence on their size. It is curious that in 1921, in the Proceedings of the National Academy of Sciences of the United States, a work on the Brownian motion of another Wiener, Norbert, the famous founder of cybernetics, was published.
The ideas of L.K. Wiener were accepted and developed by a number of scientists - Sigmund Exner in Austria (and 33 years later - his son Felix), Giovanni Cantoni in Italy, Karl Wilhelm Negeli in Germany, Louis Georges Gouy in France, three Belgian priests - Jesuits Carbonelli, Delso and Tirion and others. Among these scientists was the later famous English physicist and chemist William Ramsay. It gradually became clear that the smallest grains of matter were being hit from all sides by even smaller particles, which were no longer visible through a microscope - just as waves rocking a distant boat are not visible from the shore, while the movements of the boat itself are visible quite clearly. As they wrote in one of the articles in 1877, “...the law of large numbers no longer reduces the effect of collisions to average uniform pressure; their resultant will no longer be equal to zero, but will continuously change its direction and its magnitude.”
Qualitatively, the picture was quite plausible and even visual. A small twig or a bug should move in approximately the same way, pushed (or pulled) in different directions by many ants. These smaller particles were actually in the vocabulary of scientists, but no one had ever seen them. They were called molecules; Translated from Latin, this word means “small mass.” Amazingly, this is exactly the explanation given to a similar phenomenon by the Roman philosopher Titus Lucretius Carus (c. 99–55 BC) in his famous poem On the Nature of Things. In it, he calls the smallest particles invisible to the eye the “primordial principles” of things.
The principles of things first move themselves,
Following them are bodies from their smallest combination,
Close, as it were, in strength to the primary principles,
Hidden from them, receiving shocks, they begin to strive,
Themselves to move, then encouraging larger bodies.
So, starting from the beginning, the movement little by little
It touches our feelings and becomes visible too
To us and in the specks of dust that move in the sunlight,
Even though the tremors from which it occurs are imperceptible...
Subsequently, it turned out that Lucretius was wrong: it is impossible to observe Brownian motion with the naked eye, and dust particles in a sunbeam that penetrated into a dark room “dance” due to vortex movements of the air. But outwardly both phenomena have some similarities. And only in the 19th century. It became obvious to many scientists that the movement of Brownian particles is caused by random impacts of the molecules of the medium. Moving molecules collide with dust particles and other solid particles that are in the water. The higher the temperature, the faster the movement. If a speck of dust is large, for example, has a size of 0.1 mm (the diameter is a million times larger than that of a water molecule), then many simultaneous impacts on it from all sides are mutually balanced and it practically does not “feel” them - approximately the same as a piece of wood the size of a plate will not “feel” the efforts of many ants that will pull or push it in different directions. If the dust particle is relatively small, it will move in one direction or the other under the influence of impacts from surrounding molecules.
Brownian particles have a size of the order of 0.1–1 μm, i.e. from one thousandth to one ten-thousandth of a millimeter, which is why Brown was able to discern their movement because he was looking at tiny cytoplasmic grains, and not the pollen itself (which is often mistakenly written about). The problem is that the pollen cells are too large. Thus, in meadow grass pollen, which is carried by the wind and causes allergic diseases in humans (hay fever), the cell size is usually in the range of 20 - 50 microns, i.e. they are too large to observe Brownian motion. It is also important to note that individual movements of a Brownian particle occur very often and over very short distances, so that it is impossible to see them, but under a microscope, movements that have occurred over a certain period of time are visible.
It would seem that the very fact of the existence of Brownian motion clearly proved molecular structure matter, however, even at the beginning of the 20th century. There were scientists, including physicists and chemists, who did not believe in the existence of molecules. The atomic-molecular theory only slowly and with difficulty gained recognition. Thus, the leading French organic chemist Marcelin Berthelot (1827–1907) wrote: “The concept of a molecule, from the point of view of our knowledge, is uncertain, while another concept - an atom - is purely hypothetical.” The famous French chemist A. Saint-Clair Deville (1818–1881) spoke even more clearly: “I do not accept Avogadro’s law, nor the atom, nor the molecule, for I refuse to believe in what I can neither see nor observe.” And the German physical chemist Wilhelm Ostwald (1853–1932), laureate Nobel Prize, one of the founders of physical chemistry, back in the early 20th century. resolutely denied the existence of atoms. He managed to write a three-volume chemistry textbook in which the word “atom” is never even mentioned. Speaking on April 19, 1904, with a large report at the Royal Institution to members of the English Chemical Society, Ostwald tried to prove that atoms do not exist, and “what we call matter is only a collection of energies collected together in a given place.”
