Why thermal motion. thermal motion

 Theory: Atoms and molecules are in continuous thermal motion, move randomly, constantly change direction and velocity modulus due to collisions.

The higher the temperature, the higher the speed of the molecules. As the temperature decreases, the speed of the molecules decreases. There is a temperature that is called "absolute zero" - the temperature (-273 ° C) at which stops thermal motion molecules. But "absolute zero" is unattainable.
Brownian motion- chaotic movement of microscopic particles of solid matter visible suspended in a liquid or gas, caused by the thermal movement of particles of a liquid or gas. This phenomenon was first observed in 1827 by Robert Brown. He studied the pollen of plants, which was in the aquatic environment. Brown noticed that pollen shifts all the time over time, and the higher the temperature, the faster the rate of pollen shift. He suggested that the movement of pollen is due to the fact that water molecules hit the pollen and make it move.

Diffusion is the process of mutual penetration of molecules of one substance into the gaps between the molecules of another substance.

An example of Brownian motion is
1) random movement of pollen in a drop of water
2) random movement of midges under the lantern
3) dissolution solids in liquids
4) penetration nutrients from soil to plant roots
Decision: from the definition of Brownian motion, it is clear that the correct answer is 1. Pollen moves randomly due to the fact that water molecules hit it. The erratic movement of midges under the lamp is not suitable, since the midges themselves choose the direction of movement, the last two answers are examples of diffusion.
Answer: 1.

Oge assignment in physics (I will solve the exam): Which of the following statements is(are) correct?
A. Molecules or atoms in a substance are in continuous thermal motion, and one of the arguments in favor of this is the phenomenon of diffusion.
B. Molecules or atoms in matter are in continuous thermal motion, and the proof of this is the phenomenon of convection.
1) only A
2) only B
3) both A and B
4) neither A nor B
Decision: Diffusion is the process of mutual penetration of molecules of one substance into the gaps between the molecules of another substance. The first statement is correct, the Convention is a transfer internal energy with layers of liquid or gas, it turns out that the second statement is not true.
Answer: 1.

Oge assignment in physics (fipi): 2) A lead ball is heated in a candle flame. How does the volume of the balloon change during heating? average speed the movement of its molecules?
Establish a correspondence between physical quantities and their possible changes.
For each value, determine the appropriate nature of the change:
1) increases
2) decreases
3) does not change
Write in the table the selected numbers for each physical quantity. Numbers in the answer may be repeated.
Solution (Thanks to Milena) : 2) 1. The volume of the ball will increase due to the fact that the molecules will start moving faster.
2. The speed of molecules when heated will increase.
Answer: 11.

Exercise demo version OGE 2019: One of the provisions of the molecular-kinetic theory of the structure of matter is that "particles of matter (molecules, atoms, ions) are in continuous chaotic motion." What do the words "continuous movement" mean?
1) Particles always move in a certain direction.
2) The movement of particles of matter does not obey any laws.
3) The particles all move together in one direction or the other.
4) The movement of molecules never stops.
Decision: Molecules are moving, due to collisions, the speed of the molecules is constantly changing, so we cannot calculate the speed and direction of each molecule, but we can calculate the root mean square speed of the molecules, and it is related to temperature, as the temperature decreases, the speed of the molecules decreases. It is calculated that the temperature at which the movement of molecules will stop is -273 °C (the lowest possible temperature in nature). But it is not achievable. so the molecules never stop moving.

Themes USE codifier: thermal motion of atoms and molecules of matter, Brownian motion, diffusion, interaction of particles of matter, experimental evidence of atomistic theory.

The great American physicist Richard Feynman, the author of the famous Feynman Lectures on Physics, wrote the following remarkable words:

– If, as a result of some global catastrophe, all the accumulated scientific knowledge would be destroyed and only one phrase would pass to the coming generations of living beings, then what statement, composed of least quantity words, would bring the most information? I think that is atomic hypothesis(you can call it not a hypothesis, but a fact, but this does not change anything): all bodies are made up of atoms of small bodies that are in constant motion, attract at a short distance, but repel if one of them is pressed closer to the other. That one sentence... contains an incredible amount of information about the world, you just have to apply a little imagination and a little consideration to it.

These words contain the essence of the molecular-kinetic theory (MKT) of the structure of matter. Namely, the main provisions of the MKT are the following three statements.

