Magnetic field and its characteristics

Lecture outline:

Magnetic field, its properties and characteristics.

Magnetic field- the form of existence of matter surrounding moving electric charges (current-carrying conductors, permanent magnets).

This name is due to the fact that, as the Danish physicist Hans Oersted discovered in 1820, it has an orienting effect on the magnetic needle. Oersted's experiment: a magnetic needle was placed under a current-carrying wire, rotating on a needle. When the current was turned on, it was installed perpendicular to the wire; when the direction of the current changed, it turned in the opposite direction.

Basic properties of the magnetic field:

generated by moving electric charges, current-carrying conductors, permanent magnets and an alternating electric field;

acts with force on moving electric charges, current-carrying conductors, and magnetized bodies;

an alternating magnetic field generates an alternating electric field.

From Oersted's experience it follows that the magnetic field is directional and must have a vector force characteristic. It is designated and called magnetic induction.

The magnetic field is represented graphically using magnetic field lines or magnetic induction lines. Magnetic power lines These are the lines along which iron filings or the axes of small magnetic needles are located in a magnetic field. At each point of such a line the vector is directed along a tangent.

Magnetic induction lines are always closed, which indicates the absence of magnetic charges in nature and the vortex nature of the magnetic field.

Conventionally, they leave the north pole of the magnet and enter the south. The density of the lines is chosen so that the number of lines per unit area perpendicular to the magnetic field is proportional to the magnitude of the magnetic induction.

N

Magnetic solenoid with current

The direction of the lines is determined by the right screw rule. A solenoid is a coil with current, the turns of which are located close to each other, and the diameter of the turn is much less than the length of the coil.

The magnetic field inside the solenoid is uniform. A magnetic field is called uniform if the vector is constant at any point.

The magnetic field of a solenoid is similar to the magnetic field of a bar magnet.

WITH
A current-carrying solenoid is an electromagnet.

Experience shows that for a magnetic field, as for an electric field, superposition principle: the induction of a magnetic field created by several currents or moving charges is equal to the vector sum of the induction of magnetic fields created by each current or charge:

The vector is entered in one of 3 ways:

a) from Ampere’s law;

b) by the effect of a magnetic field on a current-carrying frame;

c) from the expression for the Lorentz force.

A imper experimentally established that the force with which a magnetic field acts on an element of a conductor with current I located in a magnetic field is directly proportional to the force

current I and the vector product of the element of length and magnetic induction:

- Ampere's law

N
The direction of the vector can be found according to the general rules of the vector product, from which the rule of the left hand follows: if the palm of the left hand is positioned so that the magnetic lines of force enter it, and the 4 extended fingers are directed along the current, then the bent thumb will show the direction of the force.

The force acting on a wire of finite length can be found by integrating over the entire length.

When I = const, B=const, F = BIlsin

If  =90 0, F = BIl

Magnetic field induction- vector physical quantity, numerically equal to the force acting in a uniform magnetic field on a conductor of unit length with unit current, located perpendicular to the magnetic lines of force.

1T is the induction of a uniform magnetic field, in which a force of 1N acts on a conductor 1m long with a current of 1A, located perpendicular to the magnetic lines of force.

So far we have considered macrocurrents flowing in conductors. However, according to Ampere's assumption, in any body there are microscopic currents caused by the movement of electrons in atoms. These microscopic molecular currents create their own magnetic field and can rotate in the fields of macrocurrents, creating an additional magnetic field in the body. The vector characterizes the resulting magnetic field created by all macro- and microcurrents, i.e. at the same macrocurrent, the vector in different environments has different values.

The magnetic field of macrocurrents is described by the magnetic intensity vector.

For a homogeneous isotropic medium

,

 0 = 410 -7 H/m - magnetic constant,  0 = 410 -7 N/A 2,

 is the magnetic permeability of the medium, showing how many times the magnetic field of macrocurrents changes due to the field of microcurrents of the medium.

Magnetic flux. Gauss's theorem for magnetic flux.

Vector flow(magnetic flux) through the site dS called a scalar quantity equal to

where is the projection onto the direction of the normal to the site;

 - angle between vectors and.

Directional surface element,

Vector flux is an algebraic quantity,

If - when leaving the surface;

If - upon entering the surface.

The flux of the magnetic induction vector through an arbitrary surface S is equal to

For a uniform magnetic field =const,

1 Wb - magnetic flux passing through a flat surface with an area of ​​1 m2, located perpendicular to a uniform magnetic field, the induction of which is 1 T.

The magnetic flux through the surface S is numerically equal to the number of magnetic field lines crossing this surface.

Since magnetic induction lines are always closed, for a closed surface the number of lines entering the surface (Ф 0), therefore, the total flux of magnetic induction through a closed surface is zero.

- Gauss's theorem: The flux of the magnetic induction vector through any closed surface is zero.

This theorem is a mathematical expression of the fact that in nature there are no magnetic charges on which magnetic induction lines begin or end.

The Biot-Savart-Laplace law and its application to the calculation of magnetic fields.

The magnetic field of direct currents of various shapes was studied in detail by Fr. scientists Biot and Savard. They found that in all cases, magnetic induction at an arbitrary point is proportional to the current strength and depends on the shape, size of the conductor, the location of this point in relation to the conductor and on the environment.

The results of these experiments were summarized by Fr. mathematician Laplace, who took into account the vector nature of magnetic induction and hypothesized that the induction at each point is, according to the principle of superposition, the vector sum of the inductions of elementary magnetic fields created by each section of this conductor.

