Fundamentals of the molecular kinetic theory of the structure of matter
The fundamentals of molecular kinetic theory were developed by M.V. Lomonosov, L. Boltzmann, J. Maxwell and others. This theory is based on the following provisions:
1. All substances consist of tiny particles - molecules. Molecules of complex substances consist of even smaller particles - atoms. Different combinations of atoms create types of molecules. An atom consists of a positively charged nucleus surrounded by a negatively charged electron shell. The mass of molecules and atoms is measured in atomic mass units (a.m.u.). The diameter of atoms and molecules is of the order of 10 - 10 cm. The amount of substance that contains the number of particles (atoms or molecules) equal to the number of atoms in 0.012 kg of carbon isotope C is called we pray.
Molecules collide with each other, changing speed both in magnitude and direction. In this case, a redistribution of their total kinetic energy occurs. A body consisting of molecules is considered as a system of moving and interacting particles. Such a system of molecules has energy consisting of the potential energy of interaction between particles and the kinetic energy of particle motion. This energy is called internal energy of bodies. The amount of internal energy transferred between bodies during heat exchange is called amount of heat (Joule, cal). Joule - SI. 1 cal = 4.18 J. Atoms and molecules are in continuous motion, which is called thermal. The main property of thermal movement is its uninterrupted nature (chaoticity). To quantitatively characterize the intensity of thermal motion, the concept of body temperature is introduced. The more intense the thermal movement of molecules in a body, the higher its temperature. When two bodies come into contact, energy moves from the more heated body to the less heated one and eventually settles state of thermal equilibrium.
From the point of view of molecular kinetic concepts temperature is a quantity characterizing the average kinetic energy of the translational motion of molecules or atoms. The unit of heat temperature measurement is degree.(One hundredth of the difference between the boiling and freezing points of pure water at atmospheric pressure). The Kelvin absolute temperature scale was introduced into physics. A degree Celsius is equal to a degree Kelvin. At a temperature of – 273 C, the translational motion of gas molecules (absolute zero) should stop, i.e. the system (body) has the lowest possible energy.
The basic principles of the molecular kinetic theory of the structure of matter are confirmed by numerous experiments and phenomena (diffusion, Brownian motion, mixing of liquids, compressibility of various substances, dissolution of solids in liquids, etc.). Modern experimental methods - X-ray diffraction analysis, observations using an electron microscope and others - have enriched our understanding of the structure of matter. In a gas, the distances between molecules are relatively large, and the attractive forces are negligible. Gas molecules always tend to be evenly distributed throughout the entire volume they occupy. The gas exerts pressure on the walls of the vessel in which it is located. This pressure is caused by the impacts of moving molecules. When studying the kinetic theory of gas, the so-called ideal gas. A gas in which we neglect the forces of intermolecular interaction and the volume of gas molecules. Assuming that during collisions, the molecules of an ideal gas are like absolutely elastic balls.
The term “temperature” appeared at a time when physicists thought that warm bodies consisted of more of a specific substance - caloric - than the same bodies, but cold ones. And temperature was interpreted as a value corresponding to the amount of caloric in the body. Since then, the temperature of any body has been measured in degrees. But in fact it is a measure of the kinetic energy of moving molecules, and, based on this, it should be measured in Joules, in accordance with the System of Units C.
The concept of “absolute zero temperature” comes from the second law of thermodynamics. According to it, the process of heat transfer from a cold body to a hot one is impossible. This concept was introduced by the English physicist W. Thomson. For his achievements in physics, he was given the title of nobility “Lord” and the title “Baron Kelvin”. In 1848, W. Thomson (Kelvin) proposed using a temperature scale in which he took absolute zero temperature, corresponding to extreme cold, as the starting point, and took degrees Celsius as the division value. The Kelvin unit is 1/27316 of the temperature of the triple point of water (about 0 degrees C), i.e. temperature at which pure water immediately exists in three forms: ice, liquid water and steam. temperature is the lowest possible low temperature at which the movement of molecules stops and it is no longer possible to extract thermal energy from a substance. Since then, the absolute temperature scale has been named after him.
