Ways to change internal energy - Hypermarket of knowledge. Internal energy and ways to change it

How to change the mechanical energy of the body? Yes, very simple. Change its location or give it acceleration. For example, kick the ball or lift it higher off the ground.

In the first case, we will change its kinetic energy, in the second, potential. But what about internal energy? How to change the internal energy of the body? To begin with, let's figure out what it is. Internal energy is the kinetic and potential energy of all the particles that make up the body. In particular, the kinetic energy of particles is the energy of their motion. And the speed of their movement, as you know, depends on temperature. That is, the logical conclusion is that by raising the body temperature, we will increase its internal energy. The easiest way to increase body temperature is through heat transfer. When bodies with different temperatures come into contact, the colder body heats up at the expense of the warmer one. The warmer body in this case cools down.

A simple daily example: a cold spoon in a cup of hot tea heats up very quickly, while the tea cools down a little. An increase in body temperature is possible in other ways. What do we all do when our face or hands freeze outside? We three of them. When rubbed, objects heat up. Also, objects heat up during impacts, pressure, that is, in other words, during interaction. Everyone knows how fire was made in ancient times - they either rubbed pieces of wood against each other, or knocked flint on another stone. Also in our time, flint lighters use the friction of a metal rod on flint.

So far, we have been talking about changing the internal energy by changing the kinetic energy of its constituent particles. What about the potential energy of these same particles? As you know, the potential energy of particles is the energy of their relative position. Thus, to change the potential energy of the particles of the body, we need to deform the body: compress, twist, and so on, that is, change the location of the particles relative to each other. This is achieved by influencing the body. We change the speed of individual parts of the body, that is, we do work on it.

Examples of changes in internal energy

Thus, all cases of influence on the body in order to change its internal energy are achieved in two ways. Either by transferring heat to it, that is, by heat transfer, or by changing the speed of its particles, that is, by performing work on the body.

Examples of changes in internal energy- this is almost all the processes taking place in the world. The internal energy of particles does not change in the case when absolutely nothing happens to the body, which, you will agree, is extremely rare - the law of conservation of energy is in effect. There is something going on around us all the time. Even with objects that at first glance nothing happens, in fact, there are various changes imperceptible to us: slight changes in temperature, small deformations, and so on. The chair sags under our weight, the temperature of the book on the shelf slightly changes with each movement of air, not to mention the drafts. Well, as for living bodies, it is clear without words that something is happening inside them all the time, and the internal energy changes almost at every moment of time.

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. Potential energy is possessed by bodies interacting with each other. 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 the body and on its position relative to the body with which it interacts.

If the 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 cork, the bottom of which is covered with water (Fig. 67), then after some time the cork will fly out of the jar and fog will form in the jar.

This is due to the fact that in the air in the jar there is water vapor, which is formed during the evaporation of water. 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 temperature of the air in the bank has dropped.

The reason for this is the following. The cork flew out of the can, because the air in there acted on it with a certain force. The air at the exit of the cork did the work. It is known that a body can perform work if it has energy. Therefore, the air in the jar has energy.

When the air did work, its temperature dropped, and its state 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. Therefore, the work was done not due to mechanical, but due to other energy. This energy is internal energy the air in the can.

3. The internal energy of a body is the sum of the kinetic energy of the movement of its molecules and the potential energy of their interaction.

Molecules have kinetic energy \((E_k) \) because they are in motion, and potential energy \((E_p) \) because they interact.

Internal energy is denoted by the letter \ (U \) . The unit of internal energy is 1 joule (1 J).

\[ U=E_k+E_p \]

4. The greater the speed of movement of molecules, the higher the temperature of the body, therefore, internal energy depends on body temperature. To transfer a substance from a solid state to a liquid state, for example, to turn ice into water, you need to bring energy to it. Therefore, 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. So, the internal energy of a ball lying on the table and on the floor is the same, as well as a ball that is motionless 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, the change in body temperature can be used to judge the change in its internal energy.

