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Chapter 2. ELECTROMAGNETIC INDUCTION

So far, we have considered electric and magnetic fields that do not change with time. It was found that the electrostatic field is created by motionless charged particles, and the magnetic field is created by moving ones, i.e. electric current. Now let's get acquainted with electric and magnetic fields, which change over time.

Most important fact, which has been discovered, is the closest relationship between electric and magnetic fields. It turned out that a time-varying magnetic field generates electric field, and the changing electric field is magnetic. Without this connection between the fields, the variety of manifestations of electromagnetic forces would not be as extensive as it is actually observed. There would be no radio waves or light.

§ 8 DISCOVERY OF ELECTROMAGNETIC INDUCTION

In 1821, M. Faraday wrote in his diary: "Turn magnetism into electricity." After 10 years, this problem was solved by him.

It is no coincidence that the first decisive step in the discovery of new properties of electromagnetic interactions was made by the founder of the ideas about the electromagnetic field M. Faraday, who was confident in the unified nature of electrical and magnetic phenomena. Thanks to this, he made a discovery that became the basis for the design of generators of all power plants in the world, which convert mechanical energy into electric current energy. (Sources operating on other principles: galvanic cells, batteries, etc., provide an insignificant fraction of the generated electrical energy.)

Electric current, argued M. Faraday, is able to magnetize a piece of iron. Could a magnet in turn cause an electric current? Long time this connection could not be found. It was difficult to think of the main thing, namely: a moving magnet, or a magnetic field changing in time, can excite electricity in a coil.

What kind of accidents could prevent the discovery, shows the following fact. Almost simultaneously with Faraday, the Swiss physicist Colladon tried to get an electric current in a coil using a magnet. In the course of his work, he used a galvanometer, the light magnetic needle of which was placed inside the coil of the device. So that the magnet does not have a direct effect on the arrow, the ends of the coil, where Colladon introduced the magnet, hoping to get a current in it, were brought out into adjoining room and there are connected to a galvanometer. Having inserted the magnet into the coil, Colladon went into the next room and was disappointed to be convinced that the galvanometer showed no current. If only he could watch the galvanometer all the time, and ask someone to work on the magnet, a remarkable discovery would be made. But this did not happen. A magnet at rest relative to a coil causes no current in it.

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Lesson topic:

Discovery of electromagnetic induction. magnetic flux.

Target: introduce students to the phenomenon of electromagnetic induction.

During the classes

I. Organizational moment

II. Knowledge update.

1. Frontal survey.

• What is Ampère's hypothesis?
• What is magnetic permeability?
• What substances are called para- and diamagnets?
• What are ferrites?
• Where are ferrites used?
• How do you know that there is a magnetic field around the Earth?
• Where are the North and South magnetic poles of the Earth?
• What processes take place in the Earth's magnetosphere?
• What is the reason for the existence of a magnetic field near the Earth?

2. Analysis of experiments.

Experiment 1

The magnetic needle on the stand was brought to the lower and then to the upper end of the tripod. Why does the arrow turn to the lower end of the tripod from either side with the south pole, and to the upper end - the north end?(All iron objects are in the Earth's magnetic field. Under the influence of this field, they are magnetized, and the lower part of the object detects the north magnetic pole, and the top - the south.)

Experiment 2

In a large cork stopper, make a small groove for a piece of wire. Lower the cork into the water, and put the wire on top, placing it along the parallel. In this case, the wire, together with the cork, is rotated and installed along the meridian. Why?(The wire has been magnetized and is set in the Earth's field like a magnetic needle.)

III. Learning new material

There are magnetic forces between moving electric charges. Magnetic interactions are described based on the concept of a magnetic field that exists around moving electric charges. Electric and magnetic fields are generated by the same sources - electric charges. It can be assumed that there is a connection between them.

In 1831, M. Faraday confirmed this experimentally. He discovered the phenomenon of electromagnetic induction (slides 1.2).

Experiment 1

We connect the galvanometer to the coil, and we will put forward from it permanent magnet. We observe the deviation of the galvanometer needle, a current (induction) has appeared (slide 3).

The current in the conductor occurs when the conductor is in the area of ​​\u200b\u200bthe alternating magnetic field (slide 4-7).

