Essay
in the discipline "Physics"
Topic: “Discovery of the phenomenon electromagnetic induction»
Completed:
Student of group 13103/1
Saint Petersburg
2. Faraday's experiments. 3
3. Practical use phenomena of electromagnetic induction. 9
4. List of used literature... 12
Electromagnetic induction is 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 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 (EMF) does not depend on what is causing the change in flow - the change in the flow itself magnetic field or movement of a circuit (or part of it) in a magnetic field. The electric current caused by this emf is called induced current.
In 1820, Hans Christian Oersted showed that an electric current flowing through a circuit causes a magnetic needle to deflect. If electric current generates magnetism, then the appearance of electric current must be associated with magnetism. This idea captured the English scientist M. Faraday. “Convert magnetism into electricity,” he wrote in his diary in 1822.
Michael Faraday
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 in him all his life as the main features of his scientific activity Physical and chemical experiments Faraday began to do this 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 next 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 in 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.
Faraday's experiments
Obsessed with ideas about the inextricable connection and interaction of the forces of nature, Faraday tried to prove that just as Ampere could create magnets with the help of electricity, so it was possible to create electricity with the help of magnets.
His logic was simple: mechanical work easily turns into heat; on the contrary, heat can be converted into mechanical work (say, steam engine). In general, among the forces of nature, the following relationship most often occurs: if A gives birth to B, then B gives birth to A.
If Ampere obtained magnets with the help of electricity, then, apparently, it is possible to “obtain electricity from ordinary magnetism.” Arago and Ampère set themselves the same task in Paris, and Colladon in Geneva.
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 when under ordinary conditions The presence of an electrified body is enough for its influence to excite electricity in any other body. At the same time, it was known that a wire through which current passes and which also represents an electrified body does not have any effect on other wires placed nearby.
What caused this exception? This is the question that interested Faraday and the solution of which led him to the most important discoveries in the field of induction electricity. Faraday carried out many experiments and kept pedantic notes. He devotes a paragraph to each small study in his laboratory notes (published in London in full in 1931 under the title “Faraday’s Diary”). Faraday’s ability to work is evidenced by the fact that the last paragraph of the “Diary” is marked with the number 16041. Faraday’s brilliant skill as an experimenter, obsession, and clear philosophical position could not but be rewarded, but it took eleven long years to wait for the result.
Apart from his intuitive conviction in the universal connection of phenomena, nothing actually supported him in his search for “electricity from magnetism.” Moreover, like his teacher Davy, he relied more on his experiences than on mental constructs. Davy taught him:
– A good experiment is more valuable than the profundity of a genius like Newton.
And yet, it was Faraday who was destined for great discoveries. A great realist, he spontaneously broke the empiricist shackles that Davy had once imposed on him, and at these moments a great insight dawned on him - he acquired the ability to make the deepest generalizations.
The first glimmer of luck appeared only on August 29, 1831. On this day, Faraday was testing a simple device in the laboratory: an iron ring with a diameter of about six inches, wrapped in two pieces of insulated wire. When Faraday connected a battery to the terminals of one winding, his assistant, artillery sergeant Andersen, saw the needle of the galvanometer connected to the other winding twitch.
It twitched and calmed down, although the direct current continued to flow through the first winding. Faraday carefully examined all the details of this simple installation - everything was in order.
But the galvanometer needle stubbornly stood at zero. Out of frustration, Faraday decided to turn off the current, and then a miracle happened - while opening the circuit, the galvanometer needle swung again and froze at zero again!
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, 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.
It was here that Ampere’s great ideas - the connection between electric current and magnetism - were revealed to Faraday in all their clarity. After all, the first winding into which he supplied current immediately became a magnet. If we consider it like a magnet, then the experiment on August 29 showed that magnetism seems to give birth to electricity. Only two things remained strange in this case: why did the surge of electricity when the electromagnet was turned on quickly fade away? And moreover, why does the splash appear when the magnet is turned off?
The next day, August 30, a new series of experiments. The effect is clearly expressed, but nevertheless completely incomprehensible.
