Through the circuit can occur: 1) in the case of a fixed 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 induction EMF in both cases is determined by the law (2.1), 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 magnetic field induction increase, then it will increase with time and magnetic flux through the surface bounded by the coil. 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 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 stationary 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 a phenomenon 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 and a fixed conductor is not so much in the appearance of an induction current, but in the occurrence electric field that drives electric charges.

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 unlike the case of a stationary electric field, the work vortex field in charge displacement q on a closed path is not equal to zero. After all, 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 induction EMF in this conductor.

Induction currents in massive conductors. especially large numerical value inductive currents reach 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 separate plates no longer gives the desired effect, since large Foucault currents arise in the caled plate.

In § 7 it was noted that there are magnetic insulators - ferrites. 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, 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.

1. What is the nature of external forces that cause the appearance of an induction current in a fixed conductor!
2. What is the difference between a vortex electric field and an electrostatic or stationary one!
3. What are Foucault currents!
4. What are the advantages of ferrites compared to conventional ferromagnets!

Myakishev G. Ya., Physics. Grade 11: textbook. for general education institutions: basic and profile. levels / G. Ya. Myakishev, B. V. Bukhovtsev, V. M. Charugin; ed. V. I. Nikolaev, N. A. Parfenteva. - 17th ed., revised. and additional - M.: Education, 2008. - 399 p.: ill.

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The purpose of the lesson: to form the concept that the EMF of induction can occur either in a fixed conductor placed in a changing magnetic field, or in a moving conductor in a constant magnetic field; the law of electromagnetic induction is valid in both cases, and the origin of the EMF is different.

During the classes

Checking homework by frontal questioning and problem solving

1. What value changes in proportion to the rate of change of the magnetic flux?

2. Work, what forces does the induction EMF create?

3. Formulate and write down the formula for the law of electromagnetic induction.

4. There is a minus sign in the law of electromagnetic induction. Why?

5. What is the EMF of induction in a closed loop of wire, the resistance of which is 0.02 Ohm, and the induction current is 5 A.

Solution. Ii = ξi /R; ξi= Ii R; ξi= 5 0.02= 0.1 B

Learning new material

Consider how the induction emf arises in fixed conductor, placed in an alternating magnetic field. The easiest way to understand this

An example of the operation of a transformer.

One coil is closed to the AC network, if the second coil is closed, then a current appears in it. The electrons in the secondary wires will move. What forces move free electrons? The magnetic field cannot do this, since it acts only on moving electric charges.

Free electrons are set in motion by the action of an electric field, which was created by an alternating magnetic field.

Thus, we have come to the concept of a new fundamental property fields: changing in time, the magnetic field generates an electric field. This conclusion was made by J. Maxwell.

Thus, in the phenomenon of electromagnetic induction, the main thing is the creation of an electric field by a magnetic field. This field sets free charges in motion.

The structure of this field is different than that of the electrostatic one. It has nothing to do with electrical charges. Tension lines do not start at positive charges and end at negative charges. Such lines have no beginning and end - they are closed lines similar to the lines of magnetic field induction. This is a vortex electric field.

The induction emf in a fixed conductor placed in an alternating magnetic field is equal to the work of the vortex electric field moving charges along this conductor.

Toki Foucault (French physicist)

The benefits and harms of induction currents in massive conductors.

Where are ferrites used? Why do they not generate eddy currents?

Consolidation of the studied material

– Explain the nature of extraneous forces acting in motionless conductors.

– Difference between electrostatic and vortex electric fields.

– Pros and cons of Foucault currents.

- Why do not eddy currents occur in ferrite cores?

- Calculate the EMF of induction in the conductor circuit if the magnetic flux has changed in 0.3 s by 0.06 Wb.

Solution. ξi= – ΔФ/Δt; ξi= – 0.06/0.3 = 0.2 B

Summing up the lesson

Homework: § 12, rep. § 11, exercise 2 No. 5, 6.

