Magnetic field is called a special type of matter, different from a substance, through which the effect of a magnet on other bodies is transmitted.
A magnetic field arises in the space surrounding moving electric charges and permanent magnets. It only affects moving charges. Under the influence of electromagnetic forces, moving charged particles are deflected
From its original path in a direction perpendicular to the field.
Magnetic and electric fields are inseparable and together form a single electromagnetic field. Every change electric field leads to the appearance of a magnetic field, and, conversely, any change in the magnetic field is accompanied by the appearance of an electric field. The electromagnetic field propagates at the speed of light, that is, 300,000 km / s.
The action of permanent magnets and electromagnets on ferromagnetic bodies, the existence and indissoluble unity of the poles of magnets and their interaction is well known (unlike poles attract, like poles repel). Similarly
with the earth's magnetic poles, the poles of magnets are called north and south.
The magnetic field is graphically depicted by magnetic lines of force, which set the direction of the magnetic field in space (Fig. 1). These lines have no beginning or end, i.e. are closed.
The lines of force of the magnetic field of a straight conductor are concentric circles that enclose the wire. The stronger the current, the stronger the magnetic field around the wire. With distance from the wire with current, the magnetic field weakens.
In the space surrounding a magnet or electromagnet, the direction from north pole to south. The more intense the magnetic field, the higher the density of lines of force.
The direction of the magnetic field lines is determined gimlet rule:.
Rice. 1. Magnetic field of magnets:
a - direct; b - horseshoe
Rice. 2. Magnetic field:
a - straight wire; b - inductive coil
If the screw is screwed in in the direction of the current, then the magnetic magnetic lines of force will be directed along the course of the screw (Fig. 2 a)
To obtain a stronger magnetic field, inductive coils with a wire winding are used. In this case, the magnetic fields of the individual turns of the inductive coil are added and their lines of force merge into a common magnetic flux.
Magnetic lines of force come out of the inductive coil
at the end where the current is directed counterclockwise, that is, this end is the north magnetic pole (Fig. 2, b).
When the direction of the current in the inductive coil changes, the direction of the magnetic field will also change.
Just as a resting electric charge acts on another charge through an electric field, electricity acts on another current by magnetic field... The action of a magnetic field on permanent magnets is reduced to its action on charges moving in the atoms of a substance and creating microscopic circular currents.
The doctrine of electromagnetism based on two provisions:
- the magnetic field acts on moving charges and currents;
- a magnetic field arises around currents and moving charges.
Interaction of magnets
Permanent magnet(or magnetic needle) is oriented along the Earth's magnetic meridian. The end that points to the north is called north pole(N) and the opposite end is south pole(S). Approaching two magnets to each other, we note that their like poles repel, and opposite ones attract ( rice. 1 ).
If we divide the poles by cutting the permanent magnet into two parts, then we will find that each of them will also have two poles, that is, it will be a permanent magnet ( rice. 2 ). Both poles - north and south - are inseparable from each other, equal.
The magnetic field created by the Earth or permanent magnets is depicted, like an electric field, by magnetic lines of force. A picture of the lines of force of the magnetic field of a magnet can be obtained by placing a sheet of paper above it, on which iron filings are poured in an even layer. Once in a magnetic field, the sawdust is magnetized - each of them has a north and south pole. The opposite poles tend to get closer to each other, but this is hampered by the friction of sawdust on the paper. If you tap the paper with your finger, the friction will decrease and the sawdust will be attracted to each other, forming chains that represent the lines of the magnetic field.
On rice. 3 shows the location in the field of a direct magnet of sawdust and small magnetic arrows indicating the direction of the magnetic field lines. This direction is taken as the direction of the north pole of the magnetic needle.
Oersted's experience. Magnetic field current 
V early XIX v. Danish scientist Oersted made an important discovery by discovering electric current acting on permanent magnets ... He placed a long wire near the magnetic needle. When passing a current through the wire, the arrow turned, trying to position itself perpendicular to it ( rice. 4 ). This could be explained by the occurrence of a magnetic field around the conductor.
