The ICT is based on three important points:
- all substances consist of tiny particles (atoms, molecules, electrons, ions);
- particles of matter are in continuous chaotic motion (it is often called thermal motion);
- particles of matter interact with each other.
Formation of basic concepts of statistical physics.
Macroscopic bodies are large bodies made up of a huge number of molecules.
Thermal phenomena are phenomena associated with the heating or cooling of bodies.
Thermal movement of molecules is a random and chaotic movement of molecules.
The possibility of mechanical crushing of substances, dissolution of substances in water, diffusion, compression and expansion of gases.
Diffusion is the phenomenon of penetration of molecules of one substance between the molecules of another substance. Brownian motion of small particles suspended in a liquid under the influence of molecular impacts
Some force is required to break a solid, while solid and liquid bodies are difficult to compress.
Drops of liquid placed in close proximity to each other merge.
Experimental confirmation of MCT.
The first position of the ICB
1. Assumption about molecular structure substances were confirmed only indirectly. Place a very small drop of oil on the surface of the water. The oil stain will spread over the surface of the water, but the area of \u200b\u200bthe oil film cannot exceed a certain value. It is natural to assume that the maximum film area corresponds to an oil layer one molecule thick. For example, a 1 mm drop of olive oil3 spreads over an area of \u200b\u200bno more than 1 m2 ... Hence it follows that the size of the oil molecule is about 10-9 m.
2. Another confirmation is the experiment of Bridgman: the oil poured into a steel vessel is squeezed under ultra-high pressure, and it is noticed that oil droplets have appeared on the walls of the vessel. Conclusion: the oil consists of the smallest particles that were able to pass through the gaps between the particles of the steel vessel.
The second position of the MCT proves the phenomenon of diffusion - the mutual penetration of molecules of one substance into the gaps of another substance.
1. To make sure that the molecules are moving, you can quite simply: drop a drop of perfume in one end of the room, and in a few seconds this smell will spread throughout the room. In the air around us, molecules rush at the speed of artillery shells - hundreds of meters per second.
The diffusion rate increases with increasing temperature.
2. At the beginning of the 19th century, the English botanist Brown, observing the particles of pollen suspended in water through a microscope, noticed that these particles were in "eternal dance". The reason for the so-called "Brownian motion" was understood only 50 years after its discovery: individual impacts of liquid molecules on a particle do not cancel each other out if this particle is small enough. Since then, Brownian motion has been regarded as a clear experimental confirmation of the thermal motion of molecules.
MOLECULAR PHYSICS
THERMAL PHENOMENA
Basic principles of molecular kinetic theory
and their empirical confirmation.
Lesson objectives:
1. To acquaint students with the main provisions of the molecular kinetic theory and their experimental confirmation.
2. Continue working on the development of memory, attention, speech, thinking, interest in physics through the demonstration of experiments.
3. Continue the formation of will, perseverance, desire for knowledge,
responsible attitude to study.
Lesson type : a lesson in learning new material.
Demonstrations: 1. Fragment of the video film "Brownian motion".
2. Diffusion in liquids and gases.
3. Interaction of body particles.
Lesson plan:
- Presentation of new material.
- Control questions on the stated topic.
- Solving quality problems.
- Homework
Outline of presentation of new material: 1. Introduction.
2. Historical background.
3. The main provisions of the ICB.
Lesson flow: (Slide number 1)
- Presentation of new material.
1. Introduction.
We live in a world of macroscopic bodies. Mechanics studies the movement of macroscopic bodies - the movement of some bodies relative to others in space over time. But she is unable to explain why there are solids, liquids and gases and why these bodies can pass from one state to another.
In mechanics, they speak of forces as the causes of speed changes, without clarifying the nature of these forces. It remains unclear why elastic forces appear when bodies are compressed, why friction forces arise. These and many other questions can be answered by studying the section "Molecular Physics".
After mechanical movement, the most noticeable phenomena are associated with the heating or cooling of bodies, with a change in their temperature. These phenomena are called thermal. Thermal phenomena occur inside bodies and are entirely determined by the thermal motion of the particles that make up this body.
The significance of thermal phenomena.The habitual appearance of our planet exists and can exist only in a rather narrow temperature range. If the temperature exceeded 100 ° C, then on Earth at the usual atmospheric pressure there would be no rivers, seas and oceans, there would be no water at all, All water would turn into steam. If the temperature dropped by several tens of degrees, the oceans would turn into glaciers.
