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

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Lesson objectives:

development of interests and abilities of students based on the transfer of knowledge and experience of cognitive and creative activities;
students’ understanding of such important concepts as energy, internal energy, heat transfer and its types: thermal conductivity, radiation, convection;
developing students' ideas about the fundamental laws of nature using the example of the law of conservation of energy.

Tasks:

students acquire knowledge about internal energy, ways of changing it, familiarity with the terms: heat transfer, thermal conductivity, radiation;
developing in students the ability to observe natural phenomena, conduct experimental research, and draw conclusions;
students’ mastery of such general scientific concepts as a natural phenomenon, an empirically established fact, the result of an experiment.

Lesson type: combined.

Demos:

transformation of mechanical energy (using the example of the movement of a rubber ball and Maxwell's pendulum);
transformation of mechanical energy into internal energy (using the example of a lead ball falling onto a lead plate);
change in internal energy according to Figs 4 and 5 of the textbook (Perishkin A.V. Physics-8), heating a coin in a candle flame and when rubbing it against a wooden ruler, heating lead with hammer blows;
experiments according to Fig. 6-9 in the textbook (Peryshkin A.V. Physics-8);
experiments on Fig. 10.11 in the textbook (Peryshkin A.V. Physics-8)
observation of convection in gases using the example of observing convection flows from a burning candle in projection onto an illuminated screen;
demonstration of lamps that use the phenomenon of convection;
heating the air in the heat receiver by radiation;
demonstration of the absorption capacity of various substances.

During the classes

Note:

The materials presented in this presentation include several topics important for the further study of thermal phenomena, designed for use in several lessons and when explaining a new topic, and during general repetition in the 8th grade and when studying molecular physics in the 10th grade.

It is advisable to consolidate the acquired knowledge on the topic using examples of problems that are sufficiently represented in collections of problems in physics:

A.V. Peryshkin Collection of problems in physics grades 7-9, ed. "Exam" M., 2013.
IN AND. Lukashik, E.V. Ivanova Collection of problems in physics grades 7-9, ed. "Enlightenment" JSC "Moscow Textbooks", M., 2001.
and others.

Therefore this presentation can be used partially and (or) completely in a lesson, depending on the goals and objectives of this lesson. For example, when learning new material.

Explanation of new material:

When starting to formulate the concept of internal energy, it is necessary to invite students to remember what they know about the mechanical energy of bodies.

Questions for students:

When are bodies said to have energy?
What types of mechanical energy are distinguished?
What bodies have kinetic energy and what does it depend on?
What does the potential energy of a body depend on?
Give examples of the transformation of mechanical energy.

(Slides 2-5)

Slide 2

Slide 3

Slide 4

Slide 5

The formation of the concept of internal energy is based on the idea of an apparent “violation” of the law of conservation of energy when a lead ball collides with a lead plate.

Experience No. 1. Impact of a lead ball on a lead plate. Based on the “violation” of the law of conservation of energy and the study of the state of the lead ball after the impact, they conclude that all bodies have energy, which is called internal energy (slide 6-8).

Slide 6

Slide 7

Slide 8

Next, it is necessary to explain to students the difference between internal energy and mechanical energy of bodies. It is important to conclude that the internal energy of bodies does not depend on the mechanical energy of the body, but depends on the temperature of the body and the state of aggregation of the substance. In other words, the internal energy of a body is determined by the speed of movement of the particles that make up the body and their relative position.

The next stage of studying new material is studying ways to change the internal energy of the body. Experiments can clearly demonstrate that the internal energy of a body can be changed by doing work (on the body and the body itself) and by heat transfer.

These are the following experiments:

1. Changing internal energy by doing work on the body.

Experience No. 2. Rub the coin on a wooden ruler, palms of your hands together. Students conclude: the internal energy of the body has increased.

Experience No. 3. Take an air flint. During rapid compression, the air heats up so significantly that the ether vapor located in the cylinder under the piston ignites. Students conclude: the internal energy of the body has increased.

2. Change in internal energy when work is performed by the body itself.

Experience No. 4. We pump air into a thick-walled glass vessel, closed with a stopper, through a special hole in it. After some time, the cork will fly out of the vessel. At the moment when the cork flies out of the vessel, it is necessary to draw students' attention to the formation of fog in the glass vessel, which indicates a decrease in the temperature of the air and water vapor in it. Students conclude: the internal energy of the body has decreased.

3. Change in internal energy by heat transfer.

Based on experiences from everyday life (a spoon dipped into hot tea heats up, a turned off hot iron in the room cools down).

