Presentation on the topic “Brownian motion. Presentation "Brownian motion. The structure of matter" in physics - project, report Download presentation on the topic Brownian motion

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

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Brownian motion.
Completed by: Bakovskaya Julia and Vozniak Albina, students of the 10th grade Checked by: Tsypenko L.V., teacher of physics in 2012

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Brownian motion - in natural science, the random movement of microscopic, visible, suspended in a liquid (or gas) solid particles (dust particles, particles of plant pollen, and so on), caused by the thermal motion of particles of a liquid (or gas). The concepts of "Brownian motion" and "thermal motion" should not be confused: Brownian motion is a consequence and evidence of the existence of thermal motion.

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

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Discovery of Brownian motion
This phenomenon was discovered by R. Brown in 1827, when he was conducting research on plant pollen. The Scottish botanist Robert Brown (sometimes his surname is transcribed as 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 devoted many years to studying them. Described plants brought from Indonesia and Central Africa. Studied plant physiology, first described the nucleus in detail plant cell. Petersburg Academy of 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, 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." Now, in order to repeat Brown's observation, it is enough to have a not very powerful 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 vividly than in a liquid: small patches of ash or soot (depending on the source of the smoke) are visible scattering light, which continuously jump back and forth. It is also possible to observe Brownian motion in ink solution: at a magnification of 400x, the motion of particles is already easily distinguishable. As is often the case in science, many years later, historians discovered that back in 1670, the inventor of the microscope, the Dutchman Anthony 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, so the discovery is rightly attributed to Brown, who first studied and described it in detail.

Yuldasheva Lolita

Biography of Robert Brown, experience with pollen, causes of Brownian motion.

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Presentation in physics "Brownian motion" by a 7th grade student of GBOU secondary school No. 1465 named after Admiral N.G. Kuznetsova Yuldasheva Lolita Physics teacher: L.Yu. Kruglova

Brownian motion

Biography of Robert Brown (1773-1858) British (Scottish) botanist of the late XVIII - first half of XIX century, morphologist and plant taxonomist, discoverer of "Brownian motion". Born December 21, 1773 in Montrose in Scotland, studied in Aberdeen, studied medicine and botany at the University of Edinburgh in 1789-1795. In 1795 he entered the Northern Regiment of the Scottish Militia, with whom he was in Ireland. Here he collected local plants and met the botanist Sir Joseph Banks. hard work natural sciences won him the friendship of Banks, on whose recommendation he was appointed botanist on an expedition sent in 1801 on the ship Investigator (eng. Investigator) under the command of Captain Flinders to explore the coast of Australia. Together with the artist Ferdinand Bauer, he visited parts of Australia, then Tasmania and the Bass Strait Islands. Most of all he was interested in the flora and fauna of these countries. In 1805 Brown returned to England, bringing with him about 4,000 species of Australian plants, many birds and minerals for the Banks collection; he spent several years developing this rich material, such as no one had ever brought from distant countries. Described plants brought from Indonesia and Central Africa. Studied plant physiology, first described in detail the nucleus of a plant cell. Petersburg Academy of Sciences made him an honorary member. But the name of the scientist is now widely known not because of these works. Member of the Royal Society of London (since 1810). From 1810 to 1820, Robert Brown was in charge of the Linnean Library and the vast collections of his patron Banks, President of the Royal Society of London. In 1820 he became librarian and curator of the botanical department of the British Museum, where, after the death of Banks, the collections of the latter were transferred.

Robert Brown's experience Brown, in the quiet of his London office in 1827, studied the obtained plant specimens through a microscope. The turn came to pollen, which is, in fact, fine grains. Dropping a drop of water on the cover glass, Brown brought in a certain amount of pollen. Looking through the microscope, Brown discovered that something strange was happening in the focal plane of the microscope. Pollen particles constantly moved in a chaotic way, not allowing the researcher to see them. Brown decided to tell his colleagues about his observations. Brown's published article had a title typical of that leisurely time: “A Brief Report of Microscopic Observations Conducted on Particles in June and August, 1827, Contained in Plant Pollen; and on the existence of active molecules in organic and inorganic bodies.

Brownian motion 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 germ 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 larger or smaller quantities, these molecules.”

I must say 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 further writes: "Throughout the study, I continued to use the same lenses with which I began work, in order to give more persuasiveness to my statements and to make them as accessible as possible to ordinary observations."

Now, in order 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 vividly than in a liquid: small patches of ash or soot (depending on the source of the smoke) are visible scattering light, which continuously jump back and forth. Qualitatively, the picture was quite plausible and even visual. A small twig or bug should move in approximately the same way, which are pushed (or pulled) in different directions by many ants. These smaller particles were actually in the lexicon of scientists, only no one had ever seen them. They called them molecules; translated from Latin, this word means "small mass."

Brownian particle trajectories

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, that he examined tiny cytoplasmic grains, and not the pollen itself (which is often mistakenly reported). The fact 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 small 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 unambiguously proved the molecular structure of matter, but 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 gained recognition only slowly and with difficulty.

Brownian motion and diffusion. The movement of Brownian particles looks very much like 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 motion of molecules were established in the simplest case of the gaseous state of matter. It turned out that the molecules in gases move very quickly - at the speed of a bullet, but they cannot “fly away” far, as they very often collide with other molecules. For example, oxygen and nitrogen molecules in the air, moving at an average speed of about 500 m/s, experience more than a billion collisions every second. Therefore, the path of the molecule, if it could be traced, would be a complex broken line. A similar trajectory is described by Brownian particles if their position is fixed at certain time intervals. Both diffusion and Brownian motion are a consequence of the chaotic thermal motion of molecules and therefore are 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 randomly shifts to one side or the other. Calculations have shown that for a particle with a size of 0.1 microns, one movement occurs in three billionths of a second over a distance of only 0.5 nm (1 nm = m). According to the apt expression of one author, this is reminiscent of the movement of 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: it is not the movements of individual particles that are observed, but their huge masses, it is only necessary to ensure that convection is not superimposed on diffusion - the mixing of matter as a result of vortex flows (such flows are easy to notice by dropping a drop of a colored solution, for example, ink, into a glass of hot water).

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

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