Temperature is measured on different scales
The most commonly used temperature scale is called the Celsius scale. It is based on two points: the temperature of the phase transition of water from liquid to steam and water to ice. A. Celsius in 1742 proposed dividing the distance between reference points into 100 intervals, and taking water as zero, with the freezing point as 100 degrees. But the Swede K. Linnaeus suggested doing the opposite. Since then, water has frozen at zero degrees A. Celsius. Although it should boil exactly at Celsius. Absolute zero Celsius corresponds to minus 273.16 degrees Celsius.
There are several more temperature scales: Fahrenheit, Reaumur, Rankin, Newton, Roemer. They have different division prices. For example, the Reaumur scale is also built on the reference points of boiling and freezing of water, but it has 80 divisions. The Fahrenheit scale, which appeared in 1724, is used in everyday life only in some countries of the world, including the USA; one is the temperature of the mixture of water ice and ammonia and the other is the temperature of the human body. The scale is divided into one hundred divisions. Zero Celsius corresponds to 32 Conversion of degrees to Fahrenheit can be done using the formula: F = 1.8 C + 32. Reverse conversion: C = (F - 32)/1.8, where: F - degrees Fahrenheit, C - degrees Celsius. If you are too lazy to count, go to an online service for converting Celsius to Fahrenheit. In the box, enter the number of degrees Celsius, click "Calculate", select "Fahrenheit" and click "Start". The result will appear immediately.
Named after the English (more precisely Scottish) physicist William J. Rankin, who was a contemporary of Kelvin and one of the creators of technical thermodynamics. There are three important points in his scale: the beginning is absolute zero, the freezing point of water is 491.67 degrees Rankine and the boiling point of water is 671.67 degrees. The number of divisions between the freezing of water and its boiling for both Rankine and Fahrenheit is 180.
Most of these scales are used exclusively by physicists. And 40% of American high school students surveyed today said that they do not know what absolute zero temperature is.
In the world around us, various kinds of physical phenomena occur that are directly related to change in body temperature. Since childhood, we have known that cold water, when heated, first becomes barely warm and only after a certain time becomes hot.
With such words as “cold”, “hot”, “warm”, we define different degrees of “heating” of bodies, or, in the language of physics, different temperatures of bodies. The temperature of warm water is slightly higher than the temperature of cool water. If you compare the temperature of summer and winter air, the difference in temperature is obvious.
Body temperature is measured using a thermometer and expressed in degrees Celsius (°C).
As is known, diffusion occurs faster at higher temperatures. It follows from this that the speed of movement of molecules and temperature are deeply interrelated. If you increase the temperature, the speed of movement of molecules will increase, if you decrease it, it will decrease.
Thus, we conclude: body temperature directly depends on the speed of movement of molecules.
Hot water consists of exactly the same molecules as cold water. The difference between them is only in the speed of movement of the molecules.
Phenomena that relate to heating or cooling of bodies and temperature changes are called thermal. These include heating or cooling air, melting metal, and melting snow.
Molecules, or atoms, which are the basis of all bodies, are in endless chaotic motion. The number of such molecules and atoms in the bodies around us is enormous. A volume equal to 1 cm³ of water contains approximately 3.34 · 10²² molecules. Any molecule has a very complex trajectory of movement. For example, gas particles moving at high speeds in different directions can collide with each other and with the walls of the container. Thus, they change their speed and continue moving again.
Figure 1 shows the random movement of paint particles dissolved in water.
Thus, we draw another conclusion: The chaotic movement of particles that make up bodies is called thermal motion.
Chaoticity is the most important feature of thermal motion. One of the most important proofs of molecular motion is diffusion and Brownian motion.(Brownian motion is the movement of tiny solid particles in a liquid under the influence of molecular impacts. As observation shows, Brownian motion cannot stop).
In liquids, molecules can vibrate, rotate, and move relative to other molecules. If we take solids, then their molecules and atoms vibrate around certain average positions.
