The phenomenon of electromagnetic induction. Discovery, experience, application

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S arrow pointing. The current was, however, weak, but the principle found made it possible to subsequently build powerful generators. Without them, electricity would still be a luxury available to few people.
An electric current arises in a conducting closed loop if the loop is in an alternating magnetic field or moves in a time-constant field so that the number of magnetic induction lines penetrating the loop changes. This phenomenon is called electromagnetic induction.

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

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

Description of the experiments of Michael Faraday wooden block two copper wires are wound. 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 effect was observed 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 the experiments of Michael Faraday Another experiment consisted of recording current surges at the ends of a coil into which a permanent magnet was inserted. Faraday called such bursts "waves of electricity"

induced emf The induced emf that causes current surges ("waves of electricity") depends not on the magnitude of the magnetic flux, but on the rate of its change.

1. Determine the direction of the induction lines 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 moves into the ring, then Ф>0, if it moves out, then Ф 0, if it moves out, then Ф 0, if it moves out, then Ф 0, if it moves out, then Ф 0 , if extends, then F
3. Determine the direction of the induction lines magnetic field B created by the induction current (if Ф>0, then lines B and B are directed in opposite sides; 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 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 F

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 induced emf? Operating principle of which electrical apparatus based on the law of electromagnetic induction?

The phenomenon of electromagnetic induction was discovered by Mile Faraday in 1831. Even 10 years earlier, Faraday was thinking about a way to turn magnetism into electricity. He believed that the magnetic field and electric field must be connected somehow.

Discovery of electromagnetic induction

For example, using an electric field you can magnetize iron object. It should probably be possible to generate electric current using a magnet.

First, Faraday discovered the phenomenon of electromagnetic induction in conductors that are motionless relative to each other. When a current appeared in one of them, a current was also induced in the other coil. Moreover, in the future it disappeared, and appeared again only when the power to one coil was turned off.

After some time, Faraday proved through experiments that when a coil without current moves in a circuit relative to another, the ends of which are supplied with voltage, an electric current will also arise in the first coil.

The next experiment was the introduction of a magnet into the coil, and at the same time a current appeared in it. These experiments are shown in the following figures.

Faraday formulated the main reason for the appearance of current in a closed circuit. In a closed conducting circuit, current arises when the number of magnetic induction lines that penetrate this circuit changes.

The greater this change, the stronger the induced current. It does not matter how we achieve a change in the number of magnetic induction lines. For example, this can be done by moving a circuit in a non-uniform magnetic field, as happened in the experiment with a magnet or moving a coil. And we can, for example, change the current strength in a coil adjacent to the circuit, and the magnetic field created by this coil will change.

Statement of the law

Let's sum it up summary. The phenomenon of electromagnetic induction is the phenomenon of the occurrence of current in a closed circuit, when the magnetic field in which this circuit is located changes.

For a more precise formulation of the law of electromagnetic induction, it is necessary to introduce a quantity that would characterize the magnetic field - the flux of the magnetic induction vector.

Magnetic flux

The magnetic induction vector is designated by the letter B. It will characterize the magnetic field at any point in space. Now consider a closed loop bounding a surface with area S. Let's place it in a uniform magnetic field.

There will be a certain angle a between the normal vector to the surface and the magnetic induction vector. Magnetic fluxФ through a surface of area S is called physical quantity, equal to the product of the magnitude of the magnetic induction vector by the surface area and the cosine of the angle between the magnetic induction vector and the normal to the contour.

Ф = B*S*cos(a).

The product B*cos(a) is the projection of the vector B onto the normal n. Therefore, the form for magnetic flux can be rewritten as follows:

The unit of magnetic flux is the weber. Indicated by 1 Wb. A magnetic flux of 1 Wb is created by a magnetic field with an induction of 1 T through a surface area of ​​1 m^2, which is located perpendicular to the magnetic induction vector.

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

Most important fact, which was 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 fields, the variety of manifestations of electromagnetic forces would not be as extensive as they actually are. 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 taken by the founder of the concept of the electromagnetic field - Faraday. Faraday was confident in the unified nature of electrical and magnetic phenomena. Thanks to this, he made a discovery, which subsequently formed the basis for the design of generators for all power plants in the world, converting mechanical energy into electrical energy. (Other sources: galvanic cells, batteries, etc. - provide an insignificant share of the generated energy.)

An electric current, Faraday reasoned, can magnetize a piece of iron. Couldn't a magnet, in turn, cause an electric current?

For a long time this connection could not be discovered. It was difficult to figure out the main thing, namely: only a moving magnet or a time-varying magnetic field can excite an electric current in a coil.

