The internal energy of a body is the internal energy of an ideal gas. Internal energy of an ideal gas - features, theory and formula

You see a rocket taking off. She does the work - lifts the astronauts and cargo. The kinetic energy of the rocket increases, as the rocket gains more and more speed as it rises. The potential energy of the rocket also increases, as it rises higher and higher above the Earth. Therefore, the sum of these energies, that is the mechanical energy of the rocket also increases.

We remember that when the body does work, its energy decreases. However, the rocket does work, but its energy does not decrease, but increases! What is the solution to the contradiction? It turns out that in addition to mechanical energy, there is another type of energy - internal energy. It is by reducing the internal energy of the burning fuel that the rocket performs mechanical work and, in addition, increases its mechanical energy.

Not only combustible, but also hot bodies have internal energy that can easily be converted into mechanical work. Let's do an experiment. We heat a weight in boiling water and put it on a tin box attached to a pressure gauge. As the air in the box warms up, the fluid in the pressure gauge will begin to move (see figure).

The expanding air does work on the fluid. Due to what energy does this happen? Of course, due to the internal energy of the kettlebell. Therefore, in this experiment we observe the conversion of the internal energy of the body into mechanical work. Note that the mechanical energy of the weight in this experiment does not change - it is always equal to zero.

So, internal energy- this is such an energy of the body, due to which mechanical work can be performed, while not causing a decrease in the mechanical energy of this body.

The internal energy of any body depends on many reasons: the type and state of its substance, the mass and temperature of the body, and others. All bodies have internal energy: large and small, hot and cold, solid, liquid and gaseous.

The most easily used for human needs is the internal energy of only, figuratively speaking, hot and combustible substances and bodies. These are oil, gas, coal, geothermal sources near volcanoes and so on. In addition, in the 20th century, man learned to use the internal energy of the so-called radioactive substances. These are, for example, uranium, plutonium and others.

Look at the right side of the diagram. The popular literature often mentions thermal, chemical, electrical, atomic (nuclear) and other types of energy. All of them, as a rule, are varieties of internal energy, since they can be used to perform mechanical work without causing a loss of mechanical energy. We will consider the concept of internal energy in more detail in the further study of physics.

their interactions.

Internal energy is included in balance of energy transformations in nature. After the discovery of internal energy was formulated law of conservation and transformation of energy. Consider the mutual transformation of mechanical and internal energies. Let a lead ball lie on a lead plate. Let's lift it up and let it go. When we lifted the ball, we informed it of potential energy. When the ball falls, it decreases, because the ball falls lower and lower. But with increasing speed, the kinetic energy of the ball gradually increases. The ball's potential energy is converted into kinetic energy. But then the ball hit the lead plate and stopped. Both its kinetic and potential energies relative to the plate became equal to zero. Examining the ball and the plate after the impact, we will see that their state has changed: the ball is slightly flattened, and a small dent has formed on the plate; when we measure their temperature, we find that they have warmed up.

Heating means an increase in the average kinetic energy of the body's molecules. During deformation, the relative position of the particles of the body changes, and therefore their potential energy also changes.

Thus, it can be argued that as a result of the impact of the ball on the plate, the mechanical energy that the ball possessed at the beginning of the experiment is converted into internal energy of the body.

It is not difficult to observe the reverse transition of internal energy into mechanical energy.

For example, if you take a thick-walled glass vessel and pump air into it through a hole in the cork, then after some time the cork will fly out of the vessel. At this point, fog forms in the vessel. The appearance of fog means that the air in the vessel has become colder and, consequently, its internal energy has decreased. This is explained by the fact that the compressed air in the vessel, pushing out the cork (i.e., expanding), did the work by reducing its internal energy. The kinetic energy of the cork has increased due to the internal energy of the compressed air.

Thus, one of the ways to change the internal energy of a body is the work done by the molecules of the body (or other bodies) on the given body. The way to change the internal energy without doing work is heat transfer.

Internal energy of an ideal monatomic gas.

Since the molecules of an ideal gas do not interact with each other, their potential energy is considered to be zero. The internal energy of an ideal gas is determined only by the kinetic energy of the random translational motion of its molecules. To calculate it, you need to multiply the average kinetic energy of one atom by the number of atoms . Given that k N A = R, we obtain the value of the internal energy of an ideal gas:

.