The magnetic lines of force of the field created by a straight conductor with current are concentric circles located in a plane perpendicular to it, with centers at the point through which the current passes ( rice. 5 ). The direction of the lines is determined by the right screw rule:
If the screw is rotated in the direction of the field lines, it will move in the direction of the current in the conductor. .
The strength characteristic of the magnetic field is vector of magnetic induction B ... At each point, it is directed tangentially to the field line. The lines of the electric field begin at positive charges and end at negative ones, and the force acting on the charge in this field is directed tangentially to the line at each point. In contrast to the electric, the lines of the magnetic field are closed, which is due to the absence of "magnetic charges" in nature.
The magnetic field of the current is fundamentally no different from the field created by a permanent magnet. In this sense, the analogue of a flat magnet is a long solenoid - a coil of wire, the length of which is much greater than its diameter. The diagram of the lines of the magnetic field created by him, shown in rice. 6 , is similar to that for a flat magnet ( rice. 3 ). The circles indicate the cross-sections of the wire that forms the solenoid winding. Currents flowing through the wire from the observer are indicated by crosses, and currents in the opposite direction to the observer are indicated by dots. The same designations are adopted for the lines of the magnetic field when they are perpendicular to the plane of the drawing ( rice. 7 a, b).
The direction of the current in the solenoid winding and the direction of the magnetic field lines inside it are also related by the right-hand screw rule, which in this case is formulated as follows:
If you look along the axis of the solenoid, then the current flowing clockwise creates a magnetic field in it, the direction of which coincides with the direction of movement of the right screw ( rice. eight )
Based on this rule, it is easy to figure out that the solenoid shown on rice. 6 , the north pole is its right end, and the south pole is the left.
The magnetic field inside the solenoid is uniform - the magnetic induction vector has a constant value there (B = const). In this respect, the solenoid is like a flat capacitor, inside which a uniform electric field is created.
Force acting in a magnetic field on a conductor with current
It was experimentally found that a force acts on a conductor with a current in a magnetic field. In a uniform field, a straight conductor of length l, through which a current I flows, located perpendicular to the field vector B, experiences a force: F = I l B .
The direction of the force is determined left hand rule:
If the four outstretched fingers of the left hand are placed in the direction of the current in the conductor, and the palm is perpendicular to vector B, then the left thumb will indicate the direction of the force acting on the conductor (rice. nine ).
It should be noted that the force acting on a conductor with a current in a magnetic field is not directed tangentially to its lines of force, like an electric force, but is perpendicular to them. The magnetic force does not act on a conductor located along the lines of force.
The equation F = IlB allows you to quantify the induction of the magnetic field.
Attitude 
does not depend on the properties of the conductor and characterizes the magnetic field itself.
The modulus of the magnetic induction vector B is numerically equal to the force acting on a conductor of unit length located perpendicular to it, through which a current of one ampere flows.
In the SI system, the unit of magnetic field induction is tesla (T):
A magnetic field. Tables, diagrams, formulas
(Interaction of magnets, Oersted's experiment, vector of magnetic induction, direction of vector, principle of superposition. Graphic representation of magnetic fields, lines of magnetic induction. Magnetic flux, energy characteristic of the field. Magnetic forces, Ampere force, Lorentz force. Movement of charged particles in a magnetic field. Magnetic properties of matter, Ampere's hypothesis)
When connected to two parallel conductors of electrical current, they will attract or repel, depending on the direction (polarity) of the connected current. This is due to the appearance of a special kind of matter around these conductors. This matter is called the magnetic field (MF). Magnetic force is the force with which the conductors act on each other.
The theory of magnetism originated in antiquity, in the ancient civilization of Asia. In Magnesia, a special breed was found in the mountains, pieces of which could be attracted to each other. According to the name of the place, this breed was called "magnets". The bar magnet contains two poles. At the poles, its magnetic properties.
A magnet hanging on a string will show the sides of the horizon with its poles. Its poles will be turned north and south. The compass device operates on this principle. The opposite poles of two magnets attract, and the like poles repel.
Scientists have found that a magnetized arrow near a conductor deflects when an electric current passes through it. This suggests that an MP is formed around it.
The magnetic field affects:
Moving electric charges.
Substances called ferromagnets: iron, cast iron, their alloys.
