The textbook of physics for grade IX gives a brief excursion into the history of the discovery of the law in question. The review should be supplemented. We are talking about a fundamental law of nature, and you need to reveal all its aspects in the process of becoming. The story of Faraday's process of searching for the law is especially instructive, and there is no need to spare time here.
Michael Faraday was born in 1791 in the vicinity of London in the family of a blacksmith. His father did not have the means to pay for his studies, and at the age of 13 Faraday was forced to start studying bookbinding. Luckily, he was apprenticed to a bookstore owner. An inquisitive boy eagerly read, and not easy literature. He was attracted by the natural science articles in the Encyclopædia Britannica, he studied Mars' Discourses on Chemistry. In 1811, Faraday began attending public lectures on physics by the well-known London educator Tatum.
The turning point in Faraday's life was 1812. A client of the bookstore owner, a member of the Royal Institute, Dance recommended that the young man listen to the lectures of the famous chemist Gamfrn Davy. Faraday followed good advice; he listened eagerly and took careful notes. On the advice of the same Dance, he processed the notes and sent them to Davy, adding a request for an opportunity for research work. In 1813, Faraday received a job as a laboratory assistant in the chemical laboratory of the Royal Institute, which was led by Davy.
In the beginning, Faraday is a chemist. He quickly takes the path of independent creativity, and Devi's pride often has to suffer from the success of the student. In 1820, Faraday learned about Oersted's discovery, and since then his thoughts have absorbed electricity and magnetism. He begins his famous experimental research, which led to the transformation of physical thinking. In 1823, Faraday was elected a member of the Royal Society of London, and then appointed director of the physical and chemical laboratories of the Royal Institute. The greatest discoveries were made within the walls of these laboratories. Faraday's life, outwardly monotonous, is striking in its creative tension. It is evidenced by the three-volume work "Experimental Research on Electricity", which reflects step by step the creative path of a genius.
In 1820, Faraday posed a fundamentally new problem: "to transform magnetism into electricity." This was shortly after the discovery of the magnetic action of currents. In Oersted's experiment, an electric current acts on a magnet. Since, according to Faraday, all the forces of nature are interconvertible, it is possible, on the contrary, to excite an electric current by magnetic force.
Faraday liquefies gases, makes fine chemical analyzes, discovers new chemical properties of substances. But his mind is relentlessly occupied with the problem posed. In 1822, he describes an attempt to detect a "state" due to current flow: "to polarize a beam of light from a lamp by reflection and try to find out whether water located between the poles of a voltaic battery in a glass vessel will have a depolarizing effect ..." Faraday hoped thus obtain some information about the properties of the current. But the experience did not give anything. Next comes 1825. Faraday publishes the article "Electromagnetic current (under the influence of a magnet)", in which he expresses the following thought. If the current acts on the magnet, then it must experience a reaction. “For various reasons,” writes Faraday, “the assumption was made that the approach of the pole of a strong magnet would reduce the electric current.” And he describes an experience that realizes this idea.
A diary dated November 28, 1825 describes a similar experience. The battery of galvanic cells was connected by a wire. Parallel to this wire was another (the wires were separated by a double layer of paper), the ends of which were connected to the galvanometer. Faraday seemed to reason like this. If the current is the movement of an electric fluid and this movement acts on a permanent magnet - a set of currents (according to Ampère's hypothesis), then the moving fluid in one conductor should make the motionless one move in the other, and the galvanometer should fix the current. The “various considerations” that Faraday wrote about when presenting the first experiment boiled down to the same thing, only there the reaction of an electric fluid moving in a conductor from the molecular currents of a permanent magnet was expected. But the experiments gave a negative result.
The solution came in 1831, when Faraday suggested that induction should occur with and non-stationary process. This was the key idea that led to the discovery of the phenomenon of electromagnetic induction.
It is possible that a message received from America forced him to turn to the idea of ​​\u200b\u200bchanging the current. The news came from the American physicist Joseph Henry (1797 - 1878).
In his youth, Henry showed neither exceptional ability nor interest in science. He grew up in poverty, was a farmhand, an actor. Just like Faraday, he educates himself. He began studying at the age of 16 at the Albany Academy. In seven months, he acquired so much knowledge that he got a job as a teacher in a rural school. Henry then worked for chemistry professor Beck as a lecture assistant. He combined work with studies at the academy. After completing the course, Henry was appointed engineer and inspector on the Erie Canal. A few months later, he left this lucrative position, accepting an invitation to the post of professor of mathematics and physics at Albany. At this time, the English inventor William Sturgeon (1783 - 1850) reported on his invention of a horseshoe magnet capable of lifting a steel body weighing up to four kilograms.
Henry became interested in electromagnetism. He immediately found a way to increase lift to a ton. This was achieved by a new technique at that time: instead of insulating the body of the magnet, the wire was insulated. A way to create multilayer windings has been discovered. Back in 1831, Henry showed the possibility of building an electric motor, invented an electromagnetic relay, and with its help demonstrated the transmission of electrical signals over a distance, anticipating Morse's invention (Morse's telegraph appeared in 1837).
Like Faraday, Henry set himself the task of obtaining an electric current using a magnet. But this was the statement of the problem of the inventor. And the search was guided by bare intuition. The discovery took place a few years before Faraday's experiments. The setting of Henry's key experiment is shown in Figure 9. Here everything is the same as it has been shown so far. Only we prefer a more convenient accumulator to a galvanic cell, and instead of torsion balances we use a galvanometer.
But Henry did not tell anyone about this experience. “I should have printed this sooner,” he said contritely to his friends, “but I had so little time! I wanted to bring the results into some kind of system.”(emphasis mine.- AT. D.). And the lack of regular education and even more - the utilitarian-inventive spirit of American science played a bad role. Henry, of course, did not understand and did not feel the depth and importance of the new discovery. Otherwise, of course, he would have informed the scientific world about the greatest fact. Keeping silent about the induction experiments, Henry immediately sent a message when he managed to lift a whole ton with an electromagnet.
