Internal energy can be changed in two ways.
If work is done on a body, its internal energy increases.
If the body itself does the work, its internal energy decreases.
There are three simple (elementary) types of heat transfer:
Thermal conductivity
Convection
Convection is the phenomenon of heat transfer in liquids or gases, or granular media by flows of matter. There is a so-called natural convection, which occurs spontaneously in a substance when it is unevenly heated in a gravitational field. With such convection, the lower layers of the substance heat up, become lighter and float, and the upper layers, on the contrary, cool, become heavier and sink down, after which the process is repeated again and again.
Thermal radiation or radiation is the transfer of energy from one body to another in the form of electromagnetic waves due to their thermal energy.
Internal energy of an ideal gas
Based on the definition of an ideal gas, it does not have a potential component of internal energy (there are no molecular interaction forces, except shock). Thus, the internal energy of an ideal gas represents only the kinetic energy of motion of its molecules. Previously (equation 2.10) it was shown that the kinetic energy of the translational motion of gas molecules is directly proportional to its absolute temperature.
Using the expression for the universal gas constant (4.6), we can determine the value of the constant α.
Thus, the kinetic energy of translational motion of one molecule of an ideal gas will be determined by the expression.
In accordance with kinetic theory, the distribution of energy across degrees of freedom is uniform. Translational motion has 3 degrees of freedom. Consequently, one degree of freedom of movement of a gas molecule will account for 1/3 of its kinetic energy. 
For two, three and polyatomic gas molecules, in addition to the degrees of freedom of translational motion, there are degrees of freedom of the rotational motion of the molecule. For diatomic gas molecules, the number of degrees of freedom of rotational motion is 2, for three and polyatomic molecules - 3.
Since the distribution of the energy of motion of a molecule over all degrees of freedom is uniform, and the number of molecules in one kilomole of gas is equal to Nμ, the internal energy of one kilomole of an ideal gas can be obtained by multiplying expression (4.11) by the number of molecules in one kilomole and by the number of degrees of freedom of motion of a molecule of a given gas .
where Uμ is the internal energy of a kilomol of gas in J/kmol, i is the number of degrees of freedom of movement of a gas molecule.
For 1 - atomic gas i = 3, for 2 - atomic gas i = 5, for 3 - atomic and polyatomic gases i = 6.
Electric current. Conditions for the existence of electric current. EMF. Ohm's law for a complete circuit. Work and current power. Joule-Lenz law.
Among the conditions necessary for the existence of an electric current there are: the presence of free electric charges in the medium and the creation of an electric field in the medium. An electric field in a medium is necessary to create directional movement of free charges. As is known, a charge q in an electric field of intensity E is acted upon by a force F = qE, which causes free charges to move in the direction of the electric field. A sign of the existence of an electric field in a conductor is the presence of a non-zero potential difference between any two points of the conductor.
However, electrical forces cannot maintain an electric current for a long time. The directed movement of electric charges after some time leads to equalization of potentials at the ends of the conductor and, consequently, to the disappearance of the electric field in it. To maintain current in an electrical circuit, charges must be subject to forces of a non-electrical nature (external forces) in addition to Coulomb forces. A device that creates external forces, maintains a potential difference in a circuit and converts various types of energy into electrical energy is called a current source.
Conditions for the existence of electric current:
presence of free charge carriers
· presence of potential difference. these are the conditions for the occurrence of current. for the current to exist
· closed circuit
· a source of external forces that maintains the potential difference.
Any forces acting on electrically charged particles, with the exception of electrostatic (Coulomb) forces, are called extraneous forces.
Electromotive force.
Electromotive force (EMF) is a scalar physical quantity that characterizes the work of external (non-potential) forces in direct or alternating current sources. In a closed conducting circuit, the EMF is equal to the work of these forces to move a single positive charge along the circuit.
The unit of EMF, like voltage, is the volt. We can talk about electromotive force at any part of the circuit. The electromotive force of a galvanic cell is numerically equal to the work of external forces when moving a single positive charge inside the element from its negative pole to its positive one. The sign of the EMF is determined depending on the arbitrarily chosen direction of bypass of the section of the circuit where the current source is turned on.
