Broadcasting
An alternating magnetic field, excited by a changing current, creates in the surrounding space electric field, which in turn excites a magnetic field, etc. Mutually generating each other, these fields form a single alternating electromagnetic field - an electromagnetic wave. Having arisen in the place where there is a current-carrying wire, the electromagnetic field propagates through space at the speed of light -300,000 km/s.
Magnetotherapy
In the frequency spectrum different places occupy radio waves, light, x-rays and other electromagnetic radiation. They are usually characterized by continuously coupled electric and magnetic fields.
Synchrophasotrons
Currently under magnetic field understand special form matter consisting of charged particles. In modern physics, beams of charged particles are used to penetrate deep into atoms in order to study them. The force with which a magnetic field acts on a moving charged particle is called the Lorentz force.
Flow meters - counters
The method is based on the application of Faraday's law for a conductor in a magnetic field: in a flow of electrically conductive liquid moving in a magnetic field, an EMF is induced, proportional to the flow speed, converted by the electronic part into an electrical analogue/digital signal.
DC generator
In generator mode, the machine's armature rotates under the influence of an external torque. Between the stator poles there is a constant magnetic flux piercing anchor. The conductors of the armature winding move in a magnetic field and, therefore, an EMF is induced in them, the direction of which can be determined by the rule " right hand"In this case, a positive potential arises on one brush relative to the second. If a load is connected to the generator terminals, then current will flow through it.
Transformers
Transformers are widely used in transmission electrical energy over long distances, its distribution between receivers, as well as in various rectifying, amplifying, signaling and other devices.
Energy conversion in a transformer is carried out by an alternating magnetic field. A transformer is a core made of thin steel plates insulated from one another, on which two and sometimes more windings (coils) of insulated wire are placed. Winding to which a source of electrical energy is connected alternating current, is called the primary winding, the remaining windings are called secondary.
If the secondary winding of a transformer has three times more turns wound than the primary winding, then the magnetic field created in the core by the primary winding, crossing the turns of the secondary winding, will create three times the voltage in it.
By using a transformer with a reverse turns ratio, you can just as easily obtain a reduced voltage.
Subject: Usage electromagnetic induction
Lesson Objectives:
Educational:
- Continue work on developing the concept of the electromagnetic field as a type of matter and evidence of its real existence.
- Improve skills in solving qualitative and calculation problems.
Developmental: Continue working with students on...
- formation of ideas about modern physical picture of the world,
- the ability to reveal the relationship between the studied material and phenomena of life,
- expanding students' horizons
Educational: Learn to see the manifestations of the studied patterns in life around you
Demonstrations
1. Transformer
2. Fragments of the CD “Physics grades 7-11. Library of visual aids"
1) “Electricity generation”
2) “Writing and reading information on magnetic tape”
3. Presentations
1) “Electromagnetic induction - tests” (Parts I and II)
2) "Transformer"
During the classes
1. Update:
Before you consider new material, please answer the following questions:
2. Problem solving on cards, see presentation (Appendix 1) (answers: 1 B, 2 B, 3 B, 4 A, 5 B) – 5 min
3. New material.Use of electromagnetic induction
1) In the past academic year when studying the topic “Information Media” in computer science, we talked about disks, floppy disks, etc. It turns out that recording and reading information using magnetic tape is based on the use of the phenomenon of electromagnetic induction.
Recording and reproducing information using magnetic tape (Fragments of the CD “Physics grades 7-11. Library of visual aids”, “Recording and reading information on magnetic tape” - 3 min) (Appendix 2)
2) Let's consider the structure and fundamental operation of such a device as a TRANSFORMER. (see presentation Appendix 3)
The operation of a transformer is based on the phenomenon of electromagnetic induction.
TRANSFORMER - a device that converts alternating current of one voltage into alternating current of another voltage at a constant frequency.
3) In the simplest case, the transformer consists of a closed steel core, on which two coils with wire windings are placed. The one of the windings that is connected to an alternating voltage source is called primary, and the one to which the “load” is connected, i.e., devices that consume electricity, is called secondary.
a) step-up transformer
b) step-down transformer
When transferring energy to long distance– use of step-down and step-up transformers.
