Around which there is a magnetic field. How to represent a magnetic field. Magnetic field where to find it

magnetic field is called a special type of matter, different from substance, through which the action of a magnet is transmitted to other bodies.

A magnetic field occurs 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. Any change electric field leads to the appearance 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, i.e. 300,000 km/s.

Commonly known action permanent magnets and electromagnets on ferromagnetic bodies, the existence and inseparable unity of the poles of magnets and their interaction (opposite 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 visually depicted by magnetic lines of force, which set the direction of the magnetic field in space (Fig..1). These lines have neither beginning nor end, i.e. are closed.

The lines of force of the magnetic field of a straight conductor are concentric circles enclosing the wire. The stronger the current, the stronger the magnetic field around the wire. As you move away from a current-carrying wire, the magnetic field weakens.

In the space surrounding a magnet or an electromagnet, the direction from north pole to south. The stronger the magnetic field, the higher the density of field lines.

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 you screw in the screw in the direction of the current, then the magnetic magnetic lines of force will be directed along the screw (Fig. 2 a)

To obtain a stronger magnetic field, inductive coils with wire windings are used. In this case, the magnetic fields of individual turns of the inductive coil add up and their lines of force merge into a common magnetic flux.

Magnetic field lines coming out of an inductive coil

at the end where the current is directed counterclockwise, i.e. this end is 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.

Subject: Magnetic field

Prepared by: Baigarashev D.M.

Checked by: Gabdullina A.T.

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 either repel or attract.

The explanation of this phenomenon is possible from the standpoint of the appearance around the conductors of a special type of matter - a magnetic field.

The forces with which current-carrying conductors interact are called magnetic.

A magnetic field- This special kind 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 strength in the conductor and the direction of the magnetic moment of the body.

The history of magnetism goes back to ancient times, to the ancient civilizations of Asia Minor. It was on the territory of Asia Minor, in Magnesia, that they found rock, samples of which are attracted to each other. According to the name of the area, such samples began to be called "magnets". Any magnet in the form of a rod or a horseshoe has two ends, which are called poles; it is in this place that its magnetic properties are most pronounced. If you hang a 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 north pole magnet (N). The opposite pole is called south pole(S).

Magnetic poles interact with each other: like poles repel, and unlike poles attract. Similarly, the concept of an electric field surrounding an electric charge introduces the concept of a magnetic field around a magnet.

In 1820, Oersted (1777-1851) discovered that a magnetic needle located next to an electrical conductor deviates when current flows through the conductor, that is, a magnetic field is created around the current-carrying conductor. If we take a frame with current, then the external magnetic field interacts with the magnetic field of the frame and has an orienting effect on it, i.e., there is a position of the frame at which the external magnetic field has a maximum rotating effect on it, and there is a position when the torque force is zero.

The magnetic field at any point can be characterized by the vector B, which is called magnetic induction vector or magnetic induction at the point.

Magnetic induction B is a vector physical quantity, which is a 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 loop with current placed in a uniform field to the product of the current strength in the loop and its area:

The direction of the magnetic induction vector B is taken to be the direction of the positive normal to the frame, which is related to 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 electric field strength are depicted, the lines of magnetic field induction are depicted. The line of induction of the magnetic field is an imaginary line, the tangent to which coincides with the direction B at the point.

The directions 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 that 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 lines of magnetic induction of the magnetic field created by an electric current that flows through a straight conductor is determined by the rule of a gimlet or a right screw. The direction of rotation of the screw head is taken as the direction of the lines of magnetic induction, which would ensure its translational movement in the direction 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 electrostatic field lines, which start at a positive charge and end at a negative one, magnetic field lines 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 torque of 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 current-carrying conductor in a magnetic field.

A conductor with current placed in a magnetic field is subjected to the Ampère force, the value of which is determined by the following expression:

where I is the current strength in the conductor, l- the length of the conductor, B 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 by the rule of the left hand: we place the palm of the left hand so that the lines of magnetic induction enter the palm, we place four fingers in the direction of the current in the conductor, then bent thumb shows the direction of the ampere force.

Considering that I = q 0 nSv and substituting this expression into (3.21), we obtain F = q 0 nSh/B sin a. The number of particles (N) in a given volume of the conductor is N = nSl, then F = q 0 NvB sin a.