Absolutely all molecules of the body participate in the thermal movement of molecules and atoms, which is why with a change in thermal movement, the state of the body itself and its various properties also change. Thus, if you increase the temperature of ice, it begins to melt, taking on a completely different form - ice becomes liquid. If, on the contrary, you lower the temperature of, for example, mercury, then it will change its properties and turn from a liquid into a solid.
T 
The body temperature directly depends on the average kinetic energy of the molecules. We draw an obvious conclusion: the higher the temperature of a body, the greater the average kinetic energy of its molecules. And, conversely, as the body temperature decreases, the average kinetic energy of its molecules decreases.
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To study the topic “Thermal Movement” we need to repeat:
In the world around us, various kinds of physical phenomena occur that are directly related to changes in body temperature.
Since childhood, we remember that the water in the lake is first cold, then barely warm, and only after a while it becomes suitable for swimming
With such words as “cold”, “hot”, “slightly warm”, we define different degrees of “heating” of bodies, or, in the language of physics, different temperatures of bodies.
If you compare the temperature in the lake in summer and late autumn, the difference is obvious. The temperature of warm water is slightly higher than the temperature of ice water.
As is known, diffusion occurs faster at higher temperatures. It follows from this that the speed of movement of molecules and temperature are deeply interrelated.
Conduct an experiment: Take three glasses and fill them with cold, warm and hot water, and now put a tea bag in each glass and observe how the color of the water changes? Where will this change occur most intensely?
If you increase the temperature, the speed of movement of molecules will increase, if you decrease it, it will decrease. Thus, we conclude: body temperature directly depends on the speed of movement of molecules.
Hot water consists of exactly the same molecules as cold water. The difference between them is only in the speed of movement of the molecules.
Phenomena that relate to heating or cooling of bodies and temperature changes are called thermal. These include heating or cooling not only liquid bodies, but also gaseous and solid air.
More examples of thermal phenomena: metal melting, snow melting.
Molecules, or atoms, which are the basis of all bodies, are in endless chaotic motion. The movement of molecules in different bodies occurs differently. Gas molecules move randomly at high speeds along a very complex trajectory.When they collide, they bounce off each other, changing the magnitude and direction of the velocities.
Liquid molecules oscillate around equilibrium positions (since they are located almost close to each other) and relatively rarely jump from one equilibrium position to another. The movement of molecules in liquids is less free than in gases, but more free than in solids.
In solids, molecules and atoms vibrate around certain average positions.
As the temperature increases, the particle speed increases, That's why The chaotic movement of particles is usually called thermal.
Interesting:
What is the exact height of the Eiffel Tower? And this depends on the ambient temperature!
The fact is that the height of the tower varies by as much as 12 centimeters.
and the temperature of the beams can reach up to 40 degrees Celsius.
And as you know, substances can expand under the influence of high temperature.
Chaoticity is the most important feature of thermal motion. One of the most important evidence of the movement of molecules is diffusion and Brownian motion. (Brownian motion is the movement of tiny solid particles in a liquid under the influence of molecular impacts. As observation shows, Brownian motion cannot stop). Brownian movement was discovered by the English botanist Robert Brown (1773-1858).
Absolutely all molecules of the body participate in the thermal movement of molecules and atoms, which is why with a change in thermal movement, the state of the body itself and its various properties also change.
Let's remember how the properties of water change with temperature changes.
Body temperature directly depends on the average kinetic energy of molecules. We draw an obvious conclusion: the higher the temperature of a body, the greater the average kinetic energy of its molecules. And, conversely, as the body temperature decreases, the average kinetic energy of its molecules decreases.
Temperature - a quantity that characterizes the thermal state of the body or, in other words, a measure of the “heating” of the body.
The higher the temperature of a body, the greater the average energy of its atoms and molecules.
Temperature is measured thermometers, i.e. temperature measuring instruments
Temperature is not directly measured! The measured value is temperature dependent!
Currently, there are liquid and electric thermometers.
In modern liquid thermometers, this is the volume of alcohol or mercury. The thermometer measures your own temperature! And, if we want to measure the temperature of some other body using a thermometer, we must wait some time until the temperatures of the body and the thermometer are equal, i.e. thermal equilibrium will occur between the thermometer and the body. A home thermometer “thermometer” needs time to give an accurate reading of the patient’s temperature.
