In physics, the concept electromotive force(abbreviated - EMF) is used as the main energy characteristic of current sources.

Electromotive Force (EMF)

Electromotive force (EMF) - the ability of the energy source to create and maintain a potential difference on the clamps.

EMF- measured in volts

The voltage at the source terminals is always less EMF by the voltage drop.

Electromotive force

U RH = E – U R0

U RH is the voltage at the source terminals. Measured with the external circuit closed.

E - EMF - measured at the factory.

Electromotive force (EMF) is a physical quantity, which is equal to the quotient of the division of the work that, when moving an electric charge, is performed by external forces in a closed circuit, to this charge itself.

It should be noted that electromotive force in the current source also occurs in the absence of the current itself, that is, when the circuit is open. This situation is usually called "idle", and the value itself EMF when it is equal to the difference in those potentials that are available at the terminals of the current source.

Chemical electromotive force

Chemical electromotive force is present in batteries, galvanic batteries in the course of corrosion processes. Depending on the principle on which the operation of a particular power source is built, they are called either batteries or galvanic cells.

One of the main distinguishing characteristics of galvanic cells is that these current sources are, so to speak, disposable. During their functioning, those active substances, due to which electrical energy is released, decompose almost completely as a result of chemical reactions. That is why if the galvanic cell is completely discharged, then it is no longer possible to use it as a current source.

Unlike galvanic cells, batteries are reusable. This is possible because the chemical reactions that take place in them are reversible.

electromagnetic electromotive force

electromagnetic EMF occurs during the operation of such devices as dynamos, electric motors, chokes, transformers, etc.

Its essence is as follows: when conductors are placed in a magnetic field and they are moved in it in such a way that the magnetic lines of force intersect, guidance occurs. EMF. If the circuit is closed, then an electric current occurs in it.

In physics, the phenomenon described above is called electromagnetic induction. electromotive force, which is induced in this case, is called EMF induction.

It should be noted that pointing EMF Induction occurs not only in those cases when the conductor moves in a magnetic field, but also when it remains stationary, but at the same time the magnitude of the magnetic field itself changes.

Photoelectric electromotive force

This variety electromotive force occurs when there is either an external or internal photoelectric effect.

In physics, the photoelectric effect (photoelectric effect) means that group of phenomena that occurs when light acts on a substance, and at the same time electrons are emitted in it. This is called the external photoelectric effect. If, however, it appears electromotive force or the electrical conductivity of a substance changes, then they speak of an internal photoelectric effect.

Now, both external and internal photoelectric effects are very widely used to design and manufacture a huge number of such light radiation receivers that convert light signals into electrical ones. All these devices are called photocells and are used both in technology and in various scientific research. In particular, photocells are used to make the most objective optical measurements.

Electrostatic driving force

As for this type electromotive force, then it, for example, occurs during mechanical friction that occurs in electrophore units (special laboratory demonstration and auxiliary devices), it also takes place in thunderclouds.

Wimshurst generators (this is another name for electrophore machines) use such a phenomenon as electrostatic induction for their operation. During their operation, electric charges accumulate at the poles, in Leyden jars, and the potential difference can reach very substantial values ​​(up to several hundred thousand volts).

The nature of static electricity is that it occurs when, due to the loss or acquisition of electrons, intramolecular or intraatomic equilibrium is disturbed.

Piezoelectric electromotive force

This variety electromotive force occurs when either squeezing or stretching of substances called piezoelectrics occurs. They are widely used in designs such as piezoelectric sensors, crystal oscillators, hydrophones, and some others.

It is the piezoelectric effect that underlies the operation of piezoelectric sensors. They themselves belong to the sensors of the so-called generator type. In them, the input is the applied force, and the output is the amount of electricity.

As for devices such as hydrophones, their operation is based on the principle of the so-called direct piezoelectric effect, which piezoceramic materials have. Its essence lies in the fact that if sound pressure is applied to the surface of these materials, then a potential difference appears on their electrodes. Moreover, it is proportional to the magnitude of the sound pressure.

One of the main areas of application of piezoelectric materials is the production of quartz oscillators, which have quartz resonators in their design. Such devices are designed to receive oscillations of a strictly fixed frequency, which are stable both in time and with temperature changes, and also have a very low level of phase noise.