Faraday senses that a discovery is somewhere nearby.
“Now I am again studying electromagnetism and I think that I have hit upon a successful thing, but I cannot yet confirm this. It may very well be that after all my labors I will end up with seaweed instead of fish.”
By the next morning, September 24, Faraday had prepared a lot various devices, in which the main elements were no longer windings with electric current, but permanent magnets. And there was an effect too! The arrow deviated and immediately rushed to the spot. This slight movement occurred during the most unexpected manipulations with the magnet, sometimes seemingly by accident.
The next experiment is October 1st. Faraday decides to go back to the very beginning - to two windings: one with current, the other connected to the galvanometer. The difference with the first experiment is the absence of a steel ring - core. The splash is almost unnoticeable. The result is trivial. It is clear that a magnet without a core is much weaker than a magnet with a core. Therefore, the effect is less pronounced.
Faraday is disappointed. For two weeks he does not go near the devices, thinking about the reasons for the failure.
“I took a cylindrical magnetic bar (3/4 inch in diameter and 8 1/4 inches long) and inserted one end of it into the spiral of copper wire(220 feet long) connected to a galvanometer. Then I quickly pushed the magnet inside the spiral to its entire length, and the galvanometer needle experienced a push. Then I just as quickly pulled the magnet out of the spiral, and the arrow swung again, but in the opposite direction. These swings of the needle were repeated every time the magnet was pushed or pushed out.”
The secret is in the movement of the magnet! The impulse of electricity is determined not by the position of the magnet, but by the movement!
This means that “an electric wave arises only when a magnet moves, and not due to the properties inherent in it at rest.”
Rice. 2. Faraday's experiment with a coil
This idea is incredibly fruitful. If the movement of a magnet relative to a conductor creates electricity, then apparently the movement of a conductor relative to a magnet should generate electricity! Moreover, this “electric wave” will not disappear as long as the mutual movement of the conductor and magnet continues. This means that it is possible to create an electric current generator that can operate for as long as desired, as long as the mutual movement of the wire and magnet continues!
On October 28, Faraday installed a rotating copper disk between the poles of a horseshoe magnet, from which, using sliding contacts (one on the axis, the other on the periphery of the disk), it was possible to remove electrical voltage. This was the first electric generator, created by human hands. Thus a new source was found electrical energy, in addition to the previously known ones (friction and chemical processes), is induction, and the new kind This energy is inductive electricity.
Experiments similar to Faraday's, as already mentioned, were carried out in France and Switzerland. Professor Colladon of the Academy of Geneva was a sophisticated experimenter (he, for example, made precise measurements of the speed of sound in water on Lake Geneva). Perhaps, fearing the shaking of the instruments, he, like Faraday, removed the galvanometer from the rest of the installation if possible. Many argued that Colladon observed the same fleeting movements of the needle as Faraday, but, expecting a more stable, long-lasting effect, did not attach due importance to these “random” bursts...
Indeed, the opinion of most scientists of that time was that the reverse effect of “creating electricity from magnetism” should apparently have the same stationary character as the “direct” effect - “formation of magnetism” due to electric current. The unexpected "fleetingness" of this effect confused many, including Colladon, and these many paid for their prejudice.
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.
Rice. 3. Faraday's experiment with an iron ring
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: in the wire that wrapped around the iron, a current was always excited at the moment of magnetization and demagnetization of the 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 such outstanding scientists of the time as Arago himself, Ampère, Poisson, Babage and Herschel were strenuously looking for this explanation. 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 approach and removal of the source from magnetic pole optional. To excite currents, it is enough to cross the lines of magnetic force in a known manner.
Rice. 4. “Magnetic Force Lines”
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, the copper circle, rotated in horizontal plane, gave 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 the coil approaches the 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.
Rice. 5. Lenz's rule
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 the complete mathematical theory 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 ring lines electric field, 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 earthly 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...
