What is thermal motion? What concepts are associated with it? Thermal motion: internal energy.

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, it increases thermal motion molecules, the properties of matter 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 is slightly flattened, there is a dent in the surface, and both of them are slightly heated. 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. Temperature warm water slightly warmer than cold 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 we take solids, then in them the molecules and atoms vibrate around 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, to lower the temperature, for example, mercury, then it will change its properties and from a liquid, it will turn 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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 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, which is called "absolute zero" - the temperature (-273 ° C) at which the thermal movement of molecules stops. But "absolute zero" is unattainable.
Brownian motion is the random movement of microscopic particles of solid matter visible suspended in a liquid or gas, caused by the thermal motion 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 brownian motion is an
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 true, the Convention is the transfer of 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.

§ 1. Thermal motion. temperature In the world around us, various physical phenomena occur that are associated with the heating and cooling of bodies. We know that when cold water is heated, it first becomes warm and then hot. With such words as "cold", "warm" and "hot", we point to a different degree of heating of bodies, or, as they say in physics, to a different temperature of bodies. Temperature hot water above cold temperature. The air temperature in summer is higher than in winter. Examples of thermal phenomena:
a - melting ice; b - freezing of water Body temperature is measured with a thermometer and expressed in degrees Celsius (°C). You already know that diffusion at a higher temperature is faster. This means that the speed of movement of molecules and temperature are related. When the temperature rises, the speed of movement of molecules increases, when it decreases, it decreases. Therefore, body temperature depends on the speed of movement of molecules. Warm water is made up of the same molecules as cold water. The difference between them lies only in the speed of movement of molecules. Phenomena associated with heating or cooling bodies, with a change in temperature, are called thermal. Such phenomena include, for example, heating and cooling of air, melting of ice, melting of metals, etc. Melting of metal The molecules or atoms that make up bodies are in continuous random motion. Their number in the bodies around us is very large. So, in a volume equal to 1 cm3 of water, there are about 3.34 1022 molecules. Each molecule moves along a very complex trajectory. This is due to the fact that, for example, gas particles moving at high speeds in different directions collide with each other and with the walls of the vessel. As a result, they change their speed and continue moving again. Figure 1 shows the trajectories of microscopic particles of paint dissolved in water. Rice. 1. The trajectory of the movement of paint microparticles dissolved in water Since its temperature is related to the speed of movement of the body's molecules, the random movement of particles is called thermal motion. In liquids, molecules can oscillate, rotate, and move relative to each other. AT solids molecules and atoms vibrate around some average positions. All molecules of the body participate in thermal motion, therefore, with a change in the nature of thermal motion, the state of the body and its properties also change. So, when the temperature rises, the ice begins to melt, turning into a liquid. If the temperature of, for example, mercury is lowered, then it turns from a liquid into a solid. Model crystal lattice ice Body temperature is closely related to the average kinetic energy of molecules. The higher the body temperature, the greater the average kinetic energy of its molecules. As the temperature of a body decreases, the average kinetic energy of its molecules decreases.