1. Any substance consists of the smallest particles of molecules and atoms. They are located discretely in space, that is, at certain distances from each other.
2. Atoms or molecules of a substance are in a state of random movement (this movement is called thermal movement), which never stops.
3. Atoms or molecules of a substance interact with each other by forces of attraction and repulsion, which depend on the distances between the particles.

These provisions are a generalization of numerous observations and experimental facts. Let's take a closer look at these provisions and give their experimental justification.

For example, is a water molecule consisting of two hydrogen atoms and one oxygen atom. Dividing it into atoms, we will no longer deal with a substance called "water". Further, by dividing the atoms and into component parts, we get a set of protons, neutrons and electrons and thereby lose the information that at first these were hydrogen and oxygen.

Atoms and molecules are called simply particles substances. What exactly is a particle - an atom or a molecule - in each specific case is not difficult to establish. If it's about chemical element, then the particle will be an atom; if considered complex substance, then its particle is a molecule consisting of several atoms.

Further, the first proposition of the MKT states that particles of matter do not fill space continuously. The particles are arranged discretely, that is, at separate points. Between the particles there are gaps, the size of which can vary within certain limits.

In favor of the first position of the MKT is the phenomenon thermal expansion tel. Namely, when heated, the distances between the particles of the substance increase, and the dimensions of the body increase. On cooling, on the contrary, the distances between the particles decrease, as a result of which the body contracts.

A striking confirmation of the first position of the MKT is also diffusion- mutual penetration of adjoining substances into each other.

For example, in fig. 1 shows the process of diffusion in a liquid. The particles of the solute are placed in a glass of water and are located first in the upper left part of the glass. Over time, the particles move (as they say, diffuse) from an area of ​​high concentration to an area of ​​low concentration. In the end, the concentration of particles becomes the same everywhere - the particles are evenly distributed throughout the entire volume of the liquid.

Rice. 1. Diffusion in a liquid

How to explain diffusion from the point of view of molecular-kinetic theory? Very simply: particles of one substance penetrate into the gaps between the particles of another substance. Diffusion goes the faster, the larger these gaps - therefore, gases are most easily mixed with each other (in which the distances between particles are many more sizes particles themselves).

Thermal motion of atoms and molecules

Recall once again the wording of the second provision of the MKT: the particles of matter perform random motion (also called thermal motion) that never stops.

Experimental confirmation of the second position of the MKT is again the phenomenon of diffusion, because the mutual penetration of particles is possible only with their continuous movement! But the most striking evidence of the eternal chaotic motion of particles of matter is Brownian motion. This is the name of the continuous erratic movement brownian particles- dust particles or grains (cm in size) suspended in a liquid or gas.

Brownian motion got its name in honor of the Scottish botanist Robert Brown, who saw through a microscope the continuous dance of pollen particles suspended in water. As proof that this movement takes forever, Brown found a piece of quartz with a cavity filled with water. Despite the fact that water got there many millions of years ago, the motes that got there continued their movement, which was no different from what was observed in other experiments.

The reason for Brownian motion is that a suspended particle experiences uncompensated impacts from liquid (gas) molecules, and due to the chaotic motion of molecules, the magnitude and direction of the resulting impact are absolutely unpredictable. Therefore, a Brownian particle describes complex zigzag trajectories (Fig. 2).

Rice. 2. Brownian motion

By the way, Brownian motion can also be considered as proof of the very fact of the existence of molecules, i.e., it can also serve as an experimental substantiation of the first position of the MKT.

Interaction of particles of matter

The third position of the MKT speaks of the interaction of particles of matter: atoms or molecules interact with each other by forces of attraction and repulsion, which depend on the distances between the particles: as the distances increase, the forces of attraction begin to predominate, and as the distances decrease, the repulsive forces.

The validity of the third position of the MKT is evidenced by the elastic forces arising from the deformations of bodies. When a body is stretched, the distances between its particles increase, and the forces of attraction of particles to each other begin to prevail. When a body is compressed, the distances between particles decrease, and as a result, repulsive forces predominate. In both cases, the elastic force is directed in the direction opposite to the deformation.

Another confirmation of the existence of forces of intermolecular interaction is the presence of three aggregate states of matter.

