Molecular physics. Fundamentals of molecular physics and thermodynamics
What does he study? molecular physics
Molecular physics studies macroscopic processes in bodies from the point of view of the atomic-molecular structure of matter. She views heat as the random movement of microparticles. Generally speaking, this branch of physics also pays attention to the properties and structure of individual molecules and atoms. Molecular physics is often called the molecular kinetic theory of matter (MKT).
In the 19th century, during the birth of MCT, at a time when the very existence of molecules and atoms was questioned, the strict separation of molecular physics from thermodynamics was justified. It was necessary to separate reliable facts from false hypotheses. But when the 20th century irrefutably proved and found methods for the structural study of matter, MCT lost its hypothetical character, which it had at the beginning of its inception. The hypothetical element of molecular physics remains only in relation to simplified models, which it still uses to describe and explain certain phenomena.
The need for such models has not lost its relevance, since we still do not have complete information about molecular structure tel. However, it must be said that now a clear separation of MCT from thermodynamics has become irrelevant. Currently, when presenting certain principles of thermodynamics, they use conclusions that were obtained in MCT and vice versa. It is said that MCT and thermodynamics complement each other.
Study the processes that take place in large systems very difficult due to the huge number of particles and their small sizes. It is almost impossible to consider each particle separately; statistical quantities are introduced: average speed particles, their concentration, particle mass. There is a need to establish a mathematical connection (equations) between micro parameters that relate to individual particles (the mass of a molecule, its speed, etc.) and macro parameters that describe the system as a whole (temperature, pressure) A formula characterizing the state of the system taking into account microscopic and macroscopic parameters is called the basic equation of the molecular kinetic theory of gases).
Statistical method
Definition
The method that molecular physics often uses when considering objects or phenomena is called statistical. The statistical method consists of studying the properties of macroscopic systems based on analysis, using methods of mathematical statistics, laws of thermal motion large number particles that form these systems.
The simplest but most informative model, which we will often use in MCT, will be the ideal gas model. In such a gas, molecules move freely (isolated from other molecules), only occasionally colliding with each other or with the walls of the container.
Example 1
Task: B starting moment time t=0 gas temperature T occupies half-space $x
Let us write the kinetic equation taking into account the fact that there are no collisions of molecules:
$\frac(\partial f)(\partial t)+\overrightarrow(v)\frac(\partial f)(\partial \overrightarrow(r))=0$ (1.1)
the general solution to equation (1.1) is $:\ f=f\left(\overrightarrow(r)-\overrightarrow(v)t,\ \overrightarrow(v)\right).\ $ We use the initial condition, write: $f= f_0\left(v\right)\ for\ v_x>\frac(x)(t),\ f=0\ for\ v_x
where $f_0\left(v\right)$ is the Maxwell velocity distribution of molecules ($dN_(v_xv_yv_z)=Nf\left(v\right)dv_x(dv)_y(dv)_z$).
Gas Density:
\,\]
where $S\left(\varepsilon \right)=\frac(2)(\sqrt(\pi ))\int\nolimits^(\varepsilon )_0(e^(-y^2)dy)$, $N_0 \ $ - initial density.
It should be noted that if we neglect collisions of molecules, then the resulting formulas are correct only in the region $\left|x\right|\ll l.$
Answer: The molecular density distribution, if the molecules do not collide with each other, is determined by the formula: $N\left(t,x\right)=\int\nolimits^(\infty )(\int\nolimits_(-\infty )(\int \nolimits^(\infty )_(\frac(x)(t))(f_0\left(v\right)m^3dv_xdv_ydv_z)=\frac(N_0)(2)))\left,$
where $S\left(\varepsilon \right)=\frac(2)(\sqrt(\pi ))\int\nolimits^(\varepsilon )_0(e^(-y^2)dy)$.
Example 2
Assignment: In Fig. Figure 1 shows the process in an ideal gas at constant volume and variable mass. How does the mass of gas change in this process?