Laplace formulated a law in 1820, which was called the Biot-Savart-Laplace law: each element of a current-carrying conductor creates a magnetic field, the induction vector of which at some arbitrary point K is determined by the formula:

- Biot-Savart-Laplace law.

From the Biot-Sauvar-Laplace law it follows that the direction of the vector coincides with the direction of the vector product. The same direction is given by the rule of the right screw (gimlet).

Considering that,

Conductor element co-directed with current;

Radius vector connecting to point K;

The Biot-Savart-Laplace law is of practical importance because allows you to find at a given point in space the induction of the magnetic field of a current flowing through a conductor of finite dimensions and arbitrary shape.

For a current of arbitrary shape, such a calculation is a complex mathematical problem. However, if the current distribution has a certain symmetry, then the application of the superposition principle together with the Biot-Savart-Laplace law makes it possible to calculate specific magnetic fields relatively simply.

Let's look at some examples.

A. Magnetic field of a straight conductor carrying current.

for a conductor of finite length:

for a conductor of infinite length:  1 = 0,  2 = 

B. Magnetic field at the center of the circular current:

=90 0 , sin=1,

Oersted experimentally discovered in 1820 that circulation in a closed loop surrounding a system of macrocurrents is proportional to the algebraic sum of these currents. The proportionality coefficient depends on the choice of system of units and in SI is equal to 1.

C
Circulation of a vector is called a closed loop integral.

This formula is called circulation theorem or total current law:

circulation of the magnetic field strength vector along an arbitrary closed circuit is equal to the algebraic sum of the macrocurrents (or total current) covered by this circuit. his characteristics In the space surrounding currents and permanent magnets, a force arises field, called magnetic. Availability magnetic fields is revealed...

The magnetic field has long 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 undeniable facts.

Let's look at everything in order. So, how does any magnetic field operate and form? That's right, from electric current. And current, according to physics textbooks, is a directional flow of charged particles, isn’t it? So, when a current passes through any conductor, a certain type of matter begins to act around it - a magnetic field. A magnetic field can be created by a 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 influence the flow of charged particles, and they change the initial direction of movement perpendicular to the field itself.

A magnetic field can also 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 a special structure in rotating bions in a region of space. An ordinary moving electric charge can make them rotate and move. Bions transmit any possible interactions in this region of space. Therefore, a moving charge attracts one pole of all bions and makes them rotate. Only he can bring them out of their state of rest, nothing else, because other forces will not be able to influence them.

In an electric field there are charged particles that move very quickly and can travel 300,000 km in just a second. Light has the same speed. A magnetic field cannot exist 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 one. This law is also reverse.

We talk a lot about the magnetic field here, but how can we 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 detect it using various devices. But in order to study something, you need to have at least some idea about it. It is also often necessary to depict a magnetic field in diagrams. To make it easier to understand, 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 using small metal filings and an ordinary magnet. Let's pour these sawdust onto a flat surface and expose them to 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 the magnetic field. All forces and, accordingly, lines of force are continuous and closed in this place.

A magnetic needle has similar characteristics and properties to a compass, and is used to determine the direction of lines of force. If it falls into the zone of action of a magnetic field, we can see the direction of action of the forces from its north pole. Then let us highlight several conclusions from here: the top of an ordinary permanent magnet, from which the lines of force emanate, is designated 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 have a fairly wide application, 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 such as magnetic permeability and induction are inextricably linked with it. To explain all the reasons for the appearance of a magnetic field, we must rely on real scientific facts and confirmation. Otherwise, in more complex problems, an incorrect approach may violate the integrity of the theory.

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

Introduction

What is a magnetic field? Everyone has heard about it, everyone has seen how the magnetized compass needle always turns with the same end towards the north magnetic pole, and with its other end always turns towards the south magnetic pole. What distinguishes a person from the smartest animal is that he is curious and wants to know why this happens, how it works, what happens this way. It was to explain what was happening around him that ancient man invented the gods. Spirits, gods in the minds of people were the factors that explained everything that a person saw, heard, on what luck in hunting and in war depended, who moved the Sun across the sky, who made a thunderstorm, shed rain and poured snow, in general, everything that exists , everything that happens. Imagine, a little grandson comes up to his grandfather, points to lightning and asks: what is this, why does fire fly from a cloud to the ground, and who is knocking so loudly there in the clouds? If the grandfather answered: I don’t know, then the grandson looked at him with regret and began to respect him less. But when the grandfather said that it was the god Yarilo who rode a chariot through the clouds and shot fiery arrows at bad people, the grandson listened and respected his grandfather even more. He began to be less afraid of thunder and lightning, because he knew that he was good, so Yarilo would not shoot at him.

In early childhood, when I began to play pranks, Grandma Anna would say: “Shurka, look, don’t play pranks, otherwise God will hit you with a pebble.” And at the same time she pointed to the icon in the red corner on the shelf-goddess. I became quiet for a while, looking warily at the stern man drawn on the board, but one day I doubted his ability to throw stones. He put a stool on the bench, climbed onto it and looked at the shelf behind the icon. I didn’t see any stones there, and when the grandmother began to frighten me once again, he laughed and said: “He doesn’t have any stones, and in general he’s drawn and can’t throw. And there’s no need to scare me, my God, I’m not little anymore.” In the same way, our distant ancestor once doubted that it was Yarilo who was riding in the sky and shooting arrows. It was then that rational knowledge arose, when people doubted the omnipotence of the gods. But what did they replace them with? And they replaced the gods with the laws of nature, and began to firmly believe in these laws. But where man cannot explain what is happening with the laws of nature, he left room for the gods. That is why religion and science coexist in society to this day.