Thermionic electromotive force

This variety electromotive force occurs when thermal emission of charged particles occurs from the surface of heated electrodes. Thermionic emission is used quite widely in practice, for example, the operation of almost all radio tubes is based on it.

Thermoelectric electromotive force

This variety EMF occurs when at different ends of dissimilar conductors or simply in different parts of the circuit, the temperature is distributed very non-uniformly.

thermoelectric electromotive force used in devices such as pyrometers, thermocouples and refrigeration machines. Sensors whose operation is based on this phenomenon are called thermoelectric, and are, in fact, thermocouples consisting of electrodes soldered together, made of different metals. When these elements are either heated or cooled, a EMF, which is proportional to the change in temperature.

At the ends of the conductor, and hence the current, it is necessary to have external forces of a non-electric nature, with the help of which the separation of electric charges occurs.

Third party forces any forces acting on electrically charged particles in a circuit are called, with the exception of electrostatic (i.e., Coulomb).

Third-party forces set in motion charged particles inside all current sources: in generators, at power plants, in galvanic cells, batteries, etc.

When the circuit is closed, an electric field is created in all conductors of the circuit. Inside the current source, the charges move under the action of external forces against the Coulomb forces (electrons move from a positively charged electrode to a negative one), and in the rest of the circuit they are driven by an electric field (see figure above).

In current sources, in the process of working to separate charged particles, different types of energy are converted into electrical energy. According to the type of converted energy, the following types of electromotive force are distinguished:

- electrostatic- in an electrophore machine, in which mechanical energy is converted into electrical energy during friction;

- thermoelectric- in a thermoelement, the internal energy of a heated junction of two wires made of different metals is converted into electrical energy;

- photovoltaic— in a photocell. Here, light energy is converted into electrical energy: when some substances are illuminated, for example, selenium, copper oxide (I), silicon, a loss of a negative electric charge is observed;

- chemical- in galvanic cells, batteries, and other sources in which chemical energy is converted into electrical energy.

Electromotive Force (EMF)- characteristic of current sources. The concept of EMF was introduced by G. Ohm in 1827 for DC circuits. In 1857, Kirchhoff defined EMF as the work of external forces during the transfer of a unit electric charge along a closed circuit:

ɛ \u003d A st / q,

where ɛ - EMF of the current source, A st- the work of external forces, q is the amount of charge transferred.

The electromotive force is expressed in volts.

We can talk about the electromotive force in any part of the circuit. This is the specific work of external forces (the work of moving a unit charge) not in the entire circuit, but only in this area.

Internal resistance of the current source.

Let there be a simple closed circuit consisting of a current source (for example, a galvanic cell, battery or generator) and a resistor with resistance R. The current in a closed circuit is not interrupted anywhere, therefore, it also exists inside the current source. Any source represents some resistance to current. It's called current source internal resistance and is marked with the letter r.

In the generator r- this is the resistance of the winding, in a galvanic cell - the resistance of the electrolyte solution and electrodes.

Thus, the current source is characterized by the values ​​of EMF and internal resistance, which determine its quality. For example, electrostatic machines have a very high EMF (up to tens of thousands of volts), but at the same time their internal resistance is huge (up to hundreds of Mohms). Therefore, they are unsuitable for receiving high currents. In galvanic cells, the EMF is only approximately 1 V, but the internal resistance is also small (approximately 1 ohm or less). This allows them to receive currents measured in amperes.

Electromagnetic induction - the generation of electric currents by magnetic fields that change over time. The discovery of this phenomenon by Faraday and Henry introduced a certain symmetry to the world of electromagnetism. Maxwell in one theory managed to collect knowledge about electricity and magnetism. His research predicted the existence of electromagnetic waves before experimental observations. Hertz proved their existence and opened the era of telecommunications to mankind.

Faraday and Lenz laws

Electric currents create magnetic effects. Is it possible for a magnetic field to generate an electric one? Faraday discovered that the desired effects arise due to changes in the magnetic field over time.

When a conductor is crossed by an alternating magnetic flux, an electromotive force is induced in it, causing an electric current. The system that generates the current can be a permanent magnet or an electromagnet.

The phenomenon of electromagnetic induction is governed by two laws: Faraday's and Lenz's.

Lenz's law allows you to characterize the electromotive force with respect to its direction.

Important! The direction of the induced emf is such that the current it causes tends to oppose the cause that creates it.

