The phenomenon of electromagnetic induction was discovered by M. Faraday in 1831. The phenomenon can be observed in the following experiments. Let's take a coil with a large number of turns (solenoid), close it with a galvanometer, and we will push a permanent magnet along the axis from one of its ends. In this case, an electric current will appear in the solenoid, which will be detected by the deviation of the galvanometer needle. This current will stop when the magnet stops moving. If you remove the magnet from the solenoid, then the solenoid will again have a current, but in the opposite direction. The same phenomenon will take place if the magnet is left stationary and the solenoid is moved. Instead of a magnet, you can take a second solenoid (Fig. 51), through which a constant current flows formula "src="http://hi-edu.ru/e-books/xbook785/files/I2.gif" border="0" align ="absmiddle" alt=".

The phenomenon of electromagnetic induction is as follows: in any closed conducting circuit, when the flux of magnetic induction changes through the area bounded by this circuit, an electric current arises. This current is called inductive.

The occurrence of an induction current in a closed circuit is due to the appearance in this circuit under the influence of a time-varying flow of a certain electromotive force of an EMF. The magnitude of this EMF was first associated with the rate of change in the flux of magnetic induction by Faraday

def-e"> Faraday's Law

The minus sign in the law means that the emf of induction always has such a direction that it interferes with the cause that causes it. This rule was established by the St. Petersburg professor E.Kh. Lenz.

If we consider the magnetic flux, the formula is" src="http://hi-edu.ru/e-books/xbook785/files/108-2.gif" border="0" align="absmiddle" alt="(Fig. 52, b), or directed opposite to it, if it increases the mark "\u003e B. The flux of magnetic induction through the area S, bounded by the frame, is equal to

formula" src="http://hi-edu.ru/e-books/xbook785/files/109-1.gif" border="0" align="absmiddle" alt="the angle between the normal to the frame and the vector B changes

formula" src="http://hi-edu.ru/e-books/xbook785/files/109-3.gif" border="0" align="absmiddle" alt="According to Faraday's law (12.1), with a changing flow through the frame, an induction current arises in it, which will change over time with a frequency equal to the frame rotation speed formula "src="http://hi-edu.ru/e-books/xbook785 /files/109-4.gif" border="0" align="absmiddle" alt="

As you can see, the induction EMF changes according to the harmonic law with the frequency formula alt="Obtaining an EMF during the rotation of a coil in a magnetic field underlies the operation of an alternator.

Origin mechanism induction current in a moving conductor can be explained using the Lorentz force F = qvB.

Under the action of the Lorentz force, charges are separated: positive ones accumulate at one end of the conductor, negative ones at the other ( fig. 53). These charges create an electrostatic Coulomb field inside the conductor. If the conductor is open, then the movement of charges under the influence of the Lorentz force will occur until the electric force balances the Lorentz force. The action of the Lorentz force is similar to the action of some electric field, this field is outside field.

The occurrence of EMF induction is also possible in a fixed circuit located in an alternating magnetic field. What is the nature of extraneous forces (of non-electrostatic origin) in this case?

Maxwell hypothesized that any alternating magnetic field excites an electric field in the surrounding space, which is the cause of the induction current in the circuit. This field is characterized by intensity (the index indicates the cause of this field - the magnetic field).

The circulation of this electric field marked "> L is not equal to zero:

formula" src="http://hi-edu.ru/e-books/xbook785/files/111-1.gif" border="0" align="absmiddle" alt="

formula" src="http://hi-edu.ru/e-books/xbook785/files/111-2.gif" border="0" align="absmiddle" alt="

formula" src="http://hi-edu.ru/e-books/xbook785/files/111-5.gif" border="0" align="absmiddle" alt="is the partial derivative of the induction B with respect to time.

For an electrostatic field mark "> Q ) the circulation along any closed contour is equal to zero:

def-e">potential.

The electric field is defined as vortex, for which the circulation along a closed loop L is not equal to zero:

mark "> I (t), then it creates a magnetic field with induction B (t), and hence the flow formula" src="http://hi-edu.ru/e-books/xbook785/files/112. gif" border="0" align="absmiddle" alt="

The phenomenon of electromagnetic induction caused by a change in current in the circuit itself is called self-induction. Its root cause is the change in current in the loop, which is easier to measure than the change in magnetic flux.