I remember how my older friends showed us kids a trick. An iron nail placed on the table moved along the table by itself, and the magician guy under the table moved his hand. The nail followed the hand. We stared at it in surprise and did not understand why the nail was moving. When I told my mother about this trick, she explained that the guy had a magnet in his hand that attracted iron to itself, that the guy under the table was not just moving his hand, but that he had a magnet in his hand. At that moment, this explanation satisfied my curiosity, but a little later I wanted to understand why a magnet at a distance - through a table board, through a layer of air - attracts iron to itself. Neither my mother nor my father could answer this question. I had to wait until school. There, in a physics lesson, the teacher explained that a magnet acts on iron through a magnetic field that it creates around itself, that a magnet has two poles - north and south, that some invisible magnetic lines of force come out of the north, which bend in an arc and enter into South Pole.

Then for the first time I thought: it means that in the world, in addition to the visible, audible and tangible, there is something invisible and intangible. Then I thought: what if God is invisible and intangible - like this magnetic field. It seems to be nowhere, but it still exists. And on icons, out of stupidity, he is depicted as a peasant. I did not know then that the philosopher Spinoza had thought of this even earlier than me, who began to consider Nature and God as one and inseparable, visible and invisible. Nature is God!

I remember I tried to imagine this magnetic field consisting of lines of force, and I did not understand anything. I did not see or hear these lines. They didn’t smell of anything, and it wasn’t very clear to me at the time to believe that there could be something around us that we couldn’t sense at all. Iron nails and filings felt the magnetic field and oriented and moved in it, but I, with my subtle senses, did not feel anything. This inferiority frankly depressed me. But not just me. A. Einstein wrote about his great surprise at the properties of the magnet he saw, which his father gave him for his birthday as a child, because he could not understand how and why these attractive properties of the magnet occur.

When the social studies teacher already in the 10th grade introduced us to the definition of matter given by V.I. Lenin: “matter is what exists around us and is given to us in sensations,” I asked her indignantly: “but we don’t feel the magnetic field, but it exists, isn’t it matter?” Yes, the senses alone are not enough to perceive all forms of matter; we also need a mind, with the help of which, if we don’t feel something, we don’t feel it, then we understand that it exists. Realizing this, I decided to study science and develop my mind, hoping that this would allow me to understand many things. But as I expanded the space of what was understandable to me, the incomprehensible did not disappear, but only moved away, and the horizon line of the incomprehensible became longer and longer, since the circle of the known increased and the length of its circumference, separating what was understood by my mind from the unknown and incomprehensible, also increased. This is the main paradox of knowledge: the more we learn and understand, the more we still do not know. Nikolai Kuzansky, who for some reason is considered a scholastic philosopher, wrote about this learned ignorance, although the truth he discovered rather suggests that he was a dialectician.

The first mentions of rocks capable of attracting iron date back to ancient times. The magnet is associated with an ancient legend about the shepherd Magnus, who once discovered that his iron staff and sandals, lined with iron nails, were attracted to an unknown stone. Since then, this stone has been called the “Magnus stone”, or magnet.

The origin and essence of the Earth's magnetic field, as well as magnetic fields in general, remains a mystery to this day. There are many hypotheses - options for explaining this phenomenon, but the truth is still “out there”. This is how physicists define the magnetic field: " Magnetic field is a force field acting on moving electric charges and on bodies with a magnetic moment, regardless of the state of their motion." And further: "A magnetic field can be created by a current of charged particles and/or magnetic moments of electrons in atoms (and magnetic moments of other particles , although to a noticeably lesser extent). In addition, it appears in the presence of a time-varying electric field." I would not say that from a logical point of view this is a brilliant definition. To say that a magnetic field is a force field is to say nothing, it is a tautology. After all, a gravitational field - also a force field, and the field of nuclear forces is a force! The indication of the influence of a magnetic field on moving electric charges says something, this is a description of one of the properties of a magnetic field. But it is not clear whether the magnetic field acts directly on particles having electric charges, or not. it acts on the magnetic fields formed by these particles, and those (transformed fields of particles) in turn act on the particles - they transfer the received impulse to them.

The English physician and physicist William Gilbert was the first to study magnetic phenomena when he wrote the work “On the Magnet, Magnetic Bodies and the Great Magnet - the Earth.” At that time it was believed that electricity and magnetism had nothing in common. But at the beginning of the 19th century. Danish scientist G.H. Oersted in 1820 experimentally proved that magnetism is one of the latent forms of electricity, and confirmed this experimentally. This experience led to an avalanche of new discoveries that were of great importance. A field appears around conductors carrying electric current, which is called magnetic. A beam of moving electrons has an effect on a magnetic needle, similar to a conductor with current (Ioffe experiment). Convection currents of electrically charged particles in their effect on the magnetic needle are similar to conduction currents (Eichenwald's experiment).

A magnetic field is created only by moving electric charges or moving electrically charged bodies, as well as permanent magnets. This makes the magnetic field different from the electric field, which is created by both moving and stationary electric charges.