Faraday represented an alternating magnetic field as a change in the number of lines of force penetrating the surface bounded by a given contour. This number depends on the induction AT magnetic field, from the contour area S and its orientation in the given field.

F \u003d BS cos a - magnetic flux.

F [Wb] Weber (slide 8)

The induction current can have different directions, which depend on whether the magnetic flux penetrating the circuit decreases or increases. The rule for determining the direction of the induced current was formulated in 1833. E. X. Lenz.

Experiment 2

We slide a permanent magnet into a light aluminum ring. The ring is repelled from it, and when extended, it is attracted to the magnet.

The result does not depend on the polarity of the magnet. Repulsion and attraction is explained by the appearance of an induction current in it.

When the magnet is pushed in, the magnetic flux through the ring increases: the repulsion of the ring in this case shows that induction current in it has such a direction in which the induction vector of its magnetic field is opposite in direction to the induction vector of the external magnetic field.

Lenz's rule:

The induction current always has such a direction that its magnetic field prevents any changes in the magnetic flux, causing appearance induction current(slide 9).

IV. Conducting laboratory work

Laboratory work on the topic "Experimental verification of the Lenz rule"

Devices and materials:milliammeter, coil-coil, arcuate magnet.

Working process

1. Prepare a table.

A new period in the development of physical science begins with the ingenious discovery by Faraday electromagnetic induction. It was in this discovery that the ability of science to enrich technology with new ideas was clearly manifested. Already Faraday himself foresaw the existence of electromagnetic waves on the basis of his discovery. On March 12, 1832, he sealed an envelope with the inscription "New Views, now to be kept in a sealed envelope in the archives of the Royal Society." This envelope was opened in 1938. It turned out that Faraday quite clearly understood that induction actions propagate with a finite speed in a wave way. "I consider it possible to apply the theory of oscillations to the propagation of electrical induction," wrote Faraday. At the same time, he pointed out that “the propagation of a magnetic effect takes time, that is, when a magnet acts on another distant magnet or a piece of iron, the influencing cause (which I will allow myself to call magnetism) spreads from magnetic bodies gradually and requires a certain time for its propagation which will obviously turn out to be very small. I also believe that electric induction propagates in exactly the same way. I believe that the propagation of magnetic forces from the magnetic pole is similar to the oscillation of a rough water surface, or sound vibrations air particles.

Faraday understood the importance of his idea and, not being able to test it experimentally, decided with the help of this envelope "to secure the discovery for himself and, thus, to have the right, in case of experimental confirmation, to declare this date the date of his discovery." So, on March 12, 1832, mankind for the first time came to the idea of ​​existence electromagnetic waves. From this date begins the history of discovery radio.

But Faraday's discovery had importance not only in the history of technology. It had a huge impact on the development of the scientific worldview. From this discovery, physics enters new object - physical field. Thus, Faraday's discovery belongs to those fundamental scientific discoveries which leave a noticeable trace in the entire history of human culture.

London blacksmith's son bookbinder was born in London on September 22, 1791. The brilliant self-taught did not even have the opportunity to finish primary school and paved the way for science himself. While studying bookbinding, he read books, especially on chemistry, he did chemical experiments. listening public lectures the famous chemist Davy, he was finally convinced that his vocation was science, and turned to him with a request to be hired at the Royal Institute. From 1813, when Faraday was admitted to the institute as a laboratory assistant, and until his death (August 25, 1867), he lived in science. Already in 1821, when Faraday received electromagnetic rotation, he set as his goal "to turn magnetism into electricity." Ten years of searching and hard work culminated in the discovery on August 29, 1871 of electromagnetic induction.

"Two hundred and three feet of copper wire in one piece were wound on a large wooden drum; another two hundred and three feet of the same wire were insulated in a spiral between the turns of the first winding, the metallic contact being removed by means of a cord. One of these spirals was connected to a galvanometer, and the other with a well-charged battery of one hundred pairs of four-inch-square-inch plates, with double copper plates. When the contact was made, there was a temporary but very slight effect on the galvanometer, and a similar weak effect took place when the contact with the battery was opened. This is how Faraday described his first experience of inducing currents. He called this kind of induction voltaic-electrical induction. He goes on to describe his main experience with the iron ring, the prototype of the modern transformer.