Practical application of the phenomenon of electromagnetic induction
1. Radio broadcasting
An alternating magnetic field excited by a changing current creates an electric field in the surrounding space, which in turn excites a magnetic field, etc. Mutually generating each other, these fields form a single alternating electromagnetic field - an electromagnetic wave. Having arisen in the place where there is a current-carrying wire, the electromagnetic field propagates through space at the speed of light -300,000 km/s.
Rice. 6. Radio
2. Magnetic therapy
In the frequency spectrum different places occupy radio waves, light, x-ray radiation and others electromagnetic radiation. They are usually characterized by continuously coupled electric and magnetic fields.
3. Synchrophasotrons
Currently, a magnetic field is understood as a special form of matter consisting of charged particles. In modern physics, beams of charged particles are used to penetrate deep into atoms in order to study them. The force with which a magnetic field acts on a moving charged particle is called the Lorentz force.
4. Flow meters
The method is based on the application of Faraday's law for a conductor in a magnetic field: in a flow of electrically conductive liquid moving in a magnetic field, an EMF is induced, proportional to the flow speed, converted by the electronic part into an electrical analogue/digital signal.
5. DC generator
In generator mode, the machine's armature rotates under the influence of an external torque. Between the stator poles there is a constant magnetic flux piercing anchor. The conductors of the armature winding move in a magnetic field and, therefore, an EMF is induced in them, the direction of which can be determined by the rule " right hand"In this case, a positive potential arises on one brush relative to the second. If a load is connected to the generator terminals, then current will flow through it.
6. Transformers
Transformers are widely used in transmitting electrical energy over long distances, distributing it between receivers, as well as in various rectifying, amplifying, signaling and other devices.
Energy conversion in a transformer is carried out by an alternating magnetic field. A transformer is a core made of thin steel plates insulated from one another, on which two and sometimes more windings (coils) of insulated wire are placed. Winding to which a source of electrical energy is connected alternating current, is called the primary winding, the remaining windings are called secondary.
If the secondary winding of a transformer has three times more turns wound than the primary winding, then the magnetic field created in the core by the primary winding, crossing the turns of the secondary winding, will create three times the voltage in it.
By using a transformer with a reverse turns ratio, you can just as easily obtain a reduced voltage.
List of used literature
1. [Electronic resource]. Electromagnetic induction.
< https://ru.wikipedia.org/>
2. [Electronic resource]. Faraday. Discovery of electromagnetic induction.
< http://www.e-reading.club/chapter.php/26178/78/Karcev_-_Maksvell.html >
3. [Electronic resource]. Discovery of electromagnetic induction.
4. [Electronic resource]. Practical application of the phenomenon of electromagnetic induction.
The phenomenon of electromagnetic induction is used primarily to convert mechanical energy into electrical energy. For this purpose they are used alternators(induction generators).
The simplest alternating current generator is a wire frame rotating uniformly at an angular velocity w=const in a uniform magnetic field with induction IN(Fig. 4.5). Magnetic induction flux penetrating a frame with an area S, is equal
When the frame rotates uniformly, the rotation angle 
, where is the rotation frequency. Then
According to the law of electromagnetic induction, the EMF induced in the frame of its rotation is
If you connect a load (electricity consumer) to the frame clamps using a brush-contact device, then alternating current will flow through it.
For industrial electricity production at power stations, they are used synchronous generators(turbogenerators, if the station is thermal or nuclear, and hydrogenerators, if the station is hydraulic). The stationary part of a synchronous generator is called stator, and rotating – rotor(Fig. 4.6). The generator rotor has a direct current winding (excitation winding) and is a powerful electromagnet. Direct current supplied to the excitation winding through a brush-contact apparatus magnetizes the rotor, and an electromagnet with north and south poles.
There are three alternating current windings located on the generator stator, which are shifted relative to each other by 120 0 and are connected to each other according to a specific connection circuit.
When the excited rotor rotates with the help of a steam or hydraulic turbine, its poles pass under the stator windings, and an electromotive force varying according to a harmonic law is induced in them. Next, the generator according to a certain scheme electrical network connects to power consumption nodes.