In gases, the molecules are separated from each other by distances significantly exceeding the dimensions of the molecules themselves (in air under normal conditions, by about 1000 times). At such distances, the forces of interaction between molecules are practically absent, therefore gases occupy the entire volume provided to them and are easily compressed.

In liquids, the spaces between molecules are comparable to the size of the molecules. The forces of molecular attraction are very tangible and ensure the preservation of volume by liquids. But these forces are not strong enough for liquids to preserve their shape - liquids, like gases, take the form of a vessel.

In solids, the forces of attraction between particles are very strong: solids retain not only volume, but also shape.

The transition of a substance from one state of aggregation to another is the result of a change in the magnitude of the forces of interaction between the particles of the substance. The particles themselves remain unchanged.

To study the topic "Thermal motion" we need to repeat:

In the world around us, various kinds of physical phenomena occur, which are directly related to changes in the temperature of bodies.

Since childhood, we remember that the water in the lake is cold at first, then barely warm, and only after a while becomes suitable for swimming.

With such words as “cold”, “hot”, “slightly warm”, we define different degrees of “heatedness” of bodies, or, in the language of physics, different temperatures of bodies.

If we compare the temperature in the lake in summer and late autumn, the difference is obvious. Temperature warm water slightly above the temperature of ice water.

As is known, diffusion at a higher temperature is faster. From this it follows that the speed of movement of molecules and temperature are deeply interconnected.

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 take place most intensively?

If you increase the temperature, then the speed of movement of molecules will increase, if you decrease it, it will decrease. Thus, we conclude: body temperature is directly related to 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 molecules.

Phenomena that are related to the heating or cooling of bodies, a change in temperature, are called thermal. These include heating or cooling, not only liquid bodies but also gaseous and solid air.

Other 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 in different ways. Molecules of gases randomly move at high speeds along a very complex trajectory.Colliding, they bounce off each other, changing the magnitude and direction of the velocities.

Liquid molecules oscillate around equilibrium positions (because 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 oscillate around certain average positions.

As the temperature rises, the speed of the particles increases, That's why the chaotic motion of particles is usually called thermal.

Interesting:

What is the exact height eiffel tower? And it depends on the ambient temperature!

The fact is that the height of the tower fluctuates 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.

Randomness is the most important feature of thermal motion. One of the most important evidence for the movement of molecules is diffusion and Brownian motion. (Brownian motion is the movement of the smallest solid particles in a liquid under the influence of molecular impacts. As observation shows, Brownian motion cannot stop). Brownian motion was discovered by the English botanist Robert Brown (1773-1858).

Absolutely all molecules of the body participate in the thermal motion of molecules and atoms, which is why with a change in thermal motion the state of the body itself, its various properties, also change.

Consider how the properties of water change with temperature.

Body temperature directly depends on the average kinetic energy of molecules. We draw an obvious conclusion: the higher the temperature of the body, the greater the average kinetic energy of its molecules. Conversely, as the body temperature decreases, the average kinetic energy of its molecules decreases.

Temperature - a value that characterizes the thermal state of the body or otherwise a measure of the "heating" of the body.

The higher the temperature of a body, the more energy its atoms and molecules have on average.

Temperature is measured thermometers, i.e. temperature measuring instruments

The temperature is not directly measured! The measured value depends on the temperature!

Currently, there are liquid and electrical thermometers.

In modern liquid thermometers is the volume of alcohol or mercury. The thermometer measures its own temperature! And, if we want to measure the temperature of some other body with a thermometer, we must wait some time until the temperatures of the body and the thermometer are equal, i.e. thermal equilibrium will come between the thermometer and the body. The home thermometer "thermometer" needs time to give more precisely meaning 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 with a thermometer and is most often expressed in terms of degrees Celsius(°C). There are also other units of measurement: Fahrenheit, Kelvin and Réaumur.

Most physicists measure temperature on the Kelvin scale. 0 degrees Celsius = 273 degrees Kelvin

What do you think determines the rate of dissolution of sugar in water? You can do a simple experiment. Take two pieces of sugar and throw one into a glass of boiling water, the other into a glass of cold water.

You will see how the sugar in boiling water will dissolve several times faster than in cold water. The cause of dissolution is diffusion. This means that diffusion occurs faster at higher temperatures. Diffusion is caused by the movement of molecules. Therefore, we conclude that molecules move faster at higher temperatures. That is, the speed of their movement depends on temperature. That is why the random chaotic motion of the molecules that make up the body is called thermal motion.

