Magnetic fields arise in nature and can be created artificially. The person noticed their useful characteristics, which he learned to apply in everyday life. What is the source of the magnetic field?

How did the theory of the magnetic field develop

The magnetic properties of some substances were noticed in antiquity, but their real study began in medieval Europe. Using small steel needles, a scientist from France Peregrine discovered the intersection of magnetic lines of force at certain points - the poles. Only three centuries later, guided by this discovery, Gilbert continued his study and subsequently defended his hypothesis that the Earth has its own magnetic field.

The rapid development of the theory of magnetism began from the beginning of the 19th century, when Ampere discovered and described the influence of an electric field on the occurrence of a magnetic field, and Faraday's discovery of electromagnetic induction also established an inverse relationship.

What is magnetic field

A magnetic field manifests itself in a forceful effect on electric charges in motion, or on bodies that have a magnetic moment.

1. Conductors through which electric current flows;
2. Permanent magnets;
3. Changing electric field.

The root cause of the appearance of a magnetic field is identical for all sources: electric micro-charges - electrons, ions or protons have their own magnetic moment or are in directional motion.

Important! Electric and magnetic fields mutually generate each other, changing over time. This relationship is determined by Maxwell's equations.

Magnetic field characteristics

The characteristics of the magnetic field are:

1. Magnetic flux, a scalar quantity that determines how many lines of force of a magnetic field pass through a given cross section. It is designated by the letter F. Calculated by the formula:

F = B x S x cos α,

where B is the vector of magnetic induction, S is the section, α is the angle of inclination of the vector to the perpendicular drawn to the plane of the section. Measurement unit - weber (Wb);

1. The vector of magnetic induction (B) shows the force acting on the charge carriers. It is directed towards the north pole, where the usual magnetic needle points. Quantitatively, the magnetic induction is measured in teslas (T);
2. Tension MP (N). Determined by the magnetic permeability of various media. In a vacuum, permeability is taken as unity. The direction of the tension vector coincides with the direction of the magnetic induction. The unit of measurement is A / m.

How to imagine a magnetic field

It is easy to see the manifestation of a magnetic field on the example of a permanent magnet. It has two poles, and depending on the orientation, the two magnets attract or repel. The magnetic field characterizes the processes occurring in this case:

1. MP is mathematically described as a vector field. It can be constructed by means of many vectors of magnetic induction B, each of which is directed towards the north pole of the compass needle and has a length that depends on the magnetic force;
2. An alternative way to represent it is to use ley lines. These lines never intersect, do not start or stop anywhere, forming closed loops. MF lines merge in more frequent areas where the magnetic field is strongest.

Important! The density of the lines of force indicates the strength of the magnetic field.

Although the MT cannot be seen in reality, the lines of force can be easily visualized in the real world by placing iron filings in the MP. Each particle acts like a tiny magnet with a north and south pole. The result is a pattern similar to lines of force. A person is not able to feel the impact of MP.

Magnetic field measurement

Since this is a vector quantity, there are two parameters for measuring MF: strength and direction. Heading is easy to measure with a compass connected to the field. An example is a compass placed in the earth's magnetic field.

Measuring other characteristics is much more difficult. Practical magnetometers appeared only in the 19th century. Most of them work by using the force that the electron senses when moving along the MP.

Very accurate measurement of low magnetic fields has become feasible since the discovery in 1988 of giant magnetoresistance in layered materials. This discovery in fundamental physics was quickly applied to magnetic hard disk technology for storing data on computers, resulting in a thousandfold increase in storage capacity in just a few years.

In conventional measurement systems, MF is measured in tests (T) or in gauss (G). 1 T = 10000 G. Gauss is often used because Tesla is too large a field.

Interesting. A small magnet on the refrigerator creates a MF equal to 0.001 T, and the Earth's magnetic field on average is 0.00005 T.