5. Internal energy can be changed by doing work. So, in the described experiment, the internal energy of air and water vapor in the jar decreased when they did the work of pushing the cork out. At the same time, the temperature of air and water vapor decreased, as evidenced by the appearance of fog.

If a piece of lead is hit several times with a hammer, then even by touch it can be determined that the piece of lead will heat up. Consequently, its internal energy, as well as the internal energy of the hammer, increased. This happened because work had been 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 hot water is poured into a glass of cold water, the temperature of the hot water will decrease and that of the cold water will increase. In this case, no work is done, but the internal energy of hot water decreases, as evidenced by the decrease in its temperature.

Since at the beginning the temperature of hot water was higher than the temperature of cold water, then the internal energy of hot water is greater. This means that hot water molecules have more kinetic energy than cold water molecules. This energy is transferred by hot water molecules to cold water molecules during collisions, and the kinetic energy of cold water molecules increases. The kinetic energy of hot water molecules decreases in this case.

In the considered example, mechanical work is not performed, the internal energy of the bodies changes by heat transfer.

Heat transfer is a method of changing the internal energy of a body when energy is transferred 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) the movement of the entire vessel with gas
3) the interaction of the vessel with gas and the Earth
4) the action on the vessel with the gas of external forces

2. The internal energy of a body depends on

A) body weight
B) the position of the body relative to the surface of the Earth
B) the speed 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) the position of the body relative to the surface of the Earth

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) increases for gases, does not change for solids and liquids
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 a certain speed
4) raise above the Earth's surface

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 a 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 a 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) Parts will decrease while water will increase
4) Details will increase while 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. The internal energy of water in an airplane

1) equal to the internal energy of the 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 on the table, internal energy

1) cups and water increased
2) cups and water decreased
3) cups decreased while water increased
4) cups increased while water decreased

10. Body temperature can be raised 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 does the internal energy of the ball, its mass and the density of the substance of the ball change in this case? For each physical quantity, determine the appropriate nature of the change. Write in the table the selected numbers for each physical quantity. Numbers in the answer may be repeated.

PHYSICAL QUANTITY
A) internal energy
B) mass
B) Density

NATURE OF THE 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 in the table the selected numbers for each physical quantity. Numbers in the answer may be repeated.

PHYSICAL QUANTITY
A) volume
B) internal energy
B) temperature

NATURE OF THE CHANGE
1) increases
2) decreases
3) does not change

Answers

The internal energy of a body is not some kind of constant. In the same body, it can change.

When the temperature rises, the internal energy of the body increases, as the average velocity of the molecules increases.

Consequently, the kinetic energy of the molecules of this body increases. Conversely, as the temperature decreases, the internal energy of the body decreases..

Thus, the internal energy of the body changes with a change in the speed of movement of molecules.

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

Rice. 3. An increase in the internal energy of the body when doing work on it

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

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

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

If the work is done by the body itself, then it internal, energy decreases.

Let's do the following experiment.

Into a thick-walled glass vessel, closed with a cork, we pump air through a special hole in it (Fig. 4).

Rice. 4. Reducing the internal energy of the body when doing work by the body itself

After a while, the cork will pop out of the vessel. At the moment when the cork pops out of the vessel, fog is formed. Its appearance means that the air in the vessel has become colder. 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. You can judge the decrease in internal energy by cooling the air in the vessel. So, the internal energy of a body can be changed by doing work.

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

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

How can the increase in internal energy be explained in these cases?

Consider the following example.

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

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

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

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

When the temperatures of the bodies equalize, heat transfer stops.

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

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

Questions

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

Exercise 2

The force of friction does work on the body. Does this change the internal energy of the body? By what signs can one judge this?
When you go down the rope quickly, your hands get hot. Explain why this is happening.

Exercise

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

Internal energy can be changed in two ways.

If work is done on a body, its internal energy increases.

Internal energy of the body(denoted as E or U) is the sum of the energies of molecular interactions and thermal motions of a molecule. The internal energy is a single-valued function of the state of the system. This means that whenever a system finds itself in a given state, its internal energy assumes the value inherent in this state, regardless of the system's history. Consequently, the change in internal energy during the transition from one state to another will always be equal to the difference between its values in the final and initial states, regardless of the path along which the transition was made.