At any point on the surface stretched over the circuit, the induction dB is proportional to the current in the circuit. If it is integrated over the entire surface, then the total magnetic flux mark "> I

mark ">L - loop inductance, proportionality factor, depending on the configuration of the loop.

Inductance shows what kind of magnetic flux permeates the surface covered by the circuit, with a current strength of 1 A in it. Its unit is Wb / A, which is called henry (Hn).

If the contour has a complex shape, for example, contains several turns, then instead of defining "> flux linkage, the formula" src = "http://hi-edu.ru/e-books/xbook785/files/112-4.gif" border ="0" align="absmiddle" alt="

the expression is valid for L = const.

Another definition of L follows from this (more important in practice): inductance shows what self-induction EMF occurs in the circuit if the rate of change in the current strength in it is 1 A / s.

For a solenoid, the magnetic flux through one turn is marked "> N turns of the solenoid (flux linkage),

mark">V =Sl - volume of the solenoid.

Comparing this expression with (12.4) , we get

formula" src="http://hi-edu.ru/e-books/xbook785/files/mu.gif" border="0" align="absmiddle" alt=".

Magnetic flux through the surface covered by contour 2 can be created by current

Denote the formula" src="http://hi-edu.ru/e-books/xbook785/files/113.gif" border="0" align="absmiddle" alt="

formula" src="http://hi-edu.ru/e-books/xbook785/files/I1.gif" border="0" align="absmiddle" alt="changes, then in circuit 2 is induced EMF of mutual induction

formula" src="http://hi-edu.ru/e-books/xbook785/files/I2.gif" border="0" align="absmiddle" alt="there is an emf of mutual induction

formula" src="http://hi-edu.ru/e-books/xbook785/files/113-3.gif" border="0" align="absmiddle" alt=" - mutual inductance of the circuits, they depend on the geometric shape, size, mutual arrangement of the contours and the magnetic permeability of the medium.

Calculate the mutual inductance of two coils wound on a common toroidal core(Fig. 55). Foucault currents, or eddy currents.

A heavy metal plate oscillating between the poles of an electromagnet stops if the direct current that feeds the electromagnet is turned on. All its energy is converted into heat released by the Foucault currents. There are no currents in a fixed plate.

Eddy currents can be significantly weakened if cuts are made in the plate that increase its resistance. In the solid cores of transformers, electric motors operating on alternating current, Foucault currents would release a significant amount of heat. Therefore, the cores are made stacked, composing them from thin plates separated by a dielectric layer.

The phenomenon of the occurrence of Foucault's induction currents underlies the operation of induction furnaces, which allow heating metals to a melting point.

Foucault currents obey Lenz's rule: their magnetic field is directed in such a way as to counteract the change in the magnetic flux that induces eddy currents. This fact is used to calm the moving parts of various devices (damping).

Eddy currents also occur in wires through which an alternating electric current flows. The direction of the eddy currents is such that they counteract the change in the primary current in the conductor. Thus, the alternating current is unevenly distributed over the cross section of the wire; it is, as it were, forced out onto the surface of the conductor. At the surface of the wire, the current density is maximum, and deep into the conductor it decreases and reaches the lowest value on its axis. This phenomenon is called the skin effect (skin - skin). The current is concentrated in the "skin" of the conductor. Therefore, at high frequencies, there is no need for large-section conductors: all the same, the current will flow only in the surface layer.

In 1831, Michael Faraday discovered that in a closed conducting circuit, when a magnetic field changes, an electric current arises, called induction.

An induction current in a metal wire coil occurs when a magnet is pushed inside the coil and when the magnet is pulled out of the coil, as well as when the current strength changes in the second coil, the magnetic field of which penetrates the first coil.

The phenomenon of the occurrence of an electric current in a closed conducting circuit with changes in the magnetic field penetrating the circuit is called electromagnetic induction. The appearance of an electric current in a closed circuit with changes in the magnetic field penetrating the circuit indicates the action of external forces of a non-electric nature in the circuit or the occurrence EMF induction.