The nature of the occurrence of the magnetic field

Magnetism and magnetic fields are manifestations of electromagnetic force. There are two possible ways how to organize the energy charge in motion and, consequently, the magnetic field.

The first is to connect the wire to a current source, a MF is formed around it.

Important! As the current (the number of charges in motion) increases, the MF increases proportionally. With distance from the wire, the field decreases depending on the distance. This is described by Ampere's law.

Some materials with higher magnetic permeability are capable of concentrating magnetic fields.

Since the magnetic field is a vector, it is necessary to determine its direction. For a normal current flowing through a straight wire, the direction can be found by the right-hand rule.

To use the rule, you must imagine that the wire is wrapped around the right hand, and the thumb indicates the direction of the current. Then the other four fingers will show the direction of the magnetic induction vector around the conductor.

The second way to create a magnetic field is to use the fact that electrons with their own magnetic moment appear in some substances. This is how permanent magnets work:

1. Although atoms often have many electrons, they generally bond so that the total magnetic field of the pair is canceled out. It is said that two electrons paired in this way have opposite spin. Therefore, in order to magnetize something, you need atoms that have one or more electrons with the same spin. For example, iron has four such electrons and is suitable for making magnets;
2. The billions of electrons in atoms can be randomly oriented, and there will be no total MF, no matter how many unpaired electrons the material has. It must be stable at low temperatures to provide an overall preferred orientation of the electrons. High magnetic permeability determines the magnetization of such substances under certain conditions outside the influence of MF. These are ferromagnets;
3. Other materials can exhibit magnetic properties in the presence of an external MF. The external field serves to align all electron spins, which disappears after the removal of the MF. These substances are paramagnets. Refrigerator door metal is an example of a paramagnet.

The earth can be represented in the form of capacitor plates, the charge of which has the opposite sign: "minus" - at the earth's surface and "plus" - in the ionosphere. Between them is atmospheric air as an insulating pad. The giant capacitor maintains a constant charge due to the influence of the Earth's MF. Using this knowledge, you can create a scheme for obtaining electrical energy from the Earth's magnetic field. True, the result will be low voltage values.

Have to take:

• grounding device;
• the wire;
• Tesla's transformer, capable of generating high-frequency oscillations and creating a corona discharge, ionizing the air.

Tesla's coil will act as an electron emitter. The whole structure is connected together, and the transformer must be raised to a considerable height to ensure a sufficient potential difference. Thus, an electrical circuit will be created through which a small current will flow. It is impossible to obtain a large amount of electricity using this device.

Electricity and magnetism dominate many worlds around humans: from the most fundamental processes in nature to cutting edge electronic devices.

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History of electricity

Electricity, a set of phenomena caused by the existence, movement and interaction of electrically charged bodies or particles. The interaction of electric charges is carried out using an electromagnetic field (in the case of stationary electric charges - an electrostatic field).

Moving charges (electric current), along with electric, also excite a magnetic field, that is, they generate an electromagnetic field through which electromagnetic interaction is carried out (the doctrine of magnetism is an integral part of the general doctrine of electricity). Electromagnetic phenomena are described by classical electrodynamics, which is based on Maxwell's equations

The laws of the classical theory of electricity cover a huge set of electromagnetic processes. Among the 4 types of interactions (electromagnetic, gravitational, strong and weak) existing in nature, electromagnetic ones take the first place in the breadth and variety of manifestations. This is due to the fact that all bodies are built of electrically charged particles of opposite signs, the interactions between which, on the one hand, are many orders of magnitude more intense than gravitational and weak ones, and on the other, they are long-range, in contrast to strong interactions. The structure of atomic shells, the cohesion of atoms into molecules (chemical forces) and the formation of condensed matter are determined by electromagnetic interaction.