The internal energy of an ideal monatomic gas is directly proportional to its temperature. If we use the Clapeyron-Mendeleev equation, then the expression for the internal energy of an ideal gas can be represented as:

.

It should be noted that, according to the expression for the average kinetic energy of one atom and due to the randomness of movement, for each of the three possible directions of movement, or each degree of freedom, along the axis X, Y and Z have the same energy.

Number of degrees of freedom is the number of possible independent directions of molecular motion.

A gas, each molecule of which consists of two atoms, is called diatomic. Each atom can move in three directions, so the total number of possible directions of movement is 6. Due to the connection between molecules, the number of degrees of freedom decreases by one, therefore the number of degrees of freedom for a diatomic molecule is five.

The average kinetic energy of a diatomic molecule is . Accordingly, the internal energy of an ideal diatomic gas is:

.

Formulas for the internal energy of an ideal gas can be generalized:

.

where i is the number of degrees of freedom of gas molecules ( i= 3 for monatomic and i= 5 for a diatomic gas).

For ideal gases, the internal energy depends only on one macroscopic parameter - temperature and does not depend on volume, since the potential energy is zero (volume determines the average distance between molecules).

For real gases, the potential energy is not zero. Therefore, the internal energy in thermodynamics in the general case is uniquely determined by the parameters characterizing the state of these bodies: the volume (V) and temperature (T).

« Physics - Grade 10 "

Thermal phenomena can be described using quantities (macroscopic parameters) measured by instruments such as a manometer and a thermometer. These devices do not respond to the impact of individual molecules. The theory of thermal processes, which does not take into account the molecular structure of bodies, is called thermodynamics. In thermodynamics, processes are considered from the point of view of the conversion of heat into other forms of energy.

What is internal energy.
What ways of changing internal energy do you know?

Thermodynamics was created in the middle of the 19th century. after the discovery of the law of conservation of energy. It is based on the concept internal energy. The very name "internal" implies consideration of the system as an ensemble of moving and interacting molecules. Let us dwell on the question of what relationship exists between thermodynamics and molecular-kinetic theory.

Thermodynamics and statistical mechanics.

The first scientific theory of thermal processes was not molecular kinetic theory, but thermodynamics.

Thermodynamics arose in the study of the optimal conditions for the use of heat to do work. This happened in the middle of the 19th century, long before the molecular-kinetic theory gained general acceptance. At the same time, it was proved that, along with mechanical energy, macroscopic bodies also have energy contained within the bodies themselves.

Now in science and technology, in the study of thermal phenomena, both thermodynamics and molecular-kinetic theory are used. In theoretical physics, molecular kinetic theory is called statistical mechanics

Thermodynamics and statistical mechanics study the same phenomena by different methods and complement each other.

thermodynamic system called a set of interacting bodies exchanging energy and matter.

Internal energy in molecular-kinetic theory.

The basic concept in thermodynamics is the concept of internal energy.

Internal energy of the body(systems) is the sum of the kinetic energy of the chaotic thermal motion of molecules and the potential energy of their interaction.

The mechanical energy of the body (system) as a whole is not included in the internal energy. For example, the internal energy of gases in two identical vessels under equal conditions is the same regardless of the movement of the vessels and their location relative to each other.

It is almost impossible to calculate the internal energy of a body (or its change), taking into account the movement of individual molecules and their positions relative to each other, due to the huge number of molecules in macroscopic bodies. Therefore, it is necessary to be able to determine the value of internal energy (or its change) depending on macroscopic parameters that can be directly measured.

Internal energy of an ideal monatomic gas.

Let us calculate the internal energy of an ideal monatomic gas.

According to the model, molecules of an ideal gas do not interact with each other, therefore, the potential energy of their interaction is zero. The entire internal energy of an ideal gas is determined by the kinetic energy of the random motion of its molecules.

To calculate the internal energy of an ideal monatomic gas with mass m, you need to multiply the average kinetic energy of one atom by the number of atoms. Considering that kN A = R, we obtain the formula for the internal energy of an ideal gas:

The internal energy of an ideal monatomic gas is directly proportional to its absolute temperature.

It does not depend on the volume and other macroscopic parameters of the system.

Change in the internal energy of an ideal gas

i.e., it is determined by the temperatures of the initial and final states of the gas and does not depend on the process.