If you transfer electricity from station generators to consumers via power lines directly (at the generator voltage, which is relatively low), then large losses of energy and voltage will occur in the network (pay attention to the ratios , ). Therefore, to transport electricity economically, it is necessary to reduce the current strength. But since the transmitted power remains unchanged, the voltage must increase by the same amount as the current decreases.
The electricity consumer, in turn, needs to reduce the voltage to the required level. Electrical devices in which the voltage increases or decreases by a given number of times are called transformers.
The operation of a transformer is also based on the law of electromagnetic induction.
Let's consider the principle of operation of a two-winding transformer (Fig. 4.7). When alternating current passes through the primary winding, an alternating magnetic field with induction appears around it IN, whose flow is also variable 
. The transformer core serves to direct the magnetic flux (the magnetic resistance of the air is high). An alternating magnetic flux, closed through the core, induces an alternating EMF in each of the windings:
Then 
In powerful transformers, the coil resistances are very small, so the voltages at the terminals of the primary and secondary windings are approximately equal to the EMF:
Where k – transformation ratio. At k1 (
) transformer is downward.
When connected to the secondary winding of a load transformer, current will flow in it. With an increase in electricity consumption, according to the law of conservation of energy, the energy supplied by the station’s generators should increase, i.e.
where
This means that by increasing the voltage using a transformer k times, it is possible to reduce the current strength in the circuit by the same number of times (at the same time, Joule losses decrease by k 2 once).
Brief conclusions
- The phenomenon of the occurrence of EMF in a closed conducting circuit located in an alternating magnetic field is called electromagnetic induction.
2. According to the law of electromagnetic induction, the induced emf in a closed conducting circuit is numerically equal and opposite in sign to the rate of change of the magnetic flux through the surface bounded by this circuit:
The minus sign reflects Lenz's rule: with any change in the magnetic flux through a closed conducting loop, an induced current arises in the latter in such a direction that its magnetic field counteracts the change in the external magnetic flux.
The essence of the phenomenon of electromagnetic induction lies not so much in the appearance of an induction current, but in the appearance of a vortex electric field. The vortex electric field is generated by an alternating magnetic field. Unlike electrostatic field the vortex electric field is not potential, its lines of force are always closed, like power lines magnetic field.
The study of the origin of electric current has always excited scientists. After in early XIX century, the Danish scientist Oersted found out that a magnetic field arises around an electric current, scientists asked the question: can a magnetic field generate an electric current and vice versa. The first scientist who succeeded was the scientist Michael Faraday.
Faraday's experiments
After numerous experiments, Faraday was able to achieve some results.
1. The occurrence of electric current
To conduct the experiment, he took a coil with big amount turns and connected it to a milliammeter (a device that measures current). The scientist moved the magnet up and down the coil.
During the experiment, an electric current actually appeared in the coil due to a change in the magnetic field around it.
According to Faraday's observations, the milliammeter needle deviated and indicated that the movement of the magnet generated an electric current. When the magnet stopped, the arrow showed the zero marking, i.e. no current circulated through the circuit.
rice. 1 Change in current strength in the coil due to movement of the reactor This phenomenon, in which a current arises under the influence of an alternating magnetic field in a conductor, is called the phenomenon of electromagnetic induction.
2.Changing the direction of induction current
In his subsequent research, Michael Faraday tried to find out what influences the direction of the resulting induced electric current. While conducting experiments, he noticed that by changing the number of coils on a coil or the polarity of magnets, the direction of the electric current that arises in a closed network changes.
3.The phenomenon of electromagnetic induction
To conduct the experiment, the scientist took two coils, which he placed close to each other. The first coil, having a large number of turns of wire, was connected to a current source and a switch that opens and closes the circuit. He connected the second similar coil to a milliammeter without connecting it to a current source.