Let us determine the force acting from the side of the magnetic field on a separate charged particle moving in a magnetic field:

This force is called the Lorentz force (1853-1928). The direction of the Lorentz force can be determined by the rule of the left hand: the palm of the left hand is positioned so that the lines of magnetic induction enter the palm, four fingers show the direction of movement of the positive charge, the thumb 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- the part of a conductor that is in a magnetic field. If the currents are in the same direction, then the conductors are attracted (Fig. 60), if the opposite direction, they are repelled. The forces acting on each conductor are equal in magnitude, opposite in direction. Formula (3.22) is the main one for determining the unit of current strength 1 ampere (1 A).

The magnetic properties of a substance are characterized by a scalar physical quantity - magnetic permeability, showing how many times the induction B of a magnetic field in a substance that completely fills the field differs in absolute value from the induction B 0 of a 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 matter move in different orbits. For simplicity, we consider these orbits to be circular, and each electron revolving around the 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, an electron in an atom has its own magnetic field, called the spin field.

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).

IN diamagnetic In 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 an atom becomes directed against the external field. The diamagnet is pushed out of the external magnetic field.

At 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 matter all these small magnets are oriented arbitrarily, and the total magnetic induction of all their fields is equal to zero. If you place a paramagnet in an external magnetic field, then all small magnets - atoms will turn in the external magnetic field like compass needles and the magnetic field in the substance increases ( n >= 1).

ferromagnetic are materials that are n"1. So-called domains, macroscopic regions of spontaneous magnetization, are created in ferromagnetic materials.

In different domains, the induction of magnetic fields has 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 shifted 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. For some values ​​of B 0, the induction stops its sharp growth. 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 as it changes.

The magnetic hysteresis loop is a closed curve (cdc`d`c), expressing 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 the induction in the material when the induction of the external magnetic field decreases from B 0s to zero; -B c and B c - coercive force - a value equal to the induction of the external magnetic field necessary 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 has to be frequently remagnetized (cores of transformers, generators, etc.).

Magnetically hard ferromagnets, which have a large coercive force, are used for the manufacture of permanent magnets.

For a long time, the magnetic field has raised many questions in humans, but even now it remains a little-known phenomenon. Many scientists tried to study its characteristics and properties, because the benefits and potential of using the field were indisputable facts.

Let's take everything in order. So, how does any magnetic field act and form? That's right, electric current. And the current, according to physics textbooks, is a stream of charged particles with a direction, isn't it? So, when a current passes through any conductor, a certain kind of matter begins to act around it - a magnetic field. The magnetic field can be created by the current of charged particles or by the magnetic moments of electrons in atoms. Now this field and matter have energy, we see it in electromagnetic forces that can affect the current and its charges. The magnetic field begins to act on the flow of charged particles, and they change the initial direction of motion perpendicular to the field itself.

Another magnetic field can be called electrodynamic, because it is formed near moving particles and affects only moving particles. Well, it is dynamic due to the fact that it has special structure in rotating bions on a region of space. An ordinary electric moving charge can make them rotate and move. Bions transmit any possible interactions in this region of space. Therefore, the moving charge attracts one pole of all bions and causes them to rotate. Only he can bring them out of a state of rest, nothing else, because other forces will not be able to influence them.

In an electric field are charged particles that move very fast and can travel 300,000 km in just a second. Light has the same speed. There is no magnetic field without an electric charge. This means that the particles are incredibly closely related to each other and exist in a common electromagnetic field. That is, if there are any changes in the magnetic field, then there will be changes in the electric field. This law is also reversed.

We talk a lot about the magnetic field here, but how can you imagine it? We cannot see it with our human naked eye. Moreover, due to the incredibly fast propagation of the field, we do not have time to fix it with the help of various devices. But in order to study something, one must have at least some idea of ​​it. It is also often necessary to depict the magnetic field in diagrams. In order to make it easier to understand it, conditional field lines are drawn. Where did they get them from? They were invented for a reason.