The simplest electrical and magnetic phenomena have been known since ancient times. Minerals were found that attract pieces of iron, and it was also discovered that amber (Greek electron, elektron, hence the term electricity), rubbed against wool, attracts light objects (electrification by friction). However, it was not until 1600 that W. Hilbert first established the distinction between electrical and magnetic phenomena. He discovered the existence of magnetic poles and their inseparability from each other, and also established that the earth is a giant magnet.

In the 17th - 1st half of the 18th centuries. numerous experiments were carried out with electrified bodies, the first electrostatic machines based on electrification by friction were built, the existence of two kinds of electric charges was established (C. Dufay), and the electrical conductivity of metals was discovered (the English scientist S. Gray). With the invention of the first capacitor - the Leyden jar (1745) - it became possible to accumulate large electrical charges. In 1747-53, Franklin expounded the first consistent theory of electrical phenomena, finally established the electrical nature of lightning and invented a lightning rod.

In the 2nd half of the 18th century. a quantitative study of electrical and magnetic phenomena began. The first measuring instruments appeared - electroscopes of various designs, electrometers. G. Cavendish (1773) and C. Coulomb (1785) experimentally established the law of interaction of stationary point electric charges (Cavendish's works were published only in 1879).

This basic law of electrostatics (Coulomb's law) for the first time made it possible to create a method for measuring electric charges by the forces of interaction between them. Coulomb also established the law of interaction between the poles of long magnets and introduced the concept of magnetic charges concentrated at the ends of magnets.

The next stage in the development of the science of electricity is associated with the discovery at the end of the 18th century. L. Galvani "animal electricity" and works A. Volta, who invented the first source of electric current - a galvanic cell (the so-called volt pillar, 1800), which creates a continuous (constant) current for a long time. In 1802, V.V. Petrov, having built a galvanic cell of much higher power, discovered an electric arc, investigated its properties and pointed out the possibility of using it for illumination, as well as for melting and welding metals. G. Davy obtained (1807) previously unknown metals sodium and potassium by electrolysis of aqueous solutions of alkalis. J., P. Joule established (1841) that the amount of heat released in a conductor by an electric current is proportional to the square of the current; this law was substantiated (1842) by the exact experiments of E.H. Lenz (Joule-Lenz law).

G. Ohm established (1826) the quantitative dependence of the electric current on the voltage in the circuit. C.F. Gauss formulated (1830) the main theorem of electrostatics.

The most fundamental discovery was made by H. Oersted in 1820; he discovered the effect of an electric current on a magnetic needle - a phenomenon that indicated the connection between electricity and magnetism. Following this, in the same year, A.M. Ampere established the law of interaction of electric currents (Ampere's law). He also showed that the properties of permanent magnets can be explained on the basis of the assumption that constant electric currents (molecular currents) circulate in the molecules of magnetized bodies. Thus, according to Ampere, all magnetic phenomena are reduced to the interactions of currents, while magnetic charges do not exist. Since the discoveries of Oersted and Ampere, the doctrine of magnetism has become an integral part of the doctrine of electricity.

From the 2nd quarter of the XIX century. the rapid penetration of electricity into technology began. In the 20s. the first electromagnets appeared. One of the first applications of electricity was the telegraph, in the 30s and 40s. electric motors and current generators were built, and in the 40s - electric lighting devices, etc. The practical use of electricity in the future increased more and more, which in turn had a significant impact on the theory of electricity.

In the 30-40s. XIX century. M. Faraday made a great contribution to the development of the science of electricity - the creator of the general doctrine of electromagnetic phenomena, in which all electrical and magnetic phenomena are considered from a single point of view. With the help of experiments, he proved that the actions of electric charges and currents do not depend on the method of their production [before Faraday, they distinguished between "ordinary" (obtained by electrification by friction), atmospheric, "galvanic", magnetic, thermoelectric, "animal" and other types of E. ].

Arago's experiment ("rotation magnetism").