The direction of the induction current in the circuit depends on whether the magnetic flux penetrating the circuit increases or decreases, as well as on the direction of the magnetic field induction vector relative to the circuit. The general rule for determining the direction of the induction current in the circuit was established in 1833 by E.Kh. Lenz.

Lenz's rule can be visualized using a light aluminum ring (Fig. 11.1). Experience shows that when a permanent magnet is introduced, the ring is repelled from it, and when removed, it is attracted to the magnet. The result of the experiments does not depend on the polarity of the magnet.

The repulsion and attraction of a solid ring is explained by the occurrence of an induction current in the ring with changes in the magnetic flux through the ring and the action of a magnetic field on the induction current. When the magnet is pushed into the ring, the induction current in it has such a direction that the magnetic field created by this current opposes the external magnetic field, and when the magnet is pushed out, the induction current in it has such a direction that the induction vector of its magnetic field coincides in direction with the induction vector of the external field .

Lenz's rule: the induction current arising in a closed circuit counteracts with its magnetic field the change in the magnetic flux by which it is caused.

Law of electromagnetic induction: The induction emf in a closed loop is equal to the modulus of the rate of change of the magnetic flux through the surface bounded by the loop:

Taking into account the Lenz rule, the law of electromagnetic induction is written as follows:

If identical changes in the magnetic flux occur in series-connected circuits, then the induction EMF in them is equal to the sum of the induction EMF in each of the circuits. Therefore, when changing the magnetic flux in the coil, consisting of n identical turns of wire, the total induction emf in n times more EMF induction in a single circuit:

The occurrence of an electric current in a closed circuit indicates that when the magnetic flux penetrating the circuit changes, forces act on free electric charges in the circuit. The wire of the circuit is motionless, free electric charges in it can be considered motionless. Only an electric field can act on stationary electric charges. Therefore, with any change in the magnetic field in the surrounding space, an electric field arises. This electric field sets in motion free electric charges in the circuit, creating an induction electric current. The electric field that occurs when the magnetic field changes is called vortex electric field.

The work of the forces of the vortex electric field on the movement of electric charges is the work of external forces, the source of the induction EMF.

A vortex electric field differs from an electrostatic one in that it is not associated with electric charges, its lines of tension are closed lines. The work of the forces of the vortex electric field during the movement of an electric charge along a closed line can be different from zero.

How does an electromotive force arise in a conductor that is in an alternating magnetic field? What is a vortex electric field, its nature and causes? What are the main properties of this field? All these questions and many more will be answered in today's lesson.

Topic: Electromagnetic induction

Lesson:Vortex electric field

Recall that Lenz's rule allows you to determine the direction of the induction current in a circuit located in an external magnetic field with a variable flux. Based on this rule, it was possible to formulate the law of electromagnetic induction.

Law of electromagnetic induction

When the magnetic flux penetrating the circuit area changes, an electromotive force arises in this circuit, numerically equal to the rate of change of the magnetic flux, taken with a minus sign.

How does this electromotive force come about? It turns out that the EMF in the conductor, which is in an alternating magnetic field, is associated with the emergence of a new object - eddy electric field.

Consider experience. There is a coil of copper wire into which an iron core is inserted in order to increase the magnetic field of the coil. The coil is connected through conductors to an alternating current source. There is also a coil of wire placed on a wooden base. An electric light bulb is connected to this coil. The material of the wire is covered with insulation. The base of the coil is made of wood, i.e., of a material that does not conduct electricity. The coil frame is also made of wood. Thus, any possibility of contact of the light bulb with the circuit connected to the current source is excluded. When the source is closed, the light bulb lights up, therefore, an electric current flows in the coil - which means that external forces in this coil do work. It is necessary to find out where third-party forces come from.

The magnetic field penetrating the plane of the coil cannot cause the appearance of an electric field, since the magnetic field acts only on moving charges. According to the electronic theory of conductivity of metals, there are electrons inside them that can move freely inside the crystal lattice. However, this motion in the absence of an external electric field is random. Such randomness leads to the fact that the total effect of the magnetic field on a current-carrying conductor is zero. In this way, the electromagnetic field differs from the electrostatic field, which also acts on stationary charges. So, the electric field acts on moving and stationary charges. However, the kind of electric field that was studied earlier is created only by electric charges. The induction current, in turn, is created by an alternating magnetic field.

