Internal body energy cannot be a constant value. It can change in any body. If you increase your body temperature, then it internal energy will increase, because the average speed of molecular movement will increase. Thus, the kinetic energy of the molecules of the body increases. And, conversely, as the temperature decreases, the internal energy of the body decreases.

We can conclude: The internal energy of a body changes if the speed of movement of the molecules changes. Let's try to determine what method can be used to increase or decrease the speed of movement of molecules. Consider the following experiment. Let's attach a brass tube with thin walls to the stand. Fill the tube with ether and close it with a stopper. Then we tie it with a rope and begin to move the rope intensively in different sides. After a certain time, the ether will boil, and the force of the steam will push out the plug. Experience demonstrates that the internal energy of the substance (ether) has increased: after all, it has changed its temperature, at the same time boiling.

The increase in internal energy occurred due to the work done when the tube was rubbed with a rope.

As we know, heating of bodies can also occur during impacts, flexion or extension, or, more simply, during deformation. In all the examples given, the internal energy of the body increases.

Thus, the internal energy of the body can be increased by doing work on the body.

If the work is performed by the body itself, its internal energy decreases.

Let's consider another experiment.

We pump air into a glass vessel that has thick walls and is closed with a stopper through a specially made hole in it.

After some time, the cork will fly out of the vessel. At the moment when the stopper flies out of the vessel, we will be able to see the formation of fog. Consequently, its formation means that the air in the vessel has become cold. The compressed air that is in the vessel does a certain amount of work when pushing the plug out. This work he performs due to his internal energy, which at the same time is reduced. Conclusions about the decrease in internal energy can be drawn based on the cooling of the air in the vessel. Thus, The internal energy of a body can be changed by performing certain work.

However, internal energy can be changed in another way, without doing work. Let's consider an example: water in a kettle that is standing on the stove is boiling. The air, as well as other objects in the room, are heated by a central radiator. IN similar cases, internal energy increases, because body temperature increases. But the work is not done. So, we conclude a change in internal energy may not occur due to the performance of a certain amount of work.

Let's look at another example.

Place a metal knitting needle in a glass of water. Kinetic energy of molecules hot water, more kinetic energy cold metal particles. The hot water molecules will transfer some of their kinetic energy to the cold metal particles. Thus, the energy of the water molecules will decrease in a certain way, while the energy of the metal particles will increase. The water temperature will drop, and the temperature of the knitting needle will slowly will increase. In the future, the difference between the temperature of the knitting needle and the water will disappear. Due to this experience, we saw a change in internal energy different bodies. We conclude: The internal energy of various bodies changes due to heat transfer.

The process of converting internal energy without performing specific work on the body or the body itself is called heat transfer.

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There are ways to change the internal energy of a body: work and heat transfer.

When work is performed, it changes in two cases: during friction and during inelastic deformation. When work is done by friction force, the internal energy increases due to a decrease mechanical energy, the rubbing bodies heat up. In the case of inelastic compression of a body, its internal energy increases due to a decrease in mechanical energy.

Heat transfer is the process of changing internal energy without doing work, while the internal energy of one body increases due to a decrease in the internal energy of another body. The energy transition comes from bodies with more high temperature to bodies with a lower temperature. There are its options: thermal conductivity, convection and radiation.
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Internal energy is not a constant value. It may change. If you increase the temperature of a body, its internal energy will increase (the average speed of molecules will increase). As the temperature decreases, the internal energy of the body decreases.

Let's consider experience.
Let's attach a brass tube with thin walls to the stand. Fill the tube with ether and close it with a stopper. We tie it with a rope and begin to intensively move the rope to the sides. After a while, the ether will boil, and the force of the steam will push out the plug. The internal energy of the substance (ether) increased: it changed its temperature, boiling. The increase in internal energy occurred due to the work being done.

Heating of bodies can also occur during impacts, flexion or extension, or deformation. The internal energy of the body increases.

The internal energy of the body can be increased by doing work on the body. If the work is performed by the body itself, its internal energy decreases.

Let's consider experience.
We pump air into a glass vessel that has thick walls and is closed with a stopper through a specially made hole in it.

After some time, the cork will fly out of the vessel. At the moment when the stopper flies out of the vessel, we can see the formation of fog. Its formation means that the air in the vessel has become cold. The compressed air that is in the vessel does a certain amount of work when pushing the plug out. He performs this work due to his internal energy, which is reduced. Conclusions about the decrease in internal energy can be drawn based on the cooling of the air in the vessel. Thus, the internal energy of the body can be changed by performing certain work.

However, internal energy can be changed in another way, without doing work.

Let's look at an example.
The water in the kettle, which is standing on the stove, is boiling. The air, as well as other objects in the room, are heated by a central radiator. In such cases, the internal energy increases, because body temperature increases. But the work is not done. This means that a change in internal energy may not occur due to the performance of certain work.

Let's look at an example.
Place a metal knitting needle in a glass of water. The kinetic energy of hot water molecules is greater than the kinetic energy of cold metal particles. The hot water molecules will transfer some of their kinetic energy to the cold metal particles. Thus, the energy of the water molecules will decrease in a certain way, while the energy of the metal particles will increase. The water temperature will drop, and the temperature of the knitting needle will slowly increase. In the future, the difference between the temperature of the knitting needle and the water will disappear. Due to this experience, we saw a change in the internal energy of various bodies. We conclude: the internal energy of various bodies changes due to heat transfer.

The process of converting internal energy without performing a certain work on the body or the body itself is called heat transfer.