Ohm's law for a complete circuit.
Let's consider the simplest complete circuit consisting of a current source and a resistor with resistance R. A current source having an emf ε has a resistance r, it is called the internal resistance of the current source. To obtain Ohm's law for a complete circuit, we use the law of conservation of energy.
Let a charge q pass through the cross section of the conductor during a time Δt. Then, according to the formula, the work done by external forces when moving a charge q is equal to . From the definition of current strength we have: q = IΔt. Hence, .
Due to the work of external forces, when current passes through the circuit, an amount of heat is released on its external and internal sections of the circuit, according to the Joule-Lenz law equal:
According to the law of conservation of energy, A st = Q, therefore Hence Thus, the emf of the current source is equal to the sum of the voltage drops in the external and internal sections of the circuit.
1.
There are two types of mechanical energy: kinetic and potential. Any moving body has kinetic energy; it is directly proportional to the mass of the body and the square of its speed. Bodies interacting with each other have potential energy. The potential energy of a body interacting with the Earth is directly proportional to its mass and the distance between
him and the surface of the Earth.
The sum of the kinetic and potential energy of a body is called its total mechanical energy. Thus, the total mechanical energy depends on the speed of movement of the body and on its position relative to the body with which it interacts.
If a body has energy, then it can do work. When work is done, the energy of the body changes. The value of work is equal to the change in energy.
2. If air is pumped into a thick-walled jar closed with a stopper, the bottom of which is covered with water (Fig. 67), then after some time the stopper will fly out of the jar and fog will form in the jar.
This is explained by the fact that there is water vapor in the air in the jar, which is formed when water evaporates. The appearance of fog means that the steam has turned into water, i.e. condensed, and this can happen when the temperature drops. Consequently, the air temperature in the jar decreased.
The reason for this is the following. The cork flew out of the jar because the air there acted on it with a certain force. The air did work when the plug came out. It is known that a body can do work if it has energy. Therefore, the air in the jar has energy.
As the air performed work, its temperature decreased and its condition changed. At the same time, the mechanical energy of the air did not change: neither its speed nor its position relative to the Earth changed. Consequently, the work was done not due to mechanical, but due to other energy. This energy is internal energy air in the jar.
3. The internal energy of a body is the sum of the kinetic energy of movement of its molecules and the potential energy of their interaction.
Molecules have kinetic energy \((E_к) \) , since they are in motion, and potential energy \((E_п) \) , since they interact.
Internal energy is denoted by the letter \(U\) . The unit of internal energy is 1 joule (1 J).
\[ U=E_к+E_п \]
4. The greater the speed of movement of molecules, the higher the body temperature, therefore, internal energy depends on body temperature. To transform a substance from a solid to a liquid state, for example, to turn ice into water, you need to supply energy to it. Consequently, water will have more internal energy than ice of the same mass, and, therefore, internal energy depends on the state of aggregation of the body.
The internal energy of a body does not depend on its movement as a whole and on its interaction with other bodies. Thus, the internal energy of a ball lying on the table and on the floor is the same, as well as a ball stationary and rolling on the floor (if, of course, we neglect the resistance to its movement).
The change in internal energy can be judged by the value of the work done. In addition, since the internal energy of a body depends on its temperature, a change in the body’s temperature can be used to judge the change in its internal energy.
5. Internal energy can be changed by doing work. Thus, in the described experiment, the internal energy of air and water vapor in the jar decreased as they performed the work of pushing out the stopper. At the same time, the temperature of the air and water vapor decreased, as evidenced by the appearance of fog.
If you hit a piece of lead several times with a hammer, you can even tell by touch that the piece of lead will heat up. Consequently, his internal energy, as well as the internal energy of the hammer, increased. This happened because work was done on a piece of lead.
If the body itself does work, then its internal energy decreases, and if work is done on it, then its internal energy increases.