This is the law of thermal equilibrium:
For any group of isolated bodies, after some time the temperatures become the same,
those. a state of thermal equilibrium occurs.
Body temperature is measured using a thermometer and is most often expressed in degrees Celsius(°C). There are also other units of measurement: Fahrenheit, Kelvin and Reaumur.
Most often, physicists measure temperature on the Kelvin scale. 0 degrees Celsius = 273 degrees Kelvin
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Goals.
- Educational.
- Give the concept of temperature as a measure of average kinetic energy; consider the history of the creation of thermometers, compare different temperature scales; to develop the ability to apply acquired knowledge to solve problems and perform practical tasks, to expand students’ horizons in the field of thermal phenomena.
- Educational.
- Developing the ability to listen to your interlocutor and express your own point of view
- Developmental.
- Development in students of voluntary attention, thinking (the ability to analyze, compare, build analogies, draw conclusions.), cognitive interest (based on a physical experiment);
- formation of ideological concepts about the knowability of the world.
DURING THE CLASSES
Hello, please sit down.
When studying mechanics, we were interested in the movement of bodies. Now we will consider phenomena associated with changes in the properties of bodies at rest. We will study heating and cooling of air, melting of ice, melting of metals, boiling of water, etc. Such phenomena are called thermal phenomena.
We know that when cold water is heated, it first becomes warm and then hot. The metal part removed from the flame gradually cools. The air surrounding the hot water radiators heats up, etc.
We use the words “cold”, “warm”, “hot” to denote the thermal state of bodies. The quantity characterizing the thermal state of bodies is temperature.
Everyone knows that the temperature of hot water is higher than the temperature of cold water. In winter, the outside air temperature is lower than in summer.
All molecules of any substance move continuously and randomly (chaotically).
The random chaotic movement of molecules is called thermal motion.
Tell me, what is the difference between thermal motion and mechanical motion?
It involves many particles with different trajectories. The movement never stops. (Example: Brownian motion)
Demonstration of the Brownian motion model
What does thermal motion depend on?
- Experiment No. 1: Dip a piece of sugar in cold water and another in hot water. Which one will dissolve faster?
- Experiment No. 2: Place 2 pieces of sugar (one larger than the other) in cold water. Which one will dissolve faster?
The question of what temperature is turned out to be very difficult. How, for example, does hot water differ from cold water? For a long time there was no clear answer to this question. Today we know that at any temperature water consists of the same molecules. Then what exactly changes in water as its temperature increases? From experience we saw that sugar will dissolve much faster in hot water. Dissolution occurs due to diffusion. Thus, Diffusion occurs faster at higher temperatures than at lower temperatures.
But the cause of diffusion is the movement of molecules. This means that there is a connection between the speed of movement of molecules and body temperature: In a body with a higher temperature, molecules move faster.
But temperature depends not only on the average speed of molecules. For example, oxygen, the average speed of molecules of which is 440 m/s, has a temperature of 20 °C, and nitrogen, with the same average speed of molecules, has a temperature of 16 °C. The lower temperature of nitrogen is due to the fact that nitrogen molecules are lighter than oxygen molecules. Thus, the temperature of a substance is determined not only by the average speed of movement of its molecules, but also by their mass. We see the same in experiment No. 2.
We know quantities that depend on both the speed and the mass of the particle. This is impulse and kinetic energy. Scientists have found that it is the kinetic energy of molecules that determines body temperature: temperature is a measure of the average kinetic energy of particles in a body; the greater this energy, the higher the body temperature.
So, when bodies heat up, the average kinetic energy of molecules increases, and they begin to move faster; When cooled, the energy of the molecules decreases and they begin to move more slowly.
Temperature is a quantity that characterizes the thermal state of the body. A measure of the “heat” of a body. The higher the temperature of a body, the greater the average energy of its atoms and molecules.
Is it possible to rely only on your sensations to judge the degree of heating of the body?
- Experiment No. 1: Touch a wooden object with one hand and a metal object with the other.