Let's try to see the magnetic field with the help of small metal filings and an ordinary magnet. We will pour these sawdust on a flat surface and introduce them into the action of a magnetic field. Then we will see that they will move, rotate and line up in a pattern or pattern. The resulting image will show the approximate effect of forces in a magnetic field. All forces and, accordingly, lines of force are continuous and closed in this place.

The magnetic needle has similar characteristics and properties with a compass, and it is used to determine the direction of the lines of force. If it falls into the zone of action of a magnetic field, we can see the direction of action of forces by its north pole. Then we will single out several conclusions from here: the top of an ordinary permanent magnet, from which the lines of force emanate, is designated by the north pole of the magnet. Whereas the south pole denotes the point where the forces are closed. Well, the lines of force inside the magnet are not highlighted in the diagram.

The magnetic field, its properties and characteristics are quite widely used, because in many problems it has to be taken into account and studied. This is the most important phenomenon in the science of physics. More complex things are inextricably linked with it, such as magnetic permeability and induction. To explain all the reasons for the appearance of a magnetic field, one must rely on real scientific facts and confirmations. Otherwise, in more complex problems, the wrong approach can violate the integrity of the theory.

Now let's give examples. We all know our planet. You say that it has no magnetic field? You may be right, but scientists say that the processes and interactions inside the Earth's core create a huge magnetic field that stretches for thousands of kilometers. But any magnetic field must have its poles. And they exist, just located a little away from the geographic pole. How do we feel it? For example, birds have developed navigation abilities, and they orient themselves, in particular, by the magnetic field. So, with his help, the geese arrive safely in Lapland. Special navigation devices also use this phenomenon.

A MAGNETIC FIELD

The magnetic field is a special kind of matter, invisible and intangible to humans,
existing independently of our consciousness.
Even in ancient times, scientists-thinkers guessed that something exists around the magnet.

Magnetic needle.

A magnetic needle is a device necessary for studying the magnetic action of an electric current.
It is a small magnet mounted on the tip of the needle, has two poles: north and south. The magnetic needle can rotate freely on the tip of the needle.
The north end of the magnetic needle always points north.
The line connecting the poles of the magnetic needle is called the axis of the magnetic needle.
A similar magnetic needle is in any compass - a device for orienteering on the ground.

Where does the magnetic field originate?

Oersted's experiment (1820) - shows how a conductor with current and a magnetic needle interact.

When the electric circuit is closed, the magnetic needle deviates from its original position, when the circuit is opened, the magnetic needle returns to its original position.

In the space around the conductor with current (and in general case around any moving electric charge) there is a magnetic field.
The magnetic forces of this field act on the needle and turn it.

In general, one can say
that a magnetic field arises around moving electric charges.
Electric current and magnetic field are inseparable from each other.

INTERESTING WHAT...

Many celestial bodies Planets and stars have their own magnetic fields.
However, our nearest neighbors - the Moon, Venus and Mars - do not have a magnetic field,
similar to earth.
___

Gilbert discovered that when a piece of iron is brought near one pole of a magnet, the other pole begins to attract more strongly. This idea was patented only 250 years after Hilbert's death.

In the first half of the 90s, when new Georgian coins appeared - lari,
local pickpockets got magnets,
because the metal from which these coins were made was well attracted by a magnet!

If you take a dollar bill around the corner and bring it to a powerful magnet
(for example, horseshoe), creating a non-uniform magnetic field, a piece of paper
deviate towards one of the poles. It turns out that the color of the dollar bill contains iron salts,
having magnetic properties, so the dollar is attracted to one of the poles of the magnet.

If brought to a carpenter's bubble level big magnet, then the bubble will move.
The fact is that the bubble level is filled with a diamagnetic liquid. When such a liquid is placed in a magnetic field, a magnetic field of the opposite direction is created inside it, and it is pushed out of the field. Therefore, the bubble in the liquid approaches the magnet.

YOU SHOULD KNOW ABOUT THEM!

The organizer of the magnetic compass business in the Russian Navy was a well-known deviator scientist,
captain of the 1st rank, author scientific papers according to the theory of the compass I.P. Belavan.
Participant world travel on the frigate "Pallada" and a participant Crimean War 1853-56 he was the first in the world to demagnetize a ship (1863)
and solved the problem of installing compasses inside an iron submarine.
In 1865 he was appointed head of the country's first Compass Observatory in Kronstadt.