This is the message that Faraday received. Perhaps it served as the last link in the chain of inferences that led to the key idea. In the experiment of 1825, two wires were separated with paper. There should have been an induction, but it was not detected due to the weakness of the effect. Henry showed that in an electromagnet the effect is greatly enhanced by the use of a multilayer winding. Therefore, the induction must increase if the inductive action is transmitted over a large length. Indeed, a magnet is a collection of currents. The excitation of magnetization in a steel rod when a current is passed through the winding is the induction of the current by the current. It increases if the path of the current through the winding becomes longer.
Such is the possible chain of Faraday's logical conclusions. Here is a full description of the first successful experience: “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 laid in a spiral between the turns of the first winding, the metallic contact everywhere being removed by means of a cord. One of these coils was connected to a galvanometer, and the other to a well-charged battery of one hundred pairs of four-inch square plates with double copper plates. When the contact was closed, there was a sudden but very weak action on the galvanometer, and a similar weak action took place when the contact with the battery was opened.
This was the first experience that gave a positive result after a decade of searching. Faraday establishes that when closing and opening, induction currents of opposite directions arise. He then proceeds to study the effect of iron on induction.
“A ring was welded from round bar, soft iron; the thickness of the metal was seven or eight inches, and the outer diameter of the ring was six inches. On one part of this ring three coils were wound, each containing about twenty-four feet of copper wire one-twentieth of an inch thick. Spirals were isolated from iron and from each other and superimposed on one another ... They could be used separately and in combination; this group is labeled BUT(Fig. 10). On the other part of the ring, about sixty feet of the same copper wire was wound in the same way, in two pieces, forming a spiral. AT, which had the same direction as the spirals BUT, but was separated from them at each end for about half an inch by bare iron.
Spiral AT connected by copper wires to a galvanometer placed at a distance of three feet from the ring. Separate spirals BUT 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 more strongly than was observed above, when using ten times more powerful spiral without iron.
Finally, Faraday makes an experiment with which the presentation of the question of electromagnetic induction is still usually started. This was an exact repetition of Henry's experience depicted in Figure 9.
The problem set by Faraday in 1820 was solved: magnetism was converted into electricity.
First, Faraday distinguishes the induction of current from current (he calls it “volta-electric induction” and current from a magnet (“magneto-electric induction”). But then he shows that all cases are subject to one general pattern.
The law of electromagnetic induction covered another group of phenomena, which later received the name of self-induction phenomena. Faraday called the new phenomenon as follows: "The inductive effect of an electric current on itself."
This question arose in connection with the following fact reported to Faraday in 1834 by Jenkin. This fact was as follows. Two plates of a galvanic battery are connected by a short wire. At the same time, the experimenter cannot get an electric shock from this wire by any tricks. But if we take the winding of an electromagnet instead of a wire, then every time the circuit is opened, a shock is felt. Faraday wrote: “At the same time, something else is observed, a phenomenon known to scientists for a long time, namely: a bright electric spark jumps at the point of separation "(my italics - V.D.).
Faraday began to examine these facts and soon discovered a number of new aspects of the phenomenon. It took him a little time to establish "the identity of phenomena with the phenomena of induction." Experiments that are still being demonstrated both in secondary and higher education in explaining the phenomenon of self-induction were set up by Faraday in 1834.
Independently, similar experiments were carried out by J. Henry, however, like experiments on induction, they were not published in a timely manner. The reason is the same: Henry did not find a physical concept that encompasses phenomena of various forms.
For Faraday, self-induction was a fact that illuminated the further path of search. Summarizing observations, he comes to conclusions of great fundamental importance. “There is no doubt that the current in one part of the wire can act by induction on other parts of the same wire that are nearby ... This is what gives the impression that the current acts on itself.”
Not knowing the nature of the current, Faraday nevertheless accurately points to the essence of the matter: “When the current acts by induction along with it, a conductive substance located along with it, then it probably acts on the electricity present in this conductive substance - it doesn’t matter whether the latter is in state of current or it is motionless; in the first case, it strengthens or weakens the current, depending on its direction in the second, it creates a current.
The mathematical expression of the law of electromagnetic induction was given in 1873 by Maxwell in his Treatise on Electricity and Magnetism. Only after that did it become the basis of quantitative calculations. So the law of electromagnetic induction should be called the Faraday-Maxwell law.
Methodical remarks. It is known that the excitation of an inductive current in a conductor moving in a constant magnetic field, and in a stationary conductor, which is in an alternating magnetic field, obeys the same law. For Faraday and Maxwell, this was obvious, since they imagined the lines of magnetic induction as real formations in the ether. When the current is turned on and off, or the current strength changes around the conductors that make up the circuit, the lines of magnetic induction move. At the same time, they cross the circuit itself, causing the phenomenon of self-induction. If there is any conductor near the circuit with a changing current, then the lines of magnetic induction, crossing it, excite the EMF of electromagnetic induction.
The materialization of the lines of force of the electric field and the lines of magnetic induction have become the property of history. However, it would be a mistake to give the lines of force only a formal character. Modern physics considers that the line of force of the electric field and the line of magnetic induction are the locus of points at which the given field has a state different from the state at other points. This state is determined by the values ​​of the vectors and at these points. When the field changes, the vectors and change, accordingly changes the configuration of the lines of force. The state of the field can move in space at the speed of light. If the conductor is in a field whose state changes, an EMF is excited in the conductor.