Thermal motion of molecules

As the temperature rises, the thermal motion of molecules increases, and the properties of the substance change. The solid melts, turning into a liquid, the liquid evaporates, turning into a gaseous state. Accordingly, if the temperature is lowered, then the average energy of the thermal motion of molecules will also decrease, and accordingly, the processes of changing the state of aggregation of bodies will occur in the opposite direction: water will condense into a liquid, the liquid will freeze, turning into a solid state. At the same time, we are always talking about the average values ​​of temperature and molecular velocity, since there are always particles with larger and smaller values ​​of these values.

Molecules in substances move, passing a certain distance, therefore, do some work. That is, we can talk about the kinetic energy of particles. As a result of their relative position there is also the potential energy of molecules. When in question about the kinetic and potential energy of bodies, then we are talking about the existence of the total mechanical energy of bodies. If the particles of the body have kinetic and potential energy, therefore, we can talk about the sum of these energies as an independent quantity.

Internal energy of the body

Consider an example. If we throw an elastic ball on the floor, then the kinetic energy of its movement is completely converted into potential energy at the moment it touches the floor, and then again goes into kinetic energy when it bounces. If we throw a heavy iron ball on a hard, inelastic surface, then the ball will land without bouncing. Its kinetic and potential energies after landing will be equal to zero. Where has the energy gone? Did she just disappear? If we examine the ball and the surface after the collision, we can see that the ball flattened a bit, a dent was left on the surface, and both of them warmed up slightly. That is, there was a change in the arrangement of the molecules of the bodies, and the temperature also increased. This means that the kinetic and potential energies of the particles of the body have changed. The energy of the body has not gone anywhere, it has passed into the internal energy of the body. Internal energy is called the kinetic and potential energy of all particles of the body. The collision of the bodies caused a change in the internal energy, it increased, and the mechanical energy decreased. This is what it consists

In the world around us, there are various kinds of physical phenomena that are directly related to change in body temperature. Since childhood, we know that cold water when heated, at first it becomes barely warm and only after certain time hot.

With such words as “cold”, “hot”, “warm”, we define different degrees of “heating” of bodies, or, speaking in the language of physics, different temperatures of bodies. The temperature of warm water is slightly higher than the temperature of cool water. If we compare the temperature of summer and winter air, the difference in temperature is obvious.

Body temperature is measured with a thermometer and is expressed in degrees Celsius (°C).

As is known, diffusion at a higher temperature is faster. From this it follows that the speed of movement of molecules and temperature are deeply interconnected. If you increase the temperature, then the speed of movement of molecules will increase, if you decrease it, it will decrease.

Thus, we conclude: body temperature is directly related to 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 molecules.

Phenomena that are related to the heating or cooling of bodies, a change in temperature, are called thermal. These include heating or cooling air, melting metal, 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 x 10²² molecules. Any molecule has a very complex trajectory of motion. For example, gas particles moving at high speeds in different directions can collide both with each other and with the walls of the vessel. Thus, they change their speed and continue moving again.

Figure #1 shows the random movement of paint particles dissolved in water.

Thus, we make one more conclusion: the chaotic movement of the particles that make up bodies is called thermal motion.

Randomness is the most important feature of thermal motion. One of the most important evidence for the movement of molecules is diffusion and Brownian motion.(Brownian motion is the movement of the smallest solid particles in a liquid under the influence of molecular impacts. As observation shows, Brownian motion cannot stop).

In liquids, molecules can oscillate, rotate, and move relative to other molecules. If you take solid bodies, then in them the molecules and atoms vibrate about some average positions.

Absolutely all molecules of the body participate in the thermal motion of molecules and atoms, which is why with a change in thermal motion the state of the body itself, its various properties, also change. Thus, if you increase the temperature of the ice, it begins to melt, while taking on a completely different form - the ice becomes a liquid. If, on the contrary, the temperature of, for example, mercury is lowered, then it will change its properties and turn from a liquid into a solid.

T body temperature directly depends on the average kinetic energy of the molecules. We draw an obvious conclusion: the higher the temperature of the body, the greater the average kinetic energy of its molecules. Conversely, as the body temperature decreases, the average kinetic energy of its molecules decreases.

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