If you pour hot water into a glass of cold water, the temperature of the hot water will decrease, and the temperature of the cold water will increase. In this case, no work is done, but the internal energy of hot water decreases, as evidenced by a decrease in its temperature.
Since at first the temperature of the hot water was higher than the temperature of the cold water, the internal energy of the hot water is greater. This means that hot water molecules have more kinetic energy than cold water molecules. Hot water molecules transfer this energy to cold water molecules during collisions, and the kinetic energy of cold water molecules increases. The kinetic energy of hot water molecules decreases.
In the example considered, mechanical work is not performed; the internal energy of the bodies changes by heat transfer.
Heat transfer is the method of changing the internal energy of a body by transferring energy from one part of the body to another or from one body to another without doing work.
Part 1
1. The internal energy of a gas in a sealed vessel of constant volume is determined by
1) chaotic movement of gas molecules
2) movement of the entire vessel with gas
3) interaction of the vessel with gas and the Earth
4) the action of external forces on a vessel with gas
2. The internal energy of a body depends on
A) body weight
B) body position relative to the Earth’s surface
B) the speed of movement of the body (in the absence of friction)
Correct answer
1) only A
2) only B
3) only B
4) only B and C
3. The internal energy of a body does not depend on
A) body temperature
B) body weight
B) body position relative to the Earth’s surface
Correct answer
1) only A
2) only B
3) only B
4) only A and B
4. How does the internal energy of a body change when it is heated?
1) increases
2) decreases
3) for gases it increases, for solids and liquids it does not change
4) does not change for gases, increases for solids and liquids
5. The internal energy of a coin increases if it
1) heat in hot water
2) immerse in water of the same temperature
3) make it move at some speed
4) raise above the surface of the Earth
6. One glass of water stands on a table in the room, and another glass of water of the same mass and the same temperature is on a shelf hanging at a height of 80 cm relative to the table. The internal energy of a glass of water on the table is
1) internal energy of water on the shelf
2) more internal energy of water on the shelf
3) less internal energy of water on the shelf
4) equal to zero
7. After the hot part is immersed in cold water, the internal energy
1) both parts and water will increase
2) both parts and water will decrease
3) the parts will decrease and the water will increase
4) parts will increase and water will decrease
8. One glass of water is on the table in the room, and another glass of water of the same mass and the same temperature is in an airplane flying at a speed of 800 km/h. Internal energy of water in an airplane
1) equal to the internal energy of water in the room
2) more internal energy of water in the room
3) less internal energy of water in the room
4) equal to zero
9. After hot water is poured into a cup standing on the table, the internal energy
1) cups and water increased
2) cups and water decreased
3) the cups decreased and the water increased
4) the cups increased and the water decreased
10. Body temperature can be increased if
A. Do work on it.
B. Give him some warmth.
Correct answer
1) only A
2) only B
3) both A and B
4) neither A nor B
11. The lead ball is cooled in the refrigerator. How do the internal energy of the ball, its mass and the density of the substance of the ball change? For each physical quantity, determine the corresponding nature of change. Write down the selected numbers for each physical quantity in the table. The numbers in the answer may be repeated.
PHYSICAL QUANTITY
A) internal energy
B) mass
B) density
NATURE OF CHANGE
1) increases
2) decreases
3) does not change
12. Air is pumped into the bottle, tightly closed with a stopper. At some point the cork flies out of the bottle. What happens to the volume of air, its internal energy and temperature? For each physical quantity, determine the nature of its change. Write down the selected numbers for each physical quantity in the table. The numbers in the answer may be repeated.
PHYSICAL QUANTITY
A) volume
B) internal energy
B) temperature
NATURE OF CHANGE
1) increases
2) decreases
3) does not change
Answers
To solve practical problems, 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. Upon completion 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. Using 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. The higher the temperature, the greater the amplitude of vibrations of the molecules of a solid body. 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.
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:
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.