1. The purpose of the lesson: to formulate the quantitative law of electromagnetic induction; students should learn what is the EMF of magnetic induction and what is magnetic flux. Lesson progress Checking homework...
2. The purpose of the lesson: to find out what causes the induction EMF in moving conductors placed in a constant magnetic field; bring students to the conclusion that a force acts on charges ...
3. The purpose of the lesson: to form an idea of ​​the magnetic field as a form of matter; expand students' knowledge of magnetic interactions. Lesson 1. Analysis control work 2. Learning new...
4. The purpose of the lesson: to form students' understanding of the electric and magnetic fields, as a single whole - the electromagnetic field. Lesson progress Checking homework by testing ...
5. The purpose of the lesson: to find out how the discovery of electromagnetic induction took place; to form the concept of electromagnetic induction, the significance of Faraday's discovery for modern electrical engineering. Course of the lesson 1. Analysis of the control work ...
6. The purpose of the lesson: to form the idea that a change in the current strength in a conductor creates a vortex will, which can either accelerate or decelerate moving electrons. During the classes...
7. The purpose of the lesson: to introduce the concept of electromotive force; get Ohm's law for a closed circuit; to give students an idea of ​​the difference between EMF, voltage and potential difference. Move...
8. The purpose of the lesson: to acquaint students with the history of the struggle between the concepts of close action and action at a distance; with flawed theories, introduce the concept of electric field strength, form the ability to depict electrical ...
9. The purpose of the lesson: based on the model of a metal conductor, to study the phenomenon of electrostatic induction; find out the behavior of dielectrics in an electrostatic field; introduce the concept of dielectric permittivity. Lesson progress Checking home...
10. The purpose of the lesson: to form students' understanding of the electric current; consider the conditions necessary for the existence of an electric current. Course of the lesson 1. Analysis of the test 2. Studying new material ...
11. The purpose of the lesson: to test students' knowledge on the topics studied, to improve the skills of solving problems of various types. Lesson progress Checking homework Students answers according to prepared at home ...
12. The purpose of the lesson: to consider the device and the principle of operation of transformers; provide evidence that electricity would never have this wide application if only in due time...
13. The purpose of the lesson: to continue the formation in students of the unity of oscillatory processes different nature. Course of the lesson 1. Analysis of the test. 2. Learning new material When studying electromagnetic oscillations...
14. The purpose of the lesson: to form the idea that magnetic fields are formed not only by electric current, but also by permanent magnets; consider the scope of permanent magnets. Our planet...
15. The purpose of the lesson: to form an idea of ​​​​the energy that an electric current has in a conductor and the energy of the magnetic field created by the current. Lesson progress Checking homework by testing ...

Lesson 15 EMF induction in moving conductors

Purpose: to find out the conditions for the occurrence of EDW in moving conductors.

During the classes

I. Organizing time

II. Repetition

What is the phenomenon of electromagnetic induction?

What conditions are necessary for the existence of the phenomenon of electromagnetic induction?

How is the direction of the induced current determined by the Lenz rule?

By what formula is the induction emf determined and what physical meaning has a minus sign in this formula?

III. Learning new material

Let's take a transformer. By including one of the windings in the AC network, we get the current in the other coil. An electric field acts on free charges.

Electrons in a fixed conductor are set in motion by an electric field, and the electric field is directly generated by an alternating magnetic field. Changing in time, the magnetic field generates an electric field. The field sets the electrons in motion in the conductor and thereby reveals itself. The electric field that occurs when the magnetic field changes has a different structure than the electrostatic one. It is not connected with charges, it does not begin anywhere and does not end anywhere. Represents closed lines. It is called the vortex electric field. But unlike a stationary electric field, the work of a vortex field along a closed path is not equal to zero.

The induction current in massive conductors is called Foucault currents.

Application: melting of metals in vacuum.

Harmful effect: useless loss of energy in the cores of transformers and in generators.

EMF when a conductor moves in a magnetic field

When moving the jumperUThe Lorentz force acts on the electrons to do work. Electrons move from C to L. The jumper is the source of the EMF, therefore,

The formula is used in any conductor moving in a magnetic field ifIf between vectorsis the angle α, then the formula is used:

Becausethen

Cause of EDCis the Lorentz force. The sign of e can be determined by the right hand rule.

IV. Consolidation of the studied material

What field is called induction or vortex electric field?

What is the source of the induction electric field?

What are Foucault currents? Give examples of their use. In what cases do you have to deal with them?

What are the characteristics of an inductive electric field compared to a magnetic field? Stationary or electrostatic field?

V. Summing up the lesson

Homework

item 12; 13.

Topic. Law of electromagnetic induction

The purpose of the lesson: to acquaint students with the law of electromagnetic induction.

Type of lesson: lesson learning new material.

LESSON PLAN

Knowledge control		1. Flux of magnetic induction. 2. The phenomenon of electromagnetic induction. 3. Lenz's rule.
Demonstrations		1. Dependence of the EMF of induction on the rate of change of the magnetic flux. 2. Fragments of the video film "The phenomenon of electromagnetic induction."
Learning new material		1. The law of electromagnetic induction. 2. Vortex electric field. 3. EMF of induction in moving conductors.
Consolidation of the studied material		1. Qualitative questions. 2. Learning to solve problems.

STUDY NEW MATERIAL

Where do the extraneous forces that act on the charges in the circuit come from? In the case of a conductor stationary relative to the observer, the cause of the appearance of extraneous forces is an alternating magnetic field. The fact is that an alternating magnetic field generates an electric field in the surrounding space - it is this field that acts on free charged particles in a conductor. But the generation of an electric field by a magnetic field occurs even where there is no leading circuit and no electric current occurs. As you can see, a magnetic field can not only transmit magnetic interactions, but also be the cause of the appearance of another form of matter - an electric field.

However, the electric field generated by an alternating magnetic field has a significant difference from the field created by charged particles.

The electric field created by an alternating magnetic field is vortex, that is, its lines of force are closed.