But even those physicists who accepted molecular theory, could not believe that so in a simple way The validity of the atomic-molecular theory was proven, so a variety of alternative reasons were put forward to explain the phenomenon. And this is quite in the spirit of science: until the cause of a phenomenon is unambiguously identified, it is possible (and even necessary) to assume various hypotheses, which should, if possible, be tested experimentally or theoretically. Thus, back in 1905, a short article by St. Petersburg physics professor N.A. Gezehus, teacher of the famous academician A.F. Ioffe, was published in the Brockhaus and Efron Encyclopedic Dictionary. Gesehus wrote that, according to some scientists, Brownian motion is caused by “light or heat rays passing through a liquid,” and boils down to “simple flows within a liquid that have nothing to do with the movements of molecules,” and these flows can be caused by “evaporation, diffusion and other reasons." After all, it was already known that a very similar movement of dust particles in the air is caused precisely by vortex flows. But the explanation given by Gesehus could easily be refuted experimentally: if you look at two Brownian particles located very close to each other through a strong microscope, their movements will turn out to be completely independent. If these movements were caused by any flows in the liquid, then such neighboring particles would move in concert.
Theory of Brownian motion.
At the beginning of the 20th century. most scientists understood the molecular nature of Brownian motion. But all explanations remained purely qualitative; no quantitative theory could withstand experimental testing. In addition, you yourself experimental results were unclear: the fantastic spectacle of non-stop rushing particles hypnotized the experimenters, and they did not know exactly what characteristics of the phenomenon needed to be measured.
Despite the apparent complete disorder, it was still possible to describe the random movements of Brownian particles by a mathematical relationship. For the first time, a rigorous explanation of Brownian motion was given in 1904 by the Polish physicist Marian Smoluchowski (1872–1917), who in those years worked at Lviv University. At the same time, the theory of this phenomenon was developed by Albert Einstein (1879–1955), a then little-known 2nd class expert at the Patent Office of the Swiss city of Bern. His article, published in May 1905 in the German journal Annalen der Physik, was entitled On the motion of particles suspended in a fluid at rest, required by the molecular kinetic theory of heat. With this name, Einstein wanted to show that the molecular kinetic theory of the structure of matter necessarily implies the existence of random motion of the smallest solid particles in liquids.
It is curious that at the very beginning of this article, Einstein writes that he is familiar with the phenomenon itself, albeit superficially: “It is possible that the movements in question are identical with the so-called Brownian molecular motion, but the data available to me regarding the latter are so inaccurate that I could not formulate a this is a definite opinion.” And decades later, already in his late life, Einstein wrote something different in his memoirs - that he did not know about Brownian motion at all and actually “rediscovered” it purely theoretically: “Not knowing that observations of “Brownian motion” have long been known, I discovered that the atomic theory leads to the existence of observable motion of microscopic suspended particles." Be that as it may, Einstein's theoretical article ended with a direct call to experimenters to test his conclusions experimentally: "If any researcher could soon answer the questions raised here questions!" – he ends his article with such an unusual exclamation.
The answer to Einstein's passionate appeal was not long in coming.
According to the Smoluchowski-Einstein theory, the average value of the squared displacement of a Brownian particle (s2) during time t is directly proportional to the temperature T and inversely proportional to the viscosity of the liquid h, the particle size r and Avogadro's constant
NA: s2 = 2RTt/6phrNA,
Where R is the gas constant. So, if in 1 minute a particle with a diameter of 1 μm moves by 10 μm, then in 9 minutes - by 10 = 30 μm, in 25 minutes - by 10 = 50 μm, etc. Under similar conditions, a particle with a diameter of 0.25 μm over the same periods of time (1, 9 and 25 min) will move by 20, 60 and 100 μm, respectively, since = 2. It is important that the above formula includes Avogadro’s constant, which thus , can be determined by quantitative measurements of the movement of a Brownian particle, which was done by the French physicist Jean Baptiste Perrin (1870–1942).