In 1831, Faraday discovered electromagnetic induction - the excitation of an electric current in a circuit located in an alternating magnetic field. This phenomenon (also observed in 1832 by J. Henry) is the foundation of electrical engineering. In 1833-34 Faraday established the laws of electrolysis; these works laid the foundation for electrochemistry. Later, trying to find the relationship between electrical and magnetic phenomena with optical ones, he discovered the polarization of dielectrics (1837), the phenomena of paramagnetism and diamagnetism (1845), the magnetic rotation of the plane of polarization of light (1845), etc.

Faraday was the first to introduce the concept of electric and magnetic fields. He rejected the concept of action at a distance, the supporters of which believed that bodies directly (through emptiness) act on each other at a distance.

According to Faraday's ideas, the interaction between charges and currents is carried out through intermediate agents: charges and currents create electric or (respectively) magnetic fields in the surrounding space, with the help of which the interaction is transmitted from point to point (the concept of short-range action). At the heart of his ideas about electric and magnetic fields was the concept of lines of force, which he considered as mechanical formations in a hypothetical medium - ether, similar to stretched elastic threads or cords.

Faraday's ideas about the reality of the electromagnetic field were not immediately recognized. The first mathematical formulation of the laws of electromagnetic induction was given by F. Neumann in 1845 in the language of the concept of action at a distance.

He also introduced the important concepts of the coefficients of self- and mutual induction of currents. The meaning of these concepts was fully revealed later, when W. Thomson (Lord Kelvin) developed (1853) the theory of electrical oscillations in a circuit consisting of a capacitor (electrical capacity) and a coil (inductance).
The creation of new devices and methods of electrical measurements, as well as a unified system of electrical and magnetic units of measurement created by Gauss and W. Weber, was of great importance for the development of the theory of electricity.

In 1846, Weber pointed out the relationship between current strength and the density of electric charges in a conductor and the speed of their ordered movement. He also established the law of interaction of moving point charges, which contained a new universal electrodynamic constant, which is the ratio of electrostatic and electromagnetic units of charge and has the dimension of speed.

In the experimental determination (Weber and F. Kohlrausch, 1856) this constant was obtained a value close to the speed of light; this was a definite indication of the connection between electromagnetic and optical phenomena.

In 1861-73, the theory of electricity was developed and completed in the works of J.C. Maxwell. Based on the empirical laws of electromagnetic phenomena and introducing the hypothesis of the generation of a magnetic field by an alternating electric field, Maxwell formulated the fundamental equations of classical electrodynamics, named after him. At the same time, he, like Faraday, considered electromagnetic phenomena as some form of mechanical processes in the ether.

The main new consequence arising from these equations is the existence of electromagnetic waves propagating at the speed of light. Maxwell's equations formed the basis of the electromagnetic theory of light. Maxwell's theory found decisive confirmation in 1886-89, when Hertz experimentally established the existence of electromagnetic waves. After its discovery, attempts were made to establish communication using electromagnetic waves, culminating in the creation of radio, and intensive research began in the field of radio engineering.

In the late XIX - early XX centuries. a new stage in the development of the theory of electricity began. Investigations of electric discharges were crowned with the discovery by J.J. Thomson of the discreteness of electric charges. In 1897 he measured the ratio of the electron charge to its mass, and in 1898 he determined the absolute value of the electron charge. H. Lorentz, relying on the discovery of Thomson and the conclusions of the molecular kinetic theory, laid the foundations of the electronic theory of the structure of matter. In the classical electronic theory, matter is considered as a collection of electrically charged particles, the motion of which is subject to the laws of classical mechanics. Maxwell's equations are obtained from the equations of electronic theory by statistical averaging.

Attempts to apply the laws of classical electrodynamics to the study of electromagnetic processes in moving media encountered significant difficulties. In an effort to resolve them, A. Einstein came (1905) to the relativity of the theory. This theory finally refuted the idea of ​​the existence of the ether, endowed with mechanical properties. After the creation of the theory of relativity, it became obvious that the laws of electrodynamics cannot be reduced to the laws of classical mechanics.