Faraday noticed that the intensity of the induced current increases when the number of field lines traversing the circuit changes faster. In other words, the EMF of electromagnetic induction is directly dependent on the speed of the moving magnetic flux.

The induction emf formula is defined as:

E \u003d - dF / dt.

The "-" sign shows how the polarity of the induced emf is related to the sign of the flux and the changing speed.

A general formulation of the law of electromagnetic induction is obtained, from which expressions for particular cases can be derived.

The movement of a wire in a magnetic field

When a wire of length l moves in a magnetic field with induction B, an EMF will be induced inside it, proportional to its linear velocity v. To calculate the EMF, the formula is used:

• in the case of conductor movement perpendicular to the direction of the magnetic field:

E \u003d - B x l x v;

• in case of movement at a different angle α:

E \u003d - B x l x v x sin α.

The induced EMF and current will be directed in the direction we find using the right hand rule: by placing your hand perpendicular to the magnetic field lines and pointing your thumb in the direction the conductor moves, you can find out the direction of the EMF from the remaining four straightened fingers.

Rotating coil

The operation of the electric power generator is based on the rotation of the circuit in the MP, which has N turns.

EMF is induced in the electrical circuit whenever the magnetic flux crosses it, in accordance with the definition of the magnetic flux Ф = B x S x cos α (magnetic induction multiplied by the surface area through which the MP passes, and the cosine of the angle formed by the vector B and the perpendicular line to the plane S).

It follows from the formula that F is subject to changes in the following cases:

• the intensity of the MF changes - the vector B;
• the area bounded by the contour varies;
• the orientation between them, given by the angle, changes.

In the first experiments of Faraday, induced currents were obtained by changing the magnetic field B. However, it is possible to induce an EMF without moving the magnet or changing the current, but simply by rotating the coil around its axis in the magnetic field. In this case, the magnetic flux changes due to a change in the angle α. The coil, during rotation, crosses the lines of the MP, an emf arises.

If the coil rotates uniformly, this periodic change results in a periodic change in magnetic flux. Or the number of MF lines of force crossed every second takes equal values ​​with equal time intervals.

Important! The induced emf changes with the orientation over time from positive to negative and vice versa. The graphical representation of the EMF is a sinusoidal line.

For the formula for the EMF of electromagnetic induction, the expression is used:

E \u003d B x ω x S x N x sin ωt, where:

• S is the area limited by one turn or frame;
• N is the number of turns;
• ω is the angular velocity with which the coil rotates;
• B – MF induction;
• angle α = ωt.

In practice, in alternators, often the coil remains stationary (stator) and the electromagnet rotates around it (rotor).

EMF self-induction

When an alternating current passes through the coil, it generates an alternating magnetic field, which has a changing magnetic flux that induces an emf. This effect is called self-induction.

Since the MP is proportional to the intensity of the current, then:

where L is the inductance (H), determined by geometric quantities: the number of turns per unit length and the dimensions of their cross section.

For the induction emf, the formula takes the form:

E \u003d - L x dI / dt.

If two coils are located side by side, then an EMF of mutual induction is induced in them, depending on the geometry of both circuits and their orientation relative to each other. When the separation of the circuits increases, the mutual inductance decreases, as the magnetic flux connecting them decreases.

Let there be two coils. Through the wire of one coil with N1 turns, current I1 flows, creating an MF passing through the coil with N2 turns. Then:

1. Mutual inductance of the second coil relative to the first:

M21 = (N2 x F21)/I1;

1. Magnetic Flux:

F21 = (M21/N2) x I1;

1. Find the induced emf:

E2 = - N2 x dФ21/dt = - M21x dI1/dt;

1. EMF is induced identically in the first coil:

E1 = - M12 x dI2/dt;

Important! The electromotive force caused by mutual inductance in one coil is always proportional to the change in electric current in the other.

Mutual inductance can be considered equal to:

M12 = M21 = M.

Accordingly, E1 = - M x dI2/dt and E2 = M x dI1/dt.

M = K √ (L1 x L2),

where K is the coupling coefficient between two inductances.

The phenomenon of mutual inductance is used in transformers - electrical devices that allow you to change the value of the voltage of an alternating electric current. The device consists of two coils wound around one core. The current present in the first one creates a changing magnetic field in the magnetic circuit and an electric current in the other coil. If the number of turns of the first winding is less than the other, the voltage increases and vice versa.