4) Operation of the transformer (conducting the experiment).
Lighting of the light in the secondary coil ( explanation of this experience);
- principle of operation welding machine (Why are the turns in the secondary coil of a step-down transformer thicker?);
- operating principle of the furnace ( The power in both coils is the same, but what about the current?)
5) Practical application of electromagnetic induction
Examples of technical use of electromagnetic induction: transformer, electric current generator - the main source of electricity.
Thanks to the discovery of electromagnetic induction, it became possible to generate cheap electrical energy. The basis for the operation of modern power plants (including nuclear ones) is induction generator.
Alternating current generator (disc fragment Fragments of the CD “Physics grades 7-11. Library of visual aids”, “Electricity generation” - 2 min) (Appendix 4)
The induction generator consists of two parts: a moving rotor and a stationary stator. Most often, the stator is a magnet (permanent or electric) that creates an initial magnetic field (it is called an inductor). The rotor consists of one or more windings in which an induced current is created under the influence of a changing magnetic field. (Another name for such a rotor is an anchor).
- detection metal objects– special detectors;
- magnetic levitation train(see page 129 of the textbook by V. A. Kasyanov “Physics - 11”)
Foucault currents (eddy currents;)
– closed induction currents arising in massive conducting bodies.
They appear either as a result of a change in the magnetic field in which a conducting body is located, or as a result of such a movement of the body when the magnetic flux penetrating this body (or any part of it) changes.
Like any other currents, eddy currents have a thermal effect on the conductor: the bodies in which such currents arise heat up.
Example: installation of electric furnaces for melting metals and microwave ovens.
4. Conclusions, assessments.
1) Electromagnetic induction, give examples of the practical application of electromagnetic induction.
2) Electromagnetic waves are the most common type of matter, and electromagnetic induction is special case manifestations of electromagnetic waves.
5. Solving problems using cards, see presentation(Appendix 5) (answers - 1B, 2A, 3A, 4B).
6. House task: P.35,36 (Textbook of physics edited by V.A. Kasyanov, grade 11)
Practical application of electromagnetic induction
The phenomenon of electromagnetic induction is used primarily to convert mechanical energy into electrical energy. For this purpose they are used alternators(induction generators).
| sin |
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| A |
| IN |
| WITH |
| T |
| F |
| Rice. 4.6 |
The excitation winding through a brush-contact apparatus magnetizes the rotor, and in this case an electromagnet with north and south poles is formed.
There are three alternating current windings located on the generator stator, which are shifted relative to each other by 120 0 and are connected to each other according to a specific connection circuit.
When the excited rotor rotates with the help of a steam or hydraulic turbine, its poles pass under the stator windings, and an electromotive force varying according to a harmonic law is induced in them. Next, the generator is connected to electricity consumption nodes according to a certain electrical network diagram.
If you transfer electricity from station generators to consumers via power lines directly (at the generator voltage, which is relatively low), then large losses of energy and voltage will occur in the network (pay attention to the ratios 
, 
). Therefore, to transport electricity economically, it is necessary to reduce the current strength. However, since the transmitted power remains unchanged, the voltage must
increase by the same amount as the current decreases.
The electricity consumer, in turn, needs to reduce the voltage to the required level. Electrical devices in which the voltage increases or decreases by a given number of times are called transformers. The operation of a transformer is also based on the law of electromagnetic induction.
| sin |
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| N |
| t |
| - |
| = |
| . |
| sin |
| sin |
| t |
| N |
| t |
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Then 
Powerful transformers have very low coil resistances,
therefore, the voltages at the terminals of the primary and secondary windings are approximately equal to the EMF:
Where k – transformation ratio. At k<1 (
) transformer is increasing, at k>1 (
) transformer is downward.
When connected to the secondary winding of a load transformer, current will flow in it. With an increase in electricity consumption, according to the law
conservation of energy should increase the energy supplied by the station generators, that is
This means that by increasing the voltage using a transformer
V k times, it is possible to reduce the current strength in the circuit by the same number of times (at the same time, Joule losses decrease by k 2 times).