If an ideal gas consists of more complex molecules than a monatomic one, then its internal energy is also proportional to the absolute temperature, but the coefficient of proportionality between U and T is different. This is explained by the fact that complex molecules not only move forward, but also rotate and oscillate about their equilibrium positions. The internal energy of such gases is equal to the sum of the energies of the translational, rotational and vibrational motions of the molecules. Therefore, the internal energy of a polyatomic gas is greater than the energy of a monatomic gas at the same temperature.

Dependence of internal energy on macroscopic parameters.

We have established that the internal energy of an ideal gas depends on one parameter - temperature.

For real gases, liquids and solids, the average potential energy of interaction of molecules not equal to zero. True, for gases it is much less than the average kinetic energy of molecules, but for solid and liquid bodies it is comparable to it.

The average potential energy of interaction of gas molecules depends on the volume of the substance, since when the volume changes, the average distance between the molecules changes. Consequently, the internal energy of a real gas in thermodynamics generally depends, along with the temperature T, on the volume V.

Can it be argued that the internal energy of a real gas depends on pressure, based on the fact that pressure can be expressed in terms of the temperature and volume of the gas.

The values ​​of macroscopic parameters (temperatures T of the volume V, etc.) unambiguously determine the state of the bodies. Therefore, they also determine the internal energy of macroscopic bodies.

The internal energy U of macroscopic bodies is uniquely determined by the parameters characterizing the state of these bodies: temperature and volume.

According to MKT, all substances are composed of particles that are in continuous thermal motion and interact with each other. Therefore, even if the body is motionless and has zero potential energy, it has energy (internal energy), which is the total energy of motion and interaction of the microparticles that make up the body. The composition of internal energy includes:

1. kinetic energy of translational, rotational and vibrational motion of molecules;
2. potential energy of interaction of atoms and molecules;
3. intraatomic and intranuclear energy.

In thermodynamics, processes are considered at temperatures at which the oscillatory motion of atoms in molecules is not excited, i.e. at temperatures not exceeding 1000 K. Only the first two components of the internal energy change in these processes. That's why

under internal energy in thermodynamics, they understand the sum of the kinetic energy of all molecules and atoms of a body and the potential energy of their interaction.

The internal energy of a body determines its thermal state and changes during the transition from one state to another. In a given state, the body has a well-defined internal energy, independent of the process as a result of which it passed into this state. Therefore, the internal energy is very often called body state function.

\(~U = \dfrac (i)(2) \cdot \dfrac (m)(M) \cdot R \cdot T,\)

where i- degree of freedom. For a monatomic gas (for example, inert gases) i= 3, for diatomic - i = 5.

From these formulas it can be seen that the internal energy of an ideal gas depends only on temperature and number of molecules and does not depend on volume or pressure. Therefore, the change in the internal energy of an ideal gas is determined only by a change in its temperature and does not depend on the nature of the process in which the gas passes from one state to another:

\(~\Delta U = U_2 - U_1 = \dfrac (i)(2) \cdot \dfrac(m)(M) \cdot R \cdot \Delta T ,\)

where ∆ T = T 2 - T 1 .

• Molecules of real gases interact with each other and therefore have potential energy W p , which depends on the distance between the molecules and, consequently, on the volume occupied by the gas. Thus, the internal energy of a real gas depends on its temperature, volume, and molecular structure.

*Derivation of the formula

Average kinetic energy of a molecule \(~\left\langle W_k \right\rangle = \dfrac (i)(2) \cdot k \cdot T\).

The number of molecules in the gas \(~N = \dfrac (m)(M) \cdot N_A\).

Therefore, the internal energy of an ideal gas

\(~U = N \cdot \left\langle W_k \right\rangle = \dfrac (m)(M) \cdot N_A \cdot \dfrac (i)(2) \cdot k \cdot T .\)

Given that k⋅N A= R is the universal gas constant, we have

\(~U = \dfrac (i)(2) \cdot \dfrac (m)(M) \cdot R \cdot T\) is the internal energy of an ideal gas.

Change in internal energy

To solve practical issues, it is not the internal energy itself that plays a significant role, but its change Δ U = U 2 - U one . The change in internal energy is calculated based on the laws of conservation of energy.