In addition to generating, transforming electricity, magnetic induction is used in other devices. For example, in magnetic levitation trains that do not move in direct contact with the rails, but a few centimeters higher due to the electromagnetic repulsion force.

Video

Topics of the USE codifier: electromotive force, current source internal resistance, Ohm's law for a complete electrical circuit.

Until now, in the study of electric current, we have considered the directed motion of free charges in external circuit, that is, in conductors connected to the terminals of the current source.

As we know, positive charge:

Goes into the external circuit from the positive terminal of the source;

Moves in an external circuit under the influence of a stationary electric field created by other moving charges;

It comes to the negative terminal of the source, completing its path in the external circuit.

Now our positive charge needs to close its trajectory and return to the positive terminal. To do this, he needs to overcome the final segment of the path - inside the current source from the negative terminal to the positive. But think about it: he doesn’t want to go there at all! The negative terminal attracts it to itself, the positive terminal repels it from itself, and as a result, an electric force acts on our charge inside the source, directed against charge movement (i.e. against the direction of the current).

third party force

However, current flows through the circuit; therefore, there is a force that “drags” the charge through the source despite the opposition of the electric field of the terminals (Fig. 1).

Rice. 1. Third party power

This force is called outside force; It is thanks to her that the current source functions. An external force has nothing to do with a stationary electric field - it is said to have non-electric origin; in batteries, for example, it arises due to the flow of appropriate chemical reactions.

Denote by the work of an external force to move the positive charge q inside the current source from the negative terminal to the positive one. This work is positive, since the direction of the external force coincides with the direction of charge movement. The work of an external force is also called current source operation.

There is no external force in the external circuit, so the work of the external force to move the charge in the external circuit is zero. Therefore, the work of an external force in moving the charge around the entire circuit is reduced to the work of moving this charge only inside the current source. Thus, this is also the work of an external force in moving the charge throughout the chain.

We see that the external force is non-potential - its work when moving a charge along a closed path is not equal to zero. It is this non-potentiality that ensures the circulation of electric current; the potential electric field, as we said earlier, cannot support a constant current.

Experience shows that work is directly proportional to the charge being moved. Therefore, the ratio is no longer dependent on the charge and is a quantitative characteristic of the current source. This relationship is indicated by:

(1)

This value is called electromotive force(EMF) current source. As you can see, EMF is measured in volts (V), so the name "electromotive force" is extremely unfortunate. But it has long been rooted, so you have to put up with it.

When you see the inscription on the battery: "1.5 V", then know that this is exactly the EMF. Is this value equal to the voltage that the battery creates in the external circuit? It turns out not! Now we will understand why.

Ohm's law for a complete circuit

Any current source has its own resistance, which is called internal resistance this source. Thus, a current source has two important characteristics: EMF and internal resistance.

Let a current source with an EMF equal to , and an internal resistance is connected to a resistor (which in this case is called external resistor, or external load, or payload). All this together is called complete chain(Fig. 2).

Rice. 2. Complete chain

Our task is to find the current in the circuit and the voltage across the resistor.

In time, a charge passes through the circuit. According to formula (1), the current source does the work:

(2)

Since the current strength is constant, the work of the source is entirely converted into heat, which is released at the resistances and. This amount of heat is determined by the Joule–Lenz law:

(3)

So, , and we equate the right parts of formulas (2) and (3) :

After reducing to we get:

So we found the current in the circuit:

(4)

Formula (4) is called Ohm's law for a complete circuit.

If you connect the source terminals with a wire of negligible resistance, then you get short circuit. In this case, the maximum current will flow through the source - short circuit current:

Due to the smallness of the internal resistance, the short-circuit current can be very large. For example, a penlight battery heats up at the same time so that it burns your hands.

Knowing the current strength (formula (4)), we can find the voltage across the resistor using Ohm's law for the circuit section:

(5)

This voltage is the potential difference between the points and (Fig. 2). The potential of the point is equal to the potential of the positive terminal of the source; the potential of the point is equal to the potential of the negative terminal. Therefore, stress (5) is also called voltage at the source terminals.

We see from formula (5) what will happen in a real circuit - after all, it is multiplied by a fraction less than one. But there are two cases where .