"A ring was welded from a round bar of soft iron; the thickness of the metal was seven-eighths of an inch, and the outer diameter of the ring was six inches. On one part of this ring three spirals were wound, each containing about twenty-four feet of copper wire, one twentieth of an inch thick. The coils were insulated from the iron and from each other... occupying about nine inches along the length of the ring They could be used singly and in combination, this group is designated A. On the other part of the ring was wound in the same way about sixty feet of copper wire in two pieces, which formed a spiral B, having the same direction as the spirals A, but separated from them at each end for about half an inch by bare iron.

Spiral B connected copper wires with a galvanometer placed at a distance of three feet from the iron. Separate coils were connected end to end so as to form a common spiral, the ends of which were connected to a battery of ten pairs of plates of four square inches. The galvanometer reacted immediately, and much stronger than was observed, as described above, using ten times more powerful spiral, but without iron; however, despite maintaining contact, the action ceased. When contact with the battery was opened, the arrow again strongly deviated, but in the opposite direction to that induced in the first case.

Faraday further investigated the effect of iron by direct experience, introducing an iron rod inside a hollow coil, in this case "the induced current had a very strong effect on the galvanometer." "A similar action was then obtained with the help of ordinary magnets". Faraday called this action magnetoelectric induction, assuming that the nature of voltaic and magnetoelectric induction is the same.

All the described experiments constitute the content of the first and second sections of the classic work of Faraday " Experimental studies on electricity", begun on November 24, 1831. In the third section of this series "On the new electrical state of matter", Faraday for the first time tries to describe the new properties of bodies manifested in electromagnetic induction. He calls this property he discovered "electrotonic state". This is the first germ of the idea field, formed later by Faraday and for the first time precisely formulated by Maxwell. The fourth section of the first series is devoted to explaining the phenomenon of Arago. Faraday correctly classifies this phenomenon as an induction phenomenon and tries with the help of this phenomenon to "obtain a new source of electricity". When a copper disk moves between the poles of a magnet, he obtained current in a galvanometer using sliding contacts.This was the first Dynamo machine. Faraday sums up the results of his experiments with the following words: "It was thus shown that it is possible to create a constant current of electricity with the help of an ordinary magnet." From his experiments on induction in moving conductors, Faraday deduced the relationship between the pole of a magnet, the moving conductor, and the direction of the induced current, i.e., "the law governing the production of electricity by magnetoelectric induction." As a result of his research, Faraday found that "the ability to induce currents manifests itself in a circle around the magnetic resultant or force axis in exactly the same way that magnetism located around a circle arises around an electric current and is detected by it" *.

* (M. Faraday, Experimental research on electricity, vol. I, Ed. AN SSSR, 1947, p. 57.)

In other words, a vortex electric field arises around an alternating magnetic flux, just as a vortex magnetic field arises around an electric current. This fundamental fact was generalized by Maxwell in the form of his two equations of the electromagnetic field.

The study of the phenomena of electromagnetic induction, in particular the inductive action of the Earth's magnetic field, is also devoted to the second series of "Investigations", begun on January 12, 1832. The third series, begun on January 10, 1833, Faraday devotes to proving the identity various kinds electricity: electrostatic, galvanic, animal, magnetoelectric (that is, obtained through electromagnetic induction). Faraday came to the conclusion that the electricity received different ways, qualitatively the same, the difference in actions is only quantitative. This was the final blow to the concept of various "fluids" of resin and glass electricity, galvanism, animal electricity. Electricity turned out to be a single, but polar entity.

Very important is the fifth series of Faraday's "Investigations", begun on June 18, 1833. Here Faraday begins his investigations of electrolysis, which led him to the establishment of the famous laws that bear his name. These studies were continued in the seventh series, which began on January 9, 1834. In this last series, Faraday proposes a new terminology: he proposes to call the poles that supply current to the electrolyte electrodes, call the positive electrode anode, and the negative cathode, particles of deposited matter going to the anode he calls anions, and the particles going to the cathode - cations. Further, he owns the terms electrolyte for degradable substances, ions and electrochemical equivalents. All these terms are firmly held in science. Faraday draws the correct conclusion from the laws he found that one can speak of some absolute quantity electricity associated with the atoms of ordinary matter. “Although we know nothing about what an atom is,” writes Faraday, “we involuntarily imagine some small particle that appears to our mind when we think about it; however, in the same or even greater ignorance we are relative to electricity, we are not even able to say whether it is a special matter or matters, or simply the movement of ordinary matter, or another kind of force or agent; nevertheless, there is a huge number of facts that make us think that the atoms of matter are somehow endowed with or connected with electrical forces, and to them they owe their most remarkable qualities, including their chemical affinity for one another.