The internal energy of a body can change in two ways:

1. When making mechanical work. a) If an external force causes deformation of the body, then the distances between the particles of which it consists change, and consequently, the potential energy of the interaction of particles changes. With inelastic deformations, in addition, the temperature of the body changes, i.e. the kinetic energy of the thermal motion of particles changes. But when the body is deformed, work is done, which is a measure of the change in the internal energy of the body. b) The internal energy of a body also changes during its inelastic collision with another body. As we saw earlier, during inelastic collision of bodies, their kinetic energy decreases, it turns into internal energy (for example, if you hit a wire lying on an anvil several times with a hammer, the wire will heat up). The measure of change in the kinetic energy of a body is, according to the kinetic energy theorem, the work of the acting forces. This work can also serve as a measure of changes in internal energy. c) The change in the internal energy of the body occurs under the action of the force of friction, since, as is known from experience, friction is always accompanied by a change in the temperature of rubbing bodies. The work of the friction force can serve as a measure of the change in internal energy.
2. With help heat transfer. For example, if a body is placed in a burner flame, its temperature will change, and therefore its internal energy will also change. However, no work was done here, because there was no visible movement of either the body itself or its parts.

The change in the internal energy of a system without doing work is called heat exchange(heat transfer).

There are three types of heat transfer: conduction, convection and radiation.

a) thermal conductivity is the process of heat exchange between bodies (or body parts) in their direct contact, due to the thermal chaotic movement of body particles. The amplitude of oscillations of the molecules of a solid body is greater, the higher its temperature. The thermal conductivity of gases is due to the exchange of energy between gas molecules during their collisions. In the case of liquids, both mechanisms work. The thermal conductivity of a substance is maximum in the solid state and minimum in the gaseous state.

b) Convection is the transfer of heat by heated flows of liquid or gas from one part of the volume they occupy to another.

c) Heat transfer at radiation carried out at a distance by means of electromagnetic waves.

Let us consider in more detail how to change the internal energy.

mechanical work

When considering thermodynamic processes, the mechanical movement of macrobodies as a whole is not considered. The concept of work here is associated with a change in the volume of the body, i.e. moving parts of the macrobody relative to each other. This process leads to a change in the distance between the particles, and also often to a change in the speed of their movement, therefore, to a change in the internal energy of the body.

isobaric process

Consider first the isobaric process. Let there be gas in a cylinder with a movable piston at a temperature T 1 (Fig. 1).

We will slowly heat the gas to a temperature T 2. The gas will expand isobarically and the piston will move from position 1 into position 2 distance Δ l. In this case, the pressure force of the gas will do work on external bodies. Because p= const, then the pressure force F = p⋅S also constant. Therefore, the work of this force can be calculated by the formula

\(~A = F \cdot \Delta l = p \cdot S \cdot \Delta l = p \cdot \Delta V,\)

where ∆ V- change in gas volume.

• If the volume of the gas does not change (isochoric process), then the work done by the gas is zero.

• The gas does work only in the process of changing its volume.

When expanding (Δ V> 0) positive work is done on the gas ( BUT> 0); under compression (Δ V < 0) газа совершается отрицательная работа (BUT < 0).

• If we consider the work of external forces A " (BUT " = –BUT), then with the expansion (Δ V> 0) gas BUT " < 0); при сжатии (ΔV < 0) BUT " > 0.

Let's write the Clapeyron-Mendeleev equation for two gas states:

\(~p \cdot V_1 = \nu \cdot R \cdot T_1, \; \; p \cdot V_2 = \nu \cdot R \cdot T_2,\)

\(~p \cdot (V_2 - V_1) = \nu \cdot R \cdot (T_2 - T_1) .\)

Therefore, at isobaric process

\(~A = \nu \cdot R \cdot \Delta T .\)

If ν = 1 mol, then at Δ Τ = 1 K we get that R is numerically equal to A.

Hence follows physical meaning of the universal gas constant: it is numerically equal to the work done by 1 mole of an ideal gas when it is heated isobarically by 1 K.

Not an isobaric process

On the chart p (V) in an isobaric process, the work is equal to the area of ​​the rectangle shaded in Figure 2, a.

If the process not isobaric(Fig. 2, b), then the function curve p = f(V) can be represented as a broken line consisting of a large number of isochores and isobars. Work on isochoric sections is equal to zero, and the total work on all isobaric sections will be equal to

\(~A = \lim_(\Delta V \to 0) \sum^n_(i=1) p_i \cdot \Delta V_i\), or \(~A = \int p(V) \cdot dV,\ )

those. will be equal to area of ​​the shaded figure.

