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 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 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 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 and 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. 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. 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 vibrations 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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Vevcherenkova A.N. physics teacher, Tobolsk

Open lesson on the topic "Electric field. Conductors and dielectrics"

Grade: 8A

Date: 09.12.16

The purpose of the lesson : To form students' ideas about the electric field and its properties, conductors and dielectrics. Work out the concepts: electrization of bodies, electric charge, interaction of charges, two types of electric charges.

Lesson type : combined

Lesson Form: mutual learning lesson

Formed Skills : observe, compare, analyze

Lesson plan :

1. Organizing time.

The teacher greets the students. Marks attendees.

1. Blackboard work. Repetition

In the last lesson, we studied the types of charges and the rules for the interaction of these charges. I offer you the following task: the interactions of charges are drawn on the board. It is necessary to determine the "sign" of the charge of the ball with a question mark.

Teacher :

So, guys, we repeated two important properties of electrified bodies: like charges repel, and unlike charges attract.

Now let's remember what kind of body is called electrified or what is static electricity?

Today in the lesson we continue to study the topic of electrification, and in order to find out the topic of today's lesson, we need to check our homework. A crossword puzzle was given to you at home. Let's check what you got.

1. Checking homework. Setting the goal and objectives of the lesson.

Questions:

What are substances made of?

Kinetic, internal, potential, what is it?

What value was measured in Russia in miles per hour?

Which element is number three in Mendeleev's periodic table??

Name the device for measuring temperature.

Name the thermal process accompanied by intense evaporation of liquid throughout the volume.

Name the unit in which time is measured.

Name the creator of the temperature scale.

Measure of inertia and gravity.

What is the name of the thermal process in which the transition from a gaseous state to a liquid occurs??

What are molecules made of?

ICE stands for ... internal combustion.

What is the process of reverse crystallization called??

Name the first chemical element in the table of D.I. Mendeleev.

Name the unit in which heat is measured..

Name one example of air convection on a huge scale.?

Keyword ELECTRIC FIELD

These keywords will be the topic of our today's lesson. (write the topic of the lesson and the number in a notebook)

Target: today in the lesson we will learn what an electric field is; What is the difference between conductors and dielectrics? Let us give examples of substances that are conductors and non-conductors of electricity.

So we know thatCharged bodies act on each other, although at first glance there is no mediator between them . Since the electrical interaction occurs not only in air, but also in vacuum.

The interaction of electric charges was studied by English physicists Michael Faraday and James Maxwell, you see these scientists on the screen.

The conclusions made by these great scientists is that there is an environment around charged bodies, thanks to whichelectrical interaction . The space surrounding one charge interacts with the space surrounding another charge and vice versa.The mediator in this interaction will be the electric field.

To learn more about this special type of matter, about what is called conductors and dielectrics, I have prepared tasks for you, you will perform these tasks in mini groups. The task is given 10 minutes, after which each group presents their answers to the questions.

1. Independent work on cards on the topic “Electric field. Conductors and non-conductors of electricity

GroupIquestions:

What devices check the presence of a charge? ______________________________

What is an electric field?____________________________________________

_____

What bodies are called conductors? ______________________________________

_______________________________________________________________________

_______________________________________________________________________

Give examples of conductors: __________________________________________

_______________________________________________________________________

GroupIIquestions:

What are dielectrics?________________________________________________

____________________________________________________________________

Give examples of dielectrics: ______________________________________

____________________________________________________________________

What is the name of the body made of dielectrics? _____________________

____________________________________________________________________

GroupIIIquestions: (use the Internet to answer questions)

What professions use knowledge about the electric field, conductors and dielectrics? _______________________________________________________________

________________________________________________________________________

________________________________________________________________________

What universities teach these professions? ___________________________________

_________________________________________________________________________

Learning new material

When answering questions, the rest of the groups write down the main points in their notebooks.

Our sense organs do not perceive the electric field (for example, we cannot touch it). But it surrounds any charged body.