The lines of the magnetic induction vector (B) are always closed and enclose a conductor with current, and the electric field strength lines begin on positive and end on negative charges; they are open. The magnetic induction lines of a permanent magnet come out of one pole, called the north (N), and enter the other, called the south (S). At first it seems that there is a complete analogy with the lines of electric field strength (E). The poles of magnets act as magnetic charges. However, if you cut a magnet, the picture is preserved, smaller magnets are obtained - but each with its own north and south poles. It is impossible to divide the magnetic poles so that one piece has the north pole and the other has the south pole, because free (discrete) magnetic charges, unlike discrete electric charges, do not exist in nature.

Magnetic fields that exist in nature vary in scale and in the effects they cause. The Earth's magnetic field, which forms the Earth's magnetosphere, extends over a distance of 70-80 thousand kilometers in the direction of the Sun and many millions of kilometers in the opposite direction. The origin of the Earth's magnetic field is associated with the movements of liquid matter that conducts electrically charged particles in the earth's core. Jupiter and Saturn have powerful magnetic fields. The magnetic field of the Sun plays a crucial role in all processes occurring on the Sun - flares, the appearance of spots and prominences, the birth of solar cosmic rays. Magnetic fields are widely used in various industries: when loading iron scrap, when cleaning flour in bakeries from metal impurities, as well as in medicine for treating patients.

What is a magnetic field

The main strength characteristic of the magnetic field is magnetic induction vector. Often, for brevity, the magnetic induction vector is simply called a magnetic field (although this is probably not the most strict use of the term). In fact, a vector is a quantity that has a direction in space, therefore, we can talk about the direction of magnetic induction and its magnitude. But to say that a magnetic field is only the direction of magnetic induction does not explain much. There is another characteristic of the magnetic field - vector potential. As the main characteristic of the magnetic field in a vacuum, it is not the magnetic induction vector that is chosen, but the vector magnetic field strength. In a vacuum, these two vectors coincide, but in matter they do not, but from a systematic point of view, it should be considered the main characteristic of the magnetic field vector potential.

A magnetic field can be called a special type of matter through which interaction occurs between moving charged particles or bodies with a magnetic moment. Magnetic fields are a necessary (in the context of the special theory of relativity) consequence of the existence of electric fields. Magnetic and electric fields together form an electromagnetic field, the manifestations of which are, in particular, light and all other electromagnetic waves. From the point of view of quantum field theory, magnetic interaction - as a special case of electromagnetic interaction - is carried by a fundamental massless boson - a photon (a particle that can be represented as a quantum excitation of an electromagnetic field), often (for example, in all cases of static fields) virtual. A magnetic field is created (generated) by a current of charged particles, or an electric field changing over time, or the particles’ own magnetic moments (the latter, for the sake of uniformity of the picture, can be formally reduced to electric currents).

In my opinion, these definitions are very vague. It is clear that the magnetic field is not emptiness, but a special type of matter - part of the real world. It is clear that the magnetic field is inextricably linked with the movement of electric charges - electric current. But how a magnetic field and an electric field form a single electromagnetic field is not clear. Most likely, there is some kind of unified field, which, depending on the circumstances, manifests itself either as a magnetic field or as an electric one. Just like some kind of hermaphrodite, who in certain circumstances can be a boy, and in other circumstances - a girl.

The force acting on an electrically charged particle moving in a magnetic field is called the Lorentz force. This force is always directed perpendicular to the vector particle speed - v and vector potential of the magnetic field - B. This force is proportional to the charge of the particle q, its speed v, perpendicular to the direction of the magnetic field vector B and is proportional to the magnitude of the magnetic field induction B. Let me explain to those who have completely forgotten school physics: force is the reason that causes the acceleration of the movement of bodies. Here the force acts not on the mass of the particle, but on its charge. This is how the Lorentz force differs from the gravitational force, which acts on the mass of particles (bodies), since the mass of a body is its gravitational charge.

The magnetic field also acts on a current-carrying conductor. The force acting on a conductor carrying current is called the Ampere force. This force consists of the forces acting on individual electrical charges moving inside the conductor. This is the current strength, measured in amperes.

When two magnets interact, their like poles repel and their opposite poles attract. However, detailed analysis shows that this is in fact not a completely correct description of the phenomenon. It is not clear why dipoles can never be separated within this model. The experiment shows that no isolated body actually has a magnetic charge of the same sign. Every magnetized body has two poles - north and south. A magnetic dipole placed in a non-uniform magnetic field is subject to a force that tends to rotate it so that the magnetic moment of the dipole is codirectional (coincident in direction) with the magnetic field in which this magnetic dipole was placed.

In 1831, Michael Faraday discovered that a closed conductor, when placed in a changing magnetic field, produces an electric current. This phenomenon is called electromagnetic induction.

M. Faraday discovered that the electromotive force (EMF) arising in a closed conducting circuit is proportional to the rate of change of the magnetic flux passing through the part of the electrical circuit located in this magnetic field. The magnitude (EMF) does not depend on what is causing the flux change - a change in the magnetic field itself or the movement of part of the circuit in the magnetic field. The electric current caused by EMF is called induced current. This discovery made it possible to create electric current generators and, in essence, create our electric civilization. Who would have thought in the 30s of the 19th century that M. Faraday’s discovery was an epochal civilizational discovery that determined the future of mankind?