At isothermal process (T= const) the work is equal to the area of ​​the shaded figure shown in Figure 2, c.

It is possible to determine the work using the last formula only if it is known how the gas pressure changes with a change in its volume, i.e. the form of the function is known p = f(V).

Thus, it is clear that even with the same change in gas volume, the work will depend on the method of transition (i.e., on the process: isothermal, isobaric ...) from the initial state of the gas to the final one. Therefore, it can be concluded that

• Work in thermodynamics is a process function and not a state function.

Quantity of heat

As you know, during various mechanical processes, there is a change in mechanical energy W. The measure of change in mechanical energy is the work of forces applied to the system:

\(~\DeltaW = A.\)

During heat transfer, a change in the internal energy of the body occurs. The measure of change in internal energy during heat transfer is the amount of heat.

Quantity of heat is a measure of the change in internal energy during heat transfer.

Thus, both work and the amount of heat characterize the change in energy, but are not identical to internal energy. They do not characterize the state of the system itself (as internal energy does), but determine the process of energy transition from one form to another (from one body to another) when the state changes and essentially depend on the nature of the process.

The main difference between work and heat is that

• the work characterizes the process of changing the internal energy of the system, accompanied by the transformation of energy from one type to another (from mechanical to internal);
• the amount of heat characterizes the process of transfer of internal energy from one body to another (from more heated to less heated), not accompanied by energy transformations.

Heating (cooling)

Experience shows that the amount of heat required to heat a body with a mass m temperature T 1 to temperature T 2 is calculated by the formula

\(~Q = c \cdot m \cdot (T_2 - T_1) = c \cdot m \cdot \Delta T,\)

where c- specific heat capacity of a substance (table value);

\(~c = \dfrac(Q)(m \cdot \Delta T).\)

The SI unit of specific heat is the joule per kilogram-Kelvin (J/(kg K)).

Specific heat c is numerically equal to the amount of heat that must be imparted to a body of mass 1 kg in order to heat it by 1 K.

In addition to the specific heat capacity, such a quantity as the heat capacity of the body is also considered.

Heat capacity body C numerically equal to the amount of heat required to change the body temperature by 1 K:

\(~C = \dfrac(Q)(\Delta T) = c \cdot m.\)

The SI unit of heat capacity of a body is the joule per Kelvin (J/K).

Vaporization (condensation)

To change a liquid into a vapor at a constant temperature, the amount of heat required is

\(~Q = L\cdot m,\)

where L- specific heat of vaporization (table value). When steam condenses, the same amount of heat is released.

The SI unit for specific heat of vaporization is the joule per kilogram (J/kg).

Melting (crystallization)

In order to melt a crystalline body with a mass m at the melting point, it is necessary for the body to report the amount of heat

\(~Q = \lambda \cdot m,\)

where λ - specific heat of fusion (table value). During the crystallization of a body, the same amount of heat is released.

The SI unit for specific heat of fusion is the joule per kilogram (J/kg).

fuel combustion

The amount of heat that is released during the complete combustion of fuel mass m,

\(~Q = q \cdot m,\)

where q- specific heat of combustion (table value).

The SI unit for specific heat of combustion is the joule per kilogram (J/kg).

Literature

Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: Proc. allowance for institutions providing general. environments, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsia i vykhavanne, 2004. - C. 129-133, 152-161.

Topics of the USE codifier Keywords: internal energy, heat transfer, types of heat transfer.

The particles of any body - atoms or molecules - perform a chaotic incessant motion (the so-called thermal motion). Therefore, each particle has some kinetic energy.

In addition, the particles of matter interact with each other by the forces of electrical attraction and repulsion, as well as through nuclear forces. Therefore, the entire system of particles of a given body also has potential energy.

The kinetic energy of the thermal motion of particles and the potential energy of their interaction together form a new type of energy that is not reduced to the mechanical energy of the body (i.e., the kinetic energy of the motion of the body as a whole and the potential energy of its interaction with other bodies). This type of energy is called internal energy.

The internal energy of a body is the total kinetic energy of the thermal motion of its particles plus the potential energy of their interaction with each other.

The internal energy of a thermodynamic system is the sum of the internal energies of the bodies included in the system.

Thus, the internal energy of the body is formed by the following terms.

1. Kinetic energy of continuous chaotic motion of body particles.
2. Potential energy of molecules (atoms), due to the forces of intermolecular interaction.
3. Energy of electrons in atoms.
4. Intranuclear energy.