To understand what is a characteristic of a magnetic field, many phenomena should be defined. At the same time, you need to remember in advance how and why it appears. Learn what is a force field. It is also important that such a field can occur not only in magnets. In this regard, it does not hurt to mention the characteristics of the earth's magnetic field.

Emergence of the field

To begin with, it is necessary to describe the appearance of the field. After that, you can describe the magnetic field and its characteristics. It appears during the movement of charged particles. Can affect especially conductive conductors. The interaction between a magnetic field and moving charges, or conductors through which current flows, occurs due to forces called electromagnetic.

The intensity or power characteristic of the magnetic field at a certain spatial point is determined using magnetic induction. The latter is denoted by the symbol B.

Graphical representation of the field

The magnetic field and its characteristics can be represented graphically using induction lines. This definition is called lines, the tangents to which at any point will coincide with the direction of the vector y of the magnetic induction.

These lines are included in the characteristics of the magnetic field and are used to determine its direction and intensity. The higher the intensity of the magnetic field, the more data lines will be drawn.

What are magnetic lines

The magnetic lines of straight conductors with current have the shape of a concentric circle, the center of which is located on the axis of this conductor. The direction of the magnetic lines near the conductors with current is determined by the gimlet rule, which sounds like this: if the gimlet is located so that it will be screwed into the conductor in the direction of the current, then the direction of rotation of the handle corresponds to the direction of the magnetic lines.

For a coil with current, the direction of the magnetic field will also be determined by the gimlet rule. It is also required to rotate the handle in the direction of the current in the turns of the solenoid. The direction of the lines of magnetic induction will correspond to the direction forward movement gimlet.

It is the main characteristic of the magnetic field.

Created by one current, under equal conditions, the field will differ in its intensity in different environments due to the different magnetic properties in these substances. The magnetic properties of the medium are characterized by absolute magnetic permeability. It is measured in henries per meter (g/m).

The characteristic of the magnetic field includes the absolute magnetic permeability of the vacuum, called the magnetic constant. The value that determines how many times the absolute magnetic permeability of the medium will differ from the constant is called the relative magnetic permeability.

Magnetic permeability of substances

This is a dimensionless quantity. Substances with a permeability value of less than one are called diamagnetic. In these substances, the field will be weaker than in vacuum. These properties are present in hydrogen, water, quartz, silver, etc.

Media with a magnetic permeability greater than unity are called paramagnetic. In these substances, the field will be stronger than in vacuum. These media and substances include air, aluminum, oxygen, platinum.

In the case of paramagnetic and diamagnetic substances, the value of magnetic permeability will not depend on the voltage of the external, magnetizing field. This means that the value is constant for a particular substance.

Ferromagnets belong to a special group. For these substances, the magnetic permeability will reach several thousand or more. These substances, which have the property of being magnetized and amplifying the magnetic field, are widely used in electrical engineering.

Field strength

To determine the characteristics of the magnetic field, together with the magnetic induction vector, a value called the magnetic field strength can be used. This term defines the intensity of the external magnetic field. The direction of the magnetic field in a medium with the same properties in all directions, the intensity vector will coincide with the magnetic induction vector at the field point.

The strong magnetic properties of ferromagnets are explained by the presence in them of arbitrarily magnetized small parts, which can be represented as small magnets.

In the absence of a magnetic field, a ferromagnetic substance may not have pronounced magnetic properties, since the domain fields acquire different orientations, and their total magnetic field is zero.

According to the main characteristic of the magnetic field, if a ferromagnet is placed in an external magnetic field, for example, in a coil with current, then under the influence of the external field, the domains will turn in the direction of the external field. Moreover, the magnetic field at the coil will increase, and the magnetic induction will increase. If the external field is sufficiently weak, then only a part of all domains whose magnetic fields approach the direction of the external field will flip over. As the strength of the external field increases, the number of rotated domains will increase, and at a certain value of the external field voltage, almost all parts will be rotated so that the magnetic fields are located in the direction of the external field. This state called magnetic saturation.