* (M. Faraday, Experimental research on electricity, vol. I, Ed. AN SSSR, 1947, p. 335.)

Thus, Faraday clearly expressed the idea of ​​"electrification" of matter, atomic structure electricity, and the atom of electricity, or, as Faraday puts it, "the absolute quantity of electricity," turns out to be "as determined in its action, like any of those quantities which, remaining connected with the particles of matter, inform them of their chemical affinity. Elementary electric charge, as shown further development physics, can indeed be determined from Faraday's laws.

The ninth series of Faraday's "Investigations" was of great importance. This series, begun on December 18, 1834, dealt with the phenomena of self-induction, extra currents of closing and opening. Faraday points out in describing these phenomena that although they have features inertia, however, the phenomenon of self-induction is distinguished from mechanical inertia by the fact that they depend on forms conductor. Faraday notes that "extra current is identical with ... induced current" * . As a result, Faraday had an idea of ​​the very broad meaning of the process of induction. In the eleventh series of his investigations, begun on November 30, 1837, he states: "Induction plays the most general role in all electrical phenomena, apparently participating in each of them, and in reality bears the features of the first and essential beginning "**. In particular, according to Faraday, any charging process is an induction process, bias opposite charges: "substances cannot be charged absolutely, but only relatively, according to a law identical with induction. Every charge is supported by induction. All phenomena voltage include the beginning of inductions" ***. The meaning of these statements of Faraday is that any electric field ("voltage phenomenon" - in Faraday's terminology) is necessarily accompanied by an induction process in the medium ("displacement" - in Maxwell's later terminology). This process is determined by the properties of the medium , its "inductance", in Faraday's terminology, or "dielectric permittivity", in modern terminology. Faraday's experience with a spherical capacitor determined the permittivity of a number of substances with respect to air. These experiments strengthened Faraday in the idea of ​​the essential role of the medium in electromagnetic processes.

* (M. Faraday, Experimental research on electricity, vol. I, Ed. AN SSSR, 1947, p. 445.)

** (M. Faraday, Experimental research on electricity, vol. I, Ed. AN SSSR, 1947, p. 478.)

*** (M. Faraday, Experimental research on electricity, vol. I, Ed. AN SSSR, 1947, p. 487.)

The law of electromagnetic induction was significantly developed by the Russian physicist of the St. Petersburg Academy Emil Khristianovich Lenz(1804-1865). On November 29, 1833, Lenz reported to the Academy of Sciences his research "On determining the direction of galvanic currents excited by electrodynamic induction." Lenz showed that Faraday's magnetoelectric induction is closely related to Ampère's electromagnetic forces. "The proposition by which the magnetoelectric phenomenon is reduced to the electromagnetic one is as follows: if a metal conductor moves in the vicinity of a galvanic current or a magnet, then a galvanic current is excited in it in such a direction that if this conductor were stationary, then the current could cause it to move in the opposite direction; it is assumed that the conductor at rest can only move in the direction of motion or in the opposite direction" * .

* (E. X. Lenz, Selected Works, Ed. AN SSSR, 1950, pp. 148-149.)

This principle of Lenz reveals the energy of induction processes and played an important role in Helmholtz's work on establishing the law of conservation of energy. Lenz himself derived from his rule the well-known principle of reversibility in electrical engineering electromagnetic machines: if you rotate the coil between the poles of the magnet, it generates a current; on the contrary, if a current is sent to it, it will rotate. An electric motor can be turned into a generator and vice versa. Studying the action of magnetoelectric machines, Lenz discovers in 1847 the armature reaction.