Suppose that the electrons in a conductor are brought into orderly motion by some new kind of electric field. And this electric field is generated not by electric charges, but by an alternating magnetic field. Faraday and Maxwell came up with a similar idea. The main thing in this idea is that a time-varying magnetic field generates an electric one. A conductor with free electrons present in it makes it possible to detect this field. This electric field sets the electrons in the conductor in motion. The phenomenon of electromagnetic induction consists not so much in the appearance of an inductive current, but in the appearance of a new kind of electric field, which sets in motion electric charges in a conductor (Fig. 1).

The vortex field is different from the static one. It is not generated by immobile charges, therefore, the lines of intensity of this field cannot begin and end on a charge. According to research, the lines of the vortex field strength are closed lines, similar to the lines of magnetic field induction. Therefore, this electric field is vortex - the same as the magnetic field.

The second property concerns the work of the forces of this new field. Studying the electrostatic field, we found out that the work of the forces of the electrostatic field in a closed loop is zero. Since when the charge moves in one direction, the displacement and the acting force are co-directed and the work is positive, then when the charge moves in the opposite direction, the movement and the acting force are oppositely directed and the work is negative, the total work will be equal to zero. In the case of a vortex field, the work done in a closed loop will be nonzero. So, when a charge moves along a closed line of an electric field that has a vortex character, the work in different sections will maintain a constant sign, since the force and displacement in different sections of the trajectory will maintain the same direction relative to each other. The work of the forces of the vortex electric field to move the charge along a closed loop is nonzero, therefore, the vortex electric field can generate an electric current in a closed loop, which coincides with the experimental results. Then it can be argued that the force acting on the charges from the vortex field is equal to the product of the transferred charge and the strength of this field.

This force is an outside force that does work. The work of this force, related to the value of the transferred charge, is the EMF of induction. The direction of the intensity vector of the eddy electric field at each point of the intensity lines is determined by the Lenz rule and coincides with the direction of the induction current.

In a fixed circuit, located in an alternating magnetic field, an induction electric current arises. The magnetic field itself cannot be a source of extraneous forces, since it can only act on orderly moving electric charges. There can be no electrostatic field, since it is generated by fixed charges. After assuming that a time-varying magnetic field generates an electric field, we learned that this variable field is of a vortex nature, i.e., its lines are closed. The work of the vortex electric field in a closed loop is nonzero. The force acting on the transferred charge from the side of the vortex electric field is equal to the value of this transferred charge, multiplied by the strength of the vortex electric field. This force is that third-party force that leads to the emergence of an EMF in the circuit. The electromotive force of induction, i.e., the ratio of the work of external forces to the value of the transferred charge, is equal to the rate of change of the magnetic flux taken with a minus sign. The direction of the intensity vector of the vortex electric field at each point of the intensity lines is determined by the Lenz rule.

1. Kasyanov V.A., Physics 11th grade: Textbook. for general education institutions. - 4th ed., stereotype. - M.: Bustard, 2004. - 416 p.: ill., 8 p. col. incl.
2. Gendenstein L.E., Dick Yu.I., Physics 11. - M .: Mnemosyne.
3. Tikhomirova S.A., Yarovsky B.M., Physics 11. - M.: Mnemosyne.
   

1. Electronic textbook of physics ().
2. Cool physics ().
3. Xvatit.com().

1. How to explain the fact that a lightning strike can melt fuses, disable sensitive electrical appliances and semiconductor devices?
2. * When the ring was opened in the coil, an EMF of self-induction of 300 V arose. What is the strength of the vortex electric field in the turns of the coil if their number is 800, and the radius of the turns is 4 cm?

The electric field that occurs when the magnetic field changes has a completely different structure than the electrostatic one. It is not connected directly with electric charges, and its lines of tension cannot begin and end on them. They generally do not begin or end anywhere, but are closed lines, similar to the lines of magnetic field induction. This is the so-called vortex electric field. The question may arise: why, in fact, is this field called electric? After all, it has a different origin and a different configuration than the static electric field. The answer is simple: the vortex field acts on the charge q in the same way as the electrostatic one, and we considered and still consider this the main property of the field. The force acting on the charge is still F= qE, Where E- intensity of the vortex field.