The case when the field is constant and the conductor moves in this field is not described by Maxwell's theory. Einstein first noticed this. His seminal work "On the Electrodynamics of Moving Bodies" just begins with a discussion of the insufficiency of Maxwell's theory at this point. The phenomenon of EMF excitation in a conductor moving in a constant magnetic field can be included in the framework of the electromagnetic field theory if it is supplemented with the principle of relativity and the principle of constancy of the speed of light.

After the discoveries of Oersted and Ampère, it became clear that electricity has a magnetic force. Now it was necessary to confirm the influence of magnetic phenomena on electrical ones. This problem was brilliantly solved by Faraday.

Michael Faraday (1791-1867) was born in London, one of the poorest parts of it. His father was a blacksmith, and his mother was the daughter of a tenant farmer. When Faraday reached school age, he was sent to elementary school. The course taken by Faraday here was very narrow and limited only to teaching reading, writing, and the beginning of counting.

A few steps from the house where the Faraday family lived, there was a bookstore, which was also a bookbinding establishment. This is where Faraday got to, having completed the course of elementary school, when the question arose about choosing a profession for him. Michael at that time was only 13 years old. Already in his youth, when Faraday had just begun his self-education, he strove to rely solely on facts and verify the reports of others with his own experiences.

These aspirations dominated him all his life as the main features of his scientific activity. Faraday began to make physical and chemical experiments as a boy at the first acquaintance with physics and chemistry. Once Michael attended one of the lectures of Humphry Davy, the great English physicist.

Faraday made a detailed note of the lecture, bound it, and sent it to Davy. He was so impressed that he offered Faraday to work with him as a secretary. Soon Davy went on a trip to Europe and took Faraday with him. For two years they visited the largest European universities.

Returning to London in 1815, Faraday began working as an assistant in one of the laboratories of the Royal Institution in London. At that time it was one of the best physical laboratories in the world. From 1816 to 1818 Faraday published a number of small notes and small memoirs on chemistry. Faraday's first work on physics dates back to 1818.