While conducting an experiment, Faraday noticed that when an electrical circuit is closed, an induced current appears, which can be seen by the movement of the milliammeter needle. When the circuit was opened, the milliammeter also showed that there was an electric current in the circuit, but the readings were exactly the opposite. When the circuit was closed and the current circulated evenly, there was no current in the electrical circuit according to the data of the milliammeter.
https://youtu.be/iVYEeX5mTJ8
Conclusion from experiments
As a result of Faraday's discovery, the following hypothesis was proven: electric current appears only when the magnetic field changes. It has also been proven that changing the number of turns in a coil changes the value of the current (increasing the number of coils increases the current). Moreover, an induced electric current can appear in a closed circuit only in the presence of an alternating magnetic field.
What does induction electric current depend on?
Based on all of the above, it can be noted that even if there is a magnetic field, this will not lead to the generation of electric current unless the field is alternating.
So what does the magnitude of the induction field depend on?
- Number of turns on the coil;
- The rate of change of the magnetic field;
- The speed of the magnet.
Magnetic flux is a quantity that characterizes a magnetic field. By changing, the magnetic flux leads to a change in the induced electric current.
Fig.2 Change in current strength when moving a) the coil in which the solenoid is located; b) a permanent magnet, inserting it into a coil Faraday's law
Based on his experiments, Michael Faraday formulated the law of electromagnetic induction. The law is that, when a magnetic field changes, it leads to the emergence of an electric current. Current also indicates the presence of an electromotive force, electromagnetic induction (EMF).
Speed magnetic current changing entails a change in current speed and EMF.
Faraday's law: The emf of electromagnetic induction is equal in number and opposite in sign to the rate of change of the magnetic flux that passes through the surface bounded by the contour
Loop inductance. Self-induction.
A magnetic field is created when current flows in a closed circuit. The current strength affects the magnetic flux and induces EMF.
Self-induction is a phenomenon in which an induced emf occurs when the current strength in the circuit changes.
Self-induction varies depending on the shape of the circuit, its size and the environment containing it.
As the electric current increases, the self-inductive current of the circuit can slow it down. When it decreases, the self-induction current, on the contrary, does not allow it to decrease so quickly. Thus, the circuit begins to have its own electrical inertia, slowing down any change in current.
Application of induced emf
The phenomenon of electromagnetic induction has practical applications in generators, transformers and motors running on electricity.
In this case, current for these purposes is obtained in the following ways:
- Change of current in the coil;
- Movement of magnetic field through permanent magnets and electromagnets;
- Rotation of turns or coils in a constant magnetic field.
The discovery of electromagnetic induction by Michael Faraday made a great contribution to science and to our everyday life. This discovery served as an impetus for further discoveries in the field of studying electromagnetic fields and has wide application in modern life of people.
We already know that an electric current moving through a conductor creates a magnetic field around it. Based on this phenomenon, man invented and widely uses a wide variety of electromagnets. But the question arises: if electric charges When moving, they cause the appearance of a magnetic field, but doesn’t it work the other way around?
That is, can a magnetic field cause the occurrence of an electric current in a conductor? In 1831, Michael Faraday established that an electric current arises in a closed conducting electrical circuit when a magnetic field changes. Such a current is called an induction current, and the phenomenon of the occurrence of a current in a closed conducting circuit when the magnetic field penetrating this circuit changes is called electromagnetic induction.
The phenomenon of electromagnetic induction
The name “electromagnetic” itself consists of two parts: “electro” and “magnetic”. Electrical and magnetic phenomena are inextricably linked with each other. And if electric charges, moving, change the magnetic field around them, then the magnetic field, changing, will inevitably force the electric charges to move, forming an electric current.
In this case, it is the changing magnetic field that causes the generation of electric current. A constant magnetic field will not cause the movement of electric charges, and accordingly, no induced current will be generated. A more detailed examination of the phenomenon of electromagnetic induction, the derivation of formulas and the law of electromagnetic induction refers to the ninth grade course.
Application of electromagnetic induction
In this article we will talk about the use of electromagnetic induction. The operation of many motors and current generators is based on the use of the laws of electromagnetic induction. The principle of their operation is quite simple to understand.