If the magnetic flux is created by a uniform magnetic field concentrated in a long narrow cylindrical tube with a radius of r 0 (Fig. 5.8), then from symmetry considerations it is obvious that the lines of the electric field strength lie in planes perpendicular to the lines B and are circles. In accordance with the Lenz rule, with increasing magnetic

induction lines of tension E form a left screw with the direction of magnetic induction B.

Unlike a static or stationary electric field, the work of a vortex field on a closed path is not equal to zero. Indeed, when a charge moves along a closed line of electric field strength, the work on all sections of the path has the same sign, since the force and displacement coincide in direction. A vortex electric field, like a magnetic field, is not potential.

The work of the vortex electric field in moving a single positive charge along a closed fixed conductor is numerically equal to the induction EMF in this conductor.

If an alternating current flows through the coil, then the magnetic flux penetrating the coil changes. Therefore, an EMF of induction occurs in the same conductor through which the alternating current flows. This phenomenon is called self-induction.

With self-induction, the conducting circuit plays a dual role: a current flows through it, causing induction, and an induction EMF appears in it. A changing magnetic field induces an EMF in the very conductor through which the current flows, creating this field.

At the moment of current rise, the intensity of the eddy electric field, in accordance with the Lenz rule, is directed against the current. Therefore, at this moment, the vortex field prevents the current from rising. On the contrary, at the moment the current decreases, the vortex field supports it.

This leads to the fact that when a circuit containing a source of constant EMF is closed, a certain value of current strength is not set immediately, but gradually over time (Fig. 5.13). On the other hand, when the source is turned off, the current in closed circuits does not stop instantly. The resulting EMF of self-induction can exceed the EMF of the source, since the change in current and its magnetic field occurs very quickly when the source is turned off.

The phenomenon of self-induction can be observed in simple experiments. Figure 5.14 shows a parallel connection of two identical lamps. One of them is connected to the source through a resistor R, and the other in series with the coil L with an iron core. When the key is closed, the first lamp flashes almost immediately, and the second - with a noticeable delay. The self-induced emf in the circuit of this lamp is large, and the current does not immediately reach its maximum value. The appearance of an EMF of self-induction upon opening can be observed in an experiment with a circuit shown schematically in Figure 5.15. When the key is opened in the coil L EMF of self-induction appears, which maintains the initial current. As a result, at the moment of opening, a current flows through the galvanometer (dashed arrow), directed against the initial current before opening (solid arrow). Moreover, the current strength when the circuit is opened exceeds the strength of the current passing through the galvanometer when the key is closed. This means that the EMF of self-induction ξ. more emf ξis cell batteries.

The phenomenon of self-induction is similar to the phenomenon of inertia in mechanics. So, inertia leads to the fact that under the action of force the body does not instantly acquire a certain speed, but gradually. The body cannot be instantly slowed down, no matter how great the braking force. In the same way, due to self-induction, when the circuit is closed, the current strength does not immediately acquire a certain value, but increases gradually. Turning off the source, we do not stop the current immediately. Self-induction maintains it for some time, despite the presence of circuit resistance.

Further, in order to increase the speed of the body, according to the laws of mechanics, work must be done. When braking, the body itself does positive work. In the same way, to create a current, you need to do work against the vortex electric field, and when the current disappears, this field itself does positive work.

This is not just a superficial analogy. It has a deep inner meaning. After all, current is a collection of moving charged particles. With an increase in the speed of electrons, the magnetic field created by them changes and generates a vortex electric field that acts on the electrons themselves, preventing an instantaneous increase in their speed under the action of an external force. When braking, on the contrary, the vortex field tends to keep the electron velocity constant (Lenz's rule). Thus, the inertness of electrons, and hence their mass, is at least partially electromagnetic in origin. Mass cannot be completely electromagnetic, since there are electrically neutral particles that have mass (neutrons, etc.)

Inductance.

The module B of the magnetic induction created by the current in any closed circuit is proportional to the strength of the current. Since the magnetic flux F is proportional to B, then F ~ B ~ I.

It can therefore be argued that

Where L- coefficient of proportionality between the current in the conductive circuit and the magnetic flux created by it, penetrating this circuit. the value L called the inductance of the circuit or its coefficient of self-induction.

