The phenomenon of electromagnetic induction lies in the fact that with any change in the magnetic flux penetrating the circuit of a closed conductor, an electric current is formed in this conductor, which exists throughout the entire process of changing the magnetic flux. The phenomenon of electromagnetic induction can be detected in the following situations:
1. with relative movement of the coil and magnet;
2. when the magnetic field induction changes in a circuit that is located perpendicular to the magnetic field lines.
In this picture the coil A, which is included in the current source circuit, is inserted into another coil WITH which is connected to the galvanometer. When closing and opening the coil circuit A in a reel WITH an induction current is formed. Induction current also occurs when the current in the coil changes WITH or when the coils move relative to each other;
3. when changing the position of a circuit located in a constant magnetic field.
Current in the circuit can also appear when the circuit rotates in the field of a permanent magnet (Fig. A), and when the magnet itself rotates inside the circuit (Fig. b).
The discovery of electromagnetic induction is one of the most significant discoveries of the 19th century. It caused the emergence and rapid development of electrical engineering and radio engineering.
Based on the phenomenon of electromagnetic induction, powerful electrical energy generators were founded, in the development of which scientists and technicians from different countries took part. Among them were Russian scientists: Emilius Khristianovich Lenz, Boris Semenovich Jacobi, Mikhail Iosifovich Dolivo-Dobrovolsky and others, who made a great contribution to the development of electrical engineering.
Electromagnetic induction was discovered by Faraday in 1831.
To demonstrate this phenomenon, let us take a stationary magnet and a wire coil, the ends of which will be connected to a galvanometer. If the coil is brought closer to one of the poles of the magnet, then during movement the galvanometer needle deflects - an electric current is excited in the coil. When the coil moves in the opposite direction, the direction of the current is reversed. The same thing happens if you rotate the magnet 180 degrees without changing the direction of movement of the coil.
The excitation of an electric current when a conductor moves in a magnetic field is explained by the action of the Lorentz force that occurs when the conductor moves.
Let's consider the case when two parallel wires AB and CD are closed, and on the right are open. The conductive bridge BC can slide freely along the wires. When the bridge moves to the right with speed v, electrons and positive ions move with it. Each moving charge in a magnetic field is acted upon by the Lorentz force 
. It acts downward on positive ions and upward on negative ones. As a result, electrons will begin to move upward along the bridge, i.e. An electric current will flow through it, directed downward. Having redistributed the charges, they will create an electric field, which will excite currents in other parts of the ABCD circuit.
The Lorentz force F in experiment plays the role of an external force that excites an electric current.
02. Electromotive force of induction(EMF) is a scalar physical quantity that characterizes the work of external forces in direct or alternating current sources.
The minus sign is placed because it is a third-party field 
directed against positive circuit bypass.
The value lv is the increment in the area of the ABCD contour per unit time, or the rate of increment of this area. Therefore it is equal to 
Basic law of electromagnetic induction. (Differential form of the law of electromagnetic induction)
When a closed wire moves in a magnetic field, an electromotive force is excited in it, proportional to the rate of increase of the magnetic flux penetrating the wire circuit.
03. Lenz's rule (Le Chatelier's principle)
The induced current always has such a direction that it weakens the action of the cause that excites this current.
Let us take a closed coil of wire in a magnetic field, the positive direction of its circuit making up a right-handed system with the direction of the field. Let us assume that the magnetic flux F increases. Then, according to the formula 
, value 
will be negative, and the induced current in the coil will flow in the negative direction. Such a current, weakening the external magnetic field, will prevent the increase in magnetic flux. 
Let now the magnetic flux Ф decrease. Then the value 
will become positive, and the induced current in the coil will flow in a positive direction and will prevent the magnetic field and magnetic flux from decreasing.
04. Wire inductance.
Let's consider a thin closed wire through which a direct current I flows. Inside the wire, parallel to its axis, we draw an arbitrary closed mathematical contour s and set the positive direction on it. If there are no ferrimagnetic bodies in space, then the magnitude of B (magnetic field of the current) and Ф (magnetic flux) will be proportional to the current. 
here is the current strength in the Gaussian system of units, and is the current strength in the SGSM system.
Self-inductance, or self-inductance coefficient of a wire. It does not depend on the current strength, it is determined only by the size and configuration of the wire itself.
Electromagnetic induction- the phenomenon of the occurrence of electric current in a closed circuit when the magnetic flux passing through it changes. Electromagnetic induction was discovered by Michael Faraday on August 29, 1831. He discovered that the electromotive force (EMF) arising 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 is causing the flux change - a change in the magnetic field itself or the movement of the circuit (or part of it) in the magnetic field. The electric current caused by this emf is called induced current.