The internal energy of a body cannot be measured directly. Only the change in internal energy can be determined:

This formula is a mathematical expression of the first law of thermodynamics

For quasi-static processes, the following relationship holds:

Temperature measured in Kelvin

Entropy, measured in joules/kelvin

Pressure measured in pascals

Chemical potential

Number of particles in systems

Heat of combustion of fuel. conditional fuel. The amount of air needed to burn the fuel.

The quality of a fuel is judged by its calorific value. To characterize solid and liquid fuels, the specific calorific value is used, which is the amount of heat released during the complete combustion of a unit mass (kJ / kg). For gaseous fuels, the volumetric calorific value is used, which is the amount of heat released during the combustion of a unit volume (kJ / m3). In addition, gaseous fuel in some cases is estimated by the amount of heat released during the complete combustion of one mole of gas (kJ / mol).

The heat of combustion is determined not only theoretically, but also empirically, by burning a certain amount of fuel in special devices called calorimeters. The heat of combustion is estimated by the increase in water temperature in the colorimeter. The results obtained by this method are close to the values calculated from the elemental composition of the fuel.

Question 14Change in internal energy during heating and cooling. The work of gas with a change in volume.

The internal energy of the body depends on the average kinetic energy of its molecules, and this energy, in turn, depends on temperature. Therefore, by changing the body temperature, we also change its internal energy. When a body is heated, its internal energy increases, and when it cools, it decreases.

The internal energy of the body can be changed without doing work. So, for example, it can be increased by heating a kettle of water on the stove or by lowering a spoon into a glass of hot tea. The fireplace in which the fire is kindled, the roof of the house illuminated by the sun, etc. are heated. An increase in the temperature of bodies in all these cases means an increase in their internal energy, but this increase occurs without doing work.

Change in internal energy body without doing work is called heat transfer. Heat transfer occurs between bodies (or parts of the same body) that have different temperatures.

How, for example, does heat transfer occur when a cold spoon comes into contact with hot water? First, the average speed and kinetic energy of the hot water molecules exceed the average speed and kinetic energy of the metal particles from which the spoon is made. But in those places where the spoon comes into contact with water, the hot water molecules begin to transfer part of their kinetic energy to the particles of the spoon, and they begin to move faster. In this case, the kinetic energy of water molecules decreases, and the kinetic energy of the particles of the spoon increases. Along with the energy, the temperature also changes: the water gradually cools down, and the spoon heats up. The change in their temperature occurs until it becomes the same for both the water and the spoon.

Part of the internal energy transferred from one body to another during heat exchange is denoted by a letter and is called the amount of heat.

Q is the amount of heat.

The amount of heat should not be confused with temperature. Temperature is measured in degrees, and the amount of heat (like any other energy) is measured in joules.

When bodies with different temperatures come into contact, the hotter body gives off a certain amount of heat, and the colder body receives it.

Work at isobaric gas expansion. One of the main thermodynamic processes that take place in most heat engines is the process of gas expansion with the performance of work. It is easy to determine the work done during the isobaric expansion of a gas.

If during the isobaric expansion of gas from volume V1 to volume V2 the piston moves in the cylinder at a distance l (Fig. 106), then the work A "performed by the gas is equal to

Where p is the gas pressure, is the change in its volume.

Work with an arbitrary gas expansion process. An arbitrary process of gas expansion from volume V1 to volume V2 can be represented as a set of alternating isobaric and isochoric processes.

Work with isothermal gas expansion. Comparing the areas of the figures under the sections of the isotherm and isobar, we can conclude that the expansion of gas from volume V1 to volume V2 at the same initial value of gas pressure is accompanied in the case of isobaric expansion by more work.

Work with gas compression. When the gas expands, the direction of the gas pressure force vector coincides with the direction of the displacement vector, so the work A "performed by the gas is positive (A" > 0), and the work A of external forces is negative: A \u003d -A "< 0.