In 1908, Perrin began quantitative observations of the motion of Brownian particles under a microscope. He used an ultramicroscope, invented in 1902, which made it possible to detect the smallest particles by scattering light onto them from a powerful side illuminator. Perrin obtained tiny balls of almost spherical shape and approximately the same size from gum, the condensed sap of some tropical trees (it is also used as yellow watercolor paint). These tiny beads were suspended in glycerol containing 12% water; viscous liquid prevented the appearance of internal flows in it that would blur the picture. Armed with a stopwatch, Perrin noted and then sketched (of course, on a greatly enlarged scale) on a graphed sheet of paper the position of the particles at regular intervals, for example, every half minute. By connecting the resulting points with straight lines, he obtained intricate trajectories, some of which are shown in the figure (they are taken from Perrin's book Atomy, published in 1920 in Paris). Such a chaotic, disorderly movement of particles leads to the fact that they move in space quite slowly: the sum of the segments is much greater than the displacement of the particle from the first point to the last. 
Consecutive positions every 30 seconds of three Brownian particles - gum balls with a size of about 1 micron. One cell corresponds to a distance of 3 µm.
Consecutive positions every 30 seconds of three Brownian particles - gum balls with a size of about 1 micron. One cell corresponds to a distance of 3 µm. If Perrin could determine the position of Brownian particles not after 30, but after 3 seconds, then the straight lines between each neighboring points would turn into the same complex zigzag broken line, only on a smaller scale.
Using the theoretical formula and his results, Perrin obtained a fairly accurate value for Avogadro’s number for that time: 6.8.1023. Perrin also used a microscope to study the vertical distribution of Brownian particles (see AVOGADRO'S LAW) and showed that, despite the action of gravity, they remain suspended in solution. Perrin also owns other important works. In 1895 he proved that cathode rays are negative electric charges(electrons), in 1901 he first proposed a planetary model of the atom. In 1926 he was awarded the Nobel Prize in Physics.
The results obtained by Perrin confirmed Einstein's theoretical conclusions. It made a strong impression. As the American physicist A. Pais wrote many years later, “you never cease to be amazed at this result, obtained in such a simple way: it is enough to prepare a suspension of balls, the size of which is large compared to the size of simple molecules, take a stopwatch and a microscope, and you can determine Avogadro’s constant!” One might also be surprised: descriptions of new experiments on Brownian motion still appear in scientific journals (Nature, Science, Journal of Chemical Education) from time to time! After the publication of Perrin’s results, Ostwald, a former opponent of atomism, admitted that “the coincidence of Brownian motion with the requirements of the kinetic hypothesis... now gives the most cautious scientist the right to talk about experimental proof of the atomic theory of matter. Thus, the atomic theory has been elevated to the rank of a scientific, well-founded theory.” He is echoed by the French mathematician and physicist Henri Poincaré: “The brilliant determination of the number of atoms by Perrin completed the triumph of atomism... The atom of chemists has now become a reality.”
Brownian motion and diffusion.
The movement of Brownian particles is very similar in appearance to the movement of individual molecules as a result of their thermal motion. This movement is called diffusion. Even before the work of Smoluchowski and Einstein, the laws of molecular motion were established in the simplest case of the gaseous state of matter. It turned out that molecules in gases move very quickly - at the speed of a bullet, but they cannot fly far, since they very often collide with other molecules. For example, oxygen and nitrogen molecules in the air, moving at an average speed of approximately 500 m/s, experience more than a billion collisions every second. Therefore, the path of the molecule, if it were possible to follow it, would be a complex broken line. Brownian particles also describe a similar trajectory if their position is recorded at certain time intervals. Both diffusion and Brownian motion are a consequence of the chaotic thermal motion of molecules and are therefore described by similar mathematical relationships. The difference is that molecules in gases move in a straight line until they collide with other molecules, after which they change direction. A Brownian particle, unlike a molecule, does not perform any “free flights”, but experiences very frequent small and irregular “jitters”, as a result of which it chaotically shifts in one direction or the other. Calculations have shown that for a particle 0.1 µm in size, one movement occurs in three billionths of a second over a distance of only 0.5 nm (1 nm = 0.001 µm). As one author aptly puts it, this is reminiscent of moving an empty beer can in a square where a crowd of people has gathered.
Diffusion is much easier to observe than Brownian motion, since it does not require a microscope: movements are observed not of individual particles, but of their huge masses, you just need to ensure that diffusion is not superimposed by convection - mixing of matter as a result of vortex flows (such flows are easy to notice, placing a drop of a colored solution, such as ink, into a glass of hot water).