Permanent magnets are bodies that have a common magnetic moment of charged particles (electrons).
1 - South pole of the magnet
2 - North pole of magnet
3 - MP by the example of metal filings
4 - Direction of the magnetic field
Lines of force appear when a permanent magnet approaches a paper sheet on which a layer of iron filings is poured. The figure clearly shows the locations of the poles with oriented lines of force.
Sources of magnetic field
- Time-varying electric field.
- Mobile charges.
- Permanent magnets.
Since childhood, we have known permanent magnets. They were used as toys that attracted various metal parts. They were attached to the refrigerator, they were embedded in various toys.
Electric charges that are in motion tend to have more magnetic energy than permanent magnets.
Properties
- The main hallmark and the property of the magnetic field is relativity. If you leave a charged body motionless in a certain frame of reference, and place a magnetic needle next to it, it will point to the north, and at the same time it will not "feel" an extraneous field, except for the field of the earth. And if the charged body begins to move near the arrow, then an MP will appear around the body. As a result, it becomes clear that the MF is formed only when a certain charge moves.
- A magnetic field is capable of influencing and influencing an electric current. It can be detected by monitoring the movement of charged electrons. In a magnetic field, particles with a charge will deflect, conductors with a flowing current will move. The frame with the current supply connected will begin to rotate, and the magnetized materials will move a certain distance. The compass arrow is most often colored in blue color... It is a strip of magnetized steel. The compass is always oriented to the north, since the Earth has an MP. The whole planet is like a big magnet with its poles.
The magnetic field is not perceived by human organs, and can only be recorded with special devices and sensors. It can be of a variable and permanent type. An alternating field is usually created by special inductors that operate on alternating current. A constant field is formed by a constant electric field.
rules
Consider the basic rules for depicting a magnetic field for various conductors.
Gimlet rule
The line of force is drawn in a plane that is located at an angle of 90 0 to the path of current movement in such a way that at each point the force is directed tangentially to the line.
To determine the direction of the magnetic forces, you need to remember the rule of the right-hand gimbal.
The drill should be positioned along the same axis with the current vector, the handle should be rotated so that the drill would move in the direction of its direction. In this case, the orientation of the lines is determined by rotating the gimbal handle.
Ring gimbal rule
The translational movement of the gimbal in the conductor, made in the form of a ring, shows how the induction is oriented, the rotation coincides with the current flow.
The lines of force have their continuation inside the magnet and cannot be open.
The magnetic fields of different sources are summed up with each other. In doing so, they create a common field.
Magnets with the same poles repel, and those with different ones attract. The value of the strength of interaction depends on the distance between them. As the poles approach, the force increases.
Magnetic field parameters
- Concatenation of threads ( Ψ ).
- The vector of magnetic induction ( V).
- Magnetic flux ( F).
The intensity of the magnetic field is calculated by the size of the magnetic induction vector, which depends on the force F, and is formed by the current I along a conductor having a length l: B = F / (I * l).
Magnetic induction is measured in Tesla (T), in honor of the scientist who studied the phenomena of magnetism and was engaged in their calculation methods. 1 T is equal to the induction of the magnetic flux by the force 1 N at length 1m straight conductor at an angle 90 0 to the direction of the field, with a current of one ampere:
1 T = 1 x H / (A x m).
Left hand rule
The rule finds the direction of the magnetic induction vector.
If the palm of the left hand is placed in the field so that the magnetic field lines enter the palm from the North Pole at 90 0, and 4 fingers are placed along the current flow, the thumb will show the direction of the magnetic force.
If the conductor is at a different angle, then the force will directly depend on the current and the projection of the conductor onto a plane at right angles.
The force does not depend on the type of conductor material and its cross section. If there is no conductor, and the charges move in a different medium, then the force will not change.
When the direction of the magnetic field vector in one direction of the same magnitude, the field is called uniform. Different environments affect the size of the induction vector.
Magnetic flux
The magnetic induction passing over a certain area S and limited to this area is a magnetic flux.
If the area has a slope at a certain angle α to the induction line, the magnetic flux decreases by the size of the cosine of this angle. Its largest value is formed when the area is located at a right angle to the magnetic induction:
F = B * S.