The process given in Fig. 2 can be represented analytically as:
where $b$, $a$ are constants, $p$ is pressure, $T$ is thermodynamic temperature.
The process in the problem occurs at a constant volume, but we cannot call it isochoric, since the mass is variable. As a basis for the solution, we use the equation of state of an ideal gas (in the form of the Mendeleev-Clayperon equation):
where V is the volume of the gas, $m$ is the mass of the gas, $\mu $ is the molar mass of the gas, $R$ is the universal gas constant.
Let us express the gas mass from (2.2), we obtain:
Let's take into account that $V=const$, $\mu =const$ in a given process, then we write:
Let us substitute equation (2.1), which defines the process, for pressure, and obtain proportionality:
Based on proportionality (2.5), we see that during the process shown in Fig. 1, if the gas temperature increases, the gas mass decreases.
Answer: In a given process, the mass of the gas decreases.
Experimental substantiation of the main provisions of the ICT:
Molecular kinetic theory– the doctrine of the structure and properties of matter, using the idea of the existence of atoms and molecules as the smallest particles chemical substance. MCT is based on three strictly experimentally proven statements:
· Matter consists of particles - atoms and molecules, between which there are spaces;
· These particles are in chaotic motion, the speed of which is affected by temperature;
· Particles interact with each other.
The fact that a substance really consists of molecules can be proven by determining their sizes: A drop of oil spreads over the surface of the water, forming a layer whose thickness is equal to the diameter of the molecule. A drop with a volume of 1 mm 3 cannot spread more than 0.6 m 2:
There are also other ways to prove the existence of molecules, but there is no need to list them: modern instruments (electron microscope, ion projector) allow you to see individual atoms and molecules.
Molecular interaction forces. a) the interaction is electromagnetic in nature; b) short-range forces are detected at distances comparable to the size of molecules; c) there is a distance when the forces of attraction and repulsion are equal (R 0), if R>R 0, then the forces of attraction prevail, if R The action of molecular attractive forces is revealed in an experiment with lead cylinders sticking together after cleaning their surfaces. Molecules and atoms in solid perform random oscillations relative to positions in which the forces of attraction and repulsion from neighboring atoms are balanced. IN liquids molecules not only oscillate around the equilibrium position, but also make jumps from one equilibrium position to the next; these molecular jumps are the reason for the fluidity of a liquid, its ability to take the shape of a vessel. IN gases usually the distances between atoms and molecules are on average much larger than the sizes of the molecules; repulsive forces do not act over long distances, so gases are easily compressed; There are practically no attractive forces between gas molecules, therefore gases have the property of expanding indefinitely. Mass and size of molecules. Avogadro's constant: Any substance consists of particles, therefore amount of substance is considered to be proportional to the number of particles. The unit of quantity of a substance is mole
. Mole equal to the amount of substance in a system containing the same number of particles as there are atoms in 0.012 kg of carbon. The ratio of the number of molecules to the amount of substance is called Avogadro's constant: Avogadro's constant is The amount of a substance can be found as the ratio of the number of atoms or molecules of the substance to Avogadro’s constant: Molar mass is a quantity equal to the ratio of the mass of a substance to the amount of substance: Molar mass can be expressed in terms of the mass of the molecule: To determine molecular masses you need to divide the mass of a substance by the number of molecules in it: Brownian motion: Brownian motion– thermal movement of particles suspended in a gas or liquid. The English botanist Robert Brown (1773 - 1858) in 1827 discovered the random movement of solid particles visible through a microscope in a liquid. This phenomenon was called Brownian motion. This movement does not stop; with increasing temperature its intensity increases. Brownian motion is the result