Based on all the examples and experiments, a general conclusion is drawn: the internal energy of a body can change (increase or decrease) over time during the heat exchange of a given body with the surrounding bodies and when mechanical work is performed (slide 9).

Slide 9

When explaining the mechanisms and methods of heat transfer, it is necessary to draw students' attention to the fact that heat transfer always occurs in a certain direction: from a body with a higher temperature to a body with a lower temperature, which essentially leads students to the idea of the second law of thermodynamics.

Slide 10

Consideration of various types of heat transfer begins with thermal conductivity. To study this phenomenon, consider experience No. 5 with heating of a metal rod (see textbook Peryshkin A.V. Physics-8) Based on the results of the experiment, students establish the fact of heat transfer from one part of the body to another and explain it.

Then the concept of good and bad heat conductors is introduced. Visually demonstrate on simple experiments No. 6, No. 7, No. 8 described in the textbook (A.V. Peryshkin Physics-8) the different thermal conductivities of substances and consider the use in technology, everyday life and nature of the properties of bodies to conduct heat differently (slide 11-13).

Slide 11

Slide 12

Slide 13

The study of the phenomenon of convection begins with the following statement: experience No. 9: a test tube filled with water is heated on an alcohol lamp in the upper part of the test tube. At the same time, the water remains cold at the bottom of the test tube, and boils at the top. Students conclude that water has poor thermal conductivity. But! Question for students: How is water heated, for example, in a kettle? Why?

We will get answers to these questions if we do the following experience No. 10: We will heat a flask with water from below on an alcohol lamp, at the bottom of which there is a crystal of potassium permanganate, which colors the convection currents.

To demonstrate convection in gases, you can use a projector and observe the convection currents coming from a burning candle in the projection on the screen.

As examples of convection in nature, the formation of day and night breezes is considered, and in technology - the formation of draft in chimneys, convection in water heating, water cooling of an internal combustion engine (slide 14-15).

Slide 14

Slide 15

The concept of radiation as one of the methods of heat transfer can be started by asking the question: “Can the energy of the Sun be transferred to the Earth by thermal conduction? Convection? Students conclude that it cannot and, therefore, there is another way to transfer heat.

You can continue your acquaintance with radiation by putting experience No. 11 by heating a heat sink connected to a liquid pressure gauge and located at some distance to the side of the electric stove

Students are asked the question: what causes the air in the heat receiver to heat up? After all, thermal conductivity and convection are excluded here. A problematic situation arises, as a result of the discussion of which students come to the conclusion that in this case there is a special type of transmission - radiation - heat transfer using invisible rays.

Next on experiment No. 12 find out that bodies with different surfaces have different abilities to absorb energy. To do this, they use a heat sink, one of which has a shiny metal surface, the other is black and rough.

To conclude the explanation, we can give examples of radiation in nature and technology (slide 16-17).

Slide 16

Particles of any body, atoms or molecules, undergo chaotic, continuous motion (the so-called thermal motion). 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) due to the forces of intermolecular interaction.

3. Energy of electrons in atoms.

4. Intranuclear energy.

IN In the case of the simplest model of an ideal gas substance, an explicit formula can be obtained for the internal energy.

8.1 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 motion of its atoms. This energy can be found by multiplying the number of gas atoms N by the average kinetic energy E of one atom:

U=NE=N		kT = NA

U = 3 2 m RT:

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, because when the volume changes, the relative arrangement of the particles and, as a consequence, the potential energy of their interaction changes.

8 For a polyatomic gas, one also has to take into account the rotation of molecules and vibrations of atoms within the molecules.

8.2 Status function

The most important property of internal energy is that it is a function of the state of the 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 on 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.

8.3 Change in internal energy: work done

If work is done on a 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 frictional force on the board.

If the work is done by the body itself, then the internal energy of the body decreases. Let, for example, compressed air in a heat-insulated vessel under a piston expand

and lifts a certain load, thereby doing work9. During this process, the air will cool, its molecules striking after the moving piston, giving 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 away from the 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.

8.4 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.

9 The process in a thermally insulated vessel is called adiabatic. We will study the adiabatic process by looking at the first law of thermodynamics.

There are three types of heat transfer: conduction, convection and thermal radiation. Now we will look at them in more detail.

8.5 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. 18)10.

Rice. 18. 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, wool or fur are poor heat conductors. 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.

8.6 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 a cold one

10 Image from website educationalelectronicsusa.com.

air11, 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 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 shown12 in Fig. 19.

Rice. 19. 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 have somewhere 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 devices are placed at the bottom of the room. For the same reason, the kettle is placed 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.