Based on the experiences of his predecessors and combining several of his own experiences, by September 1821, Michael had printed the "Success Story of Electromagnetism". Already at that time, he made up a completely correct concept of the essence of the phenomenon of deflection of a magnetic needle under the action of a current.

Having achieved this success, Faraday left his studies in the field of electricity for ten years, devoting himself to the study of a number of subjects of a different kind. In 1823, Faraday made one of the most important discoveries in the field of physics - he first achieved the liquefaction of a gas, and at the same time established a simple but valid method for converting gases into a liquid. In 1824, Faraday made several discoveries in the field of physics.

Among other things, he established the fact that light affects the color of glass, changing it. The following year, Faraday again turns from physics to chemistry, and the result of his work in this area is the discovery of gasoline and sulfuric naphthalene acid.

In 1831, Faraday published a treatise On a Special Kind of Optical Illusion, which served as the basis for a beautiful and curious optical projectile called the "chromotrope". In the same year, another treatise by the scientist "On vibrating plates" was published. Many of these works could by themselves immortalize the name of their author. But the most important of Faraday's scientific works are his researches in the field of electromagnetism and electric induction.

Strictly speaking, the important branch of physics, which treats the phenomena of electromagnetism and inductive electricity, and which is currently of such great importance for technology, was created by Faraday out of nothing.

By the time Faraday finally devoted himself to research in the field of electricity, it was established that, under ordinary conditions, the presence of an electrified body is sufficient for its influence to excite electricity in any other body. At the same time, it was known that the wire through which the current passes and which is also 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. As usual, Faraday began a series of experiments that were supposed to clarify the essence of the matter.

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 elements, and the ends of the other to a sensitive galvanometer. When the current was passed through the first wire,

Faraday turned all his attention to the galvanometer, expecting to notice from its oscillations the appearance of a current in the second wire as well. However, there was nothing of the kind: the galvanometer remained calm. Faraday decided to increase the current and introduced 120 galvanic cells into the circuit. The result is the same. Faraday repeated this experiment dozens of times, all with the same success.

Anyone else in his place would have left the experiment, convinced that the current passing through the wire has no effect on the adjacent wire. But Faraday always tried to extract from his experiments and observations everything that they could give, and therefore, not having received a direct effect on the wire connected to the galvanometer, he began to look for side effects.

He immediately noticed that the galvanometer, remaining completely calm during the entire passage of the current, comes into oscillation at the very closing of the circuit and at its opening. It turned out that at the moment when the current is passed into the first wire, and also when this the second wire is also excited by a current, which in the first case has the opposite direction to the first current and is the same with it in the second case and lasts only one instant.

These secondary instantaneous currents, caused by the influence of primary ones, were called inductive by Faraday, and this name has been preserved for them until now. 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 (commutator), to constantly interrupt and again conduct the primary current coming from the battery through the first wire, due to which in the second wire is continuously excited by more and more 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), - induction, and a new type of this energy - induction electricity.

Continuing his experiments, Faraday further discovered that a simple approximation of a wire twisted into a closed curve to another, along which a galvanic current flows, is enough to excite an inductive current in the direction opposite to the galvanic current in a neutral wire, that the removal of a neutral wire again excites an inductive current in it. the current is already in the same direction as the galvanic current flowing along a fixed wire, and that, finally, these inductive currents are excited only during the approach and removal of the wire to the conductor of the galvanic current, and without this movement, the currents are not excited, no matter how close the wires are to each other .

Thus, a new phenomenon was discovered, similar to the above-described phenomenon of induction during the closing and termination of the galvanic current. These discoveries in turn gave rise to new ones. If it is possible to produce an inductive current by closing and stopping the galvanic current, would not the same result be obtained from the magnetization and demagnetization of iron?

The work of Oersted and Ampère had already established the relationship between magnetism and electricity. It was known that iron became a magnet when an insulated wire was wound around it and a galvanic current passed through it, and that the magnetic properties of this iron ceased as soon as the current ceased.

Based on this, Faraday came up with this kind of experiment: two insulated wires were wound around an iron ring; moreover, one wire was wound around one half of the ring, and the other around the other. A current from a galvanic battery was passed through one wire, and the ends of the other were connected to a galvanometer. And so, when the current closed or stopped, and when, consequently, the iron ring was magnetized or demagnetized, the galvanometer needle oscillated rapidly and then quickly stopped, that is, all the same instantaneous inductive currents were excited in the neutral wire - this time: already under the influence of magnetism.