Diffusion is convenient to observe in thick gels. Such a gel can be prepared, for example, in a penicillin jar by preparing a 4–5% gelatin solution in it. The gelatin must first swell for several hours, and then it is completely dissolved with stirring by lowering the jar into hot water. After cooling, a non-flowing gel is obtained in the form of a transparent, slightly cloudy mass. If, using sharp tweezers, you carefully insert a small crystal of potassium permanganate (“potassium permanganate”) into the center of this mass, the crystal will remain hanging in the place where it was left, since the gel prevents it from falling. Within a few minutes, a violet-colored ball will begin to grow around the crystal; over time, it becomes larger and larger until the walls of the jar distort its shape. The same result can be obtained using a crystal of copper sulfate, only in this case the ball will turn out not purple, but blue.
It is clear why the ball turned out: the MnO4– ions formed during the dissolution of the crystal go into solution (the gel is mainly water) and, as a result of diffusion, move evenly in all directions, while gravity has practically no effect on the diffusion rate. Diffusion in liquid is very slow: it will take many hours for the ball to grow several centimeters. In gases, diffusion is much faster, but still, if the air were not mixed, the smell of perfume or ammonia would spread in the room for hours.
Brownian motion theory: random walks.
The Smoluchowski–Einstein theory explains the laws of both diffusion and Brownian motion. We can consider these patterns using the example of diffusion. If the speed of a molecule is u, then, moving in a straight line, it will cover a distance L = ut in time t, but due to collisions with other molecules, this molecule does not move in a straight line, but continuously changes the direction of its movement. If it were possible to sketch the path of a molecule, it would be fundamentally no different from the drawings obtained by Perrin. From such figures it is clear that, due to chaotic motion, the molecule is displaced by a distance s, significantly less than L. These quantities are related by the relation s =, where l is the distance that the molecule flies from one collision to another, the average free path. Measurements have shown that for air molecules at normal atmospheric pressure l ~ 0.1 μm, which means that at a speed of 500 m/s a nitrogen or oxygen molecule will fly a distance of L = 5000 km in 10,000 seconds (less than three hours), and will shift from its original position by only s = 0.7 m ( 70 cm), which is why substances move so slowly due to diffusion even in gases.
The path of a molecule as a result of diffusion (or the path of a Brownian particle) is called a random walk. Witty physicists reinterpreted this expression as drunkard's walk - “the path of a drunkard.” Indeed, the movement of a particle from one position to another (or the path of a molecule undergoing many collisions) resembles the movement of a drunk person. Moreover, this analogy also allows one to deduce quite simply the basic equation of such a process is based on the example of one-dimensional motion, which is easy to generalize to three-dimensional.
Suppose a tipsy sailor came out of a tavern late at night and headed along the street. Having walked the path l to the nearest lantern, he rested and went... either further, to the next lantern, or back, to the tavern - after all, he does not remember where he came from. The question is, will he ever leave the zucchini, or will he just wander around it, now moving away, now approaching it? (Another version of the problem states that there are dirty ditches at both ends of the street, where the streetlights end, and asks whether the sailor will be able to avoid falling into one of them.) Intuitively, it seems that the second answer is correct. But it is incorrect: it turns out that the sailor will gradually move further and further away from the zero point, although much more slowly than if he walked only in one direction. Here's how to prove it.
Having walked for the first time to the nearest lantern (to the right or to the left), the sailor will find himself at a distance s1 = ± l from the starting point. Since we are only interested in its distance from this point, but not its direction, we will get rid of the signs by squaring this expression: s12 = l2. After some time, the sailor, having already completed N “wanderings”, will be at a distance
SN = from start. And having passed again (in one direction) to the nearest lamp, at a distance sN+1 = sN ± l, or, using the square of the displacement, s2N+1 = s2N ±2sN l + l2. If the sailor repeats this movement many times (from N to N + 1), then as a result of averaging (he takes the Nth step to the right or to the left with equal probability), the term ±2sNl will be reduced, so that (angle brackets indicate the averaged value).
Since s12 = l2, then
S22 = s12 + l2 = 2l2, s32 = s22 + l2 = 3ll2, etc., i.e. s2N = Nl2 or sN =l. The total distance traveled L can be written both as the product of the sailor’s speed and the travel time (L = ut), and as the product of the number of wanderings and the distance between the lanterns (L = Nl), therefore, ut = Nl, whence N = ut/l and finally sN = . Thus, we obtain the dependence of the displacement of the sailor (as well as the molecule or Brownian particle) on time. For example, if there are 10 m between the lamps and a sailor walks at a speed of 1 m/s, then in an hour he common path will be L = 3600 m = 3.6 km, while the displacement from the zero point in the same time will be equal to only s = 190 m. In three hours it will cover L = 10.8 km, and will shift by s = 330 m and etc.