8.7 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 at work here is 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 time13. 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. . .

11 The same process, but on a much grander scale, constantly occurs in nature: this is how the wind arises.

12 Images from physics.arizona.edu.

13 This will be discussed in more detail in electrodynamics, in the topic about electromagnetic induction.

As a result of the development of this process, an electromagnetic wave propagates in space, with electric and magnetic fields linked to 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: 300,000 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 only note that visible light is a tiny range of this scale. Below it lie the frequencies of infrared radiation, above the frequency 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 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 (6000 C) 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 (Fig. 20)14.

Rice. 20. Three types of heat transfer: thermal conductivity, convection, radiation

14 Images from beodom.com.

The internal energy of a body is not some kind of constant value. It can change in the same body.

As the temperature rises, the internal energy of a body increases, since the average speed of movement of molecules increases.

Consequently, the kinetic energy of the molecules of this body increases. As the temperature decreases, on the contrary, the internal energy of the body decreases.

Thus, the internal energy of a body changes when the speed of movement of molecules changes.

Let's try to figure out how to increase or decrease the speed of movement of molecules. To do this, let's do the following experiment. Let's attach a thin-walled brass tube to a stand (Fig. 3). Pour some ether into the tube and close it with a stopper. Then we will wrap the tube with a rope and begin to quickly move it in one direction, then in the other. After some time, the ether will boil and the steam will push out the plug. Experience shows that the internal energy of the ether has increased: after all, it has heated up and even boiled.

Rice. 3. Increasing the internal energy of the body when doing work on it

The increase in internal energy occurred as a result of work done when rubbing the tube with a rope.

Heating of bodies also occurs during impacts, extension and bending, i.e., during deformation. The internal energy of the body in all the above examples increases.

Hence, The internal energy of a body can be increased by doing work on the body.

If the body itself does the work, then it internal energy decreases.

Let's do the following experiment.

We pump air into a thick-walled glass vessel, closed with a stopper, through a special hole in it (Fig. 4).

Rice. 4. Decrease in the internal energy of the body when work is performed by the body itself

After some time, the cork will pop out of the vessel. The moment the cork pops out of the container, fog is formed. Its appearance means that the air in the vessel has become colder. The compressed air in the vessel, pushing out the plug, does work. He does this work at the expense of his internal energy, which decreases. The decrease in internal energy can be judged by the cooling of the air in the vessel. So, The internal energy of a body can be changed by doing work.

The internal energy of the body can be changed in another way, without doing work. For example, water boils in a kettle placed on the stove. The air and various objects in the room are heated by the central heating radiator, the roofs of houses are heated by the rays of the sun, etc. In all these cases, the temperature of bodies increases, which means their internal energy increases. But the work is not done.

Means, a change in internal energy can occur not only as a result of work done.

How can we explain the increase in internal energy in these cases?

Consider the following example.

Place a metal knitting needle in a glass of hot water. The kinetic energy of hot water molecules is greater than the kinetic energy of cold metal particles. Hot water molecules, when interacting with cold metal particles, will transfer part of their kinetic energy to them. As a result of this, the energy of water molecules will decrease on average, and the energy of metal particles will increase. The water temperature will decrease and the temperature of the metal spoke will gradually increase. After some time, their temperatures will equalize. This experience demonstrates a change in the internal energy of bodies.

So, The internal energy of bodies can be changed by heat transfer.

The process of changing internal energy without doing work on the body or the body itself is called heat transfer.

Heat transfer always occurs in a certain direction: from bodies with a higher temperature to bodies with a lower temperature.

When body temperatures equalize, heat transfer stops.

The internal energy of a body can be changed in two ways: by performing mechanical work or by heat transfer.

Heat transfer, in turn, can be carried out: 1) thermal conductivity; 2) convection; 3) radiation.

Questions

Using Figure 3, tell how the internal energy of a body changes when work is done on it.
Describe an experiment showing that a body can do work using internal energy.
Give examples of changes in the internal energy of a body by heat transfer.
Explain, based on the molecular structure of the substance, the heating of a knitting needle immersed in hot water.
What is heat transfer?
What are two ways to change the internal energy of the body?

Exercise 2

The friction force does work on the body. Does the internal energy of the body change? By what signs can we judge this?
When rappelling quickly, your hands get hot. Explain why this happens.

Exercise

Place the coin on a piece of plywood or a wooden board. Press the coin to the board and move it quickly in one direction or the other. Notice how many times you need to move the coin to make it warm, hot. Draw a conclusion about the connection between the work performed and the increase in the internal energy of the body.