§ Heat capacity, the amount of heat consumed to change the temperature by 1°C. According to a more strict definition, heat capacity- thermodynamic quantity determined by the expression:
§ where Δ Q- the amount of heat imparted to the system and causing its temperature to change by Delta; T. Finite difference ratio Δ Q/ΔТ is called average heat capacity, the ratio of infinitesimal quantities d Q/dT- true heat capacity. Since d Q is not a complete differential of the state function, then heat capacity depends on the transition path between two states of the system. Distinguish heat capacity system as a whole (J/K), specific heat capacity[J/(g K)], molar heat capacity[J/(mol K)]. All formulas below use molar quantities heat capacity.
Question 32:
Internal energy can be changed in two ways.
The amount of heat (Q) is the change in the internal energy of a body that occurs as a result of heat transfer.
The amount of heat is measured in SI units in joules.
[Q] = 1J.
The specific heat capacity of a substance shows how much heat is needed to change the temperature of a unit mass of a given substance by 1°C.
SI unit of specific heat capacity:
[c] = 1 J/kg °C.
Question 33:
33
The first law of thermodynamics is the amount of heat received by a system to change its internal energy and do work on external bodies. dQ=dU+dA, where dQ is the elementary amount of heat, dA is the elementary work, dU is the increment of internal energy. Application of the first law of thermodynamics to isoprocesses
Among the equilibrium processes occurring with thermodynamic systems, the following stand out: isoprocesses, in which one of the main state parameters remains constant.
Isochoric process (V=const). Diagram of this process (isochore) in coordinates p, V is depicted as a straight line parallel to the ordinate axis (Fig. 81), where the process 1-2
there is isochoric heating, and 1
-3 -
isochoric cooling. In an isochoric process, the gas does not do work on external bodies, 
Isothermal process (T=const). As already indicated in § 41, the isothermal process is described by the Boyle-Mariotte law
, in order for the temperature not to decrease during gas expansion, an amount of heat equivalent to the external work of expansion must be supplied to the gas during an isothermal process.
Question 34:
34 Adiabatic is a process in which there is no heat exchange ( dQ= 0)between the system and the environment. All fast processes can be classified as adiabatic processes. For example, the process of propagation of sound in a medium can be considered an adiabatic process, since the speed of propagation of a sound wave is so high that the exchange of energy between the wave and the medium does not have time to occur. Adiabatic processes are used in internal combustion engines (expansion and compression of the combustible mixture in cylinders), in refrigeration units, etc.
From the first law of thermodynamics ( dQ= d U+dA) for an adiabatic process it follows that 
p /С V =γ , we find
By integrating the equation in the range from p 1 to p 2 and, accordingly, from V 1 to V 2, and potentiating, we arrive at the expression
Since states 1 and 2 are chosen arbitrarily, we can write
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 it with a rope and begin to intensively move the rope 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 specific 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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Physics lesson in 8th grade on the topic: "Internal energy. Ways of changing internal energy"
Lesson objectives:
- Formation of the concept of “internal energy of the body” based on MCT of the structure of matter.
- Familiarization with ways to change the internal energy of the body.
- Formation of the concept of “heat transfer” and the ability to apply knowledge of MCT of the structure of matter in explaining thermal phenomena.
- Developing interest in physics through demonstration of interesting examples of the manifestation of thermal phenomena in nature and technology.
- Justification of the need to study thermal phenomena to apply this knowledge in everyday life.
- Development of information and communication competencies of students.
Lesson type. Combined lesson.
Type of lesson. Lesson - presentation
Lesson format.Interactive conversation, demonstration experiment, story, independent work
Forms of student work.Teamwork, individual work, group work.
Equipment: electronic presentation “Internal energy. Methods of changing internal energy", computer, projector.
Lesson progress
Organizational moment.Good afternoon Today in the lesson we will get acquainted with another type of energy, find out what it depends on and how it can be changed.
Updating knowledge.
- Repetition of basic concepts: energy, kinetic and potential energy, mechanical work.
Learning new material.