Encyclopedic YouTube
-
1 / 5
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 selected contour, = ∬ S B → ⋅ d S → , (\displaystyle =\iint \limits _(S)(\vec (B))\cdot d(\vec (S)),)- magnetic flux through the surface limited by this contour.The minus sign in the formula reflects Lenz's rule, named after the Russian physicist E. H. Lenz:
An induced current arising in a closed conducting circuit has such a direction that the magnetic field it creates counteracts the change in magnetic flux that caused the current.For a coil located 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)))- electric field strength, B → (\displaystyle (\vec (B)))- magnetic induction , S (\displaystyle S\ )- an arbitrary surface, - its boundary. Integration loop ∂ S (\displaystyle \partial S) implied 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 a change in the field itself over time without changing (moving) the boundaries of the circuit (for taking into account the latter, see below).
If, say, the magnetic field is constant, and the magnetic flux changes due to the movement of the boundaries of the circuit (for example, with an increase in its area), then the resulting EMF is generated by the forces holding 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 a moving conductor).
Potential form
When expressing the magnetic field through 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 entirely only by a change in the magnetic field, that is, by 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 details
Since the magnetic induction vector, by definition, is expressed through 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, reversing 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-hand 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 an irrotational part, we can write
E → = − ∂ A → ∂ t , (\displaystyle (\vec (E))=-(\frac (\partial (\vec (A)))(\partial t)),)and in the general case
E → = − ∇ φ − d A → d t . (\displaystyle (\vec (E))=-\nabla \varphi -(\frac (d(\vec (A)))(dt)).) In 1831, triumph came: he discovered the phenomenon of electromagnetic induction. The setup in which Faraday made his discovery involved Faraday making a ring of soft iron approximately 2 cm wide and 20 cm in diameter and winding many turns of copper wire on each half of the ring. The circuit of one winding was closed by a wire, in its turns there was a magnetic needle, removed enough so that the effect of magnetism created in the ring did not affect. Current from a battery of galvanic cells was passed through the second winding. 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 needle deviated in one direction when the current was turned on and in the other when the current was interrupted. M. Faraday established that it is possible to “convert magnetism into electricity” using 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, a message from M. Faraday appeared in the press about his discovery of electromagnetic induction.
M. Faraday sought to use the phenomenon he discovered to obtain a new source of electricity.
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 are talking about acquiring basic household conveniences.
In medieval Europe they 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 route are still the most convenient way to 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 buoyant 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 a huge amount of resources, and, above all, electricity. Any production or large-scale scientific research was carried out where three components were available: qualified personnel, the material itself with which to work and cheap electricity.
And this was possible where natural forces could impart a large torque to the rotor: rivers with large elevation differences, valleys with strong winds, faults with excess geomagnetic energy.
It is interesting that the modern method of generating 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 all the time and an electric current is generated.
Of course, the best material for the magnet and conductors has been selected, and the technology of the entire 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.
Michael Faraday (1791-1867) was born in London, in one of its poorest parts. His father was a blacksmith, and his mother was the daughter of a tenant farmer. When Faraday reached school age, he was sent to primary school. The course Faraday took here was very narrow and was limited only to learning to read, write and begin to count.
A few steps from the house in which the Faraday family lived, there was a bookshop, which was also a bookbinding establishment. This is where Faraday ended up, having completed his primary school course, when the question arose about choosing a profession for him. Michael was only 13 years old at this time. Already in his youth, when Faraday was just beginning his self-education, he sought to rely exclusively on facts and verify the messages of others with his own experiences.
These aspirations dominated him all his life as the main features of his scientific activity. Faraday began to carry out physical and chemical experiments as a boy at his first acquaintance with physics and chemistry. One day 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 invited Faraday to work with him as a secretary. Soon Davy went on a trip to Europe and took Faraday with him. Over the course of 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 physics laboratories in the world. From 1816 to 1818, Faraday published a number of small notes and short 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 published “The History of the Advances of Electromagnetism.” Already at this time, he formed a completely correct concept of the essence of the phenomenon of deflection of a magnetic needle under the influence of 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 was the first to liquefy gas, and at the same time established a simple but effective method for converting gases into 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 turned from physics to chemistry, and the result of his work in this area was the discovery of gasoline and sulfur-naphthalene acid.
In 1831, Faraday published a treatise “On a Special Kind of Optical Illusion,” which served as the basis for an excellent 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 themselves immortalize the name of their author. But the most important of Faraday's scientific works are his studies in the field 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.
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 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. As was his custom, Faraday began a series of experiments designed 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 cells, and the ends of the other to a sensitive galvanometer. When current was passed through the first wire,
Faraday turned all his attention to the galvanometer, expecting to notice from 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.
He immediately noticed that the galvanometer, remaining completely calm during the entire passage of current, begins to oscillate when the circuit itself is closed and when it is opened. It turned out that at the moment when a current is passed into the first wire, and also when this transmission stops, at 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), - induction, and a new type of this energy - inductive electricity.