In turn, the magnetic field can be created and changed (weakened or strengthened) by an alternating electric field created by electric currents in the form of streams of charged particles. The microscopic structure of a substance placed in an alternating magnetic field affects the strength of the current arising in it. Some structures weaken the resulting electric current, while others enhance it to varying degrees. One of the first studies of the magnetic properties of matter was carried out by Pierre Curie. In this regard, substances with respect to their magnetic properties are divided into two main groups:

1. Ferromagnetic substances are substances in which, below a certain critical temperature (Curie point), a long-range ferromagnetic order of the magnetic moments of the particles of the substance is established.

2. Antiferromagnets - substances in which an antiferromagnetic order has been established for the magnetic moments of particles of matter - atoms or ions: the magnetic moments of particles of matter are directed oppositely and are equal in strength.

There are also diamagnetic substances and paramagnetic substances.

Diamagnets are substances that are magnetized against the direction of an external magnetic field.

Paramagnetic substances are substances that are magnetized in an external magnetic field in the direction of the external magnetic field.

Types of ordering of the magnetic moments of atoms in paramagnetic (a), ferromagnetic (b) and antiferromagnetic (c) substances. Figure from the site: http://encyclopaedia.biga.ru/enc/science_and_technology/ MAGNITI_I_MAGNITNIE_SVOSTVA_VESHCHESTVA.html

The groups of substances listed above mainly include ordinary solid, liquid and gaseous substances. Superconductors and plasmas differ significantly from them in their interaction with the magnetic field. The magnetic field of ferromagnets (for example, iron) is noticeable over considerable distances. The magnetic properties of paramagnets are similar to the properties of ferromagnets, but are much less pronounced - at a shorter distance. Diamagnets do not attract, but are repelled by a magnet; the force acting on diamagnets is directed opposite to that acting on ferromagnets and paramagnets. According to Lenz's rule, the magnetic field of an electric current induced in a magnetic field is directed so as to counteract the change in the magnetic flux inducing this current. I would like to note that the interaction of an alternating magnetic field and the electric current and electric field induced by it corresponds to Le Chatelier’s principle. This is nothing more than auto-inhibition of the process, inherent in all processes occurring in the real world.

According to Le Chatelier's principle, every process occurring in the world gives rise to a process that has the opposite direction and inhibits the process that causes it. In my opinion, this is one of the main laws of the universe, which for some reason is not given due attention by either physicists or philosophers.

All substances have magnetic properties to a greater or lesser extent. If two conductors with electric currents are placed in any medium, then the strength of the magnetic interaction between the currents changes. The induction of a magnetic field created by electric currents in a substance differs from the induction of a magnetic field created by the same currents in a vacuum. A physical quantity that shows how many times the magnetic field induction in a homogeneous medium differs in magnitude from the magnetic field induction in a vacuum is called magnetic permeability. Vacuum has maximum magnetic permeability.

The magnetic properties of substances are determined by the magnetic properties of atoms - electrons, protons and neutrons that make up the atoms. The magnetic properties of protons and neutrons are almost 1000 times weaker than the magnetic properties of electrons. Therefore, the magnetic properties of a substance are mainly determined by the electrons that make up its atoms.

One of the most important properties of an electron is the presence of not only an electric, but also a magnetic field. The electron's own magnetic field, which supposedly arises when it rotates around its axis, is called a spin field (spin - rotation). But the electron also creates a magnetic field due to its movement around the atomic nucleus, which can be likened to a circular microcurrent. The spin fields of electrons and magnetic fields caused by their orbital motions determine a wide range of magnetic properties of substances.

Behavior of a paramagnetic (1) and a diamagnetic (2) in a non-uniform magnetic field. Figure from the site: http://physics.ru/courses/op25part2/content/chapter1/section/paragraph19/theory.html

Substances are extremely diverse in their magnetic properties. For example, platinum, air, aluminum, ferric chloride are paramagnetic, and copper, bismuth, water are diamagnetic. Paramagnetic and diamagnetic samples placed in a non-uniform magnetic field between the poles of an electromagnet behave differently - paramagnetic materials are drawn into the region of a strong field, and diamagnetic materials, on the contrary, are pushed out of it. Para- and diamagnetism is explained by the behavior of electron orbits in an external magnetic field. In the absence of an external field, atoms of diamagnetic substances have their own magnetic fields of electrons and the fields created by their orbital motion completely compensated. The occurrence of diamagnetism is associated with the action of the Lorentz force on electron orbits. Under the influence of this force, the nature of the orbital motion of electrons changes and the compensation of magnetic fields is disrupted. The resulting own magnetic field of the atom turns out to be directed opposite to the direction of the induction of the external field.

In atoms of paramagnetic substances, the magnetic fields of electrons are not completely compensated, and the atom turns out to be similar to a small circular current. In the absence of an external field, these circular microcurrents are oriented randomly, so that the total magnetic induction is zero. The external magnetic field has an orienting effect - microcurrents tend to orient themselves so that their own magnetic fields are directed in the direction of the induction of the external field. Due to the thermal motion of atoms, the orientation of microcurrents is never complete. As the external field increases, the orientation effect increases, so that the induction of the paramagnetic sample's own magnetic field increases in direct proportion to the induction of the external magnetic field. The total induction of the magnetic field in the sample consists of the induction of the external magnetic field and the induction of its own magnetic field that arose during the magnetization process.

Atoms of any substance have diamagnetic properties, but in many cases their diamagnetism is masked by a strong paramagnetic effect. The phenomenon of diamagnetism was discovered by M. Faraday in 1845.