A change in the magnetic field can be caused, for example, by moving a magnet. Therefore, if you move a magnet inside a closed circuit by any external influence, then a current will arise in this circuit. This way you can create a current generator.
If, on the contrary, you pass current from a third-party source through the circuit, then the magnet located inside the circuit will begin to move under the influence of the magnetic field formed by the electric current. This way you can assemble an electric motor.
The current generators described above convert mechanical energy into electrical energy in power plants. Mechanical energy is the energy of coal, diesel fuel, wind, water and so on. Electricity travels through wires to consumers and is converted back into mechanical energy in electric motors.
Electric motors of vacuum cleaners, hair dryers, mixers, coolers, electric meat grinders and other numerous devices that we use every day are based on the use of electromagnetic induction and magnetic forces. There is no need to talk about the use of these same phenomena in industry; it is clear that it is everywhere.
The phenomenon of electromagnetic induction is used primarily to convert mechanical energy into electrical energy. For this purpose they are used alternators(induction generators). The simplest alternating current generator is a wire frame rotating uniformly at an angular velocity w= const in a uniform magnetic field with induction IN(Fig. 4.5). Magnetic induction flux penetrating a frame with an area S, is equal
When the frame rotates uniformly, the rotation angle 
, where is the rotation frequency. Then
According to the law of electromagnetic induction, the emf induced in the frame at
its rotation,
If you connect a load (electricity consumer) to the frame clamps using a brush-contact device, then alternating current will flow through it.
For industrial electricity production at power stations, they are used synchronous generators(turbogenerators, if the station is thermal or nuclear, and hydrogenerators, if the station is hydraulic). The stationary part of a synchronous generator is called stator, and rotating – rotor(Fig. 4.6). The generator rotor has a direct current winding (excitation winding) and is a powerful electromagnet. Direct current supplied to
The excitation winding through a brush-contact apparatus magnetizes the rotor, and in this case an electromagnet with north and south poles is formed.
There are three alternating current windings located on the generator stator, which are shifted relative to each other by 120 0 and are connected to each other according to a specific connection circuit.
When the excited rotor rotates with the help of a steam or hydraulic turbine, its poles pass under the stator windings, and an electromotive force varying according to a harmonic law is induced in them. Next, the generator is connected to electricity consumption nodes according to a certain electrical network diagram.
If you transfer electricity from station generators to consumers via power lines directly (at the generator voltage, which is relatively low), then large losses of energy and voltage will occur in the network (pay attention to the ratios , ). Therefore, to transport electricity economically, it is necessary to reduce the current strength. However, since the transmitted power remains unchanged, the voltage must
increase by the same amount as the current decreases.
The electricity consumer, in turn, needs to reduce the voltage to the required level. Electrical devices in which the voltage increases or decreases by a given number of times are called transformers. The operation of a transformer is also based on the law of electromagnetic induction.
Let's consider the principle of operation of a two-winding transformer (Fig. 4.7). When alternating current passes through the primary winding, an alternating magnetic field with induction appears around it IN, whose flow is also variable
The transformer core serves to direct the magnetic flux (the magnetic resistance of the air is high). An alternating magnetic flux, closed through the core, induces an alternating EMF in each of the windings:
Powerful transformers have very low coil resistances,
therefore, the voltages at the terminals of the primary and secondary windings are approximately equal to the EMF:
Where k – transformation ratio. At k<1 (
) transformer is increasing, at k>1 (
) transformer is downward.
When connected to the secondary winding of a load transformer, current will flow in it. With an increase in electricity consumption, according to the law
conservation of energy should increase the energy supplied by the station generators, that is
This means that by increasing the voltage using a transformer
V k times, it is possible to reduce the current strength in the circuit by the same number of times (at the same time, Joule losses decrease by k 2 times).