The product ul in the resulting formula can be compared with the diffusion coefficient, which, as shown by the Irish physicist and mathematician George Gabriel Stokes (1819–1903), depends on the particle size and the viscosity of the medium. Based on similar considerations, Einstein derived his equation.
The theory of Brownian motion in real life.
The theory of random walks has important practical applications. They say that in the absence of landmarks (the sun, stars, highway noise or railway etc.) a person wanders in the forest, across a field in a snowstorm or in thick fog in circles, all the time returning to his original place. In fact, he does not walk in circles, but approximately the same way molecules or Brownian particles move. He can return to his original place, but only by chance. But he crosses his path many times. They also say that people frozen in a snowstorm were found “some kilometer” from the nearest housing or road, but in reality the person had no chance of walking this kilometer, and here’s why.
To calculate how much a person will shift as a result of random walks, you need to know the value of l, i.e. the distance a person can walk in a straight line without any landmarks. This value was measured by Doctor of Geological and Mineralogical Sciences B.S. Gorobets with the help of student volunteers. He, of course, did not leave them in a dense forest or on a snow-covered field, everything was simpler - the student was placed in the center of an empty stadium, blindfolded and asked to walk to the end of the football field in complete silence (to exclude orientation by sounds). It turned out that on average the student walked in a straight line for only about 20 meters (the deviation from the ideal straight line did not exceed 5°), and then began to deviate more and more from the original direction. In the end, he stopped, far from reaching the edge.
Let now a person walk (or rather, wander) in the forest at a speed of 2 kilometers per hour (for a road this is very slow, but for a dense forest it is very fast), then if the value of l is 20 meters, then in an hour he will cover 2 km, but will move only 200 m, in two hours - about 280 m, in three hours - 350 m, in 4 hours - 400 m, etc. And moving in a straight line at such a speed, a person would walk 8 kilometers in 4 hours , therefore, in the safety instructions for field work there is the following rule: if landmarks are lost, you need to stay in place, set up a shelter and wait for the end of bad weather (the sun may come out) or for help. In the forest, landmarks - trees or bushes - will help you move in a straight line, and each time you need to stick to two such landmarks - one in front, the other behind. But, of course, it is best to take a compass with you...
« Physics - 10th grade"
Remember the phenomenon of diffusion from the basic school physics course.
How can this phenomenon be explained?
Previously, you learned what it is 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 constituent components.
But the most obvious evidence of the movement of molecules can be obtained by observing through a microscope the smallest particles of any substance suspended in water. solid. These particles undergo random motion, which is called Brownian.
Brownian motion 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 moss spores suspended in water through a microscope.
Later he looked at other small particles, including stone particles from Egyptian pyramids. Nowadays, to observe Brownian motion, they use particles of gum gum paint, which is insoluble in water. These particles move randomly. The most amazing and unusual thing for us is that this movement never stops. We are accustomed to the fact that any moving body stops sooner or later. Brown initially thought that the moss spores were showing 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 trajectory of particles is much more complex.
Explanation of Brownian motion.
Brownian motion can only be explained on the basis of molecular kinetic theory.
“Few phenomena can captivate an observer as much as Brownian motion. Here the observer is allowed to look behind the scenes of what is happening in nature. A new world opens up before him - a non-stop bustle of a huge number of particles. The smallest particles quickly fly through the field of view of the microscope, almost instantly changing the direction of movement. Larger particles move more slowly, but they also constantly change the direction of movement. Large particles are practically crushed in place. Their protrusions clearly show the rotation of particles around their axis, which constantly changes direction in space. There is no trace of system or order anywhere. The dominance of blind chance - that’s the strong, overwhelming impression this picture makes on the observer.” R. Paul (1884-1976).
The reason for 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 the Brownian particle, for example, to the left and to 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 particle's motion.
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 of Perrin's experiments is as follows. It is known that the concentration of gas molecules in the atmosphere decreases with altitude. If there were no thermal motion, then all the molecules would fall to the Earth and the atmosphere would disappear. However, if there were no attraction to the Earth, then due to thermal motion the molecules would leave the Earth, since gas is capable of unlimited expansion. As a result of the action of these opposing factors, a certain distribution of molecules in height is established, i.e., the concentration of molecules decreases quite quickly with height. Moreover, the greater the mass of molecules, the faster their concentration decreases with height.