In the case of the simplest model of matter - an ideal gas - an explicit formula can be obtained for the internal energy.

Internal energy of a monatomic ideal gas

The potential energy of interaction between particles of an ideal gas is zero (recall that in the ideal gas model we neglect the interaction of particles at a distance). Therefore, the internal energy of a monatomic ideal gas is reduced to the total kinetic energy of the translational (for a polyatomic gas one must also take into account the rotation of molecules and vibrations of atoms within molecules) of its atoms. This energy can be found by multiplying the number of gas atoms by the average kinetic energy of one atom:

We see that the internal energy of an ideal gas (whose mass and chemical composition are unchanged) is a function of only its temperature. For a real gas, liquid or solid, the internal energy will also depend on the volume - after all, when the volume changes, the relative position of the particles changes and, as a result, the potential energy of their interaction.

Status function

The most important property of internal energy is that it is state function thermodynamic system. Namely, the internal energy is uniquely determined by a set of macroscopic parameters characterizing the system and does not depend on the "prehistory" of the system, i.e. on the state in which the system was before and how specifically it ended up in this state.

So, during the transition of a system from one state to another, the change in its internal energy is determined only by the initial and final states of the system and does not depend from the path of transition from the initial state to the final. If the system returns to its original state, then the change in its internal energy is zero.

Experience shows that there are only two ways to change the internal energy of the body:

Performing mechanical work;
heat transfer.

Simply put, you can heat the kettle in only two fundamentally different ways: rub it with something or put it on fire :-) Let's consider these methods in more detail.

Change in internal energy: doing work

If work is done above body, the internal energy of the body increases.

For example, a nail after being hit with a hammer heats up and deforms a little. But temperature is a measure of the average kinetic energy of the particles of a body. Heating a nail indicates an increase in the kinetic energy of its particles: in fact, the particles are accelerated by a hammer blow and by friction of the nail against the board.

Deformation is nothing but the displacement of particles relative to each other; After the impact, the nail undergoes compression deformation, its particles approach each other, repulsive forces between them increase, and this leads to an increase in the potential energy of the nail particles.

So, the internal energy of the nail has increased. This was the result of work done on it - the work was done by the hammer and the force of friction on the board.

If the work is done by ourselves body, then the internal energy of the body decreases.

Let, for example, compressed air in a thermally insulated vessel under a piston expand and lift a certain load, thereby doing work (the process in a thermally insulated vessel is called adiabatic. We will study the adiabatic process by considering the first law of thermodynamics). During such a process, the air will be cooled - its molecules, hitting after the moving piston, give it part of their kinetic energy. (In the same way, a football player, stopping a fast-flying ball with his foot, makes a movement with his foot from ball and extinguishes its speed.) Therefore, the internal energy of the air decreases.

Air, thus, does work due to its internal energy: since the vessel is thermally insulated, there is no energy inflow to air from any external sources, and air can draw energy to do work only from its own reserves.

Change in internal energy: heat transfer

Heat transfer is the process of transfer of internal energy from a hotter body to a colder one, not associated with the performance of mechanical work.. Heat transfer can be carried out either by direct contact of the bodies, or through an intermediate medium (and even through a vacuum). Heat transfer is also called heat exchange.

There are three types of heat transfer: conduction, convection and thermal radiation.

Now we will look at them in more detail.

Thermal conductivity

If you put an iron rod with one end into the fire, then, as we know, you cannot hold it in your hand for a long time. Getting into the region of high temperature, iron atoms begin to vibrate more intensively (i.e., acquire additional kinetic energy) and inflict stronger blows on their neighbors.

The kinetic energy of neighboring atoms also increases, and now these atoms impart additional kinetic energy to their neighbors. So, from section to section, heat gradually spreads along the rod - from the end placed in the fire to our hand. This is thermal conductivity (Fig. 1) (Image from educationalelectronicsusa.com).

Rice. 1. Thermal conductivity

Thermal conductivity is the transfer of internal energy from more heated parts of the body to less heated ones due to thermal motion and the interaction of body particles..

The thermal conductivity of different substances is different. Metals have high thermal conductivity: silver, copper and gold are the best conductors of heat. The thermal conductivity of liquids is much less. Gases conduct heat so badly that they already belong to heat insulators: due to the large distances between them, gas molecules interact weakly with each other. That is why, for example, double frames are made in windows: a layer of air prevents heat from escaping).