Even narrower temperature ranges are necessary to maintain the life of warm-blooded animals. The temperature of animals and humans is maintained by the internal mechanisms of thermoregulation at a strictly defined level. It is enough for the temperature to rise by a few tenths of a degree, and we feel unhealthy. A change of several degrees leads to the death of the organism.
A change in temperature affects all properties of bodies. So, when heated or cooled, the sizes of bodies and volumes of liquids change. The mechanical properties of bodies change significantly, for example, elasticity, resistance electric current, magnetic properties, etc.
All of the above and many others thermal phenomena obey certain laws, which we will study in the section "Molecular Physics". Let's start the study of the section with the topic "Basic principles of the molecular kinetic theory and their experimental confirmation."
(Slide number 2) 2. Historical background.
MKT explains thermal phenomena, the properties of bodies on the basis of the idea that all bodies consist of chaotically moving particles.
Historical reference:
In the V century BC. e. the ancient Greek scientist Democritus put forward an atomistic hypothesis: everything in the world consists of atoms; there is a void between the atoms. Arguments in favor of the teachings of Democritus can be found in the famous poem of the ancient Roman poet Lucrucius Kara "On the nature of things":
... clothes are damp on the seashore,
And in the sun it dries up.
However, one cannot see
How moisture settles on it and how it disappears.
It means that water is split into such tiny parts,
That they are inaccessible to our eyes.
IV century BC e. Aristotle - rejected the hypothesis of Democritus.
One and a half thousand years after the appearance of the atomistic hypothesis in medieval France, a decree was issued prohibiting the spread of the doctrine of atoms on pain of death. The Church destroys all the germs of the new and progressive that do not fit into the system of religious ideas about the world.
Only in the 17th century. a consistent molecular kinetic theory began to develop. A great contribution to the development of this theory was made by the great Russian scientist M.V. Lomonosov. He explained the basic properties of gas by the disordered movement of molecules. For the first time he explained the nature of heat.
3. The main provisions of the ICB.
The ICT is based on three important points:(Slide number 3)
- all substances consist of the smallest particles (atoms, molecules, electrons, ions);
- particles of matter are in continuous chaotic motion (it is often called thermal motion);
- particles of matter interact with each other.
4. Experimental confirmation of MCT.(Slide number 4)
First position
1. The assumption about the molecular structure of the substance was confirmed only indirectly. The sizes of molecules and atoms are so small that it is impossible to distinguish them in an ordinary microscope. Therefore, even in the 19th century, many scientists still doubted the existence of molecules. Today, technology has reached a level where even individual atoms can be viewed using ion and electron microscopes. It is quite easy to verify the existence of molecules and estimate their size. Place a very small drop of oil on the surface of the water. The oil stain will spread over the surface of the water, but the area of \u200b\u200bthe oil film cannot exceed a certain value. It is natural to assume that the maximum film area corresponds to an oil layer one molecule thick. For example, a 1 mm drop of olive oil3 spreads over an area of \u200b\u200bno more than 1 m2 ... Hence it follows that the size of the oil molecule is about 10-9 m.
2. Another confirmation is the experiment of Bridgman: the oil poured into a steel vessel is squeezed under ultra-high pressure, and you notice that oil droplets have appeared on the walls of the vessel. Conclusion: the oil consists of the smallest particles that were able to pass through the gaps between the particles of the steel vessel.
The second position proves the phenomenon of diffusion - the mutual penetration of molecules of one substance into the gaps of another substance.
1. To make sure that the molecules are moving, you can quite simply: drop a drop of perfume in one end of the room, and in a few seconds this smell will spread throughout the room. In the air around us, molecules rush at the speed of artillery shells - hundreds of meters per second.
In liquids, diffusion is slower. Poured into a glass vessel water solution copper sulfate. This solution is deep blue in color. On top of the solution into the vessel, very carefully, so as not to mix the liquids, pour clean water... Copper sulfate is heavier than water and therefore remains at the bottom of the vessel. At the beginning of the experiment, a sharp boundary is visible between the two liquids. Let's leave the vessel alone. After a few days, you will notice that the interface between the fluids has blurred. And in two weeks this border will disappear altogether, and a homogeneous liquid of pale blue color will be in the vessel. So, the cause of diffusion is the continuous and disordered movement of particles of matter. During diffusion, particles of one substance penetrate into the gaps between particles of another substance, and the substances are mixed.