Using the law of electromagnetic induction and expression (5.7.1), we obtain the equality:

(5.7.2)

From formula (5.7.2) it follows that inductance- this is a physical quantity numerically equal to the EMF of self-induction that occurs in the circuit when the current strength changes by 1 A per 1 s.

Inductance, like electrical capacitance, depends on geometric factors: the size of the conductor and its shape, but does not depend directly on the current strength in the conductor. Except

geometry of the conductor, the inductance depends on the magnetic properties of the medium in which the conductor is located.

The SI unit of inductance is called the henry (H). The inductance of the conductor is 1 Gn, if in it, when the current strength changes by 1 A behind 1s EMF of self-induction occurs 1 V:

Another special case of electromagnetic induction is mutual induction. Mutual induction is called the occurrence of an inductive current in a closed circuit(coil) when changing the current strength in the adjacent circuit(coil). The circuits are fixed relative to each other, as, for example, the coils of a transformer.

Quantitatively, mutual induction is characterized by the coefficient of mutual induction, or mutual inductance.

Figure 5.16 shows two circuits. When changing the current strength I 1 in the circuit 1 in contour 2 there is an inductive current I 2 .

The flux of magnetic induction Ф 1.2, created by the current in the primary circuit and penetrating the surface limited by the second circuit, is proportional to the current strength I 1:

The coefficient of proportionality L 1, 2 is called mutual inductance. It is similar to the inductance L.

The induction emf in the second circuit, according to the law of electromagnetic induction, is equal to:

The coefficient L 1.2 is determined by the geometry of both circuits, the distance between them, their mutual arrangement and the magnetic properties of the environment. The mutual inductance is expressed L 1,2, as well as the inductance L, in henry.

If the current strength changes in the second circuit, then induction EMF occurs in the first circuit

When the current strength changes in the conductor, a vortex electric field arises in the latter. This field slows down the electrons as the current increases and speeds them up as the current decreases.

The energy of the magnetic field of the current.

When a circuit containing a source of constant EMF is closed, the energy of the current source is initially spent on creating a current, i.e., on setting the electrons of the conductor in motion and forming a magnetic field associated with the current, and also partly on increasing the internal energy of the conductor, i.e., on its heating. After a constant value of the current strength is established, the energy of the source is spent exclusively on the release of heat. The current energy does not change.

To create a current, it is necessary to expend energy, i.e., it is necessary to do work. This is explained by the fact that when the circuit is closed, when the current begins to increase, a vortex electric field appears in the conductor, acting against the electric field that is created in the conductor due to the current source. In order for the current to become equal to I, the current source must do work against the forces of the vortex field. This work goes to increase the energy of the current. The vortex field does negative work.

When the circuit is opened, the current disappears and the vortex field does positive work. The energy stored by the current is released. This is detected by a powerful spark that occurs when a circuit with a large inductance is opened.

An expression for the energy of the current I flowing through a circuit with inductance L can be written on the basis of the analogy between inertia and self-induction.

If self-induction is similar to inertia, then the inductance in the process of creating a current should play the same role as the mass when increasing the speed of a body in mechanics. The role of the speed of a body in electrodynamics is played by the current strength I as a quantity that characterizes the movement of electric charges. If so, then the energy of the current W m can be considered a quantity similar to the kinetic energy of the body - in mechanics, and write in the form.

Magnetic flux Ф= BS cos . A change in the magnetic flux through the circuit can occur: 1) in the case of a stationary conducting circuit placed in a time-varying field; 2) in the case of a conductor moving in a magnetic field, which may not change with time. The value of the EMF of induction in both cases is determined by the law of electromagnetic induction, but the origin of this EMF is different.

Consider first the first case of the occurrence of an induction current. Let us place a circular coil of wire with radius r in a time-varying uniform magnetic field (Fig. 2.8).

Let the induction of the magnetic field increase, then the magnetic flux through the surface bounded by the coil will also increase with time. According to the law of electromagnetic induction, an inductive current will appear in the coil. When changing the induction of the magnetic field according to a linear law, the induction current will be constant.

What forces make the charges in the coil move? The magnetic field itself, penetrating the coil, cannot do this, since the magnetic field acts exclusively on moving charges (this is what it differs from the electric one), and the conductor with the electrons in it is motionless.