Topic 17. Basics of Maxwell's theory for electromagnetic field. Electromagnetic waves
In the 60s XIX century English scientist J. Maxwell (1831-1879) generalized the experimentally established laws of electric and magnetic fields and created a complete unified electromagnetic field theory. It allows you to decide the main problem of electrodynamics: find the characteristics of the electromagnetic field of a given system of electric charges and currents.
Maxwell hypothesized that any alternating magnetic field excites a vortex electric field in the surrounding space, the circulation of which is the cause of the emf of electromagnetic induction in the circuit:
(5.1)
Equation (5.1) is called Maxwell's second equation. The meaning of this equation is that a changing magnetic field generates a vortex electric field, and the latter in turn causes a changing magnetic field in the surrounding dielectric or vacuum. Since the magnetic field is created by an electric current, then, according to Maxwell, the vortex electric field should be considered as a certain current,
which occurs both in a dielectric and in a vacuum. Maxwell called this current displacement current.
Displacement current, as follows from Maxwell's theory
and Eichenwald's experiments, creates the same magnetic field as the conduction current.
In his theory, Maxwell introduced the concept apparent current, equal to the sum
conduction and displacement currents. Therefore, the total current density
According to Maxwell, the total current in a circuit is always closed, that is, at the ends of the conductors only the conduction current breaks, and in the dielectric (vacuum) between the ends of the conductor there is a displacement current that closes the conduction current.
Having introduced the concept of total current, Maxwell generalized the theorem on the circulation of a vector (or):
(5.6)
Equation (5.6) is called Maxwell's first equation in integral form. It represents a generalized law of total current and expresses the basic position of electromagnetic theory: displacement currents create the same magnetic fields as conduction currents.
The unified macroscopic theory of the electromagnetic field created by Maxwell made it possible from a unified point of view not only to explain electrical and magnetic phenomena, but to predict new ones, the existence of which was subsequently confirmed in practice (for example, the discovery of electromagnetic waves).
Summarizing the provisions discussed above, we present the equations that form the basis of Maxwell’s electromagnetic theory.
1. Theorem on the circulation of the magnetic field strength vector:
This equation shows that magnetic fields can be created either by moving charges (electric currents) or by alternating electric fields.
2. The electric field can be both potential () and vortex (), therefore the total field strength 
. Since the circulation of the vector is zero, then the circulation of the total intensity vector electric field
This equation shows that the sources of the electric field can be not only electric charges, but also time-varying magnetic fields.
3. 
,
4. 
where is the volumetric charge density inside a closed surface; – specific conductivity of the substance.
For stationary fields ( E= const , B= const) Maxwell's equations take the form
that is, the sources of the magnetic field in this case are only
conduction currents, and the sources of the electric field are only electric charges. In this particular case, the electric and magnetic fields are independent of each other, which makes it possible to study separately permanent electric and magnetic fields.
Using the known ones from vector analysis Stokes and Gauss theorems, one can imagine complete system Maxwell's equations in differential form(characterizing the field at each point in space):
(5.7)
It is obvious that Maxwell's equations not symmetrical relative to electric and magnetic fields. This is due to the fact that in nature
There are electric charges, but there are no magnetic charges.
Maxwell's equations are the most general equations for electrical
and magnetic fields in quiescent media. They play the same role in the doctrine of electromagnetism as Newton's laws do in mechanics.
Electromagnetic wave called an alternating electromagnetic field propagating in space with a finite speed.
The existence of electromagnetic waves follows from Maxwell's equations, formulated in 1865 based on a generalization of the empirical laws of electrical and magnetic phenomena. An electromagnetic wave is formed due to the mutual connection of alternating electric and magnetic fields - a change in one field leads to a change in the other, that is, the faster the magnetic field induction changes over time, the greater the electric field strength, and vice versa. Thus, for the formation of intense electromagnetic waves, it is necessary to excite electromagnetic oscillations of a sufficiently high frequency. Phase speed electromagnetic waves is determined
electrical and magnetic properties of the environment:
In a vacuum ( 
) the speed of propagation of electromagnetic waves coincides with the speed of light; in matter 
, That's why The speed of propagation of electromagnetic waves in matter is always less than in vacuum.