When compressing gas the direction of the external force vector coincides with the direction of movement, therefore the work A of the external forces is positive (A > 0), and the work A "performed by the gas is negative (A"< 0).

adiabatic process. In addition to isobaric, isochoric and isothermal processes, adiabatic processes are often considered in thermodynamics.

An adiabatic process is a process that occurs in a thermodynamic system in the absence of heat exchange with surrounding bodies, i.e., under the condition Q = 0.

Question 15 Conditions for the equilibrium of the body. Moment of power. Types of balance.

Equilibrium, or balance, of a number of related phenomena in the natural and human sciences.

A system is considered to be in a state of equilibrium if all influences on this system are compensated by others or are absent altogether. A similar concept is sustainability. Equilibrium can be stable, unstable or indifferent.

Typical examples of balance:

1. Mechanical equilibrium, also known as static equilibrium, is the state of a body at rest, or moving uniformly, in which the sum of the forces and moments acting on it is zero.

2. Chemical equilibrium - a position in which a chemical reaction proceeds to the same extent as the reverse reaction, and as a result there is no change in the amount of each component.

3. The physical balance of people and animals, which is maintained by understanding its necessity and, in some cases, by artificially maintaining this balance [source not specified 948 days].

4. Thermodynamic equilibrium - the state of the system in which its internal processes do not lead to changes in macroscopic parameters (such as temperature and pressure).

R equality to zero of the algebraic sum moments of forces also does not mean that the body is necessarily at rest. For several billion years, the rotation of the Earth around its axis continues with a constant period precisely because the algebraic sum of the moments of forces acting on the Earth from other bodies is very small. For the same reason, a spinning bicycle wheel continues to rotate at a constant frequency, and only external forces stop this rotation.

Types of balance. In practice, an important role is played not only by the fulfillment of the equilibrium condition for bodies, but also by the qualitative characteristic of equilibrium, called stability. There are three types of balance of bodies: stable, unstable and indifferent. The equilibrium is called stable if, after small external influences, the body returns to its original state of equilibrium. This happens if, with a slight displacement of the body in any direction from the initial position, the resultant of the forces acting on the body becomes non-zero and is directed towards the equilibrium position. In stable equilibrium is, for example, a ball at the bottom of the recess.

The general condition for the equilibrium of a body. Combining the two conclusions, we can formulate a general condition for the equilibrium of a body: a body is in equilibrium if the geometric sum of the vectors of all forces applied to it and the algebraic sum of the moments of these forces about the axis of rotation are equal to zero.

Question 16Vaporization and condensation. Evaporation. Boiling liquid. Dependence of liquid boiling on pressure.

Vaporization - the property of dropping liquids to change their state of aggregation and turn into steam. Vaporization that occurs only on the surface of a dropping liquid is called evaporation. Vaporization over the entire volume of a liquid is called boiling; it occurs at a certain temperature, depending on the pressure. The pressure at which a liquid boils at a given temperature is called saturated vapor pressure pnp, its value depends on the type of liquid and its temperature.

Evaporation- the process of transition of a substance from a liquid state to a gaseous state (steam). The evaporation process is the reverse of the condensation process (transition from a vapor to a liquid state. Evaporation (vaporization), the transition of a substance from a condensed (solid or liquid) phase to a gaseous (steam); first-order phase transition.

Condensation - it is the reverse process of evaporation. During condensation, the vapor molecules return to the liquid. In a closed vessel, a liquid and its vapor can be in a state of dynamic equilibrium when the number of molecules leaving the liquid is equal to the number of molecules returning to the liquid from the vapor, that is, when the rates of evaporation and condensation are the same. Such a system is called a two-phase system. A vapor that is in equilibrium with its liquid is called saturated. The number of molecules emitted from a unit surface area of a liquid in one second depends on the temperature of the liquid. The number of molecules returning from vapor to liquid depends on the concentration of vapor molecules and on the average rate of their thermal motion, which is determined by the temperature of the vapor.