of pressure fluctuations (a noticeable deviation from the average value). The reason for the Brownian motion of a particle is that the impacts of liquid molecules on the particle do not cancel each other out. Ideal gas: In a rarefied gas, the distance between the molecules is many times greater than their size. In this case, the interaction between molecules is negligible and the kinetic energy of the molecules is much greater than the potential energy of their interaction. To explain the properties of a substance in a gaseous state, instead of a real gas, its physical model is used - an ideal gas. The model assumes: The distance between molecules is slightly larger than their diameter; Molecules are elastic balls; There are no attractive forces between molecules; When molecules collide with each other and with the walls of the vessel, repulsive forces act; The movement of molecules obeys the laws of mechanics. The basic equation of MKT of an ideal gas: The basic MKT equation allows one to calculate the gas pressure if the mass of the molecule, the average value of the square of the velocity and the concentration of the molecules are known. Ideal gas pressure lies in the fact that molecules, when colliding with the walls of a vessel, interact with them according to the laws of mechanics as elastic bodies. When a molecule collides with the wall of a vessel, the projection of the velocity v x velocity vector onto the OX axis, perpendicular to the wall, changes its sign to the opposite, but remains constant in magnitude. Therefore, as a result of collisions of a molecule with a wall, the projection of its momentum onto the OX axis changes from mv 1x = -mv x to mv 2x =mv x. A change in the momentum of a molecule upon collision with a wall is caused by a force F 1 acting on it from the side of the wall. The change in momentum of the molecule is equal to the momentum of this force: During a collision, according to Newton's third law, the molecule acts on the wall with a force F 2, equal in magnitude to the force F 1 and directed oppositely. There are many molecules, and each one transfers the same impulse to the wall upon collision. In a second they transmit impulse Considering that not all molecules have the same speed, the force acting on the wall will be proportional to the mean square of the speed. Since the molecules move in all directions, the average values of the squares of the projected velocities are equal. Therefore, the mean square of the velocity projection is: Denoting the average value of the kinetic energy of the translational motion of ideal gas molecules: Temperature and its measurement: The basic MKT equation for an ideal gas establishes a connection between an easily measured macroscopic parameter - pressure - and such microscopic gas parameters as average kinetic energy and molecular concentration. But by measuring only pressure, we cannot find out either the average kinetic energy of individual molecules or their concentration. Consequently, to find the microscopic parameters of a gas, measurements of some other physical quantity related to the average kinetic energy of the molecules are needed. This quantity is temperature
. Any macroscopic body or group of macroscopic bodies, under constant external conditions, spontaneously passes into a state of thermal equilibrium. Thermal equilibrium – This is a state in which all macroscopic parameters remain unchanged for as long as desired. Temperature characterizes the state of thermal equilibrium of a system of bodies: all bodies of the system that are in thermal equilibrium with each other have the same temperature
. To measure temperature, you can use the change in any macroscopic quantity depending on temperature: volume, pressure, electrical resistance, etc. Most often in practice, the dependence of the volume of liquid (mercury or alcohol) on temperature is used. When calibrating a thermometer, the temperature of melting ice is usually taken as the reference point (0); the second constant point (100) is considered the boiling point of water at normal atmospheric pressure (Celsius scale). Since different liquids expand differently when heated, the scale thus established will depend to some extent on the properties of the liquid in question. Of course, 0 and 100°C will coincide for all thermometers, but 50°C will not coincide. 1.What does molecular physics study? What is MKT?