Diffusion is slowest in solids. In one experiment, smoothly polished plates of lead and gold were placed one on top of the other and compressed with a load. After five years, gold and lead penetrated each other by 1 mm.
The diffusion rate increases with increasing temperature.
Diffusion is of great importance in the life processes of humans, animals and plants. For example, it is thanks to diffusion that oxygen from the lungs penetrates the human blood, and from the blood - into the tissues.
2. At the beginning of the 19th century, the English botanist Brown, observing the particles of pollen suspended in water through a microscope, noticed that these particles were in "eternal dance". The reason for the so-called "Brownian motion" was understood only 50 years after its discovery: individual impacts of liquid molecules on a particle do not cancel each other out if this particle is small enough. Since then, Brownian motion has been regarded as a clear experimental confirmation of the thermal motion of molecules.
Attention to the screen. Watch an excerpt from the video Brownian Motion.
(Slide number 5)
Let us prove the third proposition.
(Slide number 6)
Let's experiment.
1. To get some idea of \u200b\u200bthe magnitude of the forces of interaction between molecules, try to break a steel or nylon thread with a cross section of 1 mm2 ... Few will be able to do this, but the forces of attraction of molecules in a small cross-section of the thread "resist" the efforts of your whole body!
2. If you tightly press lead cylinders with well-cleaned ends to each other, they "adhere" so firmly that a kilogram weight can be suspended from them (see figure). This experience also indicates the presence of forces of intermolecular attraction.
If the molecules were not attracted to each other, there would be no liquids or solids - they would simply crumble into separate molecules. On the other hand, if the molecules were only attracted, they would “stick together” into extremely dense clumps, and gas molecules would “stick” to them when they hit the walls of the vessel. The interaction of molecules is electrical in nature. Although molecules, in general, are electrically neutral, the distribution of positive and negative electric charges in them is such that at large distances (compared to the size of the molecules themselves), molecules are attracted, and at small distances, they are repelled.
(Slide number 7)
The figure shows the qualitative dependence of the forces of intermolecular interaction on the distance r between molecules, whereF about and F p - respectively, the forces of repulsion and attraction,F - their resulting. Repulsive forces are considered positive,and the forces of mutual attraction -negative.
At a distance r \u003d r about net forceF \u003d 0, i.e. the forces of attraction and repulsion balance each other. Thus, the distance r0 corresponds to the equilibrium distance between the molecules at which they would be and the absence of thermal motion. Whenr r 0 repulsive forces prevail (F\u003e 0), atr\u003e r 0 - forces of attraction(F ABOUT). At distances r\u003e 10-9 m there are practically no intermolecular forces of interaction(F → 0).
(Slide number 8)
A striking example of the different interactions of molecules is that a substance can be in different states of aggregation. For example: ice, water and water vapor.
Ice, water and water vapor are made up of the same molecules. The difference lies in the speed of the molecules, their mutual arrangement and the forces of interaction between them.
- Answer the security questions on the stated topic.
(Slide number 9)
- What is the purpose of the ICB?
- What are the main provisions of the ICT.
- List the evidence you know of the existence of molecules.
- What is the diffusion phenomenon?
- What is the essence of Brownian motion?
- What experiments prove that the forces of attraction and repulsion act between the molecules of solids and liquids?
- Solve quality problems. (Slide number 10.11)
1. Why is the smell of freshly spilled perfume found on the other side of the room only after a few minutes, although the speed of movement of molecules at room temperature is several hundred meters?
2. Two glass plates are difficult to separate from each other if there is a little water between them. If the glasses are dry, then they are easily separated from each other. Why?
3. Why polishing rubbing surfaces can lead not to a decrease in friction, but, on the contrary, to an increase?
4. What is the process of dissolving sugar in water based on?
5. What can you say about the size, composition and forces of interaction of molecules of the same substance in different states? Explain the answer.
6. Water can be easily removed from a clean glass surface. It is almost impossible to remove fat from the same surface. How can this be explained from a molecular point of view?
7. How to explain that the dust does not fall even from the surface facing down?
8. Why is there a crunch when breaking a twig?
- Homework:§ 57,58,60,61 R. # 450 - 453.
Having started experimental work on creating high pressures in 1908, by 1933 Percy Bridgeman with the help of his devices reached the pressure 12 000 atmospheres (for comparison: the pressure in the barrel of a conventional gun is hundreds of atmospheres).