Electromagnetic waves are transverse waves
–
oscillations of the vectors and occur in mutually perpendicular planes, and the vectors and form a right-handed system. From Maxwell’s equations it also follows that in an electromagnetic wave the vectors and always oscillate in the same phases, and the instantaneous values E And N at any point are related by the relation
Equations flat electromagnetic wave in vector form:
(6.66)
| y |
| z |
| x |
| Rice. 6.21 |
To characterize the transfer of energy by any wave in physics, a vector quantity called energy flux density. It is numerically equal to the amount of energy transferred per unit time through a unit area perpendicular to the direction in which
the wave spreads. The direction of the vector coincides with the direction of energy transfer. The energy flux density value can be obtained by multiplying the energy density by the wave speed
The energy density of the electromagnetic field is composed of the energy density of the electric field and the energy density of the magnetic field:
(6.67)
Multiplying the energy density of an electromagnetic wave by its phase velocity, we obtain the energy flux density
(6.68)
The vectors and are mutually perpendicular and form a right-handed system with the direction of wave propagation. Therefore the direction
vector 
coincides with the direction of energy transfer, and the modulus of this vector is determined by relation (6.68). Therefore, the energy flux density vector of an electromagnetic wave can be represented as a vector product
(6.69)
The vector is called Umov-Poynting vector.
Oscillations and waves
Topic 18. Free harmonic oscillations
Movements that have varying degrees of repetition are called fluctuations.
If the values physical quantities, changing during the movement, are repeated at equal intervals of time, then such a movement is called periodic (the movement of the planets around the Sun, the movement of the piston in the cylinder of an internal combustion engine, etc.). An oscillatory system, regardless of its physical nature, is called oscillator. An example of an oscillator is an oscillating weight suspended from a spring or string.
Full swingcall one complete cycle of oscillatory movement, after which it is repeated in the same order.
According to the method of excitation, vibrations are divided into:
· free(own), occurring in a system presented to itself near the equilibrium position after some initial impact;
· forced, occurring under periodic external influence;
· parametric, occurring when any parameter of the oscillatory system changes;
· self-oscillations, occurring in systems that independently regulate the flow of external influences.
Any oscillatory movement is characterized amplitude A - the maximum deviation of the oscillating point from the equilibrium position.
Oscillations of a point that occur with a constant amplitude are called undamped, and oscillations with gradually decreasing amplitude – fading.
The time during which a complete oscillation occurs is called period(T).
Frequency Periodic oscillations are the number of complete oscillations performed per unit of time. Unit of vibration frequency - hertz(Hz). Hertz is the frequency of oscillations whose period is equal to 1 s: 1 Hz = 1 s –1.
Cyclicor circular frequency periodic oscillations is the number of complete oscillations performed during time 2p with: 
.
=rad/s.
Today we will talk about the phenomenon of electromagnetic induction. Let us reveal why this phenomenon was discovered and what benefits it brought.
Silk
People have always strived to live better. Some might think that this is a reason to accuse humanity of greed. But often we're talking about about acquiring basic household conveniences.
IN medieval Europe knew how to make wool, cotton and linen fabrics. And even at that time, people suffered from an excess of fleas and lice. At the same time, Chinese civilization has already learned how to masterfully weave silk. Clothes made from it kept bloodsuckers away from human skin. The insects' legs slid over the smooth fabric, and the lice fell off. Therefore, the Europeans wanted to dress in silk at all costs. And the merchants thought that this was another opportunity to get rich. Therefore, the Great Silk Road was built.
This was the only way to deliver the desired fabric to suffering Europe. And so many people were involved in the process that cities arose as a result, empires fought over the right to levy taxes, and some parts of the path are still the most convenient way get to the right place.
Compass and star
Mountains and deserts stood in the way of caravans with silk. It happened that the character of the area remained the same for weeks and months. Steppe dunes gave way to similar hills, one pass followed another. And people had to somehow navigate in order to deliver their valuable cargo.