At small space-time intervals, the quantum properties of the electromagnetic field, which are not taken into account by the classical theory of electricity, become significant. The quantum theory of electromagnetic processes - quantum electrodynamics - was created in the second quarter of the XX century. The quantum theory of matter and field already goes beyond the theory of electricity, studies more fundamental problems concerning the laws of motion of elementary particles and their structure.

With the discovery of new facts and the creation of new theories, the importance of the classical doctrine of electricity did not diminish, only the limits of applicability of classical electrodynamics were determined. Within these limits, Maxwell's equations and classical electronic theory remain valid, being the foundation of the modern theory of electricity.

Classical electrodynamics forms the basis of most sections of electrical engineering, radio engineering, electronics and optics (with the exception of quantum electronics). With the help of her equations, a huge number of theoretical and applied problems were solved. In particular, numerous problems of plasma behavior in laboratory conditions and in space are solved using Maxwell's equations.

One of the first drawings of a magnetic field (René Descartes, 1644). Although magnets and magnetism were known much earlier, the study of the magnetic field began in 1269, when the French scientist Peter Peregrine (knight Pierre of Mericourt) noted the magnetic field on the surface of a spherical magnet using steel needles and determined that the resulting magnetic field lines intersected in two points, which he called "poles" by analogy with the poles of the Earth. Nearly three centuries later, William Gilbert Colchester used the work of Peter Peregrine and for the first time definitely declared that the Earth itself is a magnet. Published 1600, work by Gilbert "De Magnete", laid the foundations of magnetism as a science.

In 1750, John Michell stated that magnetic poles attract and repel according to the inverse square law. Charles-Augustin de Coulomb tested this claim experimentally in 1785 and stated bluntly that the North and South Poles cannot be separated. Based on this force between the poles, Simeon Denis Poisson (1781-1840) created the first successful model of a magnetic field, which he presented in 1824. In this model, the magnetic H-field is produced by the magnetic poles and magnetism occurs due to several pairs (north / south) of magnetic poles (dipoles).

Three consecutive discoveries have challenged this "foundation of magnetism." First, in 1819, Hans Christian Oersted discovered that an electric current creates a magnetic field around itself. Then, in 1820, André-Marie Ampere showed that parallel wires carrying current in the same direction attract each other. Finally, Jean-Baptiste Biot and Felix Savard in 1820 discovered a law called the Biot-Savard-Laplace law, which correctly predicted the magnetic field around any energized wire.

Expanding on these experiments, Ampere published his own successful model of magnetism in 1825. In it, he showed the equivalence of electric current in magnets, and instead of magnetic charge dipoles of the Poisson model, he proposed the idea that magnetism is associated with constantly flowing current loops. This idea explained why a magnetic charge could not be isolated. In addition, Ampere deduced a law named after him, which, like the Bio-Savart-Laplace law, correctly described the magnetic field created by direct current, and also introduced the theorem on the circulation of the magnetic field. Also in this work, Ampere introduced the term "electrodynamics" to describe the relationship between electricity and magnetism. In 1831, Michael Faraday discovered electromagnetic induction when he discovered that an alternating magnetic field generates electricity. He created a definition for this phenomenon, which is known as Faraday's law of electromagnetic induction. Later, Franz Ernst Neumann proved that for a moving conductor in a magnetic field, induction is a consequence of the action of Ampere's law. At the same time, he introduced the vector potential of the electromagnetic field, which, as was later shown, was equivalent to the basic mechanism proposed by Faraday. In 1850 Lord Kelvin, then known as William Thomson, designated the difference between the two magnetic fields as fields H and B... The first was applicable to the Poisson model and the second to the Ampere induction model. In addition, he deduced as H and B connected to each other. Between 1861 and 1865, James Clerk Maxwell developed and published Maxwell's equations that explained and combined electricity and magnetism in classical physics. The first collection of these equations was published in an article in 1861 entitled "On Physical Lines of Force"... These equations were found to be valid, albeit incomplete. Maxwell completed his equations in his later work of 1865 "Dynamic theory of the electromagnetic field" and determined that light is electromagnetic waves. Heinrich Hertz experimentally confirmed this fact in 1887. Although the magnetic field strength of a moving electric charge implied in Ampere's law was not explicitly stated, in 1892 Hendrik Lorentz derived it from Maxwell's equations. In this case, the classical theory of electrodynamics was basically completed.