Ferromagnets can be strongly magnetized in a magnetic field; their magnetic permeability is very high. The group under consideration includes four chemical elements: iron, nickel, cobalt, gadolinium. Of these, iron has the greatest magnetic permeability. Ferromagnetic materials can be various alloys of these elements, for example, ceramic ferromagnetic materials - ferrites.

For each ferromagnet there is a certain temperature (the so-called temperature or Curie point), above which the ferromagnetic properties disappear and the substance becomes paramagnetic. For iron, for example, the Curie temperature is 770°C, for cobalt 1130°C, and for nickel 360°C.

Ferromagnetic materials are either magnetically soft or magnetically hard. Soft magnetic ferromagnetic materials are almost completely demagnetized when the external magnetic field becomes zero. Soft magnetic materials include, for example, pure iron, electrical steel and some alloys. These materials are used in alternating current devices in which continuous magnetization reversal occurs, that is, a change in the direction of the magnetic field (transformers, electric motors, etc.).

Magnetically hard materials retain their magnetization to a large extent even after they are removed from the magnetic field. Examples of magnetically hard materials include carbon steel and a number of special alloys. Magnetically hard materials are mainly used to make permanent magnets.

A characteristic feature of the magnetization process of ferromagnets is hysteresis, that is, the dependence of magnetization on the sample’s history. The magnetization curve B (B0) of a ferromagnetic sample is a loop of a complex shape, which is called a hysteresis loop.

Dependence of the magnetic permeability of a ferromagnet on the induction of an external magnetic field. The ferromagnet is magnetized quickly at first, but having reached a maximum, it becomes magnetized more and more slowly. Figure from the site: http://physics.ru/courses/op25part2/content/chapter1/section/paragraph19/theory.html

A typical hysteresis loop for a magnetically hard ferromagnetic material. At point 2, magnetic saturation is achieved. Section 1-3 determines the residual magnetic induction, and section 1-4 determines the coercive force, which characterizes the ability of the sample to resist demagnetization. Figure from the site: http://encyclopaedia.biga.ru/enc/science_and_technology/ MAGNITI_I_MAGNITNIE_SVOSTVA_VESHCHESTVA.html

The nature of ferromagnetism can be understood on the basis of quantum concepts. Ferromagnetism is explained by the presence of electrons' own (spin) magnetic fields. In crystals of ferromagnetic materials, conditions arise under which, due to the strong interaction of the spin magnetic fields of neighboring electrons, their parallel orientation becomes energetically favorable. As a result of such interaction, spontaneously magnetized regions appear inside the ferromagnetic crystal. These areas are called domains. Each domain is a small permanent magnet.

Illustration of the magnetization process of a ferromagnetic sample: a - a substance in the absence of an external magnetic field: its individual atoms, which are small magnets, are located chaotically; b - magnetized substance: under the influence of an external field, atoms are oriented relative to each other in a certain order in accordance with the direction of the external field. Rice. from the site: http://encyclopaedia.biga.ru/enc/science_and_technology/ MAGNITI_I_MAGNITNIE_SVOSTVA_VESHCHESTVA.html

Domains in the theory of magnetism are small magnetized regions of a material in which the moments of the magnetic field of atoms are oriented parallel to each other. The domains are separated from each other by transition layers called Bloch walls. The figure shows two domains with opposite magnetic orientations and a Bloch wall between them with an intermediate orientation. Figure from the site: http://encyclopaedia.biga.ru/enc/science_and_technology/ MAGNITI_I_MAGNITNIE_SVOSTVA_VESHCHESTVA.html In the absence of an external magnetic field, the directions of the induction vectors of magnetic fields in various domains are randomly oriented in a large crystal. Such a crystal turns out to be non-magnetized. When an external magnetic field is applied, the domain boundaries shift so that the volume of domains oriented along the external field increases. With increasing induction of the external field, the magnetic induction of the magnetized substance increases. In a very strong magnetic external field, domains in which their own magnetic field coincides in direction with the external field absorb all other domains, and magnetic saturation occurs.

It should be remembered, however, that all these drawings and the domains and atoms depicted on them are just diagrams or models of real phenomena of magnetism, but not the phenomena themselves. They are used as long as they do not contradict observed facts.

A simple electromagnet designed to grip loads. The energy source is a DC battery. The field lines of the electromagnet are also shown, which can be detected by the usual method of iron filings. Figure from the site: http://encyclopaedia.biga.ru/enc/science_and_technology/ MAGNITI_I_MAGNITNIE_SVOSTVA_VESHCHESTVA.htmll

The appearance of a magnetic field in the vicinity of a conductor through which a direct electric current is passed is illustrated by an electromagnet. The current passes through a wire that is wound around a ferromagnetic rod. The magnetizing force in this case is equal to the product of the magnitude of the electric current in the coil and the number of turns in it. This power is measured in amperes. Magnetic field strength N equal to the magnetizing force per unit length of the coil. Thus, the value N measured in amperes per meter; it determines the magnetization acquired by the material inside the coil. In a vacuum magnetic induction B proportional to the magnetic field strength N. Magnetic field induction is a vector quantity, which is a force characteristic of a magnetic field. The direction of magnetic induction coincides with the direction indicated by the magnetic needle in a magnetic field, and the modulus of this vector is equal to the ratio of the modulus of the magnetic force that acts on a charged particle moving perpendicularly to the modulus of the velocity and charge of this particle. Magnetic induction according to SI is measured in teslas (T). In the GHS system, magnetic induction is measured in gauss (G). In this case, 1 T = 104 Gs.