According to MKT, all substances consist 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 movement and interaction of the microparticles that make up the body. 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 atoms in molecules, i.e. at temperatures not exceeding 1000 K. In these processes, only the first two components of the internal energy change. That's why

under internal energy in thermodynamics we 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 completely definite internal energy, independent of the process through which it passed into this state. Therefore, internal energy is often called function of body condition.

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

Where i- degree of freedom. For monatomic gas (eg noble gases) i= 3, for diatomic - i = 5.

From these formulas it is clear that the internal energy of an ideal gas depends only on temperature and number of molecules and does not depend on either volume or pressure. Therefore, the change in the internal energy of an ideal gas is determined only by the 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 Δ 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, therefore, 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 is \(~N = \dfrac (m)(M) \cdot N_A\).

Therefore, the internal energy of an ideal gas is

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

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

\(~U = \dfrac (i)(2) \cdot \dfrac (m)(M) \cdot R \cdot T\) - 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 1. 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 committing mechanical work. a) If an external force causes deformation of a body, then the distances between the particles of which it consists change, and therefore the potential energy of interaction of particles changes. During inelastic deformations, in addition, the body temperature changes, i.e. the kinetic energy of thermal motion of particles changes. But when a 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 an 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 the 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) A change in the internal energy of a body occurs under the influence of friction, since, as is known from experience, friction is always accompanied by a change in the temperature of rubbing bodies. The work done by the friction force can serve as a measure of the change in internal energy.
2. With the help heat exchange. For example, if a body is placed in the flame of a burner, its temperature will change, 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.

A 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 parts of a body) during their direct contact, caused by the thermal chaotic movement of body particles. Molecular vibration amplitude solid the more, 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 represents heat transfer by heated flows of liquid or gas from some areas of the volume they occupy to others.

c) Heat exchange at radiation carried out at a distance via electromagnetic waves.

Let us consider in more detail the ways of changing 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 body volume, i.e. movement of parts of a macrobody relative to each other. This process leads to a change in the distance between 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

Let us first consider the isobaric process. Let there be a 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 to position 2 to a distance Δ l. The gas pressure force 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 using the formula

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

where Δ 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.

• Gas performs work only in the process of changing its volume.

When expanding (Δ V> 0) gas is completed positive work (A> 0); during compression (Δ V < 0) газа совершается отрицательная работа (A < 0).

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

Let us 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, when isobaric process

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

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

It follows from this physical meaning 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 shaded rectangle 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 large quantity isochore and isobar. The work on isochoric sections is 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 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 work using the last formula only if it is known how the gas pressure changes when its volume changes, i.e. the form of the function is known p = f(V).

Thus, it is clear that even with the same change in the volume of gas, 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 state. Therefore, we can conclude that

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

Amount of heat

As is known, during various mechanical processes a change in mechanical energy occurs W. A measure of the change in mechanical energy is the work of forces applied to the system:

\(~\Delta W = A.\)

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

Amount 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 type to another (from one body to another) when the state changes and significantly depend on the nature of the process.

The main difference between work and heat is that

• work characterizes the process of changing the internal energy of a 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 mass m on temperature T 1 to temperature T 2, 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 the substance (tabular value);

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

The SI unit of specific heat capacity is 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 weighing 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 body temperature by 1 K:

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

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

Vaporization (condensation)

To transform a liquid into steam at a constant temperature, it is necessary to expend an amount of heat

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

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

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

Melting (crystallization)

In order to melt a crystalline body weighing m at the melting point, the body needs to communicate the amount of heat

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

Where λ - specific heat of fusion (tabular value). When a body crystallizes, the same amount of heat is released.

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

Fuel combustion

The amount of heat released during complete combustion of a mass of fuel m,

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

Where q- specific heat of combustion (tabular value).

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

Literature

Aksenovich L. A. Physics in high school: Theory. Assignments. Tests: Textbook. allowance for institutions providing general education. environment, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsiya i vyhavanne, 2004. - P. 129-133, 152-161.

We found out that evaporation of a liquid is possible only if there is an influx of heat to the evaporating liquid. Why is this so?

Firstly, during evaporation the internal energy of a substance increases. The internal energy of saturated vapor is always greater than the internal energy of the liquid from which this vapor was formed. The increase in the internal energy of a substance during evaporation without a change in temperature occurs mainly due to the fact that when it passes into vapor, the average distance between the molecules increases. At the same time, their mutual potential energy increases, since in order to move molecules apart over long distances, work must be expended to overcome the forces of attraction of molecules to each other.

In addition, work is done against external pressure, because steam occupies a larger volume than the liquid from which it was formed. The work done during vaporization becomes especially clear if we imagine that the liquid is evaporating in a cylinder and that the resulting steam is lifted by a light piston (Fig. 492), while doing work against atmospheric pressure. This work is easy to calculate. Let's do this calculation for water boiling at normal pressure and, therefore, at temperature. Let the piston have area . Because it's normal atmospheric pressure equal, then a force acts on the piston. If the piston rises by , work will be done. This creates pair. The vapor density at is equal to , so the vapor mass is equal to . Consequently, when steam is formed, work against external pressure will be spent .

Rice. 492. The resulting vapors lift the piston. In this case, work is done against external pressure forces

When water evaporates, (specific heat of evaporation) is consumed. Of these, as our calculation shows, they are spent on working against external pressure. Therefore, the remainder equal to represents the increase in the internal energy of steam compared to the energy of water. As you can see, for water, most of the heat during evaporation goes to increase internal energy and only a small part is spent on performing external work.

297.1. Determine the increase in internal energy during the evaporation of alcohol, if it is known that the vapor density of alcohol at the boiling point is equal to .