The stars were the first to come to the rescue. Knowing what day it was today and what constellations to expect, an experienced traveler could always determine where south was, where east was, and where to go. But there were always not enough people with sufficient knowledge. And they didn’t know how to count time accurately back then. Sunset, sunrise - that's all the landmarks. And a snow or sandstorm, cloudy weather excluded even the possibility of seeing the polar star.
Then people (probably the ancient Chinese, but scientists are still arguing about this) realized that one mineral is always located in a certain way in relation to the cardinal points. This property was used to create the first compass. The discovery of the phenomenon of electromagnetic induction was a long way off, but a start had been made.
From compass to magnet
The name “magnet” itself goes back to the toponym. The first compasses were probably made from ore mined in the hills of Magnesia. This region is located in Asia Minor. And the magnets looked like black stones.
The first compasses were very primitive. Water was poured into a bowl or other container, and a thin disk of floating material was placed on top. And a magnetized arrow was placed in the center of the disk. One end always pointed to the north, the other to the south.
It's hard to imagine that the caravan saved water for the compass while people were dying of thirst. But staying on track and allowing people, animals and goods to reach safety was more important than several individual lives.
The compasses made many journeys and encountered various natural phenomena. It is not surprising that the phenomenon of electromagnetic induction was discovered in Europe, although magnetic ore was originally mined in Asia. In this intricate way, the desire of Europeans to sleep more comfortably led to a major discovery in physics.
Magnetic or electric?
In the early nineteenth century, scientists figured out how to produce direct current. The first primitive battery was created. It was enough to send a stream of electrons through metal conductors. Thanks to the first source of electricity, a number of discoveries were made.
In 1820, the Danish scientist Hans Christian Oersted found out that the magnetic needle deviates near a conductor connected to the network. The positive pole of the compass is always located in a certain way in relation to the direction of the current. The scientist carried out experiments in all possible geometries: the conductor was above or below the arrow, they were located parallel or perpendicular. The result was always the same: the switched on current set the magnet in motion. This was how the discovery of the phenomenon of electromagnetic induction was anticipated.
But the idea of scientists must be confirmed by experiment. Immediately after Oersted's experiment, the English physicist Michael Faraday asked the question: “Do the magnetic and electric fields simply influence each other, or are they more closely related?” The scientist was the first to test the assumption that if an electric field causes a magnetized object to deviate, then the magnet should generate a current.
The experimental design is simple. Now any schoolchild can repeat it. A thin metal wire was coiled into the shape of a spring. Its ends were connected to a device that recorded the current. When a magnet moved near the coil, the device's arrow showed the voltage of the electric field. Thus, Faraday's law of electromagnetic induction was derived.
Continuation of experiments
But that's not all the scientist did. Since the magnetic and electric fields are closely related, it was necessary to find out how much.
To do this, Faraday supplied current to one winding and pushed it inside another similar winding with a radius larger than the first. Once again electricity was induced. Thus, the scientist proved: a moving charge generates both electric and magnetic fields at the same time.
It is worth emphasizing that we are talking about the movement of a magnet or magnetic field inside a closed loop of a spring. That is, the flow must change all the time. If this does not happen, no current is generated.
Formula
Faraday's law for electromagnetic induction is expressed by the formula
Let's decipher the symbols.
ε stands for emf or electromotive force. This quantity is scalar (that is, not vector), and it shows the work that certain forces or laws of nature apply to create a current. It should be noted that the work must necessarily be performed by non-electrical phenomena.
Φ is the magnetic flux through a closed loop. This value is the product of two others: the magnitude of the magnetic induction vector B and the area of the closed loop. If the magnetic field does not act strictly perpendicular to the contour, then the cosine of the angle between vector B and the normal to the surface is added to the product.
Consequences of the discovery
This law was followed by others. Subsequent scientists established the dependence of electric current intensity on power and resistance on conductor material. New properties were studied and incredible alloys were created. Finally, humanity deciphered the structure of the atom, delved into the mystery of the birth and death of stars, and revealed the genome of living beings.