Thus, here for the first time magnetism was converted into electricity. Having received these results, Faraday decided to diversify his experiments. Instead of an iron ring, he began to use an iron band. Instead of exciting magnetism in iron with a galvanic current, he magnetized the iron by touching it to a permanent steel magnet. The result was the same: in the wire wrapped around the iron, always! the current was excited at the moment of magnetization and demagnetization of iron.

Then Faraday introduced a steel magnet into the wire spiral - the approach and removal of the latter caused induction currents in the wire. In a word, magnetism, in the sense of excitation of inductive currents, acted in exactly the same way as the galvanic current.

At that time, physicists were intensely occupied with one mysterious phenomenon, discovered in 1824 by Arago and did not find an explanation, despite; that this explanation was intensively sought by such eminent scientists of the time as Arago himself, Ampère, Poisson, Babaj and Herschel.

The matter was as follows. A magnetic needle, freely hanging, quickly comes to rest if a circle of non-magnetic metal is brought under it; if the circle is then put into rotational motion, the magnetic needle begins to follow it.

In a calm state, it was impossible to discover the slightest attraction or repulsion between the circle and the arrow, while the same circle, which was in motion, pulled behind it not only a light arrow, but also a heavy magnet. This truly miraculous phenomenon seemed to the scientists of that time a mysterious riddle, something beyond the natural.

Faraday, based on his above data, made the assumption that a circle of non-magnetic metal, under the influence of a magnet, is circulated during rotation by inductive currents that affect the magnetic needle and draw it behind the magnet.

Indeed, by introducing the edge of the circle between the poles of a large horseshoe-shaped magnet and connecting the center and edge of the circle with a galvanometer with a wire, Faraday received a constant electric current during the rotation of the circle.

Following this, Faraday settled on another phenomenon that was then causing general curiosity. As you know, if iron filings are sprinkled on a magnet, they are grouped along certain lines, called magnetic curves. Faraday, drawing attention to this phenomenon, gave the foundations in 1831 to magnetic curves, the name "lines of magnetic force", which then came into general use.

The study of these "lines" led Faraday to a new discovery, it turned out that for the excitation of inductive currents, the approach and removal of the source from the magnetic pole is not necessary. To excite currents, it is enough to cross the lines of magnetic force in a known way.

Further works of Faraday in the mentioned direction acquired, from the modern point of view, the character of something completely miraculous. At the beginning of 1832, he demonstrated an apparatus in which inductive currents were excited without the help of a magnet or galvanic current.

The device consisted of an iron strip placed in a wire coil. This device, under ordinary conditions, did not give the slightest sign of the appearance of currents in it; but as soon as he was given a direction corresponding to the direction of the magnetic needle, a current was excited in the wire.

Then Faraday gave the position of the magnetic needle to one coil and then introduced an iron strip into it: the current was again excited. The reason that caused the current in these cases was terrestrial magnetism, which caused inductive currents like an ordinary magnet or galvanic current. In order to show and prove this more clearly, Faraday undertook another experiment that fully confirmed his ideas.

He reasoned that if a circle of non-magnetic metal, for example, copper, rotating in a position in which it intersects the lines of magnetic force of a neighboring magnet, gives an inductive current, then the same circle, rotating in the absence of a magnet, but in a position in which the circle will cross the lines of terrestrial magnetism, must also give an inductive current.

And indeed, a copper circle, rotated in a horizontal plane, gave an inductive current, which produced a noticeable deviation of the galvanometer needle. Faraday completed a series of studies in the field of electrical induction with the discovery, made in 1835, of "the inductive effect of current on itself."

He found out that when a galvanic current is closed or opened, instantaneous inductive currents are excited in the wire itself, which serves as a conductor for this current.

The Russian physicist Emil Khristoforovich Lenz (1804-1861) gave a rule for determining the direction of the induced current. “The induction current is always directed in such a way that the magnetic field it creates impedes or slows down the movement that causes induction,” notes A.A. Korobko-Stefanov in his article on electromagnetic induction. - For example, when the coil approaches the magnet, the resulting inductive current has such a direction that the magnetic field created by it will be opposite to the magnetic field of the magnet. As a result, repulsive forces arise between the coil and the magnet.

Lenz's rule follows from the law of conservation and transformation of energy. If induction currents accelerated the movement that caused them, then work would be created from nothing. The coil itself, after a small push, would rush towards the magnet, and at the same time the induction current would release heat in it. In reality, the induction current is created due to the work of bringing the magnet and coil closer together.

