The internal energy of a body is not some kind of constant value: for the same body it can change. When the temperature rises body, the internal energy of the body increases, as the average speed increases, and hence the kinetic energy, of the molecules of this body. As the temperature decreases, on the contrary, the internal energy of the body decreases. Thus, the internal energy of the body changes with a change in the speed of movement of its molecules. What are the ways to increase or decrease this speed? Let's turn to experience.
A thin-walled brass tube is fixed on a stand (Fig. 181), into which a little ether is poured, the tube is tightly closed with a cork. The tube is wrapped with a rope and the rope is quickly moved in one direction or the other. After a while, the ether will boil and its vapor will push the cork out. This experience shows that the internal energy of the ether has increased: after all, it has heated up and even boiled. The increase in internal energy occurred as a result of the work done when rubbing the tube with a rope.
Bodies also heat up during impacts, extension and bending, in general during deformation. In all these cases, due to the work done, the internal energy of the bodies increases.
So the internal energy bodies can be enlarged by doing work on the body. If the work is done by the body itself, then its internal energy decreases. This can be observed in the following experiment.
Take a thick-walled glass vessel, closed with a cork. Through a special hole, air is pumped into the vessel, which contains water vapor. After some time, the cork pops out of the vessel (Fig. 182). The moment the cork pops out, mist appears in the vessel. Its appearance means that the air in the vessel has become colder (remember that fog also appears on the street during a cold snap).
The compressed air in the vessel pushes out the cork and does work. He does this work at the expense of his internal energy, which at the same time decreases. We judge the decrease in energy by the cooling of the air in the vessel.
The internal energy of the body can be changed in another way.
It is known that a kettle of water standing on the stove, a metal spoon dipped into a glass of hot tea, a stove in which a fire is lit, the roof of a house illuminated by the sun are heated. In all cases, the temperature of the bodies rises, which means that their internal energy also increases. How to explain its increase?
How, for example, is a cold metal spoon dipped in hot tea heated? First, the speed and kinetic energy of hot water molecules is greater than the speed and kinetic energy of cold metal particles. In those places where the spoon comes into contact with water, hot water molecules transfer part of their kinetic energy to cold metal particles. Therefore, the speed and energy of water molecules decrease on average, and the speed and energy of metal particles increase: the water temperature decreases, and the temperature of the spoon increases - their temperatures gradually equalize. With a decrease in the kinetic energy of molecules water decreases and the internal energy of the entire water in the glass, and the internal energy of the spoon increases.
The process of changing internal energy, in which no work is done on the body, and energy is transferred from one particle to another, is called heat transfer. So, the internal energy of a body can be changed in two ways: mechanical work or heat transfer.
When the body is already heated, we cannot indicate in which of the two ways this was done. So, holding a heated steel needle in our hands, we cannot say in what way it was heated - by rubbing it or by placing it in a flame.
Questions. one. Give examples showing that the internal energy of a body increases when work is done on the body. 2. Describe an experiment showing that a body can do work due to internal energy. 3. Give examples of increasing the internal energy of the body by heat transfer. 4. Explain heat transfer based on the molecular structure of matter. 5. In what two ways can the internal energy of a body be changed?
Exercise.
Place a five-kopeck coin on a piece of plywood or a wooden board. Press the coin against the board and move it quickly, first in one direction, then in the other. Notice how many times you need to move the coin so that it becomes warm, hot. Make a conclusion about the connection between the work done and the increase in the internal energy of the body.
Any macroscopic body has energy due to its microstate. This energy called internal(denoted U). It is equal to the energy of motion and interaction of microparticles that make up the body. So, internal energy ideal gas consists of the kinetic energy of all its molecules, since their interaction in this case can be neglected. Therefore it internal energy depends only on the temperature of the gas ( u~T).
The ideal gas model assumes that the molecules are at a distance of several diameters from each other. Therefore, the energy of their interaction is much less than the energy of motion and can be ignored.
In real gases, liquids and solids, the interaction of microparticles (atoms, molecules, ions, etc.) cannot be neglected, since it significantly affects their properties. Therefore, their internal energy consists of the kinetic energy of the thermal motion of microparticles and the potential energy of their interaction. Their internal energy, apart from temperature T, will also depend on the volume V, since a change in volume affects the distance between atoms and molecules, and, consequently, the potential energy of their interaction with each other.
Internal energy is a function of the state of the body, which is determined by its temperatureTand volume V.
Internal energy uniquely determined by temperatureT and body volume V characterizing its state:U=U(T, V)
To change internal energy bodies, it is necessary to actually change either the kinetic energy of the thermal motion of microparticles, or the potential energy of their interaction (or both). As you know, this can be done in two ways - by heat transfer or as a result of doing work. In the first case, this happens due to the transfer of a certain amount of heat Q; in the second - due to the performance of work A.
In this way, the amount of heat and the work done are a measure of change in the internal energy of the body:
Δ U=Q+A.
The change in internal energy occurs due to a certain amount of heat given or received by the body or due to the performance of work.