And all these achievements required huge amount resources, and, above all, electricity. Any production or large Scientific research were carried out where three components were available: qualified personnel, the material itself with which to work, and cheap electricity.
And this was possible where the forces of nature could impart a large torque to the rotor: rivers with a large elevation difference, valleys with strong winds, faults with excess geomagnetic energy.
I wonder what modern way obtaining electricity is not fundamentally different from Faraday's experiments. The magnetic rotor spins very quickly inside a large spool of wire. The magnetic field in the winding changes and is generated all the time electricity.
Of course, selected and best material for magnet and conductors, and the technology of the whole process is completely different. But the point is one thing: the principle discovered in the simplest system is used.
After the discoveries of Oersted and Ampere, it became clear that electricity has magnetic force. Now it was necessary to confirm the influence of magnetic phenomena on electrical ones. Faraday brilliantly solved this problem.
In 1821, M. Faraday wrote in his diary: “Convert magnetism into electricity.” After 10 years, he solved this problem.
So, Michael Faraday (1791-1867) - English physicist and chemist.
One of the founders of quantitative electrochemistry. For the first time (1823) he obtained chlorine in a liquid state, then hydrogen sulfide, carbon dioxide, ammonia and nitrogen dioxide. Discovered benzene (1825), studied its physical properties and some Chemical properties. Introduced the concept of dielectric constant. Faraday's name entered the system of electrical units as a unit of electrical capacity.
Many of these works could themselves immortalize the name of their author. But the most important of scientific works Faraday's research is in the fields of electromagnetism and electrical induction. Strictly speaking, an important branch of physics that treats the phenomena of electromagnetism and inductive electricity, and which is currently of such enormous importance for technology, was created by Faraday out of nothing.
When Faraday finally devoted himself to research in the field of electricity, it was found that when under ordinary conditions The presence of an electrified body is enough for its influence to excite electricity in any other body.
At the same time, it was known that a wire through which current passes and which also represents an electrified body does not have any effect on other wires placed nearby. What caused this exception? This is the question that interested Faraday and the solution of which led him to the most important discoveries in the field of induction electricity.
Faraday wound two insulated wires parallel to each other on the same wooden rolling pin. He connected the ends of one wire to a battery of ten cells, and the ends of the other to a sensitive galvanometer. When a current was passed through the first wire, Faraday turned all his attention to the galvanometer, expecting to notice by its vibrations the appearance of a current in the second wire. However, nothing of the kind happened: the galvanometer remained calm. Faraday decided to increase the current strength and introduced 120 galvanic elements into the circuit. The result was the same. Faraday repeated this experiment dozens of times and still with the same success. Anyone else in his place would have left the experiments convinced that the current passing through a wire has no effect on the neighboring wire. But Faraday always tried to extract from his experiments and observations everything that they could give, and therefore, not receiving a direct effect on the wire connected to the galvanometer, he began to look for side effects.
electromagnetic induction electric current field
He immediately noticed that the galvanometer, remaining completely calm during the entire passage of current, began to oscillate when the circuit itself was closed, and when it was opened, it turned out that at the moment when current was passed into the first wire, and also when this transmission stopped, the second wire is also excited by a current, which in the first case has the opposite direction to the first current and the same with it in the second case and lasts only one instant. These secondary instantaneous currents, caused by the influence of the primary ones, were called inductive by Faraday, and this name has remained with them to this day.
Being instantaneous, instantly disappearing after their appearance, inductive currents would have no practical significance if Faraday had not found a way, with the help of an ingenious device (a commutator), to constantly interrupt and again conduct the primary current coming from the battery along the first wire, thanks to which the second wire is continuously excited by more and more new inductive currents, thus becoming constant. Thus, a new source of electrical energy was found, in addition to the previously known ones (friction and chemical processes), is induction, and the new kind This energy is inductive electricity.
ELECTROMAGNETIC INDUCTION(Latin inductio - guidance) - the phenomenon of generating a vortex electric field by an alternating magnetic field. If you introduce a closed conductor into an alternating magnetic field, an electric current will appear in it. The appearance of this current is called current induction, and the current itself is called induction.