Large electromagnets with iron cores and a very large number of turns, operating in continuous mode, have a large magnetizing force. They create magnetic induction in the gap between the poles of up to 6 Tesla (T). The amount of induction is limited by mechanical stress, heating of the coils and magnetic saturation of the core.

A number of giant water-cooled electromagnets (without a core) and installations for creating pulsed magnetic fields were designed by P.L. Kapitsa in Cambridge and at the Institute of Physical Problems of the USSR Academy of Sciences, as well as F. Bitter at the Massachusetts Institute of Technology. With such magnets it was possible to achieve induction of up to 50 Tesla. A relatively small electromagnet that produces fields of up to 6.2 Tesla, consumes 15 kW of electrical power and is cooled by liquid hydrogen, was developed at the Losalamos National Laboratory. Such magnetic fields are obtained at very low temperatures.

The magnetic induction vector is considered one of the physical quantities that is fundamental in the theory of electromagnetism; it can be found in a huge variety of equations, in some cases directly, and sometimes through the magnetic field strength associated with it. The only area in the classical theory of electromagnetism in which there is no magnetic induction vector is, perhaps, only pure electrostatics.

Ampere in 1825 suggested that electric microcurrents circulate in each atom of a magnet. But the electron was discovered only in 1897, and the model of the internal structure of the atom - in 1913, almost 100 years after Ampere's brilliant guess. In 1852, W. Weber suggested that each atom of a magnetic substance is a tiny magnetic dipole. The maximum or complete magnetization of a substance is achieved when all individual atomic magnets are arranged in a certain order. Weber believed that molecular or atomic “friction” helps these elementary magnets maintain their order. His theory explained the magnetization of bodies when they come into contact with a magnet and their demagnetization when impacted or heated. The “reproduction” of magnets when cutting a magnetized piece or magnetic rod into parts was also explained, when each part always had two poles. However, this theory did not explain either the origin of the elementary magnets themselves or the phenomenon of hysteresis. In 1890, Weber's theory was improved by J. Ewing, who replaced the hypothesis of atomic friction with the idea of ​​interatomic confining forces that help maintain the ordering of the elementary dipoles that make up a permanent magnet.

In 1905, P. Langevin explained the behavior of paramagnetic materials by attributing to each atom an internal uncompensated electron current. According to Langevin, it is these currents that form tiny magnets that are randomly oriented when there is no external magnetic field, but acquire an orderly orientation when one is applied. In this case, the approach to complete order corresponds to saturation of magnetization. Langevin introduced the concept of the magnetic moment of an atomic magnet, equal to the product of the “magnetic charge” and the distance between the poles. According to this theory, the weak magnetism of paramagnetic materials is explained by the weak total magnetic moment created by uncompensated electron currents.

In 1907, P. Weiss introduced the concept of a “domain,” which became an important contribution to the modern theory of magnetism. An individual domain can have linear dimensions of the order of 0.01 mm. The domains are separated from each other by so-called Bloch walls, the thickness of which does not exceed 1000 atomic sizes. Such walls represent “transition layers”, or microgradients in the magnetic nanostructure of a substance, in which a change in the direction of the magnetization of domains occurs. There are two convincing experimental confirmations of the existence of domains. In 1919, G. Barkhausen established that when an external field is applied to a sample of ferromagnetic material, its magnetization changes in small discrete portions. To identify the domain structure of a magnet using the powder figure method, a drop of a colloidal suspension of ferromagnetic powder (iron oxide) is applied to a well-polished surface of a magnetized material. Powder particles settle mainly in places of maximum inhomogeneity of the magnetic field - at the boundaries of domains. This structure can be studied under a microscope. A method for studying the magnetic field has been developed, based on the passage of polarized light through a transparent ferromagnetic material.

A free iron atom has two shells ( K And L), those closest to the nucleus are filled with electrons, with the first of them containing two and the second containing eight electrons. IN K-shell, the spin of one of the electrons is positive, and the other is negative. IN L-shell (more precisely, in its two subshells), four of the eight electrons have positive spins, and the other four have negative spins. In both cases, the electron spins within one shell are completely compensated, so that the total magnetic moment of the atom is zero. IN M-shell, the situation is different, since out of the six electrons located in the third subshell, five electrons have spins, direction

The sources of the magnetic field are moving electric charges (currents) . A magnetic field arises in the space surrounding current-carrying conductors, just as an electric field arises in the space surrounding stationary electric charges. The magnetic field of permanent magnets is also created by electric microcurrents circulating inside the molecules of a substance (Ampere's hypothesis).

To describe the magnetic field, it is necessary to introduce a force characteristic of the field, similar to the vector tensions electric field. This characteristic is magnetic induction vector The magnetic induction vector determines the forces acting on currents or moving charges in a magnetic field.
The positive direction of the vector is taken to be the direction from the south pole S to the north pole N of the magnetic needle, which is freely positioned in the magnetic field. Thus, by examining the magnetic field created by a current or a permanent magnet using a small magnetic needle, it is possible at every point in space

In order to quantitatively describe the magnetic field, it is necessary to indicate a method for determining not only
direction of the vector but and its moduleThe module of the magnetic induction vector is equal to the ratio of the maximum value
Ampere force acting on a straight conductor with current, to the current strength I in the conductor and its length Δ l :

The Ampere force is directed perpendicular to the magnetic induction vector and the direction of the current flowing through the conductor. To determine the direction of the Ampere force is usually used left hand rule: if you position your left hand so that the induction lines enter the palm, and the outstretched fingers are directed along the current, then the abducted thumb will indicate the direction of the force acting on the conductor.