The magnetic flux is measured in a unit such as "Weber", which is equal to the flow of induction by the value 1 T by area in 1 m 2.
Flux linkage
This concept is used to create a total value of the magnetic flux, which is created from a certain number of conductors located between the magnetic poles.
In the case when the same current I flows through the winding with the number of turns n, the total magnetic flux formed by all the turns is flux linkage.
Flux linkage Ψ measured in webers, and equal to: Ψ = n * Ф.
Magnetic properties
Permeability determines how much the magnetic field in a particular environment is lower or higher than the induction of the field in a vacuum. A substance is called magnetized if it forms its own magnetic field. When a substance is placed in a magnetic field, it becomes magnetized.
Scientists have identified the reason why bodies get magnetic properties. According to the hypothesis of scientists, inside substances there are electric currents of microscopic magnitude. The electron has its own magnetic moment, which has a quantum nature, moves along a certain orbit in atoms. It is these small currents that determine the magnetic properties.
If currents move randomly, then the magnetic fields caused by them are self-compensating. The external field makes the currents ordered, therefore a magnetic field is formed. This is the magnetization of the substance.
Various substances can be classified according to the properties of interaction with magnetic fields.
They are divided into groups:
Paramagnets- substances with properties of magnetization in the direction of the external field, with a low possibility of magnetism. They have a positive field strength. These substances include ferric chloride, manganese, platinum, etc.
Ferrimagnets- substances with magnetic moments unbalanced in direction and value. They are characterized by the presence of uncompensated antiferromagnetism. Field strength and temperature affect their magnetic susceptibility (various oxides).
Ferromagnets- Substances with increased positive susceptibility, depending on tension and temperature (crystals of cobalt, nickel, etc.).
Diamagnetics- have the property of magnetization in the opposite direction of the external field, that is, a negative value of the magnetic susceptibility, independent of the strength. In the absence of a field, this substance will not have magnetic properties. These substances include: silver, bismuth, nitrogen, zinc, hydrogen and other substances.
Antiferromagnets
- have a balanced magnetic moment, as a result of which a low degree of magnetization of the substance is formed. When heated, they undergo a phase transition of the substance, in which paramagnetic properties arise. When the temperature drops below a certain limit, such properties will not appear (chromium, manganese).
The considered magnets are also classified into two more categories:
Soft magnetic materials
... They have a low coercive force. In low-power magnetic fields, they can saturate. During the process of magnetization reversal, they have insignificant losses. As a result, such materials are used for the production of cores for electrical devices operating on alternating voltage (, generator,).
Magnetically hard materials. They have an increased value of the coercive force. A strong magnetic field is required to re-magnetize them. Such materials are used in the production of permanent magnets.
The magnetic properties of various substances are used in technical projects and inventions.
Magnetic circuits
The combination of several magnetic substances is called a magnetic circuit. They are similarities and are defined by similar laws of mathematics.
Electrical devices, inductances, operate on the basis of magnetic circuits. In a functioning electromagnet, the flow flows through a magnetic circuit made of a ferromagnetic material and air, which is not a ferromagnet. The combination of these components is a magnetic circuit. Many electrical devices contain magnetic circuits in their design.
It is a force field that acts on electric charges and on bodies in motion and having a magnetic moment, regardless of the state of their motion. The magnetic field is part of the electromagnetic field.
The current of charged particles or the magnetic moments of electrons in atoms create a magnetic field. Also, the magnetic field arises as a result of certain temporary changes in the electric field.
The induction vector of the magnetic field B is the main force characteristic of the magnetic field. In mathematics, B = B (X, Y, Z) is defined as a vector field. This concept serves to define and concretize the physical magnetic field. In science, the vector of magnetic induction is often simply, for brevity, referred to as a magnetic field. Obviously, such an application allows for some free interpretation of this concept.
Another characteristic of the current magnetic field is the vector potential.
