Physics questions. What does molecular physics study?

Molecular physics- a branch of physics that studies the physical properties of bodies based on consideration of their molecular 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.

Story

The first branch of molecular physics to emerge was the kinetic theory of gases. In the process of its development, classical statistical physics was created through the work of James Clerk Maxwell, Ludwig Boltzmann, and J. W. Gibbs.

Quantitative ideas about the interaction of molecules (molecular forces) began to develop in the theory of capillary phenomena. Classic works in this field by Alexi Claude Clairaut (1743), Pierre-Simon Laplace (1806), Thomas Young (1805), S. D. Poisson, Carl Friedrich Gauss (1830-1831) and others laid the foundation for the theory of surface phenomena. Intermolecular interactions were taken into account by J.D. Van der Waals (1873) when explaining physical properties real gases and liquids.

At the beginning of the 20th century, molecular physics entered a new stage of development. In the works of Jean Baptiste Perrin and Theodor Swedberg (1906), Marian Smoluchowski and Albert Einstein (1904-06), devoted to the Brownian motion of microparticles, evidence was obtained of the reality of the existence of molecules.

By X-ray methods structural analysis(and subsequently using electron diffraction and neutron diffraction methods) the structure was studied solids and liquids and its changes during phase transitions and changes in temperature, pressure and other characteristics. The doctrine of interatomic interactions based on ideas quantum mechanics developed in the works of Max Born, Fritz London and Vallière Heitler, as well as Peter Debye. The theory of transitions from one state of aggregation to another, outlined by Van der Waals and William Thomson and developed in the works of Gibbs (late 19th century), Lev Davidovich Landau and Max Volmer (1930s) and their followers, has become modern theory phase formation is an important independent branch of physics. An association statistical methods with modern ideas about the structure of matter in the works of Yakov Ilyich Frenkel, Henry Eyring (1935-1936), John Desmond Bernal and others led to the molecular physics of liquids and solids.

Problems of science

The range of issues covered by molecular physics is very wide. It examines: the structure of matter and its changes under the influence of external factors (pressure, temperature, electromagnetic field), transfer phenomena (diffusion, thermal conductivity, viscosity), phase equilibrium and phase transition processes (

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 devices (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 such 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 jumps of molecules 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:

For determining 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 MCT 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.

Read more in the article Statistical Physics

Statistical method The study of physical phenomena is based on modeling the internal structure of matter. The environment is considered as a certain physical system consisting of a large number of molecules (atoms) with given properties. Determining macroscopic characteristics and patterns based on given microscopic properties of the medium is the main task of this method.
Thus, for a set of molecules that move chaotically, one can find certain values ​​of speed, energy, and momentum that are inherent in most molecules. Such values ​​are called the most probable. It is possible to determine the average values ​​of the speed of molecules, their energy, the free path of molecules, etc., which are characteristics of the movement of a set of molecules. Using these characteristics, it is possible to determine such parameters of a macroscopic system as pressure, absolute temperature, etc.
The statistical method allows one to establish patterns in the imaginary chaos of random phenomena that are justified for the whole ensemble of phenomena, and not for each element separately, as in a dynamic pattern. The relationships established in this way are called statistical patterns.
These regularities lose their meaning with the transition to systems with a small number of particles.
Thermodynamic method
A method of describing a process that does not take into account the microscopic structure of a substance, but considers it as a continuous medium, is called thermodynamic.
The phenomenological method allows us to establish general relationships between parameters characterizing phenomena as a whole. Phenomenological laws are very general in nature, and the role of a specific environment is taken into account by using coefficients that are determined directly from experience. Using this method, in particular, the laws of ideal and real gases were established.
The phenomenological research method is used in thermodynamics - a branch of physics that, for various natural phenomena associated with thermal effects, studies the conditions for the transformation of energy from one type to another and quantitatively characterizes these transformations. Thermodynamics is based on three fundamental laws, established on the basis of a generalization of a large number of observations and experiments on fairly large (macroscopic) bodies.
The use of the phenomenological method in heat engineering, gas dynamics, rocketry, etc. turned out to be especially effective.
Considering the properties of bodies and their changes from two different positions - microscopic and macroscopic, molecular physics and thermodynamics complement each other.
The achievements of molecular physics are widely used in other natural sciences. In particular, the development of chemistry and biology is inextricably linked with its successes. In the process of development, independent sections emerged in molecular physics, for example: physical chemistry, physical kinetics, molecular biology, solid state physics.
The basic concepts of molecular physics are used in some special fields of science, in particular, in the physics of metals, polymers and plasma, crystal physics, and physical and chemical mechanics.
Molecular physics is the scientific basis of modern materials science, vacuum technology, powder metallurgy, refrigeration technology, etc.
A significant success of modern physics has been the synthesis of artificial diamond in and other superhard materials.
Achievements in molecular physics and thermodynamics form the basis for the creation of modern heat engines, refrigeration devices for liquefying gases, chemical and food production; they contribute to the further development of meteorology.