Compare the sensations
Although both objects are at the same temperature, one hand will feel cold and the other warm
- Experiment No. 2: take three vessels with hot, warm and cold water. Place one hand in a vessel with cold water and the other in a vessel with hot water. After some time, place both hands in a vessel with warm water
Compare the sensations
The hand that was in hot water now feels cold, and the hand that was in cold water now feels warm, although both hands are in the same vessel.
We have proven that our feelings are subjective. Instruments are needed to confirm them.
Instruments used to measure temperature are called thermometers. The action of such a thermometer is based on the thermal expansion of a substance. When heated, the column of the substance used in the thermometer (for example, mercury or alcohol) increases, and when cooled, it decreases. The first liquid thermometer was invented in 1631 by the French physicist J. Rey.
The body temperature will change until it comes into thermal equilibrium with the environment.
The law of thermal equilibrium: for any group of isolated bodies, after some time the temperatures become the same, i.e. a state of thermal equilibrium occurs.
It should be remembered that any thermometer always shows its own temperature. To determine the temperature of the environment, the thermometer should be placed in this environment and wait until the temperature of the device stops changing, taking a value equal to the ambient temperature. When the temperature of the environment changes, the temperature of the thermometer will also change.
A medical thermometer designed to measure human body temperature operates somewhat differently. It belongs to the so-called maximum thermometers, recording the highest temperature to which they were heated. Having measured your own temperature, you may notice that, when you find yourself in a colder environment (compared to the human body), the medical thermometer continues to show the same value. To return the mercury column to its original state, this thermometer must be shaken.
With a laboratory thermometer used to measure the temperature of the environment, this is not necessary.
Thermometers used in everyday life allow you to express the temperature of a substance in degrees Celsius (°C).
A. Celsius (1701-1744) - Swedish scientist who proposed the use of a centigrade temperature scale. On the Celsius temperature scale, zero (since the mid-18th century) is the temperature of melting ice, and 100 degrees is the boiling point of water at normal atmospheric pressure.
Let's listen to a message about the history of the development of thermometers (Presentation by Sidorova E.)
Liquid thermometers are based on the principle of changing the volume of liquid that is poured into the thermometer (usually alcohol or mercury) when the ambient temperature changes. Disadvantage: different liquids expand differently, so thermometer readings vary: Mercury -50 0 C; glycerin -47.6 0 C
We tried to make a liquid thermometer at home. Let's see what comes of it. (Video by Brykina V. Appendix 1)
We learned that there are different temperature scales. In addition to the Celsius scale, the Kelvin scale is widely used. The concept of absolute temperature was introduced by W. Thomson (Kelvin). The absolute temperature scale is called the Kelvin scale or thermodynamic temperature scale.
The unit of absolute temperature is kelvin (K).
Absolute zero is the lowest possible temperature at which nothing can be colder and it is theoretically impossible to extract thermal energy from a substance, the temperature at which the thermal movement of molecules stops
Absolute zero is defined as 0 K, which is approximately 273.15 °C
One Kelvin is equal to one degree T=t+273
Questions from the Unified State Exam
Which of the following options for measuring hot water temperature using a thermometer gives the most correct result?
1) The thermometer is immersed in water and, after removing it from the water after a few minutes, the readings are taken
2) The thermometer is lowered into the water and waited until the temperature stops changing. After this, without removing the thermometer from the water, take its readings
3) The thermometer is lowered into the water and, without removing it from the water, the readings are immediately taken
4) The thermometer is lowered into the water, then quickly removed from the water and the readings are taken
The figure shows part of the scale of a thermometer hanging outside the window. The air temperature outside is
- 18 0 C
- 14 0 C
- 21 0 C
- 22 0 C
Solve problems No. 915, 916 (“Collection of problems in physics 7-9” by V.I. Lukashik, E.V. Ivanova)
- Homework: Paragraph 28
- No. 128 D “Collection of problems in physics 7-9” V.I. Lukashik, E.V. Ivanova
Methodological support
- “Physics 8” S.V. Gromov, N.A. Motherland
- “Collection of problems in physics 7-9” V.I.Lukashik, E.V. Ivanova
- Drawings that are publicly available on the Internet