Continuing his experiments, Faraday further discovered that simply bringing a wire twisted into a closed curve close to another through which a galvanic current flows is sufficient to excite an inductive current in the neutral wire in the direction opposite to the galvanic current, and that removing the neutral wire again excites an inductive current in it. the current is already in the same direction as the galvanic current flowing along a stationary 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 when the galvanic current closes and stops. These discoveries in turn gave rise to new ones. If it is possible to cause an inductive current by short-circuiting and stopping the galvanic current, then wouldn’t the same result be obtained by magnetizing and demagnetizing iron?
The work of Oersted and Ampere had already established the relationship between magnetism and electricity. It was known that iron becomes a magnet when an insulated wire is wound around it and a galvanic current passes through it, and that the magnetic properties of this iron cease as soon as the current stops.
Based on this, Faraday came up with this kind of experiment: two insulated wires were wound around an iron ring; with one wire wrapped around one half of the ring, and the other around the other. 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 quickly oscillated and then quickly stopped, that is, 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 strip. Instead of exciting magnetism in iron by galvanic current, he magnetized the iron by touching it to a permanent steel magnet. The result was the same: always in the wire wrapped around the iron! a 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 induced currents in the wire. In a word, magnetism, in the sense of exciting induction currents, acted in exactly the same way as galvanic current.
At that time, physicists were intensely interested in one mysterious phenomenon, discovered in 1824 by Arago and which could not be explained, despite; the fact that this explanation was intensely sought by such outstanding scientists of the time as Arago himself, Ampère, Poisson, Babage and Herschel.
The point was as follows. A magnetic needle, hanging freely, quickly comes to rest if a circle of non-magnetic metal is placed under it; If the circle is then put into rotation, the magnetic needle begins to move behind 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, 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 mystery, something beyond the limits of the natural.
Faraday, based on the above data, made the assumption that a circle of non-magnetic metal, under the influence of a magnet, during rotation is run around by inductive currents, which affect the magnetic needle and drag it along the magnet.
And indeed, by introducing the edge of a circle between the poles of a large horseshoe magnet and connecting the center and edge of the circle with a galvanometer with a wire, Faraday obtained a constant electric current when the circle rotated.
Following this, Faraday focused on another phenomenon that was then arousing general curiosity. As you know, if you sprinkle iron filings on a magnet, they group along certain lines called magnetic curves. Faraday, drawing attention to this phenomenon, gave the basis 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 in order to excite induced currents, the source’s approach and distance from the magnetic pole are not necessary. To excite currents, it is enough to cross the lines of magnetic force in a known manner.
Faraday's further work in the mentioned direction acquired, from a contemporary point of view, the character of something absolutely miraculous. At the beginning of 1832, he demonstrated a device 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 it 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 earthly magnetism, which caused inductive currents like an ordinary magnet or galvanic current. To more clearly show and prove this, Faraday undertook another experiment, which fully confirmed his considerations.
He reasoned that if a circle of non-magnetic metal, such as copper, rotating in a position in which it intersects the lines of magnetic force of an adjacent magnet, produces 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 earthly magnetism, must also give an inductive current.
And indeed, a copper circle rotated in a horizontal plane produced an inductive current that produced a noticeable deflection of the galvanometer needle. Faraday ended his series of studies in the field of electrical induction with the discovery, made in 1835, of the “inductive influence 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.
Russian physicist Emil Khristoforovich Lenz (1804-1861) gave a rule for determining the direction of induction current. “The induction current is always directed in such a way that the magnetic field it creates complicates or inhibits the movement causing induction,” notes A.A. Korobko-Stefanov in his article on electromagnetic induction. - For example, when a coil approaches a magnet, the resulting induced current has such a direction that the magnetic field it creates 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 induced currents accelerated the motion that caused them, then work would be created out of nothing. The coil itself, after a slight push, would rush towards the magnet, and at the same time the induction current would release heat in it. In reality, the induced current is created due to the work of bringing the magnet and the coil closer together.
Why does induced current occur? 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 penetrated by an alternating magnetic field perpendicular to the plane of the turn. An induced current naturally arises in the coil. Maxwell interpreted this experiment exceptionally boldly and unexpectedly.
When a magnetic field changes in space, according to Maxwell, a process arises for which the presence of a wire coil has no significance. The main thing here is the emergence of closed annular electric field lines, covering a changing magnetic field. Under the influence of the resulting electric field, electrons begin to move, and an electric current arises in the coil. A coil is simply a device that detects 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 back in 1832, which then formed the basis of this invention. In general, the discovery of electromagnetic induction is not without reason considered one of the most outstanding discoveries of the 19th century - the work of millions of electric motors and electric current generators all over the world is based on this phenomenon...
Source of information: Samin D.K. “One Hundred Great Scientific Discoveries.”, M.: “Veche”, 2002.
