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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The article below will talk about internal energy and how to change it. Here we will get acquainted with the general definition of VE, with its meaning and two types of changes in state by the energy that a physical body or object possesses. In particular, the phenomenon of heat transfer and work will be considered.
Introduction
Internal energy is that part of the resource of a thermodynamic system that is not dependent on a specific reference system. It can change its meaning within the problem being studied.
Characteristics of equal value in the reference frame, in relation to which the central mass of a body/object of macroscopic dimensions is a state of rest, have the same total and internal energies. They always match each other. The set of parts that make up the total energy included in the internal energy is not constant and depends on the conditions of the problem being solved. In other words, RE is not a specific type of energy resource. It represents the totality of a number of total energy system components that vary to suit specific situations. Methods for changing internal energy are based on two basic principles: heat transfer and work.
VE is a specific concept for systems of a thermodynamic nature. It allows physicists to introduce various quantities, such as temperature and entropy, the dimension of chemical potential, and the mass of substances forming a system.
Completing of the work
There are two ways to change the internal energy of a body(s). The first is formed through the process of performing direct work on an object. The second is the phenomenon of heat transfer.
In cases where work is performed by the body itself, its internal energy indicator will decrease. When the process is completed by someone or something on the body, then its VE value will increase. In this case, there is a transformation of the mechanical energy resource into the internal type of energy that the object possesses. Everything can also flow the other way around: mechanical to internal.
Heat transfer increases the value of HE. However, if the body cools down, then the energy will decrease. With constant maintenance of heat transfer, the indicator will increase. Compression of gases serves as an example of an increase in the VE index, and their expansion (of gases) is a consequence of a decrease in the value of internal energy.
Heat transfer phenomenon
The change in internal energy by heat transfer represents an increase/decrease in energy potential. The body possesses it, without carrying out certain (in particular mechanical) work. The transferred amount of energy is called heat (Q, J), and the process itself is subject to the general ZSE. Changes in VE are always reflected by an increase or decrease in body temperature.
Both methods of changing internal energy (work and heat transfer) can be performed in relation to one object in a simultaneous manner, i.e. they can be combined.
The VE can be changed, for example, by creating friction. Here the performance of mechanical work (friction) and the phenomenon of heat exchange are clearly monitored. Our ancestors tried to make fire in a similar way. They created friction between the wood, the ignition temperature of which corresponds to 250 ° C.
A change in the internal energy of a body through work or heat transfer can occur in the same period of time, that is, these two types of means can work together. However, simple friction in a particular case will not be enough. To do this, one branch had to be sharpened. Currently, a person can get a fire by rubbing matches, the heads of which are coated with a flammable substance that ignites at 60-100 ° C. The first such products began to be created in the 30s of the 19th century. These were phosphorus matches. They are able to ignite at a relatively low temperature - 60 ° C. Currently in use which were put into production in 1855.
Energy Dependence
Speaking about ways to change internal energy, it will also be important to mention the dependence of this indicator on temperature. The fact is that the amount of this energy resource is determined by the average amount of kinetic energy concentrated in a molecule of the body, which, in turn, directly depends on the temperature. It is for this reason that a change in temperature always leads to a change in VE. It also follows from this that heating leads to an increase in energy, and cooling causes it to decrease.
Temperature and heat transfer
Methods of changing the internal energy of a body are divided into: heat transfer and mechanical work. However, it will be important to know that the amount of heat and temperature are not the same thing. These concepts should not be confused. Temperature values are defined in degrees, and the amount of heat transferred or transferred is defined in terms of joules (J).
Contact of two bodies, one of which will be hotter, always leads to the loss of heat by one (hotter) and the acquisition of it by the other (colder).
It is important to note that both methods of changing the body's VE always lead to the same results. It is impossible to determine exactly how its change was achieved based on the final state of the body.
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 transfer.
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.
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 up, 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.
Electricity. 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.
Any macroscopic body has energy, determined by its microstate. This energy called internal(denoted U). It is equal to the energy of movement 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 gas temperature ( U~T).
The ideal gas model assumes that the molecules are located 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 they internal energy consists of the kinetic energy of thermal movement of microparticles and the potential energy of their interaction. Their internal energy, except 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 is uniquely determined by temperatureT and body volume V, characterizing its state:U =U(T, V)
To change internal energy body, you need to actually change either the kinetic energy of the thermal movement of microparticles, or the potential energy of their interaction (or both together). As you know, this can be done in two ways - by heat exchange or by performing work. In the first case, this occurs due to the transfer of a certain amount of heat Q; in the second - due to the performance of work A.
Thus, the amount of heat and work done are a measure of change in the internal energy of a 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 exchange takes place, then the change internal energy occurs by receiving or releasing 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.
During melting or crystallization of solids internal energy changes due to changes in the potential energy of interaction of microparticles, because structural changes in the structure of the substance occur. In this case, the change in internal energy is equal to the heat of melting (crystallization) of the body: Δ U—Qpl =λ m, Where λ — specific heat of melting (crystallization) of a solid.
Evaporation of liquids or condensation of steam also causes changes 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 exchange) is numerically equal to the value of this work: Δ U =A.
If the change in internal energy occurs due to heat exchange, thenΔ U =Q =cm(T 2 -T 1),orΔ U = Q pl = λ m,orΔ U =Qn =rm.
Therefore, from the point of view of molecular physics: Material from the site
Internal body energy is the sum of the kinetic energy of the thermal movement 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.
Thus, 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 almost impossible to determine the full value of internal energy, so the change in internal energy is calculated Δ U, which occurs due to heat transfer and work performance.
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 there is material on the following topics:
Is it possible to unambiguously determine the internal energy of a body?
The body has energy
Physics report on internal energy
What macroparameters does the internal energy of an ideal gas depend on?
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