Having obtained record pressure values, he was able to investigate and describe:
Behavior of liquids and solids at gigantic pressures (taking into account the discoveries of other scientists, in total there are 11 types of ice, some of which were discovered by Percy Bridgman);
Change in electrical resistance at gigantic pressures, etc.
Later he created an apparatus in which he brought the pressure up to 130 000 atmospheres at 1000 degrees.
In 1940, Percy Bridgeman succeeded in obtaining synthetic pyrite crystals.
In 1946, for the complex of research carried out, he was awarded Nobel Prize in physics, we quote: "for the invention of a device that allows you to create ultrahigh pressures, and for the discoveries made in this regard in the physics of high pressures."
Percy Bridgman once remarked that it is not difficult to get new results in physics if all known experiments are carried out again under ultrahigh pressure. It should be noted that for the study of substances under abnormal conditions, several more Nobel Prizes were received by other scientists ...
Percy Williams Bridgeman
1946 Nobel Prize Laureate in Physics. The wording of the Nobel Committee: "For the invention of a device that allows you to create ultra-high pressures, and for the discoveries made in this regard in the physics of high pressures."
Our hero today is a typical American. He was born in Cambridge, but not in the one that gave us a whole galaxy of physicists from, but in the one that separates the Charles River from Boston. The city is still small - only 100 thousand people, but what kind! It is in this city that both Harvard University and the Massachusetts Institute of Technology are located.
One of the buildings of Harvard University in Cambridge (Massachusetts, USA)
Filippo Diotalevi / Flickr
Peter's parents (as those who were close to him from childhood called Percy) were not professors. His father, Raymond Landon Bridgeman, was a reporter specializing in social and political issues. Mother, Mary Ann Maria, née Williams, was described as a "simple, lively and slightly defiant" woman.
If you believe in the signs, then from the very birth life "pointed" to Peter-Percy that he needed to do physics. Born in Cambridge, then the family moved to the city with the speaking name Newton. Unsurprisingly, the teacher of the parish school in Newton advised the boy to go further down the scientific path. Naturally, Percy decided to study at Harvard. Most of his life was connected with him.
Bridgman became a bachelor in 1904. Even then, he began to engage in high pressure. The future laureate was interested in science and his reflections on it ... And nothing more. He never taught, rudely sent Harvard rector Abbott Lowell (his phrase "I'm not interested in your ... college, let me do science" became winged), and as a result, Bridgman wrote over a quarter of a thousand articles and a damn dozen monographs.
He made his first invention related to pressure back in 1905. The scientist invented a hermetically sealed method of sealing pressure vessels with gas. The solution was original: an insulating gasket made of rubber or soft metal was compressed under a pressure greater than the pressure inside the vessel (it was called the Bridgman gasket). As a result, the sealing plug automatically sealed as the pressure increased and never leaked regardless of the pressure as long as the vessel walls held. Curiously, this invention was made when Bridgman needed to fix a broken high-pressure apparatus.
Bridgeman gasket
Wikimedia Commons
As a result, Bridgman was in possession of an instrument that could study hundreds of substances under high pressure conditions. It reached 100 thousand atmospheres, and in some cases, up to 400 thousand. In fact, for the first time experimentally it was possible to study substances under the same conditions in which they are in the bowels of the Earth.
And since a new tool appeared, leading science to a completely unknown area, discoveries poured in as if from a cornucopia. Want to discover a new allotropic phosphorus modification? Excuse me! Let's try to get hot ice? Only 20 thousand atmospheres, and ice does not melt at 80 ° C!
He discovered the compressibility of atoms (starting with the compression of metallic cesium), how the molecules of liquids, including water, behave when compressed, studied the graphs of the melting point at the highest pressures. It’s even strange that the Nobel Prize came so late. By that time, Bridgeman had already managed to compress even uranium and plutonium within the framework of the Manhattan Project ... By the way, it is curious that in 1946 our hero "bypassed" another great experimenter who became famous in another Cambridge - Pyotr Leonidovich Kapitsa in the Nobel race. (We will not talk about it soon, because Kapitsa waited exactly forty years for his prize for the discovery of superfluidity of helium, which took place in 1938 ...)
Petr Kapitsa in the 1930s
Wikimedia Commons
“With the help of your original device, combined with brilliant experimental technique, you have greatly enriched our knowledge of the properties of matter at high pressures,” was how Percy Bridgman greeted at the Nobel Prize in Stockholm on December 4, 1946.