The twentieth century expanded the views on electrodynamics, thanks to the emergence of the theory of relativity and quantum mechanics. Albert Einstein, in his 1905 article, where his theory of relativity was substantiated, showed that electric and magnetic fields are part of the same phenomenon, considered in different frames of reference - a thought experiment that ultimately helped Einstein to develop special theory of relativity ... Finally, quantum mechanics was combined with electrodynamics to form quantum electrodynamics (QED).

Magnetic field Magnetic phenomenon was first observed at least 2500 years ago Compass - about 4500 years ago

Magnets It was noticed that if you bring a permanent (natural) magnet to a piece of non-magnetized iron, then the iron also becomes magnetized. After removing the magnet, a piece of iron or steel magnetized under its action loses a significant part of its magnetic properties, but still remains more or less magnetized. Thus, it turns into an artificial magnet, which has all the same properties as a natural magnet.

Magnetic field S N N F F Scientists proposed to introduce the concept of a magnetic charge, like a north and south charge, similar to the poles of a magnet. However, experimentally there was no evidence of the existence of isolated magnetic charges, which are called magnetic monopole F F S S N N S N S S N N N S S N S F F

Oersted's experience N E W S in the 19th century discovered the connection between electricity and magnetism. n Oersted's experiments. n From these experiments it followed that a magnetic needle located near a conductor with a current is acted upon by forces that tend to turn the arrow N E W S

Part II MAGNETISM, WAVE AND QUANTUM OPTICS, ATOMIC AND NUCLEAR PHYSICS Magnetic field and its characteristics The most important feature of a magnetic field is that it acts only on electric charges moving in this field. The nature of the effect of a magnetic field on a current is different depending on the shape of the conductor through which the current flows, on the location of the conductor and on the direction of the current in it. The direction associated with the current by the right-hand screw rule is taken as the positive direction of the normal.

(33. 1) (33. 2) The magnetic induction B at a given point of the uniform magnetic field is determined by the maximum torque Mmax acting on the frame with a magnetic moment pm equal to unity when the normal to the frame n is perpendicular to the direction of the field. (33.3)

Ampere force (1/2) One of the important examples of magnetic interaction of currents is the interaction of parallel currents. these phenomena were experimentally established by Ampere. If electric currents flow along two parallel conductors in the same direction, then mutual attraction of the conductors is observed. When currents flow in opposite directions, the conductors are repelled.

§ 37 The action of a magnetic field on a moving charge The force acting on an electric charge q moving in a magnetic field with speed v is called the Lorentz force and is expressed by the formula (37.1) The direction of the Lorentz force is determined by the left hand rule. (37.2) - Lorentz formula

The movement of charged particles in a magnetic field 3. - the particle moves in a straight line, along the vecror B The work of the Lorentz force is zero

Mass spectrometer mass spectrometers are devices that can be used to measure the masses of charged particles - ions or nuclei of various atoms Modern mass spectrometers allow measuring the masses of charged particles with an accuracy of more than 10–4

Charged Particle Accelerators Charged particle accelerators are devices in which beams of high-energy charged particles are generated and controlled by electric and magnetic fields. Accelerators in terms of time of action are continuous and impulse. According to the shape of the trajectory and the mechanism of particle acceleration, accelerators are divided into linear, cyclic and induction. 1. Linear accelerator:, electric field - constant 2. Linear-resonant:, electric field - variable 3. Cyclotron:, limitations by relativistic effect

4. Phazotron: E - changes, 5. Synchrotron:, - changes, 6. Synchrophasotron: and - change, 7. Betatron: - vortex,

The Hall effect is the appearance in metals (or semiconductors) with a current density, placed in a magnetic field, of an electric field in the direction perpendicular to and.