If only heat transfer takes place, then the change internal energy occurs by receiving or giving off a certain amount of heat: Δ U=Q. When heating or cooling a body, it is equal to:
Δ U=Q = cm(T 2 - T 1) =cmΔT.
When melting or crystallizing solids internal energy changes due to a change in the potential energy of the interaction of microparticles, because there are structural changes in the structure of matter. In this case, the change in internal energy is equal to the heat of fusion (crystallization) of the body: Δ U-Q pl \u003dλ m, where λ - specific heat of fusion (crystallization) of a solid body.
Evaporation of liquids or condensation of vapor also causes a change internal energy, which is equal to the heat of vaporization: Δ U=Q p =rm, where r- specific heat of vaporization (condensation) of the liquid.
Change internal energy body due to the performance of mechanical work (without heat transfer) is numerically equal to the value of this work: Δ U=A.
If a change in internal energy occurs as a result of heat transfer, thenΔ U=Q=cm(T2 —T1),orΔ U= Q pl = λ m,orΔ U=Qn =rm.
Therefore, from the point of view of molecular physics: material from the site
Internal energy of the body is the sum of the kinetic energy of the thermal motion of atoms, molecules or other particles of which it consists, and the potential energy of interaction between them; from a thermodynamic point of view, it is a function of the state of the body (system of bodies), which is uniquely determined by its macroparameters - temperatureTand volume V.
In this way, internal energy is the energy of the system, which depends on its internal state. It consists of the energy of thermal motion of all micro-particles of the system (molecules, atoms, ions, electrons, etc.) and the energy of their interaction. It is practically impossible to determine the full value of internal energy, therefore, the change in internal energy is calculated Δ u, which occurs due to heat transfer and performance of work.
The internal energy of a body is equal to the sum of the kinetic energy of thermal motion and the potential energy of interaction of its constituent microparticles.
On this page, material on the topics:
Is it possible to uniquely determine the internal energy of a body
The body has energy
Physics report on internal energy
On what macroparameters does the internal energy of an ideal gas depend
-
Back forward
Attention! The slide preview is for informational purposes only and may not represent the full extent of the presentation. If you are interested in this work, please download the full version.
Lesson Objectives:
- development of the interests and abilities of students on the basis of the transfer of knowledge and experience of cognitive and creative activities to them;
- students' understanding of such important concepts as energy, internal energy, heat transfer and its types: thermal conductivity, radiation, convection;
- the formation of students' ideas about the fundamental laws of nature on the example of the law of conservation of energy.
Tasks:
- the acquisition by students of knowledge about internal energy, ways to change it, familiarity with the terms: heat transfer, thermal conductivity, radiation;
- the formation of students' ability to observe natural phenomena, conduct experimental research, draw conclusions;
- mastery by students 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 (on the example of the movement of a rubber ball and Maxwell's pendulum);
- the transformation of mechanical energy into internal energy (for example, the fall of a lead ball on a lead plate);
- change in internal energy according to Figures 4 and 5 of the textbook (Peryshkin A.V. Physics-8), heating a coin in a candle flame and when it is rubbed against a wooden ruler, heating lead with hammer blows;
- experiments according to Fig. 6-9 in the textbook (A. V. Peryshkin, Physics-8);
- experiments on Fig. 10.11 in the textbook (Peryshkin A. V. Physics-8)
- observation of convection in gases on the example of observation of 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 sink by radiation;
- demonstration of the absorption capacity of various substances.
During the classes
Note:
The materials presented in this presentation include several topics that are important for further study of thermal phenomena, are designed for use in several lessons and when explaining a new topic, and with a general repetition in grade 8 and in the study of molecular physics in grade 10.
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 the lesson, depending on the goals and objectives of this lesson. For example, when learning new material.
Explanation of the new material:
Starting to form 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 are the types of mechanical energy?
- 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 number 1. Collision 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 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 the 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 the body is determined by the speed of movement of the particles that make up the body and their relative position.
The next stage in the study of new material is the study of ways to change the internal energy of the body. Experiments can clearly demonstrate that it is possible to change the internal energy of a body when doing work (on the body and the body itself) and during heat transfer.
These are the following experiences:
1. Change in internal energy by doing work on the body.
Experience number 2. Rub a coin on a wooden ruler, palms of hands against each other. Students conclude: the internal energy of the body has increased.
Experience number 3. Take an air flint. With rapid compression, the air heats up so significantly that the ether vapors in the cylinder under the piston ignite. Students conclude: the internal energy of the body has increased.
2. Change in internal energy when doing work by the body itself.
Experience number 4. In a thick-walled glass vessel, closed with a cork, we pump air with a pump through a special hole in it. After a while, the cork will fly out of the vessel. At the moment when the cork flies out of the vessel, it is necessary to draw the attention of students 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 in hot tea heats up, the hot iron turned off in the room cools down).
Based on all the examples and experiments, a general conclusion is made: the internal energy of the body can change (increase or decrease) with time during the heat exchange of this body with the bodies surrounding it and during the performance of mechanical work (slide 9).