Why is there an induced current? A deep explanation of the phenomenon of electromagnetic induction was given by the English physicist James Clerk Maxwell, the creator of a complete mathematical theory of the electromagnetic field.

To better understand the essence of the matter, consider a very simple experiment. Let the coil consist of one turn of wire and be pierced by an alternating magnetic field perpendicular to the plane of the turn. In the coil, of course, there is an induction current. Maxwell interpreted this experiment with exceptional courage and unexpectedness.

When the magnetic field changes in space, according to Maxwell, a process arises for which the presence of a wire coil is of no importance. The main thing here is the appearance of closed ring lines of the electric field, covering the changing magnetic field. Under the action of the emerging electric field, electrons begin to move, and an electric current arises in the coil. A coil is just a device that allows you to detect an electric field.

The essence of the phenomenon of electromagnetic induction is that an alternating magnetic field always generates an electric field with closed lines of force in the surrounding space. Such a field is called a vortex field.

Research in the field of induction produced by terrestrial magnetism gave Faraday the opportunity to express the idea of ​​a telegraph as early as 1832, which then formed the basis of this invention. In general, the discovery of electromagnetic induction is not without reason attributed to the most outstanding discoveries of the 19th century - the work of millions of electric motors and electric current generators around the world is based on this phenomenon ...

Source of information: Samin D. K. "One hundred great scientific discoveries", M.: "Veche", 2002.

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 experiments by Michael Faraday Two copper wires are wound on a wooden block. 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- the phenomenon of the occurrence of an electric current in a closed circuit with a change in the magnetic flux passing through it. Electromagnetic induction was discovered by Michael Faraday on August 29, 1831. He discovered that the electromotive force (EMF) that occurs in a closed conducting circuit is proportional to the rate of change of the magnetic flux through the surface bounded by this circuit. The magnitude of the electromotive force does not depend on what causes the change in the flux - a change in the magnetic field itself or the movement of a circuit (or part of it) in a magnetic field. The electric current caused by this EMF is called induction current.

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  According to Faraday's law of electromagnetic induction (in SI):

  E = − d Φ B d t (\displaystyle (\mathcal (E))=-((d\Phi _(B)) \over dt))- electromotive force acting along an arbitrarily chosen contour, = ∬ S B → ⋅ d S → , (\displaystyle =\iint \limits _(S)(\vec (B))\cdot d(\vec (S)))- magnetic flux through the surface bounded by this contour.

  The minus sign in the formula reflects Lenz's rule, named after the Russian physicist E. Kh. Lenz:

  The inductive current that occurs in a closed conducting circuit has such a direction that the magnetic field it creates counteracts the change in the magnetic flux that caused this current.

  For a coil in an alternating magnetic field, Faraday's law can be written as follows:

  E = − N d Φ B d t = − d Ψ d t (\displaystyle (\mathcal (E))=-N((d\Phi _(B)) \over dt)=-((d\Psi ) \over dt)) E (\displaystyle (\mathcal (E)))- electromotive force, N (\displaystyle N)- number of turns, Φ B (\displaystyle \Phi _(B))- magnetic flux through one turn, Ψ (\displaystyle \psi )- Coil flux linkage.

  vector shape

  In differential form, Faraday's law can be written as follows:

  rot E → = − ∂ B → ∂ t (\displaystyle \operatorname (rot) \,(\vec (E))=-(\partial (\vec (B)) \over \partial t))(in SI system) rot E → = − 1 c ∂ B → ∂ t (\displaystyle \operatorname (rot) \,(\vec (E))=-(1 \over c)(\partial (\vec (B)) \over \ partial t))(in the GHS system).

  In integral form (equivalent):

  ∮ ∂ S ⁡ E → ⋅ d l → = − ∂ ∂ t ∫ S B → ⋅ d s → (\displaystyle \oint _(\partial S)(\vec (E))\cdot (\vec (dl))=-( \partial \over \partial t)\int _(S)(\vec (B))\cdot (\vec (ds)))(SI) ∮ ∂ S ⁡ E → ⋅ d l → = − 1 c ∂ ∂ t ∫ S B → ⋅ d s → (\displaystyle \oint _(\partial S)(\vec (E))\cdot (\vec (dl))= -(1 \over c)(\partial \over \partial t)\int _(S)(\vec (B))\cdot (\vec (ds)))(GHS)

  Here E → (\displaystyle (\vec (E)))- intensity electric field , B → (\displaystyle (\vec (B)))- magnetic induction , S (\displaystyle S\ )- an arbitrary surface, - its boundary. Integration contour ∂ S (\displaystyle \partial S) is assumed to be fixed (immovable).