In addition to the magnetic field, charges, both moving and stationary, are also affected by an electric field. But after all, those fields that have been discussed so far (electrostatic or stationary) are created by electric charges, and the induction current appears as a result of the action of a changing magnetic field. Therefore, it can be assumed that electrons in a fixed conductor are set in motion by an electric field, and this field is directly generated by a changing magnetic field. This asserts a new fundamental property of the field: changing in time, the magnetic field generates an electric field. J. Maxwell was the first to come to this conclusion.

Now the phenomenon of electromagnetic induction appears before us in a new light. The main thing in it is the process of generating an electric field by a magnetic field. At the same time, the presence of a conductive circuit, such as a coil, does not change the essence of the process. A conductor with a supply of free electrons (or other particles) plays the role of an instrument: it only allows you to detect the emerging electric field.

The field sets in motion the electrons and the conductor and thereby reveals itself. The essence of the phenomenon of electromagnetic induction in a fixed conductor is not so much in the appearance of an induction current, but in the appearance of an electric field that sets electric charges in motion.

The electric field that occurs when the magnetic field changes has a completely different nature than the electrostatic one.

It is not connected directly with electric charges, and its lines of tension cannot begin and end on them. They generally do not start and end anywhere, but are closed lines, similar to the lines of magnetic field induction. This so-called vortex electric field(Fig. 2.9).

The faster the magnetic induction changes, the greater the electric field strength. According to Lenz's rule, with increasing magnetic induction, the direction of the electric field strength vector forms a left screw with the direction of the vector. This means that when the left-handed screw rotates in the direction of the electric field strength lines, the translational movement of the screw coincides with the direction of the magnetic induction vector. On the contrary, when the magnetic induction decreases, the direction of the intensity vector forms a right screw with the direction of the vector .

The direction of the field lines of tension coincides with the direction of the induction current. The force acting from the side of the vortex electric field on the charge q (external force) is still equal to = q. But in contrast to the case of a stationary electric field, the work of the vortex field in moving the charge q on a closed path is not equal to zero. Indeed, when a charge moves along a closed line of electric field strength, the work on all sections of the path has the same sign, since the force and displacement coincide in direction. The work of the vortex electric field when moving a single positive charge along a closed fixed conductor is numerically equal to the EMF of induction in this conductor.

Induction currents in massive conductors. Inductive currents reach a particularly large numerical value in massive conductors, due to the fact that their resistance is small.

Such currents, called Foucault currents after the French physicist who studied them, can be used to heat conductors. The device of induction furnaces, for example, microwave ovens used in everyday life, is based on this principle. This principle is also used for melting metals. In addition, the phenomenon of electromagnetic induction is used in metal detectors installed at the entrances to the buildings of air terminals, theaters, etc.

However, in many devices, the occurrence of Foucault currents leads to useless and even undesirable energy losses for heat generation. Therefore, the iron cores of transformers, electric motors, generators, etc. are made not solid, but consisting of separate plates isolated from each other. The surfaces of the plates must be perpendicular to the direction of the vortex electric field strength vector. In this case, the resistance to electric current of the plates will be maximum, and the heat release will be minimal.

Application of ferrites. Electronic equipment operates in the region of very high frequencies (millions of oscillations per second). Here, the use of coil cores from individual plates no longer gives the desired effect, since large Foucault currents arise in each plate.

When remagnetization occurs in ferrites, eddy currents do not occur. As a result, energy losses for the release of heat in them are minimized. Therefore, the cores of high-frequency transformers, magnetic antennas of transistors, etc. are made from ferrites. Ferrite cores are made from a mixture of powders of starting materials. The mixture is pressed and subjected to significant heat treatment.

With a rapid change in the magnetic field in an ordinary ferromagnet, induction currents arise, the magnetic field of which, in accordance with the Lenz rule, prevents a change in the magnetic flux in the core of the coil. Because of this, the flux of magnetic induction practically does not change and the core does not remagnetize. In ferrites, eddy currents are very small, so they can be quickly remagnetized.

Along with the potential Coulomb electric field, there is a vortex electric field. The lines of intensity of this field are closed. The vortex field is generated by a changing magnetic field.