1. Ideal current source. This is the name of a source with zero internal resistance. At , formula (5) gives .

2. Open circuit. Consider the current source itself, outside the electrical circuit. In this case, we can assume that the external resistance is infinitely large: . Then the value is indistinguishable from , and formula (5) again gives us .

The meaning of this result is simple: if the source is not connected to the circuit, then the voltmeter connected to the poles of the source will show its EMF.

Electric circuit efficiency

It's not hard to see why a resistor is called a payload. Imagine it's a light bulb. The heat generated by a light bulb is useful, because thanks to this warmth, the light bulb fulfills its purpose - it gives light.

Let us denote the amount of heat released on the payload during the time .

If the current in the circuit is , then

A certain amount of heat is also released at the current source:

The total amount of heat released in the circuit is:

Electric circuit efficiency is the ratio of useful heat to total:

The efficiency of the circuit is equal to unity only if the current source is ideal.

Ohm's law for a heterogeneous area

Ohm's simple law is valid for the so-called homogeneous section of the circuit - that is, the section on which there are no current sources. Now we will obtain more general relations, from which both Ohm's law for a homogeneous section and the Ohm's law obtained above for a complete chain follow.

The section of the circuit is called heterogeneous if it has a current source. In other words, an inhomogeneous section is a section with an EMF.

On fig. 3 shows an inhomogeneous section containing a resistor and a current source. The EMF of the source is , its internal resistance is considered to be zero (if the internal resistance of the source is , you can simply replace the resistor with a resistor ).

Rice. 3. EMF "helps" the current:

The current strength in the section is equal, the current flows from point to point. This current is not necessarily caused by a single source. The area under consideration, as a rule, is part of a circuit (not shown in the figure), and other current sources may be present in this circuit. Therefore, the current is the result of the cumulative action all sources in the circuit.

Let the potentials of the points and be equal to and , respectively. We emphasize once again that we are talking about the potential of a stationary electric field generated by the action of all sources of the circuit - not only the source belonging to this section, but also, possibly, available outside this section.

The voltage in our area is: In time, a charge passes through the section, while the stationary electric field does the work:

In addition, the positive work is done by the current source (after all, the charge has passed through it!):

The current strength is constant, therefore, the total work to advance the charge, performed on the site by a stationary electric field and external source forces, is completely converted into heat:.

We substitute here the expressions for , and the Joule–Lenz law:

Reducing by , we get Ohm's law for an inhomogeneous section of a circuit:

(6)

or, which is the same:

(7)

Notice the plus sign in front of it. We have already indicated the reason for this - the current source in this case performs positive work, "dragging" the charge inside itself from the negative terminal to the positive. Simply put, the source "helps" current flow from point to point.

We note two consequences of the derived formulas (6) and (7) .

1. If the site is homogeneous, then . Then from formula (6) we obtain - Ohm's law for a homogeneous section of the chain.

2. Suppose that the current source has an internal resistance. This, as we already mentioned, is equivalent to replacing with:

Now let's close our section by connecting the points and . We obtain the complete chain discussed above. In this case, it turns out that the previous formula will also turn into Ohm's law for a complete chain:

Thus, Ohm's law for a homogeneous section and Ohm's law for a complete circuit both follow from Ohm's law for an inhomogeneous section.

There may be another case of connection, when the source "prevents" the current from flowing through the section. Such a situation is shown in Fig. 4 . Here, the current coming from to is directed against the action of external forces of the source.

Rice. 4. EMF "interferes" with the current:

How is this possible? It's very simple: other sources available in the circuit outside the section under consideration "overpower" the source in the section and force the current to flow against. This is exactly what happens when you put the phone on charge: the adapter connected to the outlet causes the movement of charges against the external forces of the phone's battery, and the battery is thereby charged!

What will change now in the derivation of our formulas? Only one thing - the work of external forces will become negative:

Then Ohm's law for an inhomogeneous section will take the form:

(8)

where, as before, is the voltage on the section.

Let's put formulas (7) and (8) together and write Ohm's law for the section with EMF as follows:

The current flows from point to point. If the direction of the current coincides with the direction of external forces, then a “plus” is placed in front; if these directions are opposite, then "minus" is put.

Electrical circuit consists of a current source, electricity consumers, connecting wires and a key that serves to open and close the circuit and other elements (Fig. 1).

Drawings that show how to connect electrical devices in a circuit are called electrical diagrams. Devices on the diagrams are indicated by conventional signs.