The main property of an electric field is its ability to act on electric charges with some force.

The force with which an electric field acts on an electric charge introduced into it is called the electric force.

An experience: sultan + ebonite stick with fur.

Let's conduct an experiment: Let's charge an ebonite stick with the help of friction on the fur, bring it to the sultan.

What happens to the sultan? (the petals of the sultan begin to be attracted to the stick)

Why is the sultan attracted to the ebonite stick? (because a positive charge is distributed on the petals, the stick itself has a negative charge, and opposite charges attract)

What can we say about the action of the electric field near the sultan and at a distance from it?

conclusion : Near charged bodies, the action of the field is stronger, and as you move away from them, the field weakens.

Ribbons of sultans are located along the lines of force of the electric field - that is, along the lines, the tangents to which at each point of the field coincide with the vector of the force acting from the side of the field on the charge placed at this point.

1. Fixing the material.

What is the difference between the space surrounding a charged body and the space surrounding an uncharged body?(The existence of an electric field)

How can an electric field be detected?(When applying electric charge)

If you touch a charged metal ball with your finger, it loses almost all of its charge. Why?(Because man is a good conductor)

Is it enough just to touch the electrometer with a charged ebonite rod for the needle to deviate?(Yes)

Homework:§26,27,31 read.

Choice task:

1. Exercise 19;

2. Experimental task on page 78 (describe the result in a notebook);

3. ESSAY on the topic "Life without an electric field";

Bibliography:

1. Peryshkin A.V. Physics. Grade 8: textbook. for general education institutions. - 8th ed., add. – M.: Bustard, 2006. – 191.

D. G. Evstafiev,
MOU Pritokskaya secondary school, Romanovsky settlement, Aleksandrovsky district, Orenburg region

Comparison of electric and magnetic fields. Grade 11

Outline of the lesson of repetition and generalization, 11th grade

Guidelines . The lesson is held after studying the topic "Magnetic field". The main methodological approach highlighting common and distinctive features of electric and magnetic fields with filling in the table. Sufficiently developed dialectical thinking is assumed, otherwise one will have to make digressions of a philosophical nature. Comparison of electric and magnetic fields brings students to the conclusion about their relationship, on which the next topic is based - "Electromagnetic induction".

Physics and philosophy consider matter as the basis of everything that exists, which exists in different forms. It can be concentrated within a limited region of space (localized), but, on the contrary, it can be delocalized. The first state can be associated with the concept substance, the second - the concept field. Along with specific physical characteristics, these states also have common ones. For example, there is the energy of a unit volume of matter and there is the energy of a unit volume of the field. The properties of matter are inexhaustible, the process of cognition is endless. Therefore, all physical concepts must be considered in development. So, for example, modern physics, unlike classical physics, does not draw a strict boundary between field and matter. In modern physics, the field and matter are mutually transformed: matter passes into the field, and the field passes into matter. But let's not get ahead of ourselves, but remember the classification of the forms of matter. Let's look at the diagram on the board.

Try to make a short story about the forms of existence of matter according to the scheme. ( After the students answer, the teacher reminds them that The consequence of this is the similarity of the characteristics of gravite ion and electric fields, which was revealed lebut in previous lessons on the topic "Electric field" .) The conclusion suggests itself: if there is a similarity between the gravitational and electric fields, then there must be a similarity between the electric and magnetic fields. let's let's compare the properties and characteristics of the fields in the form of a table similar to the one we did when comparison of gravitational and electric fields.