Having already become a famous physicist, Bridgman declared himself as a philosopher. And very successful. Of all the Nobel laureates, about whom we have written so far, only one was almost a real philosopher (many remember his collection "Physics and Philosophy", published in the USSR). Bridgman's main book is Logic modern physics", Published in 1927. In this book, he laid the foundations for a whole new philosophical movement called operationalism (the word itself appeared in 1920 in a book by another physicist, Norman Campbell).
At the very end of his life, Bridgeman again declared himself - tragically and loudly. When he turned 79, the Nobel laureate learned that he was terminally ill. Cancer with metastases, rapid loss of strength, incipient pain. The scientist was determined to have time to pass away painlessly and not wait for the last stage, but not a single doctor wanted to help him with euthanasia. On August 20, 1961, Bridgeman shot himself in the head with a hunting rifle, leaving a bitter and angry note: “It is not very decent for society to make a person do this is do it yourself. Probably, today is the last day when I am still able to do it myself. " The Bridgeman Note is still featured in ethical debates over euthanasia.
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The essence of this method lies in the fact that the single crystals nucleating in the lower part of the crucible with the melt serve as a seed. The crucible is lowered into the colder zone of the furnace. The lower part of the crucible is conical. The growing speed is also a few mm / hour.
Scheme of the installation for growing single crystals by the Stockabberg-Bridgman method:1 - crucible with melt,2 - crystal,3 - oven,4 - refrigerator,5 - thermocouple,6 - heat shield.
Verneuil method
Verneuil's method is implemented by pouring small portions of powder mixture into a tubular furnace, where this mixture melts during falling in an oxygen-hydrogen flame and feeds a drop of melt on the surface of the seed. In this case, the seed is gradually pulled downward, and the drop remains at the same level along the height of the furnace.
Benefits :
absence of fluxes and expensive crucible materials;
no need for precise temperature control;
the ability to control the growth of a single crystal.
disadvantages :
crystals have internal stresses due to the high growth temperature;
the stoichiometry of the composition can be violated due to the reduction of components with hydrogen and the evaporation of volatile substances.
Growing speed - several mm / hour.
The figures show the principle of growing single crystals by the Verneuil method and the installation equipment.
Zone melting method
Zone melting consists in driving the melt zone along the length of the single crystal workpiece, while impurities are concentrated in the melt zone and the crystal is purified, the final part of which is then removed. Heating is carried out by induction, radiation-optical or other methods.
Diagram of the device for zone melting:1 - seed,2 - melt,3 - polycrystalline ingot, 4 - heater(the arrow shows the directionheater movement).
Induction Zone Melting System Germanium Hydrothermal Growing
The hydrothermal crystal growing method is used to grow crystals that are difficult or impossible to grow with other methods, as they most closely mimic the processes of mineral formation in nature. It is based on the fact that at high temperatures (up to 700 ° C) and pressures (up to 3000 atm.), Aqueous solutions of salts are able to actively dissolve compounds that are practically insoluble under normal conditions. For hydrothermal crystal growth, special durable steel vessels are used - autoclaves that can withstand such extreme pressures and temperatures.
The most common is a modification of the hydrothermal method called the positive temperature gradient recrystallization method. Its essence is as follows:
H 
at the bottom of the autoclave, heated from below and cooled from above, there is a dissolved substance - a charge. Above it are seeds (plates cut in a certain direction from a crystal of the grown substance). A temperature difference is created in the autoclave (the lower zone is hotter), which is facilitated by a diaphragm - a partition with holes that separates the upper and lower zones. The solution circulates between the granules of the charge, being saturated with the substance of the growing crystal. At the same time, the hydrothermal solution is heated. The hot (and therefore lighter) solution enters the top of the autoclave, where it cools.
The solubility of the crystallized substance decreases with decreasing temperature, the excess of the solute is deposited on the seeds. The cold high-density lean solution is lowered to the bottom of the autoclave and the cycle is repeated. The process is carried out until complete transfer of the charge substance to the seeds. As a result of these processes, the crystal grows. The growth rate ranges from fractions of a mm to several mm per day. The grown single crystals are usually of high quality and characteristic crystallographic faceting. grow in conditions more or less close to equilibrium.
Autoclave scheme for hydrothermal synthesis:
1 - solution, 2 - crystal, 3 - furnace, 4 - substance for crystallization
(T 1