In 1842-1843. Lenz produced a classic study "On the laws of heat generation by galvanic current" (reported on December 2, 1842, published in 1843), which he began long before Joule's similar experiments (Joule's message appeared in October 1841) and continued by him despite the publication Joule, "since the experiments of the latter may meet with some justified objections, as has already been shown by our colleague, Mr. Academician Hess" * . Lenz measures the magnitude of the current using a tangent compass - a device invented by the Helsingfort professor Johann Nerwander (1805-1848), and in the first part of his message he studies this device. In the second part of "The release of heat in wires", reported on August 11, 1843, he arrives at his famous law:

"
1. The heating of the wire by galvanic current is proportional to the resistance of the wire.
2. The heating of the wire by a galvanic current is proportional to the square of the current used for heating "**.

* (E. X. Lenz, Selected Works, Ed. AN SSSR, 1950, p. 361.)

** (E. X. Lenz, Selected Works, Ed. AN SSSR, 1950, p. 441.)

The Joule-Lenz law played an important role in establishing the law of conservation of energy. The entire development of the science of electrical and magnetic phenomena led to the idea of ​​the unity of the forces of nature, to the idea of ​​the conservation of these "forces".

Almost simultaneously with Faraday, an American physicist observed electromagnetic induction. Joseph Henry(1797-1878). Henry made a large electromagnet (1828) which, powered by a low resistance galvanic cell, supported a load of 2,000 pounds. Faraday mentions this electromagnet and indicates that with its help it is possible to obtain a strong spark when opened.

Henry for the first time (1832) observed the phenomenon of self-induction, and his priority is marked by the name of the unit of self-induction "henry".

In 1842 Henry established oscillatory character discharge of a Leiden jar. The thin glass needle with which he investigated this phenomenon was magnetized with different polarities, while the direction of the discharge remained unchanged. “The discharge, whatever its nature,” concludes Henry, “is not represented (using Franklin’s theory. - P. K.) as a single transfer of a weightless fluid from one plate to another; the discovered phenomenon makes us admit the existence of the main discharge in one direction, and then several strange backward and forward movements, each one weaker than the last, continuing until balance is reached.

Induction phenomena become the leading theme in physical research. In 1845 a German physicist Franz Neumann(1798-1895) gave a mathematical expression law of induction, summarizing the research of Faraday and Lenz.

The electromotive force of induction was expressed by Neumann as the time derivative of some function that induces the current, and the mutual configuration of the interacting currents. Neumann called this function electrodynamic potential. He also found an expression for the mutual induction coefficient. In his essay "On the Conservation of Force" in 1847, Helmholtz derives the Neumann expression for the law of electromagnetic induction from energy considerations. In the same essay, Helmholtz claims that the discharge of a capacitor is "not ... a simple movement of electricity in one direction, but ... its flow in one direction or the other between two plates in the form of oscillations that become smaller and smaller and less, until finally all living force is destroyed by the sum of the resistances.

In 1853 William Thomson(1824-1907) gave mathematical theory oscillatory discharge of the capacitor and established the dependence of the oscillation period on the parameters oscillatory circuit(Thomson's formula).

In 1858 P. Blaserna(1836-1918) took an experimental resonance curve of electrical oscillations, studying the action of a discharge-inducing circuit containing a capacitor bank and closing conductors to a side circuit, with a variable length of the induced conductor. In the same 1858 Wilhelm Feddersen(1832-1918) observed the spark discharge of a Leyden jar in a rotating mirror, and in 1862 he photographed the image of a spark discharge in a rotating mirror. Thus, the oscillatory nature of the discharge was established with complete clarity. At the same time, the Thomson formula was experimentally verified. Thus, step by step, the doctrine of electrical fluctuations, constituting the scientific foundation of electrical engineering of alternating currents and radio engineering.

The history of the discovery of electromagnetic induction. The discoveries of Hans Christian Oersted and André Marie Ampère showed that electricity has a magnetic force. The influence of magnetic phenomena on electrical phenomena was discovered by Michael Faraday. Hans Christian Oersted André Marie Ampère

Michael Faraday () "Turn magnetism into electricity," he wrote in his diary in 1822. English physicist, founder of the theory of the electromagnetic field, foreign honorary member of the St. Petersburg Academy of Sciences (1830).

