Internal energy. Work and heat transfer as ways to change the internal energy of a body

Topics of the Unified State Examination codifier: internal energy, heat transfer, types of heat transfer.

Particles of any body - atoms or molecules - perform chaotic continuous movement (the so-called thermal movement). Therefore, each particle has some kinetic energy.

In addition, particles of matter interact with each other through 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 movement 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 movement of body particles.
2. Potential energy of molecules (atoms), caused by 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) movement 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 only of its temperature. In a real gas, liquid or solid, the internal energy will also depend on the volume - after all, when the volume changes, the relative arrangement of the particles and, as a consequence, the potential energy of their interaction changes.

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 what state the system was in before and how specifically it ended up in this state.

Thus, when a system transitions 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 state. 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 a body:

Performing mechanical work;
heat transfer.

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

Change in internal energy: work done

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

For example, after hitting it with a hammer, a nail heats up and becomes slightly deformed. But temperature is a measure of the average kinetic energy of particles in a body. Heating a nail indicates an increase in the kinetic energy of its particles: in fact, the particles are accelerated by the impact of a hammer and by the friction of the nail on the board.

Deformation is nothing more than the displacement of particles relative to each other; After an impact, a nail experiences compressive deformation, its particles come closer together, the 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 being done on it - the work was done by the hammer and the friction force on the board.

If the work is done 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 this process, the air will cool - its molecules, striking 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 it from ball and dampens its speed.) Therefore, the internal energy of the air decreases.

The air, thus, does work at the expense of its internal energy: since the vessel is thermally insulated, there is no flow of energy to the air from any external sources, and the air can only draw energy to do work from its own reserves.

Change in internal energy: heat transfer

Heat transfer is the process of transferring internal energy from a hotter body to a colder one, not associated with the performance of mechanical work. Heat transfer can occur either through direct contact of 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 one end of an iron rod into the fire, then, as we know, you won’t hold it in your hand for long. Once in a region of high temperature, iron atoms begin to vibrate more intensely (i.e., they acquire additional kinetic energy) and cause stronger impacts 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 areas of the body to less heated ones due to thermal movement and interaction of body particles.

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

Therefore, porous bodies such as brick, cotton wool or fur are poor conductors of heat. They contain air in their pores. It’s not for nothing that brick houses are considered the warmest, and in cold weather people wear fur coats and jackets with a layer of down or synthetic padding.

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

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 buoyancy force from the surrounding air increases, so that the heated air begins to float to 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 spread of heat in the room is carried out by air currents.

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

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

Rice. 2. Convection

In solids, there is no convection: the interaction forces between particles are large, the particles oscillate near fixed spatial points (crystal lattice nodes), 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 emerge. If the radiator is installed under the ceiling, then no circulation will occur - the warm air will remain under the ceiling. That is why heating devices are placed at the bottom rooms. For the same reason the kettle is put on on 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 descend, and warmer air will take 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 receive energy from the Sun? Thermal conduction and convection are excluded: we are separated by 150 million kilometers of airless space.

The third type of heat transfer works here - thermal radiation. Radiation can propagate both in matter and in vacuum. How does it arise?

It turns out that electric and magnetic fields are closely related to each other and have one remarkable property. If an electric field changes with time, then it generates a magnetic field, which, generally speaking, also changes with time (this will be discussed in more detail in the sheet about 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- electric and magnetic fields “engaged” with each other. Like sound, electromagnetic waves have a speed of propagation and a frequency - in this case, this is the frequency with which the magnitude and direction of the fields fluctuate in the wave. Visible light is a special case of electromagnetic waves.

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

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

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

The source of thermal radiation is any body. 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's rays.

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

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

Internal body energy cannot be a constant value. It can change in any body. If you increase the body temperature, then its 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 a rope around it and begin to move the rope intensively in different directions. 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. 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 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 such cases, the 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. 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 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 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 specific work on the body or the body itself is called heat transfer.

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Internal body energy cannot be a constant value. It can change in any body. If you increase the body temperature, then its 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 a rope around it and begin to move the rope intensively in different directions. 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. 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 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 such cases, the 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. 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 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 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 specific work on the body or the body itself is called heat transfer.

Still have questions? Don't know how to do your homework?
To get help from a tutor, register.
The first lesson is free!

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