The following fact shows what kind of accidents could have prevented the discovery. Almost simultaneously with Faraday, the Swiss physicist Colladon tried to produce 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. So that the magnet did not have a direct effect on the needle, the ends of the coil into which Colladon pushed the magnet, hoping to receive a current in it, were brought into the next room and there connected to a galvanometer. Having inserted the magnet into the coil, Colladon walked into the next room and, with chagrin,

I made sure that the galvanometer did not show any current. If he had only to watch the galvanometer all the time and ask someone to work on the magnet, a remarkable discovery would have been made. But this did not happen. A magnet at rest relative to the coil does not generate current in it.

The phenomenon of electromagnetic induction consists in the occurrence of an electric current in a conducting circuit, which is either at rest in a time-varying magnetic field 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 accurately. Here is a description of the first experiment given by Faraday himself:

“A copper wire 203 feet long was wound on a wide wooden spool, and between its turns was wound a wire of the same length, but insulated from the first with a 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, a sudden but extremely weak action was noticed on the galvanometer, and the same was noticed when the current stopped. With the continuous passage of current through one of the spirals, it was not possible to notice either an effect on the galvanometer, or at all any inductive effect on the other spiral, despite the fact that the heating of the entire spiral connected to the battery and the brightness of the spark jumping between the coals indicated battery power" (Faraday M. "Experimental Research in Electricity", 1st series).

So, initially, induction was discovered in conductors that are motionless relative to each other when closing and opening a circuit. Then, clearly understanding that bringing current-carrying conductors closer or further away should lead to the same result as closing and opening a circuit, Faraday proved through experiments that current arises when the coils move each other.

regarding a friend. Familiar with the works of Ampere, Faraday understood that a magnet is a collection of small currents circulating in molecules. On October 17, as recorded in his laboratory notebook, an induced current was detected in the coil while the magnet was being pushed in (or pulled out). Within one month, Faraday experimentally discovered all the essential features of the phenomenon of electromagnetic induction.

Currently, everyone can repeat Faraday's experiments. To do this, you need to have two coils, a magnet, a battery of elements and a fairly sensitive galvanometer.

In the installation shown in Figure 238, an induced current occurs in one of the coils when closing or opening electrical circuit another coil, stationary relative to the first. In the installation in Figure 239, the current strength in one of the coils is changed using a rheostat. In Figure 240, a, the induction current appears when the coils move relative to each other, and in Figure 240, b - when moving permanent magnet relative to the coil.

Faraday himself already grasped the general thing on which the appearance of an induction current depends in experiments that outwardly look different.

In a closed conducting circuit, a current arises when the number of magnetic induction lines piercing the area limited by this circuit changes. And the faster the number of magnetic induction lines changes, the greater the resulting induction current. In this case, the reason for the change in the number of magnetic induction lines is completely indifferent. This may be a change in the number of magnetic induction lines penetrating the area of ​​a stationary conducting circuit due to a change in the current strength in the adjacent coil (Fig. 238), or a change in the number of induction lines due to the movement of the circuit in a non-uniform magnetic field, the density of the lines of which varies in space (Fig. 241).

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 elementary 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 after completing his course primary school, 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.

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 scientific works Faraday's research is in the fields of electromagnetism and electrical induction. Strictly speaking, an important branch of physics that treats the phenomena of electromagnetism and inductive electricity, and which is currently of such enormous importance for technology, was created by Faraday out of nothing.

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 a current was passed through the first wire, Faraday turned all his attention to the galvanometer, expecting to notice by its vibrations the appearance of a current in the second wire. However, nothing of the kind happened: the galvanometer remained calm. Faraday decided to increase the current strength and introduced 120 galvanic elements into the circuit. The result was the same. Faraday repeated this experiment dozens of times and still with the same success. Anyone else in his place would have left the experiments convinced that the current passing through a wire has no effect on the neighboring wire. But Faraday always tried to extract from his experiments and observations everything that they could give, and therefore, not receiving a direct effect on the wire connected to the galvanometer, he began to look for side effects.

He immediately noticed that the galvanometer, remaining completely calm during the entire passage of current, began to oscillate when the circuit itself was closed and when it was opened. It turned out that at the moment when a current is passed into the first wire, and also when this transmission stops, a current is also excited in the second wire, 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. Secondary instantaneous currents caused by the influence of 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 re-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 was found electrical energy, in addition to previously known (friction and chemical processes), - induction, and the new kind This energy is 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 the latter, and that magnetic properties of this iron stop 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.

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. Case I 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 5th 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 scientists of that time as a mysterious riddle, something beyond 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 magnetic curves the name “lines of magnetic force” in 1831, which later 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.

Further work Faraday's efforts 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.

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 profound 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 circuits in the surrounding space power lines. 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...