1)Molecular physics– a branch of physics that studies the physical properties of substances in various states of aggregation based on consideration of their molecular (microscopic) structure. Problems of molecular physics are solved by the methods of statistical mechanics, thermodynamics and physical kinetics; they are associated with the study of the movement and interaction of particles (atoms, molecules, ions) that make up physical bodies. 2)
Molecular kinetic theory called the doctrine of the structure and properties of matter based on the idea of the existence of atoms and molecules as the smallest particles of chemical substances. MCT explains the structure and properties of macroscopic bodies as a result of the interaction of a large number of atoms, molecules or ions of which they consist. 2.Formulate the main principles of MKT of a substance?The molecular kinetic theory is based on three main principles: 1) All substances - liquid, solid and gaseous - are formed from the smallest particles - molecules, which themselves consist of atoms ("elementary molecules"). all bodies consist of particles whose size can be neglected: atoms, molecules and ions; The molecules of a chemical substance can be simple or complex, i.e. consist of one or more atoms. Molecules and atoms are electrically neutral particles. Under certain conditions, molecules and atoms can acquire additional electrical charge and become positive or negative ions. 2) Atoms and molecules are in continuous chaotic motion. 3) Particles interact with each other through absolutely elastic collisions. 3.What experiments confirm the main provisions of MCT? 4.What is Brownian motion? Cause?Brownian motion- random movement of microscopic visible particles of a solid substance suspended in a liquid or gas, caused by the thermal movement of the particles of the liquid or gas. Brownian motion occurs due to the fact that all liquids and gases consist of atoms or molecules - tiny particles that are in constant chaotic thermal motion, and therefore continuously push the Brownian particle from different directions. It was found that large particles with sizes greater than 5 µm practically do not participate in Brownian motion (they are stationary or sediment), smaller particles (less than 3 µm) move forward along very complex trajectories or rotate. 5.What is diffusion? Examples of use?Diffusion- the process of transferring matter or energy from an area of high concentration to an area of low concentration. An example of diffusion is the mixing of gases (for example, the spread of odors) or liquids (if ink is dropped into water, the liquid will become uniformly colored after some time). Another example is associated with a solid: atoms of contacting metals mix at the contact boundary. 6.What is a mole of a substance?
Mole- a unit of measurement of the quantity of a substance in the International System of Units (SI), one of the seven basic SI units. The mole was adopted as the SI base unit by the XIV General Conference on Weights and Measures in 1971. Mole of substance- this is the amount of it that weighs in grams the same as a molecule of a substance weighs in relative atomic units, which contains 6.02 * 10^23 structural units. a branch of physics in which physics is studied. sacred properties of bodies, features of aggregative states of matter (gaseous, liquid and crystalline) and processes phase transitions depending on the molecular structure of bodies, the interaction forces of molecules (atoms, ions) and the nature of the thermal motion of these particles. M. f. closely related to statistical physics, physical kinetics And thermodynamics. Based on general theoretical representations of M. f. metal physics, polymer physics, plasma physics, physics have been developed. chemistry of dispersed systems and surface phenomena, physical-chemical. mechanics, physics of transport phenomena, etc. Biological encyclopedic dictionary Dictionary of microbiology Dictionary of microbiology Veterinary encyclopedic dictionary Physical encyclopedia Chemical encyclopedia Physical Anthropology. Illustrated explanatory dictionary Big Encyclopedic Polytechnic Dictionary Scientific and technical encyclopedic dictionary Encyclopedic Dictionary of Nanotechnology Large medical dictionary Large medical dictionary Great Soviet Encyclopedia Dictionary of synonyms MOLECULAR ECOLOGY There are often reports in the press that wildlife traders are trying to sell prohibited species or products made from endangered species of animals, under the guise of trading in completely legal goods. One of the ways to resolve 3.10. Semiconductor physics and nuclear physics From what has been said so far about the scientific interests of M. P. Bronstein, it can be understood that he paid his main attention to fundamental areas of physics. This is true, but not the whole truth. A true researcher can be captivated by any Molecular physics and heat in the 18th century If mechanics in the 18th century became a mature, well-defined field of natural science, then the science of heat essentially took only its first steps. Of course, a new approach to the study of thermal phenomena emerged back in the 17th century. Modern physics and fundamental physics First of all, let us find out the essence of the new physics, which distinguished it from the previous physics. After all, Galileo’s experiments and mathematics did not go beyond the capabilities of Archimedes, whom Galileo did not call “most divine” for nothing. What did Galileo wear? Molecular physics studies the change in the properties of substances at the molecular level depending on their state of aggregation (solid, liquid and gaseous). This section of physics is very extensive and includes many subsections.
. According to Newton's second law, the change in the momentum of a body per unit time is equal to the force acting on it: 
- the basic equation of MKT. "MOLECULAR PHYSICS" in books
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