Relationship between magnetic induction and intensity

The relationship between the magnetic induction of a ferromagnetic substance and the strength of an external field can be depicted using a graph called the magnetization curve. At the bend of the curve graph, the rate of increase in magnetic induction decreases. After a bend, where the tension reaches a certain value, saturation occurs, and the curve slightly rises, gradually acquiring the shape of a straight line. In this section, the induction is still growing, but rather slowly and only due to an increase in the strength of the external field.

The graphical dependence of these indicators is not direct, which means that their ratio is not constant, and the magnetic permeability of the material is not a constant indicator, but depends on the external field.

Changes in the magnetic properties of materials

With an increase in the current strength to full saturation in a coil with a ferromagnetic core and its subsequent decrease, the magnetization curve will not coincide with the demagnetization curve. With zero intensity, the magnetic induction will not have the same value, but will acquire some indicator called the residual magnetic induction. The situation with the lagging of magnetic induction from the magnetizing force is called hysteresis.

To completely demagnetize the ferromagnetic core in the coil, it is necessary to give a reverse current, which will create the necessary tension. For different ferromagnetic substances, a segment of different lengths is needed. The larger it is, the more energy is needed for demagnetization. The value at which the material is completely demagnetized is called the coercive force.

With a further increase in the current in the coil, the induction will again increase to the saturation index, but with a different direction of the magnetic lines. When demagnetized in reverse direction residual induction will be obtained. The phenomenon of residual magnetism is used to create permanent magnets from substances with a high residual magnetism. From substances that have the ability to remagnetize, cores are created for electrical machines and devices.

left hand rule

The force acting on a conductor with current has a direction determined by the rule of the left hand: when the palm of the virgin hand is located in such a way that the magnetic lines enter it, and four fingers are extended in the direction of the current in the conductor, the bent thumb will indicate the direction of force. This force is perpendicular to the induction vector and the current.

A current-carrying conductor moving in a magnetic field is considered a prototype of an electric motor, which changes electrical energy into mechanical.

Right hand rule

During the movement of the conductor in the magnetic field inside it is induced electromotive force, which has a value proportional to the magnetic induction, the length of the conductor involved and the speed of its movement. This dependence is called electromagnetic induction. When determining the direction of the induced EMF in the conductor, the rule is used right hand: when the right hand is positioned in the same way as in the example from the left, the magnetic lines enter the palm, and the thumb indicates the direction of movement of the conductor, the outstretched fingers indicate the direction of the induced EMF. A conductor moving in a magnetic flux under the influence of an external mechanical force is the simplest example electric generator in which mechanical energy is converted into electrical energy.

It can be formulated differently: in a closed circuit, an EMF is induced, with any change in the magnetic flux covered by this circuit, the EDE in the circuit is numerically equal to the rate of change of the magnetic flux that covers this circuit.

This form provides an average EMF indicator and indicates the dependence of the EMF not on the magnetic flux, but on the rate of its change.

Lenz's Law

You also need to remember Lenz's law: the current induced by a change in the magnetic field passing through the circuit, with its magnetic field, prevents this change. If the turns of the coil are pierced by magnetic fluxes of different magnitudes, then the EMF induced on the whole coil is equal to the sum of the EMF in different turns. The sum of the magnetic fluxes of different turns of the coil is called flux linkage. The unit of measurement of this quantity, as well as the magnetic flux, is weber.

When the electric current in the circuit changes, the magnetic flux created by it also changes. In this case, according to the law of electromagnetic induction, an EMF is induced inside the conductor. It appears in connection with a change in current in the conductor, because this phenomenon is called self-induction, and the EMF induced in the conductor is called self-induction EMF.

Flux linkage and magnetic flux depend not only on the strength of the current, but also on the size and shape of a given conductor, and the magnetic permeability of the surrounding substance.

conductor inductance

The coefficient of proportionality is called the inductance of the conductor. It denotes the ability of a conductor to create flux linkage when electricity passes through it. This is one of the main parameters electrical circuits. For certain circuits, inductance is a constant. It will depend on the size of the contour, its configuration and the magnetic permeability of the medium. In this case, the current strength in the circuit and the magnetic flux will not matter.

The above definitions and phenomena provide an explanation of what a magnetic field is. The main characteristics of the magnetic field are also given, with the help of which it is possible to define this phenomenon.