Interplanetary magnetic field

If interplanetary space were a vacuum, then the only magnetic fields in it could only be the fields of the Sun and planets, as well as a field of galactic origin that extends along the spiral branches of our Galaxy. In this case, the fields of the Sun and planets in interplanetary space would be extremely weak.
In fact, interplanetary space is not a vacuum, but is filled with ionized gas emitted by the Sun (solar wind). The concentration of this gas is 1-10 cm -3, typical velocities are between 300 and 800 km/s, the temperature is close to 10 5 K (recall that the temperature of the corona is 2×10 6 K).
solar wind– outflow of plasma from the solar corona into interplanetary space. At the level of the Earth's orbit, the average speed of solar wind particles (protons and electrons) is about 400 km/s, the number of particles is several tens per 1 cm 3.

The English scientist William Gilbert, court physician to Queen Elizabeth, was the first to show in 1600 that the Earth is a magnet, the axis of which does not coincide with the axis of rotation of the Earth. Consequently, around the Earth, like around any magnet, there is a magnetic field. In 1635, Gellibrand discovered that the earth's magnetic field was slowly changing, and Edmund Halley conducted the world's first magnetic survey of the oceans and created the world's first magnetic maps (1702). In 1835, Gauss carried out a spherical harmonic analysis of the Earth's magnetic field. He created the world's first magnetic observatory in Göttingen.

A few words about magnetic cards. Typically, every 5 years, the distribution of the magnetic field on the Earth's surface is represented by magnetic maps of three or more magnetic elements. On each of these maps, isolines are drawn along which a given element has a constant value. Lines of equal declination D are called isogons, inclinations I are called isoclines, and magnitudes of total strength B are called isodynamic lines or isodines. The isomagnetic lines of the elements H, Z, X and Y are called isolines of the horizontal, vertical, northern or eastern components, respectively.

Let's return to the drawing. It shows a circle with an angular radius of 90° - d, which describes the position of the Sun on the earth's surface. The great circle arc drawn through point P and the geomagnetic pole B intersects this circle at points H' n and H' m, which indicate the position of the Sun, respectively, at the moments of geomagnetic noon and geomagnetic midnight of point P. These moments depend on the latitude of point P. Positions The sun at local true noon and midnight are indicated by points H n and H m, respectively. When d is positive (summer in the northern hemisphere), then the morning half of the geomagnetic day is not equal to the evening. At high latitudes, geomagnetic time can be very different from true or mean time for most of the day.
Speaking about time and coordinate systems, let's also talk about taking into account the eccentricity of the magnetic dipole. The eccentric dipole has been drifting slowly outward (north and west) since 1836. Has it crossed the equatorial plane? around 1862. Its radial trajectory is located in the area of ​​Gilbert Island in the Pacific Ocean

EFFECT OF MAGNETIC FIELD ON CURRENT

Within each sector, the solar wind speed and particle density vary systematically. Rocket observations show that both parameters increase sharply at the sector boundary. At the end of the second day after passing the sector boundary, the density very quickly, and then, after two or three days, it slowly begins to increase. The solar wind speed decreases slowly on the second or third day after reaching its peak. The sector structure and the noted variations in velocity and density are closely related to magnetospheric disturbances. The sector structure is quite stable, so the entire stream structure rotates with the Sun for at least several solar revolutions, passing over the Earth approximately every 27 days.

We still remember about the magnetic field from school, but what it represents is not something that “pops up” in everyone’s memories. Let's refresh what we've covered, and perhaps tell you something new, useful and interesting.

Determination of magnetic field

A magnetic field is a force field that affects moving electric charges (particles). Thanks to this force field, objects are attracted to each other. There are two types of magnetic fields:

1. Gravitational - is formed exclusively near elementary particles and varies in its strength based on the characteristics and structure of these particles.
2. Dynamic, produced in objects with moving electric charges (current transmitters, magnetized substances).

The designation for the magnetic field was first introduced by M. Faraday in 1845, although its meaning was a little erroneous, since it was believed that both electric and magnetic influence and interaction are carried out on the basis of the same material field. Later in 1873, D. Maxwell “presented” the quantum theory, in which these concepts began to be separated, and the previously derived force field was called the electromagnetic field.

How does a magnetic field appear?

The magnetic fields of various objects are not perceived by the human eye, and only special sensors can detect it. The source of the appearance of a magnetic force field on a microscopic scale is the movement of magnetized (charged) microparticles, which are:

• ions;
• electrons;
• protons.

Their movement occurs due to the spin magnetic moment that is present in each microparticle.

Magnetic field, where can it be found?

No matter how strange it may sound, almost all objects around us have their own magnetic field. Although in the concept of many, only a pebble called a magnet has a magnetic field, which attracts iron objects to itself. In fact, the force of attraction exists in all objects, only it manifests itself in less valence.

It should also be clarified that the force field, called magnetic, appears only when electric charges or bodies are moving.

Stationary charges have an electric force field (it can also be present in moving charges). It turns out that the sources of the magnetic field are:

• permanent magnets;
• moving charges.