As noted, in order to maintain an electric current in the circuit, it is necessary that at its ends (Fig. 2) there is a constant potential difference φ A- φ b. Let at the initial time φ A > φ B , then the positive charge transfer q from a point BUT exactly AT will lead to a decrease in the potential difference between them. To maintain a constant potential difference, it is necessary to transfer exactly the same charge from B in A. If in the direction BUT → AT charges move under the action of the forces of an electrostatic field, then in the direction AT → BUT the movement of charges occurs against the forces of the electrostatic field, i.e. under the action of forces of a non-electrostatic nature, the so-called third-party forces. This condition is met in a current source that supports the movement of electric charges. In most current sources, only electrons move, in galvanic cells - ions of both signs.

Sources of electric current may be different in their design, but in any of them work is done to separate positively and negatively charged particles. The separation of charges occurs under the action outside forces. Third-party forces act only inside the current source and can be caused by chemical processes (batteries, galvanic cells), the action of light (photocells), changing magnetic fields (generators), etc.

Any current source is characterized by an electromotive force - EMF.

electromotive force ε current source is a physical scalar quantity equal to the work of external forces to move a single positive charge along a closed circuit

The SI unit of electromotive force is the volt (V).

EMF is an energy characteristic of a current source.

In the current source, in the course of work on the separation of charged particles, a transformation of mechanical, light, internal, etc. occurs. energy into electricity. Separated particles accumulate at the poles of the current source (places to which consumers are connected using terminals or clamps). One pole of the current source is charged positively, the other negatively. An electrostatic field is created between the poles of the current source. If the poles of a current source are connected by a conductor, then an electric current arises in such an electrical circuit. In this case, the nature of the field changes, it ceases to be electrostatic.

Figure 3 schematically shows the negative terminal of the current source and the section of the end of the metal wire attached to it in the form of a spherical conductor. The dotted line shows some lines of the terminal field strength before the wire is inserted into it, and the arrows show the forces acting on the free electrons of the wire located at the points marked with numbers. Electrons at different points of the cross section of the wire under the action of the Coulomb forces of the terminal field acquire motion not only along the axis of the wire. For example, an electron located at a point 1 , is involved in the "current" movement. But near points 2, 3, 4, 5 electrons have the ability to accumulate on the surface of the wire. Moreover, the surface distribution of electrons along the length of the wire will not be uniform. Therefore, connecting a wire to a current source terminal will cause some electrons to move along the wire, and some electrons to accumulate on the surface. The uneven distribution of electrons on its surface ensures the non-equipotentiality of this surface, the presence of components of the electric field strength directed along the surface of the conductor. This is the field of redistributed electrons of the conductor itself and ensures the ordered movement of other electrons. If the distribution of electrons over the surface of the conductor does not change over time, then such a field is called stationary electric field. Thus, the main role in creating a stationary electric field is played by the charges located at the poles of the current source. When the electrical circuit is closed, the interaction of these charges with the free charges of the conductor leads to the appearance of uncompensated surface charges on the entire surface of the conductor. It is these charges that create a stationary electric field inside the conductor along its entire length. This field inside the conductor is uniform, and the lines of tension are directed along the axis of the conductor (Fig. 4). The process of establishing an electric field along the conductor occurs at a speed c≈ 3 10 8 m/s.

Like an electrostatic field, it is potential. But there are significant differences between these fields:

1. electrostatic field - the field of fixed charges. The source of a stationary electric field is moving charges, and the total number of charges and the pattern of their distribution in a given space do not change over time;

2. An electrostatic field exists outside the conductor. The strength of the electrostatic field is always equal to 0 inside the volume of the conductor, and at each point of the outer surface of the conductor is directed perpendicular to this surface. A stationary electric field exists both outside and inside the conductor. The intensity of a stationary electric field is not equal to zero inside the volume of the conductor, and on the surface and inside the volume there are components of the intensity that are not perpendicular to the surface of the conductor;

3. the potentials of different points of the conductor through which the direct current passes are different (the surface and volume of the conductor are not equipotential). The potentials of all points on the surface of a conductor in an electrostatic field are the same (the surface and volume of the conductor are equipotential);

4. An electrostatic field is not accompanied by the appearance of a magnetic field, but a stationary electric field is accompanied by its appearance and is inextricably linked with it.