Slide 9
When describing 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 number 5 with heating 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 conductors of heat is introduced. Visually demonstrate on simple experiments No. 6, No. 7, No. 8, described in the textbook (A.V. Peryshkin Physics-8), different thermal conductivity of substances and consider the use in technology, everyday life and nature of the properties of bodies in different ways to conduct heat (slide 11-13).
slide 11
slide 12
slide 13
The study of the phenomenon of convection begins with the formulation of the following experience number 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 is a poor conductor of heat. But! Question to students: How is water heated, for example, in a kettle? Why?
We will get answers to these questions if we do the following experience number 10: we will heat a flask with water from below on an alcohol lamp, at the bottom of which a crystal of potassium permanganate is placed, coloring the convection currents.
To demonstrate convection in gases, you can use a projector and observe the convection flows 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 conductivity? Convection? The students conclude that it cannot, and therefore there is another way to transfer heat.
You can continue your acquaintance with radiation by putting experience number 11 for heating a heat sink connected to a liquid pressure gauge and located at some distance to the side of the electric stove
The students are asked the question: why does the air in the heat sink heat up? After all, heat conduction and convection are excluded here. A problematic situation arises, as a result of the discussion of which the students come to the conclusion that in this case it has a special type of transmission - radiation - heat transfer with the help of invisible rays.
Next on experience number 12 find out that bodies with different surfaces have different ability to absorb energy. For this, a heat sink is used, in which one surface is shiny metal, the other is black and rough.
At the end of the explanation, examples of radiation in nature and technology can be given (slide 16-17).
slide 16
Internal energy and gas work
Fundamentals of thermodynamics
Repetition. Law of conservation of total mechanical energy: the total mechanical energy of a closed system in which friction (resistance) forces do not act is conserved.
The system is called closed if all its components interact only with each other.
The performance of work and the release of energy during thermodynamic processes indicates that thermodynamic systems have a margin internal energy.
Under internal energy systems U in thermodynamics understand the sum of the kinetic energy of motion all microparticles of the system(atoms or molecules) and the potential energy of their interaction with each other. We emphasize that mechanical energy (the potential energy of a body raised under the surface of the Earth and the kinetic energy of its movement as a whole) is not included in the internal energy.
Experience shows that there are two ways to change the internal energy of a system - making a mechanical work over the system and heat exchange with other systems.
The first way to change internal energy is to perform mechanical work BUT" external forces over the system or the system itself over external bodies A (A = -A"). When work is done, the internal energy of the system changes due to the energy of an external source. So, when inflating a bicycle wheel, the system heats up due to the operation of the pump, with the help of friction, our ancestors were able to get fire, etc.
The second way to change the internal energy of the system (without doing work) is called heat exchange (heat transfer). The amount of energy received or given away by the body in such a process is called amount of heat and denoted ∆Q.
There are three types of heat transfer: conduction, convection, thermal radiation.
At thermal conductivity heat is transferred from a hotter body to a less heated body through thermal contact between them. Heat exchange can also occur between parts of the body: from a more heated part to its less heated one without the transfer of particles that make up the body.
Convection- transfer of heat by flows of moving liquid or gas from one area of the volume they occupy to another. When the kettle is heated on the stove, thermal conductivity ensures the flow of heat through the bottom of the kettle to the lower (boundary) layers of water, however, the heating of the inner layers of water is precisely the result of convection, which leads to mixing of heated and cold water.
thermal radiation- transfer of heat by means of electromagnetic waves. In this case, there is no mechanical contact between the heater and the heat receiver. For example, when you bring your hand a short distance to an incandescent lamp, you will feel its thermal radiation. The Earth receives energy from the Sun also due to thermal radiation.
Since the internal energy U is uniquely determined by the thermodynamic parameters of the system, then it is a state function. Accordingly, the change in internal energy ΔU when the state of the system changes (change in temperature, volume, pressure, transition from a liquid state to a solid state, etc.) can be found by the formula
ΔU=U 2 - U 1
where U 1 and U 2- internal energy in the first and second states. Change in internal energy ΔU does not depend on the intermediate states of the system during such a transition, but is determined only by the initial and final values of the energy.
Internal energy 1st law of thermodynamics. The sum of the kinetic energies of the chaotic movement of all particles of the body relative to the center of mass of the body (molecules, atoms) and the potential energies of their interaction with each other is called internal energy. Kinetic the energy of the particles is determined by the speed, which means - temperature body. Potential- the distance between the particles, which means - volume. Consequently: U=U (T,V) - internal energy depends on volume and temperature. U=U(T,V) For an ideal gas: U=U (T) , because we neglect the interaction at a distance. is the internal energy of an ideal monatomic gas. Internal energy is a single-valued state function (up to an arbitrary constant) and is conserved in a closed system. The reverse is not true(!) - different states can correspond to the same energy. U - internal energy N - number of atoms - average kinetic energy K - Boltzmann's constant m - mass M - molar mass R - universal gas constant Ρ density v - amount of matter Ideal gas: Joule's experiments proved the equivalence of work and the amount of heat, i.e. both quantities are a measure of energy change, they can be measured in the same units: 1 cal = 4.1868 J ≈ 4.2 J. This value is called. mechanical equivalent of heat.