§ 41 Circulation of the vector of magnetic induction for a magnetic field in vacuum Circulation of the vector of magnetic induction along a given contour is called the integral The law of total current: the circulation of the vector of magnetic induction along an arbitrary closed loop is equal to the product of the magnetic constant 0 by the algebraic sum of currents covered by this loop (41.1) The circulation of the magnetic induction vector is not zero, therefore, the magnetic field will be vortex.

Magnetic field of the solenoid and toroid On sections AB and CD On the section outside the solenoid (42.1) (42.2)

Flux of the vector of magnetic induction. Gauss's theorem for the magnetic field The flux of the vector of magnetic induction (magnetic flux) through the area d. S is called a scalar physical quantity equal to (43.1)

Gauss's theorem for a magnetic field: the flux of the magnetic induction vector through any closed surface is zero. (43.3) There are no magnetic charges in nature

Work on moving a conductor and a circuit with a current in a magnetic field (44. 1) Work on moving a conductor with a current in a magnetic field is equal to the product of the current strength and the magnetic flux crossed by the moving conductor.

(44. 2) (44. 3) (44. 4) (44. 5) (44. 6) The work of moving a closed loop with a current in a magnetic field is equal to the product of the current in the loop and the change in the magnetic flux coupled to the loop.

Phenomena of electromagnetic induction In 1831, M. Faraday discovered the phenomenon of electromagnetic induction 1. The directions of deflection of the arrow at the moment of pushing in and pulling out the magnet are opposite. 2. Deviations of the galvanometer needle are the greater, the greater the speed of the magnet relative to the coil. 3. When changing the poles of the magnet Experiment No. 1, the direction of deflection of the arrow changes. In a closed conducting circuit, when the magnetic induction flux covered by this circuit changes, an electric current arises, which is called induction.

Experiment No. 2 1. Deviations of the galvanometer needle are observed at the moment the current is switched on or off, at the moment of its increase or decrease, or when the coils move relative to each other. 2. Directions of deflection of the arrow of the galvanometer are also opposite when turning on or off the current, increasing or decreasing it, approaching and removing the coils.

Conclusion No. 1: Induction current occurs whenever there is a change in the magnetic induction flux coupled to the circuit (for example, when a conductive circuit is turned in a uniform magnetic field). Conclusion No. 2: The value of the induction current does not depend at all on the method of changing the flux of magnetic induction, but is determined only by the rate of its change. Values ​​of Faraday's discovery 1. The possibility of obtaining an electric current using a magnetic field has been proven. 2. The relationship between electrical and magnetic phenomena was established, which served as a further impetus for the development of the theory of the electromagnetic field.

Faraday's law Faraday's law of electromagnetic induction: whatever the reasons for the change in the flux of magnetic induction covered by a closed conducting loop, arising in the circuit E. D. C is equal to (46.1) Faraday's law: E. D. C electromagnetic induction in the circuit is numerically equal to and opposite in sign of the rate of change of the magnetic flux through the surface bounded by this contour.

Lenz's rule: the induction current in the circuit always has such a direction that the magnetic field it creates prevents the change in the magnetic flux that caused this induction current.

Rotation of the frame in a magnetic field (47.1) (47.2) If the frame rotates uniformly in a uniform magnetic field, then a variable EDS arises in it, changing according to a harmonic law.