Teacher . In addition to the concepts mentioned above, it should also be remembered that two typesmechanical energycan transform (transition) into each other, for example, when a body falls. Consider a freely falling ball. Obviously, when falling, its height above the surface decreases and its speed increases, this means that its potential energy decreases and its kinetic energy increases. It should be understood that these two processes do not occur separately, they are interconnected, and they say thatpotential energy turns into kinetic energy.
To understand what the internal energy of a body is, it is necessary to answer the following question: what are all bodies made of?
Students . Bodies consist of particles that continuously move chaotically and interact with each other.
Teacher . And if they move and interact, then they have kinetic and potential energy, which constitute internal energy.
Students. It turns out that all bodies have the same internal energy, which means the temperature should be the same. But this is not so.
Teacher. Of course not. Bodies have different internal energies, and we will try to find out what the internal energy of a body depends on and what does not depend on.
Definition.
Kinetic energyparticle movements andpotential energytheir interactions constituteinternal energy of the body.
Internal energy is denoted byand it is measured, like all other types of energy, in J (joules).
Consequently, we have a formula for the internal energy of the body:. Where under refers to the kinetic energy of body particles, and by– their potential energy.
Let's remember the previous lesson, in which we talked about the fact that the movement of particles of a body is characterized by its temperature, on the other hand, the internal energy of the body is related to the nature (activity) of the movement of particles. Therefore, internal energy and temperature are interrelated concepts. When the body temperature increases, its internal energy also increases, and when it decreases, it decreases.
We found out that the internal energy of the body can change. Let's consider ways to change the internal energy of the body.
You are already familiar with the concept of mechanical work of a body; it is associated with the movement of a body when a certain force is applied to it. If mechanical work is performed, then the energy of the body changes, and the same can be said specifically about the internal energy of the body. It is convenient to depict this in a diagram:
Teacher The method of increasing the internal energy of a body through friction has been known to people since ancient times. This is how people made fire. When working in workshops, for example, turning parts with a file, what can you observe? (Parts got hot). When a person is cold, he begins to shiver involuntarily. Why do you think? (When trembling, muscle contractions occur. Due to the work of muscles, the internal energy of the body increases and becomes warmer). What conclusion can be drawn from what has been said?
Students . The internal energy of a body changes when work is done. If the body itself does work, its internal energy decreases, and if work is done on it, then its internal energy increases.
Teacher . In technology, industry, and everyday practice, we constantly encounter changes in the internal energy of a body when performing work: heating of bodies during forging, during impact; performing work with compressed air or steam.
Let's relax a little, and at the same time learn some interesting facts from the history of thermal phenomena (two students give short messages prepared in advance).
Message 1. How miracles were performed.
The ancient Greek mechanic Heron of Alexandria, the inventor of the fountain that bears his name, left us a description of two ingenious ways in which the Egyptian priests deceived the people into believing in miracles.
In Figure 1 you see a hollow metal altar, and underneath it a mechanism hidden in the dungeon that moves the doors of the temple. The altar stood outside it. When a fire is lit, the air inside the altar, due to heating, puts more pressure on the water in the vessel hidden under the floor; From the vessel, water is forced out through a tube and poured into a bucket, which, when lowered, activates a mechanism that rotates the doors (Fig. 2). Amazed spectators, unaware of the installation hidden under the floor, see a “miracle” in front of them: as soon as the fire blazes on the altar, the doors of the temple, “listening to the prayers of the priest,” dissolve as if by themselves...
Exposing the “miracle” of the Egyptian priests: the doors of the temple are opened by the action of sacrificial fire.
Message 2. How miracles were performed.
Another imaginary miracle performed by the priests is shown in Fig. 3. When a flame blazes on the altar, the air, expanding, removes oil from the lower reservoir into the tubes hidden inside the figures of the priests, and then the oil miraculously itself is added to the fire... But as soon as the priest in charge of this altar quietly removed the plug from the lid reservoir - and the outpouring of oil stopped (because excess air freely escaped through the hole); The priests resorted to this trick when the offerings of the worshipers were too meager.