The vortex electric field has some features:

1) the field manifests itself through a force effect on charged particles, therefore the main characteristic of the vortex electric field is the intensity;

2) unlike the electrostatic field, the lines of intensity of the vortex electric field are closed. The direction of these lines can be determined using, for example, the left hand, as shown in the figure:

3) unlike the electrostatic field, the work of the vortex electric field along a closed trajectory is not equal to zero (the vortex electric field is non-potential).

Consider a conductor of length l moving translationally in a uniform magnetic field with induction at a speed directed at an angle to the lines of magnetic induction of the field.

Electrons moving along with a conductor in a magnetic field are affected by the Lorentz force directed along the conductor. Her module

where q 0 is the charge of a free charged particle. Under the action of this force, a separation of charges occurs - free charged particles will move to one end of the conductor, and at the other end there will be a shortage of them, that is, it will exceed the charge of the opposite sign. Therefore, in this case, the outside force is the Lorentz force. The separation of charges will lead to the appearance of an electric field, which will prevent further separation of charges. This process will stop when the Lorentz force and the force = q 0 balance each other. Therefore, inside the conductor, the electric field strength is E \u003d B sin, and the potential difference at the ends of the conductor U \u003d El \u003d B lsin. Since we are considering an open circle, the potential difference at the ends of the conductor is equal to the EMF of induction in this conductor. In this way,

If such a conductor is closed, then an electric current will pass in a circle. Thus, a conductor moving in a magnetic field can be considered as a kind of current source characterized by an EMF of induction.

QUESTION TO STUDENTS DURING THE PRESENTATION OF NEW MATERIAL

First level

1. Why does an induction current occur in stationary conductors in an alternating magnetic field?

2. What is the reason for the occurrence of an induction current when a conductor moves in a constant magnetic field?

3. What are the features of the vortex electric field?

Second level

1. What is the nature of external forces that cause the appearance of an induction current in a fixed conductor?

2. Why is the law of electromagnetic induction formulated for EMF, and not for current strength?

3. What is the nature of the EMF of induction in a conductor moving in a magnetic field?

CONFIGURATION OF THE STUDYED MATERIAL

) . Qualitative questions

1. Why do fuses sometimes blow out from a lightning strike even when the device is turned off from the socket?

2. Why is it better to take a closed conductor in the form of a coil, and not in the form of a straight wire, to detect induction current?

) . Learning to solve problems

1. Using flexible wires, a straight conductor 60 cm long is connected to a DC source with an EMF of 12 V and an internal resistance of 0.5 Ohm. The conductor moves in a uniform magnetic field with an induction of 1.6 T at a speed of 12.5 m/s perpendicular to the lines of magnetic induction. Determine the current in the conductor if the resistance of the external circuit is 2.5 ohms.

An alternating magnetic field generates induced electric field. If the magnetic field is constant, then there will be no induced electric field. Consequently, induced electric field is not related to charges, as is the case in the case of an electrostatic field; its lines of force do not begin and end on charges, but are closed on themselves, like the lines of force of a magnetic field. It means that induced electric field, like a magnetic is vortex.

If a stationary conductor is placed in an alternating magnetic field, then e is induced in it. d.s. Electrons are set in directed motion by an electric field induced by an alternating magnetic field; an induced electric current occurs. In this case, the conductor is only an indicator of the induced electric field. The field sets in motion the free electrons in the conductor and thereby reveals itself. Now it can be argued that even without a conductor this field exists, having a reserve of energy.

The essence of the phenomenon of electromagnetic induction lies not so much in the appearance of an induced current, but in the appearance of a vortex electric field.

This fundamental position of electrodynamics was established by Maxwell as a generalization of Faraday's law of electromagnetic induction.

Unlike the electrostatic field, the induced electric field is non-potential, since the work done in the induced electric field when moving a single positive charge along a closed circuit is e. d.s. induction, not zero.

The direction of the intensity vector of the vortex electric field is set in accordance with Faraday's law of electromagnetic induction and Lenz's rule. The direction of the lines of force of the vortex el. field coincides with the direction of the induction current.

Since the vortex electric field also exists in the absence of a conductor, it can be used to accelerate charged particles to speeds commensurate with the speed of light. It is on the use of this principle that the action of electron accelerators - betatrons is based.

The induction electric field has completely different properties in contrast to the electrostatic field.

The difference between a vortex electric field and an electrostatic one

1) It is not connected with electric charges;
2) The lines of force of this field are always closed;
3) The work of the forces of the vortex field on the movement of charges on a closed trajectory is not equal to zero.

electrostatic field	induction electric field (vortex electric field)
1. created by motionless electr. charges	1. caused by changes in the magnetic field
2. field lines are open - potential field	2. lines of force are closed - vortex field
3. The sources of the field are electr. charges	3. field sources cannot be specified
4. the work of the field forces in moving the test charge along a closed path = 0.	4. the work of the field forces on the movement of the test charge along a closed path = EMF of induction