Electric field	A magnetic field
Field sources
electrically charged bodies	Moving electrically charged bodies (electric currents)
Field indicators
Small pieces of paper. Electrical sleeve. Electric "sultan"	Metal filings. Closed circuit with current. Magnetic needle
Experienced Facts
Coulomb's experiments on the interaction of electrically charged bodies	Ampère's experiments on the interaction of conductors with current
Graphical characteristic
The electric field strength lines in the case of immobile charges have a beginning and an end (potential field); can be visualized (quinine crystals in oil)	The magnetic field lines are always closed (vortex field); can be visualized (metal filings)
Power characteristic
Electric field strength vector E . Size: Direction:	Magnetic field induction vector B . Size: . Direction is determined by the left hand rule
Energy characteristic
The work of the electric field of fixed charges (Coulomb force) is equal to zero when going around a closed trajectory	The work of the magnetic field (Lorentz force) is always zero
The action of the field on a charged particle
The force is always different from zero: F = qE	The force depends on the speed of the particle: it does not act if the particle is at rest, and also if
Substance and field
.

Conclusion

1. When discussing the sources of the field, to increase interest in the subject, it is good to compare two natural stones: amber and a magnet.

Amber - a warm stone of amazing beauty - has an unusual property that is conducive to philosophical constructions: it can attract! Being rubbed, it attracts dust particles, threads, pieces of paper (papyrus). It was for this property that he was given names in antiquity. So the Greeks called itelectron – attractive; Romans - harpaxom – robber, and the Persians kavuboy, i.e. capable of attracting chaff . It was considered magical, medicinal, cosmetic ...

Another stone known for thousands of years - a magnet - was considered equally mysterious and useful. In different countries, the magnet was called differently, but most of these names are translated as loving. So poetically, the ancients noted the property of pieces of a magnet to attract iron.

From my point of view, these two special stones can be considered as the first studied natural sources of electric and magnetic fields.

2. When discussing field indicators, it is useful to simultaneously demonstrate with the help of students the interaction of an electrified ebonite rod with an electric sleeve and a permanent magnet with a closed circuit with current.

3. The visualization of field lines is best demonstrated using projection onto a screen.

4. The division of dielectrics into electrets and ferroelectrics - additional material. Electrets are dielectrics that retain polarization for a long time in the absence of an external electric field and create their own electric field. In this sense, electrets are like permanent magnets that create a magnetic field. But this is another similarity with hard ferromagnets!

Ferroelectrics are crystals that have (in a certain temperature range) spontaneous polarization. With a decrease in the strength of the external field, the induced polarization is partially preserved. They are characterized by the presence of a limiting temperature - the Curie point, at which a ferroelectric becomes an ordinary dielectric. Again similarity with ferromagnets!

After working with the table, the found similarities and differences are collectively discussed. The similarity lies at the basis of a single picture of the world, the differences are explained so far at the level of different organization of matter, it is better to say - the degree of organization of matter. The mere fact that a magnetic field is found only near moving electric charges (as opposed to an electric one) makes it possible to predict more complex methods for describing the field, a more complex mathematical apparatus used to characterize the field.

Dmitry Georgievich Evstafiev - a hereditary teacher of physics (father, Georgy Sevostyanovich, a participant in the Great Patriotic War, worked for many years in the Dobrinsky secondary school, combining teaching with the duties of a school director), graduated in 1978 Physics and Mathematics of the Orenburg State Pedagogical Institute im. V.P. Chkalova majoring in Physics, teaching experience 41 years. Since 1965, he has been working at the Pritok secondary school, for several years he was its director. He was awarded three times with honorary diplomas of the Orenburg oblono. Pedagogical credo: "Do not be satisfied with what has been achieved!" Many of its graduates graduated from technical universities. Together with his wife, they raised five children, three work in schools in the Orenburg region, two study at the historical and philological faculties of the Orenburg GPU. Son Sergey is the winner of the All-Russian contest "The Best Teachers of Russia" in 2006, a teacher of computer science, works in the district center - the village of Novosergievka. Hobby is beekeeping.

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 of 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. Course of the lesson 1. Analysis of the test 2. Learning a 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 create in 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 topic 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; give evidence that electric current would never have had such a wide application, if at one time ...
13. The purpose of the lesson: to continue the formation in students of the unity of oscillatory processes of various nature. Course of the lesson 1. Analysis of the test. 2. The study of 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. Organizational moment

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 is the physical meaning of the 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.