Therefore, porous bodies, such as brick, wool or fur, are poor conductors of heat. They contain air in their pores. No wonder brick houses are considered the warmest, and in cold weather people wear fur coats and jackets with a layer of down or padding polyester.

But if the air conducts heat so poorly, then why does the room warm up from the battery?

This happens due to another type of heat transfer - convection.

Convection

Convection is the transfer of internal energy in liquids or gases as a result of circulation of flows and mixing of matter.

The air near the battery heats up and expands. The force of gravity acting on this air remains the same, but the buoyant force from the surrounding air increases, so that the heated air begins to float towards the ceiling. In its place comes cold air (the same process, but on a much grander scale, constantly occurs in nature: this is how the wind arises), with which the same thing is repeated.

As a result, air circulation is established, which serves as an example of convection - the distribution of heat in the room is carried out by air currents.

A completely analogous process can be observed in a liquid. When you put a kettle or a pot of water on the stove, the water is heated primarily due to convection (the contribution of the thermal conductivity of water is very insignificant here).

Convection currents in air and liquid are shown in fig. 2 (images from physics.arizona.edu).

Rice. 2. Convection

There is no convection in solids: the interaction forces of particles are large, the particles oscillate near fixed spatial points (the nodes of the crystal lattice), and no flows of matter can form under such conditions.

For the circulation of convection currents when heating a room, it is necessary that the heated air there was room to float. If the radiator is installed under the ceiling, then no circulation will occur - warm air will remain under the ceiling. That is why heating appliances are placed at the bottom rooms. For the same reason, they put the kettle on on the fire, as a result of which the heated layers of water, rising, give way to colder ones.

On the contrary, the air conditioner should be placed as high as possible: then the cooled air will begin to sink, and warmer air will come in its place. The circulation will go in the opposite direction compared to the movement of flows when heating the room.

thermal radiation

How does the Earth get energy from the Sun? Heat conduction and convection are excluded: we are separated by 150 million kilometers of airless space.

Here is the third type of heat transfer - thermal radiation. Radiation can propagate both in matter and in vacuum. How does it arise?

It turns out that the electric and magnetic fields are closely related to each other and have one remarkable property. If the electric field changes with time, then it generates a magnetic field, which, generally speaking, also changes with time (more on this will be discussed in the leaflet on electromagnetic induction). In turn, an alternating magnetic field generates an alternating electric field, which again generates an alternating magnetic field, which again generates an alternating electric field ...

As a result of the development of this process, electromagnetic wave- "hooked" for each other electric and magnetic fields. Like sound, electromagnetic waves have a propagation speed and frequency - in this case, this is the frequency with which the magnitudes and directions of the fields fluctuate in the wave. Visible light is a special case of electromagnetic waves.

The propagation speed of electromagnetic waves in vacuum is enormous: km/s. So, from the Earth to the Moon, light travels a little more than a second.

The frequency range of electromagnetic waves is very wide. We will talk more about the scale of electromagnetic waves in the corresponding sheet. Here we only note that visible light is a tiny range of this scale. Below it lie the frequencies of infrared radiation, above - the frequencies of ultraviolet radiation.

Recall now that atoms, being generally electrically neutral, contain positively charged protons and negatively charged electrons. These charged particles, making chaotic motion together with atoms, create alternating electric fields and thereby radiate electromagnetic waves. These waves are called thermal radiation- as a reminder that their source is the thermal motion of particles of matter.

Any body is a source of thermal radiation. In this case, the radiation carries away part of its internal energy. Having met the atoms of another body, the radiation accelerates them with its oscillating electric field, and the internal energy of this body increases. This is how we bask in the sun.

At ordinary temperatures, the frequencies of thermal radiation lie in the infrared range, so that the eye does not perceive it (we do not see how we “glow”). When a body is heated, its atoms begin to emit waves of higher frequencies. An iron nail can be red-hot - brought to such a temperature that its thermal radiation will go into the lower (red) part of the visible range. And the Sun seems to us yellow-white: the temperature on the surface of the Sun is so high that in the spectrum of its radiation there are all frequencies of visible light, and even ultraviolet, thanks to which we sunbathe.

Let's take another look at the three types of heat transfer (Figure 3) (images from beodom.com).

Rice. 3. Three types of heat transfer: conduction, convection and radiation