V scientific literature you can often find that as main characteristics magnetic field, in the absence of a magnetic medium (vacuum), the vector of the magnetic field strength is considered. Formally, such a situation is quite acceptable, since in a vacuum the vector of the magnetic field strength H and the vector of magnetic induction B coincide. At the same time, the vector of the magnetic field strength in a magnetic medium is not filled with the same physical meaning, and is a secondary value. On this basis, given the formal equality of these approaches for a vacuum, the systematic point of view considers the vector of magnetic induction is the main characteristic of the magnetic field of the current.
The magnetic field is certainly a special kind of matter. With the help of this matter, there is an interaction between possessing a magnetic moment and moving charged particles or bodies.
Special relativity considers magnetic fields as a consequence of the existence of the electric fields themselves.
Together, the magnetic and electric fields form an electromagnetic field. The manifestations of the electromagnetic field are light and electromagnetic waves.
The quantum theory of magnetic field considers magnetic interaction as a separate case of electromagnetic interaction. It is carried by a massless boson. A boson is a photon - a particle that can be thought of as the quantum excitation of an electromagnetic field.
A magnetic field is generated either by the current of charged particles, or by an electric field transforming in time space, or by the particles' own magnetic moments. For uniform perception, the magnetic moments of particles are formally reduced to electric currents.
Calculation of the value of the magnetic field.
Simple cases allow you to calculate the values of the magnetic field of a conductor with a current according to the Bio-Savart-Laplace law, or using the circulation theorem. In the same way, the value of the magnetic field can be found for a current arbitrarily distributed in a volume or space. Obviously, these laws are applicable for constant or relatively slowly changing magnetic and electric fields. That is, in cases where magnetostatics are present. More complex cases require calculating a value magnetic field current according to Maxwell's equations.
The manifestation of the presence of a magnetic field.
The main manifestation of a magnetic field is the effect on the magnetic moments of particles and bodies, on charged particles in motion. By the Lorentz force is called the force that acts on an electrically charged particle that moves in a magnetic field. This force has a constantly expressed perpendicular direction to the vectors v and B. It also has a proportional value to the charge of the particle q, the component of the velocity v, which is carried out perpendicular to the direction of the magnetic field vector B, and the value that expresses the induction of the magnetic field B. The Lorentz force according to The international system units has the following expression: F = q, in the CGS system of units: F = q / c
The cross product is displayed in square brackets.
As a result of the influence of the Lorentz force on charged particles moving along the conductor, the magnetic field can act on the conductor with current. The Ampere force is the force acting on the current-carrying conductor. The components of this force are the forces acting on individual charges that move inside the conductor.
The phenomenon of the interaction of two magnets.
The phenomenon of a magnetic field that we can meet in everyday life is called the interaction of two magnets. It is expressed in the repulsion of the same poles from each other and the attraction of opposite poles. From a formal point of view, describing the interaction between two magnets as the interaction of two monopoles is a rather useful, realizable and convenient idea. At the same time, a detailed analysis shows that in reality this is not a completely correct description of the phenomenon. The main unanswered question in this model is why monopoles cannot be split. Actually, it has been experimentally proven that any isolated body does not have a magnetic charge. Also, this model cannot be applied to a magnetic field created by a macroscopic current.
From our point of view, it is correct to assume that the force acting on a magnetic dipole located in an inhomogeneous field tends to unfold it in such a way that the magnetic moment of the dipole has the same direction as the magnetic field. However, there are no magnets that are subject to the cumulative force from the outside. uniform magnetic field current... Force that acts on a magnetic dipole with a magnetic moment m expressed by the following formula:
.
The force acting on the magnet from the side of the inhomogeneous magnetic field is expressed by the sum of all forces that are determined by this formula, and acting on the elementary dipoles that make up the magnet.
Electromagnetic induction.
In the case of a change in time of the flux of the magnetic induction vector through a closed loop, an EMF of electromagnetic induction is formed in this loop. If the circuit is stationary, it is generated by a vortex electric field, which arises as a result of changes in the magnetic field over time. When the magnetic field does not change with time and there are no changes in flux due to the movement of the conductor loop, then the EMF is generated by the Lorentz force.
Topic: Magnetic field
Prepared by: D.M. Baygarashev
Checked by: A.T. Gabdullina
A magnetic field
If two parallel conductors are connected to a current source so that an electric current passes through them, then, depending on the direction of the current in them, the conductors are either repelled or attracted.