  It should be noted that Faraday's law in this form, obviously, describes only that part of the EMF that occurs when the magnetic flux through the circuit changes due to the change in the field itself over time without changing (moving) the boundaries of the circuit (see below for taking into account the latter).

  If, say, the magnetic field is constant, and the magnetic flux changes due to the movement of the contour boundaries (for example, with an increase in its area), then the emerging EMF is generated by forces that hold the charges on the circuit (in the conductor) and the Lorentz force generated by the direct action of the magnetic field on moving (with contour) charges. At the same time, equality E = − d Φ / d t (\displaystyle (\mathcal (E))=-((d\Phi )/dt)) continues to be observed, but the EMF on the left side is no longer reduced to ∮ ⁡ E → ⋅ d l → (\displaystyle \oint (\vec (E))\cdot (\vec (dl)))(which in this particular example is generally equal to zero). In the general case (when the magnetic field changes with time and the circuit moves or changes shape), the last formula is also true, but the EMF on the left side in this case is the sum of both terms mentioned above (that is, it is generated partially by the vortex electric field, and partly by the Lorentz force and the reaction force of the moving conductor).

  Potential form

  When expressing the magnetic field in terms of the vector potential, Faraday's law takes the form:

  E → = − ∂ A → ∂ t (\displaystyle (\vec (E))=-(\partial (\vec (A)) \over \partial t))(in the absence of an irrotational field, that is, when the electric field is generated completely only by a change in the magnetic, that is, electromagnetic induction).

  In the general case, when taking into account the irrotational (for example, electrostatic) field, we have:

  E → = − ∇ φ − ∂ A → ∂ t (\displaystyle (\vec (E))=-\nabla \varphi -(\partial (\vec (A)) \over \partial t))

  More

  Since the magnetic induction vector, by definition, is expressed in terms of the vector potential as follows:

  B → = r o t A → ≡ ∇ × A → , (\displaystyle (\vec (B))=rot\ (\vec (A))\equiv \nabla \times (\vec (A)),)

  then you can substitute this expression into

  r o t E → ≡ ∇ × E → = − ∂ B → ∂ t , (\displaystyle rot\ (\vec (E))\equiv \nabla \times (\vec (E))=-(\frac (\partial ( \vec (B)))(\partial t)),) ∇ × E → = − ∂ (∇ × A →) ∂ t , (\displaystyle \nabla \times (\vec (E))=-(\frac (\partial (\nabla \times (\vec (A)) ))(\partial t)),)

  and, by interchanging the differentiation in time and spatial coordinates (rotor):

  ∇ × E → = − ∇ × ∂ A → ∂ t . (\displaystyle \nabla \times (\vec (E))=-\nabla \times (\frac (\partial (\vec (A)))(\partial t)).)

  Hence, since ∇ × E → (\displaystyle \nabla \times (\vec (E))) is completely determined by the right side of the last equation, it is clear that the vortex part of the electric field (the part that has a rotor, in contrast to the irrotational field ∇ φ (\displaystyle \nabla \varphi )) is completely determined by the expression

  − ∂ A → ∂ t . (\displaystyle -(\frac (\partial (\vec (A)))(\partial t)).)

  Those. in the absence of a vortex-free part, we can write

  E → = − ∂ A → ∂ t , (\displaystyle (\vec (E))=-(\frac (\partial (\vec (A)))(\partial t)))

  but in general

  E → = − ∇ φ − d A → d t . (\displaystyle (\vec (E))=-\nabla \varphi -(\frac (d(\vec (A)))(dt)).) 1831 came a triumph: he discovered the phenomenon of electromagnetic induction. The setup on which Faraday made his discovery was that Faraday made a soft iron ring about 2 cm wide and 20 cm in diameter and wound many turns of copper wire around each half of the ring. The circuit of one winding was closed by a wire, in its turns there was a magnetic needle, removed so that the effect of the magnetism created in the ring did not affect. A current was passed through the second winding from a battery of galvanic cells. When the current was turned on, the magnetic needle made several oscillations and calmed down; when the current was interrupted, the needle oscillated again. It turned out that the arrow deviated in one direction when the current was turned on and in the other when the current was interrupted. M. Faraday found that it is possible to "transform magnetism into electricity" with the help of an ordinary magnet.

  At the same time, the American physicist Joseph Henry also successfully conducted experiments on the induction of currents, but while he was about to publish the results of his experiments, M. Faraday's message about his discovery of electromagnetic induction appeared in the press.