Eddy currents (Foucault currents) Currents that arise in massive solid conductors and are closed in the thickness of the conductor are called eddy currents or Foucault currents. Calming (damping) moving parts of various devices

The Joule heat generated by Foucault currents is used in induction metallurgical furnaces. Skin effect Method of surface hardening of metals

Circuit inductance. Self-induction (49. 1) (49. 2) The emergence of EDS induction in a conducting circuit when the current strength changes in it is called self-induction. (49.3)

Opening and closing currents (50.1) The relaxation time is the time during which a physical quantity decreases by a factor of e.

Mutual induction (51. 1) The phenomenon of the emergence of EDS in one of the circuits when the current strength changes in the other is called mutual induction. (51.2)

Transformers The principle of operation of transformers used to increase or decrease AC voltage is based on the phenomenon of mutual induction. (52.1) (52.2) (52.3) - transformation ratio

Magnetic properties of substances Magnetic moments of electrons and atoms (54.1) (54.2) (54.3) (54.4)

Paramagnets and diamagnets Almost all substances obey the dependence can be divided into two classes: - - paramagnets, in which the magnetization of the substance increases the total magnetic field; , they are drawn into the region of a strong inhomogeneous magnetic field. - diamagnets, in which magnetization decreases the total field; diamagnets are pushed out of the region of a strong inhomogeneous field.

§ 56 Magnetization. Magnetic field in matter Magnetization is the magnitude of the magnetic moment per unit volume of matter (56.1) (56.2) (56.3)

(56.4) (56.5) (56.6) (56.7) (56.8) Paramagnets μ = 1.000072 Diamagnets μ = 0.9999967 Ferromagnets μ >> 1

(56. 9) (56. 10) Formula (56. 10) is a theorem on the circulation of the magnetic field strength vector.

Ferromagnets and their properties Ferromagnets are substances that have spontaneous magnetization, that is, they are magnetized even in the absence of an external magnetic field. 1. Ferromagnets are highly magnetic substances.

4. Hysteresis saturation point residual induction coercive force saturation point 5. Curie point - the temperature above which ferromagnetic properties disappear and the substance becomes a paramagnet.

For iron, this Curie temperature is 768 C, and for nickel - 365 C. The transition of ferromagnets to the paramagnetic state is a second-order phase transition. 6. The process of magnetization of ferromagnets is accompanied by a change in its linear dimensions and volume. This phenomenon is called magnetostriction.

The nature of ferromagnetism. P. Weiss's theory of ferromagnetism Ferromagnets below the Curie point break into a large number of small microscopic regions - domains, spontaneously magnetized to saturation. The linear dimensions of the domains are 10 -4 10 -2 cm.

Electromagnetic oscillations and waves Free harmonic oscillations in an oscillatory circuit An oscillatory circuit is a circuit consisting of coils with inductance L, capacitor C and resistor R, connected in series.

Experimental generation of electromagnetic waves Wave frequency, Hz Radiation source 103 - 10–4 3 105 - 3 1012 Oscillatory circuit Hertz vibrator Mass emitter Lamp generator Light waves: Infrared radiation 5 10–4 - 8 10–7 6 1011 - 3, 75 1014 Visible light 8 10–7 - 4 10–7 3, 75 1014 - 7, 5 1014 4 10– 7 - 10–9 7, 5 1014 - 3 1017 2 10–9 - 6 10–12 1, 5 1017 - 5 1019 Type of radiation Radio waves Ultraviolet radiation X-ray radiation - radiation Wavelength, m 5 ∙ 1019 Lamps Lasers X-ray tube Cosmic rays Radioactive decay Nuclear processes Space processes

It follows from Maxwell's equations that the alternating magnetic field is always associated with the electric field generated by it, and the alternating electric field is always associated with the magnetic field generated by it; that is, the electric and magnetic fields are inextricably linked with each other - they form a single electromagnetic field. Maxwell's theory made it possible to predict the existence of electromagnetic waves - an alternating electromagnetic field propagating in space with a finite speed.