Teacher. How familiar we all are with morning tea! It’s so nice to brew tea, pour sugar into a cup and drink a little, with a small spoon. Only one thing is bad - the spoon is too hot! What happened to the spoon? Why did her temperature rise? Why did her internal energy increase? Have we done work on it?
Students . No, they didn't.
Teacher . Let's find out why the change in internal energy occurred.
Initially, the temperature of the water is higher than the temperature of the spoon, and therefore the speed of the water molecules is greater. This means that water molecules have more kinetic energy than the particles of the metal from which the spoon is made. When they collide with metal particles, water molecules transfer part of their energy to them, and the kinetic energy of the metal particles increases, and the kinetic energy of water molecules decreases. This method of changing the internal energy of bodies is called heat transfer . In our daily life we often encounter this phenomenon. For example, in water, when lying on the ground or in snow, the body cools, which can lead to colds or frostbite. In severe frost, ducks willingly climb into the water. Why do you think? (In severe frost, the water temperature is significantly higher than the ambient air temperature, so the bird will cool less in water than in air). Heat transfer occurs in several ways, but we will talk about this in the next lesson.
Thus, there are two possible ways to change the internal energy. Which?
Students . Work performance and heat transfer.
Consolidation of the studied material.Now let's see how well you have learned the new material from today's lesson.. I will ask questions, and you will try to answer them.
Question 1 . Cold water is poured into one glass, the same amount of boiling water is poured into the other. In which glass does the water have more internal energy? (In the second, because its temperature is higher).
Question 2. Two copper bars have the same temperature, but the mass of one is 1 kg, and the other is 0.5 kg. Which of the two given bars has greater internal energy? (The first one, because its mass is greater).
Question 3. The hammer gets hot when it is struck, for example, on an anvil, and when it lies in the sun on a hot summer day. Name ways to change the internal energy of the hammer in both cases. (In the first case, work is done, and in the second, heat transfer).
Question 4 . Water is poured into a metal mug. Which of the following causes a change in the internal energy of water? (1, 3)
- Heating water on a hot stove.
- Performing work on water, bringing it into forward motion together with the mug.
- Perform work on water by mixing it with a mixer.
Teacher . And now I suggest you work on your own. (Students are divided into 6 groups, and further work will be carried out in groups). In front of you is a sheet of paper with three tasks.
Task 1. What is the reason for the change in the internal energy of bodies in the following phenomena:
- heating water with a boiler;
- cooling food placed in the refrigerator;
- ignition of a match when struck against a box;
- strong heating and combustion of artificial earth satellites when they enter the lower dense layers of the atmosphere;
- if you quickly bend the wire in the same place, first in one direction, then in the other, then this place becomes very hot;
- cooking;
- If you quickly slide down a pole or rope, you can burn your hands;
- heating the pool water on a hot summer day;
- When driving a nail, its head heats up;
- A match flares up when it is placed into a candle flame.
For two groups – with friction; the other two groups - during impact and two more groups - during compression.
Reflection.
- What new or interesting things did you learn in class today?
- How did you learn the material you covered?
- What were the difficulties? Did you manage to overcome them?
- Will the knowledge you gained in today's lesson be useful to you?
Summing up the lesson.Today we got acquainted with the basic concepts of the section “Thermal Phenomena”: internal energy and heat transfer and became familiar with the ways of changing the internal energy of bodies. The knowledge gained will help you explain and predict the course of thermal processes that you will encounter in your life.
Homework. § 2, 3. Experimental tasks:
- Use a home thermometer to measure the temperature of the water poured into a jar or bottle.
Close the vessel tightly and shake it vigorously for 10–15 minutes, then measure the temperature again.
To prevent heat transfer from your hands, wear mittens or wrap the vessel in a towel.
What method of changing internal energy did you use? Explain. - Take a rubber band tied with a ring, apply the band to your forehead and note its temperature. Holding the rubber with your fingers, vigorously stretch it several times and, when stretched, press it again to your forehead. Draw a conclusion about the temperature and the reasons that caused the change.
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