Description of Michael Faraday's experiments wooden block wound two copper wires. One of the wires was connected to a galvanometer, the other to a strong battery. When the circuit was closed, a sudden but extremely weak action was observed on the galvanometer, and the same action was noticed when the current was stopped. With the continuous passage of current through one of the spirals, it was not possible to detect deviations of the galvanometer needle

Description of Michael Faraday's Experiments Another experiment consisted in registering surges of current at the ends of a coil, inside of which a permanent magnet was inserted. Faraday called such bursts "waves of electricity"

EMF of induction The EMF of induction, which causes bursts of current ("waves of electricity"), does not depend on the magnitude of the magnetic flux, but on the rate of its change.

1. Determine the direction of the lines of induction of the external field B (they leave N and enter S). 2. Determine whether the magnetic flux through the circuit increases or decreases (if the magnet is pushed into the ring, then Ф> 0, if it is pulled out, then Ф 0, if it is pulled out, then Ф 0, if it is pulled out, then Ф 0, if it is pulled out, then Ф 0 , if extended, then Ф
3. Determine the direction of the induction lines of the magnetic field B created by the inductive current (if F>0, then the lines B and B are directed in opposite directions; if F 0, then the lines B and B are directed in opposite directions; if F 0, then the lines B and B are directed in opposite directions; if Ф 0, then lines B and B are directed in opposite directions; if Ф 0, then lines B and B are directed in opposite directions; if Ф

Questions Formulate the law of electromagnetic induction. Who is the founder of this law? What is induced current and how to determine its direction? What determines the magnitude of the EMF of induction? The principle of operation of which electrical devices is based on the law of electromagnetic induction?

Electromagnetic induction- this is a phenomenon that consists in the occurrence of an electric current in a closed conductor as a result of a change in the magnetic field in which it is located. This phenomenon was discovered by the English physicist M. Faraday in 1831. Its essence can be explained by several simple experiments.

Described in Faraday's experiments receiving principle alternating current used in induction generators generating electrical energy in thermal or hydroelectric power plants. The resistance to rotation of the generator rotor, which occurs when the induction current interacts with the magnetic field, is overcome by the operation of a steam or hydraulic turbine that rotates the rotor. Such generators convert mechanical energy into electrical energy .

Eddy currents, or Foucault currents

If a massive conductor is placed in an alternating magnetic field, then in this conductor, due to the phenomenon of electromagnetic induction, eddy induction currents arise, called Foucault currents.

Eddy currents also arise when a massive conductor moves in a constant, but inhomogeneous magnetic field in space. Foucault currents have such a direction that the force acting on them in a magnetic field slows down the movement of the conductor. A pendulum in the form of a solid metal plate made of non-magnetic material, which oscillates between the poles of an electromagnet, stops abruptly when the magnetic field is turned on.

In many cases, the heating caused by Foucault currents turns out to be harmful and has to be dealt with. The cores of transformers, the rotors of electric motors are made from separate iron plates separated by layers of an insulator that prevents the development of large induction currents, and the plates themselves are made from alloys with high resistivity.

Electromagnetic field

The electric field created by stationary charges is static and acts on the charges. D.C causes the appearance of a time-constant magnetic field acting on moving charges and currents. Electrical and magnetic field exist in this case independently of each other.

Phenomenon electromagnetic induction demonstrates the interaction of these fields, observed in substances in which there are free charges, i.e., in conductors. An alternating magnetic field creates an alternating electric field, which, acting on free charges, creates an electric current. This current, being alternating, in turn generates an alternating magnetic field, which creates an electric field in the same conductor, etc.

The combination of alternating electric and alternating magnetic fields that generate each other is called electromagnetic field . It can also exist in a medium where there are no free charges, and propagates in space in the form electromagnetic wave.

classical electrodynamics- one of highest achievements human mind. She had a huge impact on the subsequent development human civilization, predicting the existence of electromagnetic waves. This later led to the creation of radio, television, telecommunications systems, satellite navigation, as well as computers, industrial and domestic robots and other attributes of modern life.

cornerstone Maxwell's theories was the assertion that only an alternating electric field can serve as a source of a magnetic field, just as a source electric field, creating an inductive current in the conductor, is an alternating magnetic field. The presence of a conductor in this case is not necessary - an electric field also arises in empty space. The lines of an alternating electric field, similarly to the lines of a magnetic field, are closed. The electric and magnetic fields of an electromagnetic wave are equal.

Electromagnetic induction in diagrams and tables