Boiling- the process of vaporization in a liquid (transition of a substance from a liquid to a gaseous state), with the appearance of phase separation boundaries. The boiling point at atmospheric pressure is usually given as one of the main physicochemical characteristics of a chemically pure substance.

Boiling is distinguished by type:

1. boiling with free convection in a large volume;

2. boiling under forced convection;

3. as well as in relation to the average temperature of the liquid to the saturation temperature:

4. boiling of a liquid subcooled to saturation temperature (surface boiling);

5. boiling of a liquid heated to saturation temperature

Bubble

Boiling , in which steam is formed in the form of periodically emerging and growing bubbles, is called nucleate boiling. With slow nucleate boiling in a liquid (more precisely, as a rule, on the walls or at the bottom of the vessel), bubbles filled with vapor appear. Due to the intense evaporation of the liquid inside the bubbles, they grow, float, and the vapor is released into the vapor phase above the liquid. In this case, in the near-wall layer, the liquid is in a slightly overheated state, i.e., its temperature exceeds the nominal boiling point. Under normal conditions, this difference is small (on the order of one degree).

Film

When the heat flux increases to a certain critical value, the individual bubbles merge, forming a continuous vapor layer near the vessel wall, which periodically breaks through into the liquid volume. This mode is called film mode.

Internal energy can be changed in two ways.

If work is done on a body, its internal energy increases.

If the work is done by the body itself, its internal energy decreases.

In total, 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 heated unevenly in a gravitational field. With such convection, the lower layers of matter heat up, become lighter and float, while the upper layers, on the contrary, cool down, become heavier and sink down, after which the process repeats 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, there is no potential component of internal energy in it (there are no forces of interaction of molecules, except for shock). Thus, the internal energy of an ideal gas is only the kinetic energy of the movement 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), one can determine the value of the constant α.

Thus, the kinetic energy of the translational motion of one molecule of an ideal gas will be determined by the expression.

In accordance with the kinetic theory, the distribution of energy over degrees of freedom is uniform. Translational motion has 3 degrees of freedom. Therefore, one degree of freedom of motion 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 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 kilomol of a gas is Nμ, the internal energy of one kilomol of an ideal gas can be obtained by multiplying expression (4.11) by the number of molecules in one kilomol 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 kilomole of gas in J/kmol, i is the number of degrees of freedom of motion 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 an 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 environment and the creation of an electric field in the environment. The electric field in the medium is necessary to create a directed movement of free charges. As is known, a charge q in an electric field of strength E is affected by a force F = qE, which forces the free charges to move in the direction of the electric field. A sign of the existence of an electric field in the conductor is the presence of a non-zero potential difference between any two points of the conductor.

However, electric forces cannot sustain an electric current for a long time. The directed movement of electric charges after some time leads to equalization of the potentials at the ends of the conductor and, consequently, to the disappearance of the electric field in it. To maintain the current in the electric circuit, the charges, in addition to the Coulomb forces, must be affected by non-electrical forces (external 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 an electric current:

The presence of free charge carriers

the presence of a potential difference. these are the conditions for the occurrence of current. for the current to exist

a closed circuit

a source of external forces that maintains a potential difference.

Any forces acting on electrically charged particles, with the exception of electrostatic (Coulomb) forces, are called external 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 in moving a single positive charge along the circuit.

The unit of EMF, like voltage, is the volt. We can talk about the electromotive force in 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 cell from its negative pole to the positive one. The sign of the EMF is determined depending on the arbitrarily chosen direction of bypassing that section of the circuit on which the given current source is turned on.

Ohm's law for a complete circuit.

Consider the simplest complete circuit, consisting of a current source and a resistor with a 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 in time Δt. Then, according to the formula, the work of external forces when moving the charge q is equal to . From the definition of current strength, we have: q = IΔt. Hence, .

Due to the work of external forces during the passage of current in the circuit, an amount of heat is released on its external and internal sections of the circuit, according to the Joule-Lenz law equal to:

According to the law of conservation of energy A st \u003d 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.