Brownian particles participate in thermal movement. Since their interaction is negligible, the collection 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 should decrease according to the same law as the concentration of gas molecules. This law is known.
Perrin, using a high-magnification microscope with a shallow depth of field (shallow depth of field), observed Brownian particles in very thin layers of liquid. By 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 indicate the correctness of the theory of Brownian motion and that Brownian particles participate in the thermal motion of molecules.
Counting Brownian particles at different altitudes allowed Perrin to determine Avogadro's constant using a completely new method. The value of this constant coincided with the previously known one.
Small suspended particles move chaotically under the influence of impacts from liquid molecules.
In the second half of the 19th century, a serious debate about the nature of atoms flared up in scientific circles. On one side were irrefutable authorities such as Ernst Mach ( cm. Shock waves), who argued that atoms are simply mathematical functions that successfully describe observable physical phenomena and not based on reality physical basis. On the other hand, scientists of the new wave - in particular, Ludwig Boltzmann ( cm. Boltzmann's constant)—insisted that atoms were physical realities. And neither of the two sides realized that already decades before the start of their dispute, experimental results had been obtained that once and for all resolved the issue in favor of the existence of atoms as a physical reality - however, they were obtained in the discipline of natural science adjacent to physics by the botanist Robert Brown.
Back in the summer of 1827, Brown, while studying the behavior of flower pollen under a microscope (he studied the aqueous suspension of plant pollen Clarkia pulchella), suddenly discovered that individual spores make absolutely chaotic impulse movements. He determined for certain that these movements were in no way connected with the turbulence and currents of water, or with its evaporation, after which, having described the nature of the movement of particles, he honestly admitted his own powerlessness to explain the origin of this chaotic movement. However, being a meticulous experimenter, Brown established that such chaotic movement is characteristic of any microscopic particles - be it plant pollen, suspended minerals, or any crushed substance in general.
It was only in 1905 that none other than Albert Einstein first realized that this mysterious, at first glance, phenomenon serves as the best experimental confirmation of the correctness of the atomic theory of the structure of matter. He explained it something like this: a spore suspended in water is subjected to constant “bombardment” by chaotically moving water molecules. On average, molecules act on it from all sides with equal intensity and at equal intervals of time. However, no matter how small the spore is, due to purely random deviations, first it receives an impulse from the molecule that hit it on one side, then from the side of the molecule that hit it on the other, etc. As a result of averaging such collisions, it turns out that that at some moment the particle “twitches” in one direction, then, if on the other side it is “pushed” by more molecules, in the other, etc. Using the laws mathematical statistics and the molecular kinetic theory of gases, Einstein derived an equation describing the dependence of the root-mean-square displacement of a Brownian particle on macroscopic parameters. ( Interesting fact: in one of the volumes of the German journal “Annals of Physics” ( Annalen der Physik) in 1905, three articles by Einstein were published: an article with a theoretical explanation of Brownian motion, an article on the foundations of the special theory of relativity, and, finally, an article describing the theory of the photoelectric effect. It was for the latter that Albert Einstein was awarded the Nobel Prize in Physics in 1921.)
In 1908, the French physicist Jean-Baptiste Perrin (1870-1942) conducted a brilliant series of experiments that confirmed the correctness of Einstein's explanation of the phenomenon of Brownian motion. It became finally clear that the observed “chaotic” motion of Brownian particles is a consequence of intermolecular collisions. Since “useful mathematical conventions” (according to Mach) cannot lead to observable and completely real movements of physical particles, it became finally clear that the debate about the reality of atoms is over: they exist in nature. As a “prize game,” Perrin received a formula derived by Einstein, which allowed the Frenchman to analyze and estimate the average number of atoms and/or molecules colliding with a particle suspended in a liquid over a given period of time and, using this indicator, calculate the molar numbers of various liquids. This idea was based on the fact that at any given moment in time the acceleration of a suspended particle depends on the number of collisions with the molecules of the medium ( cm. Newton's laws of mechanics), and therefore on the number of molecules per unit volume of liquid. And this is nothing more than Avogadro's number (cm. Avogadro's Law) is one of the fundamental constants that determine the structure of our world.