Topic 17. Fundamentals of Maxwell's theory for the electromagnetic field. Electromagnetic waves
In the 60s XIX century English scientist J. Maxwell (1831-1879) generalized the experimentally established laws of electric and magnetic fields and created a complete unified electromagnetic field theory. It allows you to decide the main problem of electrodynamics: find the characteristics of the electromagnetic field of a given system of electric charges and currents.
Maxwell hypothesized that any alternating magnetic field excites a vortex electric field in the surrounding space, the circulation of which is the cause of the emf of electromagnetic induction in the circuit:
(5.1)
Equation (5.1) is called Maxwell's second equation. The meaning of this equation is that a changing magnetic field generates a vortex electric field, and the latter in turn causes a changing magnetic field in the surrounding dielectric or vacuum. Since the magnetic field is created by an electric current, then, according to Maxwell, the vortex electric field should be considered as a certain current,
which occurs both in a dielectric and in a vacuum. Maxwell called this current displacement current.
Displacement current, as follows from Maxwell's theory
and Eichenwald's experiments, creates the same magnetic field as the conduction current.
In his theory, Maxwell introduced the concept total current, equal to the sum
conduction and displacement currents. Therefore, the total current density
According to Maxwell, the total current in a circuit is always closed, that is, at the ends of the conductors only the conduction current breaks, and in the dielectric (vacuum) between the ends of the conductor there is a displacement current that closes the conduction current.
Having introduced the concept of total current, Maxwell generalized the theorem on the circulation of a vector (or):
(5.6)
Equation (5.6) is called Maxwell's first equation in integral form. It represents a generalized law of total current and expresses the basic position of electromagnetic theory: displacement currents create the same magnetic fields as conduction currents.
The unified macroscopic theory of the electromagnetic field created by Maxwell made it possible from a unified point of view not only to explain electrical and magnetic phenomena, but to predict new ones, the existence of which was subsequently confirmed in practice (for example, the discovery electromagnetic waves).
Summarizing the provisions discussed above, we present the equations that form the basis of Maxwell’s electromagnetic theory.
1. Theorem on the circulation of the magnetic field strength vector:
This equation shows that magnetic fields can be created either by moving charges ( electric currents), or alternating electric fields.
2. Electric field can be both potential () and vortex (), therefore the total field strength 
. Since the circulation of the vector is zero, then the circulation of the vector of the total electric field intensity
This equation shows that the sources of the electric field can be not only electric charges, but also time-varying magnetic fields.
3. 
,
where is the volumetric charge density inside a closed surface; – specific conductivity of the substance.
For stationary fields ( E= const , B= const) Maxwell's equations take the form
that is, the sources of the magnetic field in this case are only
conduction currents, and the sources of the electric field are only electric charges. In this particular case, the electric and magnetic fields are independent of each other, which makes it possible to study separately permanent electric and magnetic fields.
Using the known ones from vector analysis Stokes and Gauss theorems, one can imagine complete system Maxwell's equations in differential form(characterizing the field at each point in space):
(5.7)
It is obvious that Maxwell's equations not symmetrical relative to electric and magnetic fields. This is due to the fact that in nature
There are electric charges, but there are no magnetic charges.
Maxwell's equations are the most general equations for electrical
and magnetic fields in quiescent media. They play the same role in the doctrine of electromagnetism as Newton's laws do in mechanics.
Electromagnetic wave called an alternating electromagnetic field propagating in space with a finite speed.
The existence of electromagnetic waves follows from Maxwell's equations, formulated in 1865 based on a generalization of the empirical laws of electrical and magnetic phenomena. An electromagnetic wave is formed due to the mutual connection of alternating electric and magnetic fields - a change in one field leads to a change in the other, that is, the faster the magnetic field induction changes over time, the greater the electric field strength, and vice versa. Thus, for the formation of intense electromagnetic waves, it is necessary to excite electromagnetic oscillations of a sufficiently high frequency. Phase speed electromagnetic waves is determined
electrical and magnetic properties Wednesday:
In a vacuum (), the speed of propagation of electromagnetic waves coincides with the speed of light; in matter, therefore The speed of propagation of electromagnetic waves in matter is always less than in vacuum.