The explanation of this phenomenon is possible from the standpoint of the emergence around the conductors of a special type of matter - a magnetic field.
The forces with which conductors interact with current are called magnetic.
A magnetic field- This is a special type of matter, a specific feature of which is the action on a moving electric charge, conductors with current, bodies with a magnetic moment, with a force depending on the charge velocity vector, the direction of the current in the conductor and on the direction of the magnetic moment of the body.
The history of magnetism goes back to deep antiquity, to the ancient civilizations of Asia Minor. It was on the territory of Asia Minor, in Magnesia, that rock was found, the samples of which were attracted to each other. According to the name of the area, such samples began to be called "magnets". Any magnet in the shape of a bar or horseshoe has two ends, which are called poles; it is in this place that its magnetic properties are most pronounced. If you hang the magnet on a string, one pole will always point north. The compass is based on this principle. The north-facing pole of a free-hanging magnet is called the north pole of a magnet (N). The opposite pole is called the south pole (S).
Magnetic poles interact with each other: like poles repel, and unlike poles attract. Similarly to the concept of an electric field surrounding an electric charge, the concept of a magnetic field around a magnet is introduced.
In 1820, Oersted (1777-1851) discovered that a magnetic needle located next to an electrical conductor deflects when current flows through the conductor, i.e., a magnetic field is created around the conductor with current. If we take a frame with a current, then the external magnetic field interacts with the magnetic field of the frame and exerts an orienting effect on it, that is, there is a position of the frame in which the external magnetic field exerts a maximum rotational effect on it, and there is a position when the torque forces is zero.
The magnetic field at any point can be characterized by a vector B, which is called vector of magnetic induction or magnetic induction at the point.
Magnetic induction B is a vector physical quantity that is the force characteristic of the magnetic field at a point. It is equal to the ratio of the maximum mechanical moment of forces acting on a frame with current, placed in a uniform field, to the product of the current in the frame by its area:
For the direction of the magnetic induction vector B, the direction of the positive normal to the frame is taken, which is associated with the current in the frame by the rule of the right screw, with a mechanical moment equal to zero.
In the same way as the lines of the electric field strength are depicted, the lines of the magnetic field are depicted. The line of induction of a magnetic field is an imaginary line, the tangent to which coincides with the direction B at the point.
The direction of the magnetic field at a given point can also be defined as the direction that indicates
the north pole of the compass needle placed at this point. It is believed that the lines of induction of the magnetic field are directed from the north pole to the south.
The direction of the magnetic induction lines of the magnetic field created by the electric current that flows through the straight conductor is determined by the rule of the gimbal or the right screw. For the direction of the lines of magnetic induction, the direction of rotation of the screw head is taken, which would ensure its translational movement in the direction of the electric current (Fig. 59).
where n 01 = 4 Pi 10 -7 V s / (A m). - magnetic constant, R - distance, I - current strength in the conductor.
Unlike lines of intensity of an electrostatic field, which start at a positive charge and end at a negative one, the lines of induction of a magnetic field are always closed. No magnetic charge similar to electric charge was found.
One tesla (1 T) is taken as a unit of induction - the induction of such a uniform magnetic field, in which a maximum rotating mechanical moment of forces equal to 1 N m acts on a frame with an area of 1 m 2, through which a current of 1 A flows.
The induction of a magnetic field can also be determined by the force acting on a conductor with a current in a magnetic field.
An ampere force acts on a conductor with a current, placed in a magnetic field, the magnitude of which is determined by the following expression:
where I is the current in the conductor, l - the length of the conductor, V is the modulus of the magnetic induction vector, and is the angle between the vector and the direction of the current.
The direction of the Ampere force can be determined according to the left hand rule: we place the palm of the left hand so that the lines of magnetic induction enter the palm, place four fingers in the direction of the current in the conductor, then the bent thumb shows the direction of the Ampere force.
Taking into account that I = q 0 nSv, and substituting this expression in (3.21), we obtain F = q 0 nSh / B sin a... The number of particles (N) in a given volume of the conductor is equal to N = nSl, then F = q 0 NvB sin a.