  M. Faraday sought to use the phenomenon he had discovered in order to obtain a new source of electricity.

  So far, we have considered electric and magnetic fields that do not change with time. It was found that the electric field is created by electric charges, and the magnetic field - by moving charges, that is, by electric current. Let's move on to getting acquainted with electric and magnetic fields, which change with time.

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

  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 - Faraday. Faraday was confident in the unified nature of electrical and magnetic phenomena. Thanks to this, he made a discovery, which later formed the basis for the design of generators of all power plants in the world, converting mechanical energy into electric current energy. (Other sources: galvanic cells, batteries, etc. - provide a negligible share of the generated energy.)

  Electric current, Faraday reasoned, is capable of magnetizing a piece of iron. Could a magnet in turn cause an electric current?

  For a long time, this connection could not be found. It was difficult to think of the main thing, namely: only a moving magnet or a magnetic field changing in time can excite an electric current in the coil.

  What kind of accidents could prevent the discovery, shows the following fact. Almost simultaneously with Faraday, the Swiss physicist Colladon was trying to get an electric current in a coil using a magnet. When working, he used a galvanometer, the light magnetic needle of which was placed inside the coil of the device. To prevent the magnet from exerting a direct influence on the needle, the ends of the coil, into which Colladon pushed the magnet, hoping to get a current in it, were led into the next room and connected there to the galvanometer. Having inserted the magnet into the coil, Colladon went into the next room and, with chagrin,

  

  made sure that the galvanometer does not show current. If only he had watched the galvanometer all the time and asked someone to work on the magnet, a remarkable discovery would have been made. But this did not happen. A magnet at rest relative to a coil causes no current in it.

  The phenomenon of electromagnetic induction consists in the occurrence of an electric current in a conducting circuit, which either rests in a magnetic field that changes in time, or moves in a constant magnetic field in such a way that the number of magnetic induction lines penetrating the circuit changes. It was discovered on August 29, 1831. It is a rare case when the date of a new remarkable discovery is known so precisely. Here is a description of the first experiment given by Faraday himself:

  “A copper wire 203 feet long was wound on a wide wooden coil, and between the turns of it was wound a wire of the same length, but insulated from the first cotton thread. One of these spirals was connected to a galvanometer, and the other to a strong battery consisting of 100 pairs of plates ... When the circuit was closed, it was possible to notice a sudden, but extremely weak action on the galvanometer, and the same was noticed when the current stopped. With the continuous passage of current through one of the coils, it was not possible to note any effect on the galvanometer, or in general any inductive effect on the other coil, despite the fact that the heating of the entire coil connected to the battery, and the brightness of the spark jumping between the coals, testified to battery power "(Faraday M. "Experimental research on electricity", 1st series).

  So, initially, induction was discovered in conductors that were motionless relative to each other during the closing and opening of the circuit. Then, clearly understanding that the approach or removal of conductors with current should lead to the same result as the closing and opening of the circuit, Faraday proved through experiments that the current arises when the coils move each other.

  

  relative to a friend. Familiar with the works of Ampère, Faraday understood that a magnet is a collection of small currents circulating in molecules. On October 17, as recorded in his laboratory journal, an induction current was detected in the coil during the insertion (or withdrawal) of the magnet. Within one month, Faraday experimentally discovered all the essential features of the phenomenon of electromagnetic induction.

  At present, Faraday's experiments can be repeated by everyone. To do this, you need to have two coils, a magnet, a battery of elements and a sufficiently sensitive galvanometer.

  In the installation shown in Figure 238, an induction current occurs in one of the coils when the electrical circuit of the other coil, which is stationary relative to the first, is closed or opened. In the installation in Figure 239, a rheostat changes the current in one of the coils. In Figure 240, a, the induction current appears when the coils move relative to each other, and in Figure 240, b - when the permanent magnet moves relative to the coil.

  Faraday himself already grasped the common thing that determines the appearance of an induction current in experiments that look different outwardly.

  In a closed conducting circuit, a current arises when the number of magnetic induction lines penetrating the area bounded by this circuit changes. And the faster the number of lines of magnetic induction changes, the greater the resulting induction current. In this case, the reason for the change in the number of lines of magnetic induction is completely indifferent. This can be a change in the number of lines of magnetic induction penetrating the area of ​​​​a fixed conductive circuit due to a change in the current strength in an adjacent coil (Fig. 238), and a change in the number of lines of induction due to the movement of the circuit in an inhomogeneous magnetic field, the density of lines of which varies in space (Fig. 241).