Let us define the force acting from the side of the magnetic field on an individual charged particle moving in the magnetic field:
This force is called the Lorentz force (1853-1928). The direction of the Lorentz force can be determined according to the left hand rule: we place the palm of the left hand so that the lines of magnetic induction enter the palm, four fingers show the direction of movement of the positive charge, the big bent finger will show the direction of the Lorentz force.
The force of interaction between two parallel conductors through which currents I 1 and I 2 flow is equal to:
where l - part of a conductor in a magnetic field. If the currents are in one direction, then the conductors are attracted (Fig. 60), if they are in the opposite direction, they are repelled. The forces acting on each conductor are equal in magnitude, opposite in direction. Formula (3.22) is the basis for determining the unit of current 1 ampere (1 A).
The magnetic properties of a substance are characterized by a scalar physical quantity - magnetic permeability, which shows how many times the induction B of the magnetic field in a substance that completely fills the field differs in magnitude from the induction B 0 of the magnetic field in vacuum:
According to their magnetic properties, all substances are divided into diamagnetic, paramagnetic and ferromagnetic.
Consider the nature of the magnetic properties of substances.
Electrons in the shell of atoms of a substance move in different orbits. For simplicity, we consider these orbits to be circular, and each electron revolving around an atomic nucleus can be considered as a circular electric current. Each electron, like a circular current, creates a magnetic field, which we will call orbital. In addition, the electron in the atom has its own magnetic field, called spin.
If, when introduced into an external magnetic field with induction B 0, induction B is created inside the substance< В 0 , то такие вещества называются диамагнитными (n< 1).
V diamagnetic materials in the absence of an external magnetic field, the magnetic fields of electrons are compensated, and when they are introduced into a magnetic field, the induction of the magnetic field of the atom becomes directed against the external field. The diamagnet is pushed out of the external magnetic field.
Have paramagnetic materials, the magnetic induction of electrons in atoms is not fully compensated, and the atom as a whole turns out to be like a small permanent magnet. Usually in a substance, all these small magnets are oriented arbitrarily, and the total magnetic induction of all their fields is zero. If you place a paramagnet in an external magnetic field, then all the small magnets - atoms will turn in the external magnetic field like the arrows of a compass and the magnetic field in the substance is amplified ( n >= 1).
Ferromagnetic such materials are called in which n"1. In ferromagnetic materials, so-called domains are created, macroscopic regions of spontaneous magnetization.
In different domains, the inductions of magnetic fields have different directions (Fig. 61) and in a large crystal
mutually compensate each other. When a ferromagnetic sample is introduced into an external magnetic field, the boundaries of individual domains are displaced so that the volume of domains oriented along the external field increases.
With an increase in the induction of the external field B 0, the magnetic induction of the magnetized substance increases. At some values of B 0, the induction stops sharply increasing. This phenomenon is called magnetic saturation.
A characteristic feature of ferromagnetic materials is the phenomenon of hysteresis, which consists in the ambiguous dependence of the induction in the material on the induction of the external magnetic field when it changes.
The magnetic hysteresis loop is a closed curve (cdc`d`c), which expresses the dependence of the induction in the material on the amplitude of the induction of the external field with a periodic rather slow change in the latter (Fig. 62).
The hysteresis loop is characterized by the following values B s, B r, B c. B s - the maximum value of the induction of the material at B 0s; B r - residual induction, equal to the value of induction in the material with a decrease in the induction of the external magnetic field from B 0s to zero; -B c and B c - coercive force - a value equal to the induction of the external magnetic field required to change the induction in the material from residual to zero.
For each ferromagnet there is such a temperature (Curie point (J. Curie, 1859-1906), above which the ferromagnet loses its ferromagnetic properties.
There are two ways to bring a magnetized ferromagnet into a demagnetized state: a) heat above the Curie point and cool; b) magnetize the material with an alternating magnetic field with a slowly decreasing amplitude.
Ferromagnets with low residual induction and coercive force are called soft magnetic. They find application in devices where a ferromagnet often has to be remagnetized (cores of transformers, generators, etc.).
Hard magnetic ferromagnets with a high coercive force are used for the manufacture of permanent magnets.
