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

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Earth's magnetic field

How did the theory of the magnetic field develop

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

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

What is magnetic field

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

Sources of magnetic field:

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

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Sources of magnetic field

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

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

Magnetic field characteristics

The characteristics of the magnetic field are:

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

F = B x S x cos α,

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

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Magnetic flux

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

How to imagine a magnetic field

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

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

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

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

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Magnetic field lines

Magnetic field measurement

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

Measuring other characteristics is much more difficult. Practical magnetometers did not appear until the 19th century. Most of them work by using the force that the electron senses when moving along the MP.

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Magnetometer

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

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

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

The nature of the occurrence of the magnetic field

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

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

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

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Ampere's law

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

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

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

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Right hand rule

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

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

Earth's magnetic field

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

Have to take:

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

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Tesla coil

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

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

Video

Good day, today you will find out what is magnetic field and where it comes from.

Every person on the planet at least once, but kept magnet in hand. Starting from souvenir fridge magnets, or working magnets for collecting iron pollen and much more. As a child, it was a funny toy that was glued to ferrous metal, but not to other metals. So what is the secret of the magnet and its magnetic field.

What is magnetic field

At what point does the magnet begin to attract to itself? Around each magnet there is a magnetic field, falling into which, objects begin to be attracted to it. The size of this field can vary depending on the size of the magnet and its intrinsic properties.

Term from wikipedia:

Magnetic field is a force field acting on moving electric charges and on bodies with a magnetic moment, regardless of the state of their movement, the magnetic component of the electromagnetic field.

Where does the magnetic field come from?

The magnetic field can be created by the current of charged particles or by the magnetic moments of electrons in atoms, as well as by the magnetic moments of other particles, although to a much lesser extent.

Magnetic field manifestation

A magnetic field manifests itself in the effect on the magnetic moments of particles and bodies, on moving charged particles or conductors with. The force acting on an electrically charged particle moving in a magnetic field is called the Lorentz force, which is always directed perpendicular to the vectors v and B. It is proportional to the particle charge q, which makes up the velocity v perpendicular to the direction of the magnetic field vector B, and the magnitude of the magnetic field B.

What objects have a magnetic field

We often don’t think about it, but very many (if not all) objects around us are magnets. We are accustomed to the fact that a magnet is a pebble with a pronounced force of attraction to itself, but in fact, almost everything has an attractive force, it is simply much lower. Take, for example, our planet - we do not fly into space, although we do not hold on to the surface with anything. The field of the Earth is much weaker than the field of a magnet-pebble, therefore it keeps us only due to its enormous size - if you have ever seen how people walk on the Moon (whose diameter is four times smaller), you will clearly understand what we are talking about ... The Earth's gravity is based in large part on metal components - its crust and core - they have a powerful magnetic field. You may have heard that compasses stop pointing to the correct north near large deposits of iron ore - this is because the principle of the compass is based on the interaction of magnetic fields, and iron ore attracts its needle.

Magnetic field and its characteristics

Lecture plan:

Magnetic field, its properties and characteristics.

A magnetic field- the form of existence of matter surrounding moving electric charges (conductors with current, 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 rotating on a needle was placed under a wire with a current. When the current was turned on, it was installed perpendicular to the wire; when the direction of the current was changed, it turned in the opposite direction.

The main 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, conductors with current, magnetized bodies;

an alternating magnetic field generates an alternating electric field.

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

The magnetic field is depicted graphically using magnetic lines of force or magnetic induction lines. Magnetic power lines called the lines along which iron filings are located in the magnetic field or the axes of small magnetic arrows. At each point of such a line, the vector is directed tangentially.

The lines of magnetic induction 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 through a unit area perpendicular to the magnetic field is proportional to the value 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 a 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 that of a strip magnet.

WITH

an olenoid with a current is an electromagnet.

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

The vector is introduced in one of 3 ways:

a) from Ampere's law;

b) by the action of the magnetic field on the frame with current;

c) from the expression for the Lorentz force.

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

current I and the vector product of the length element by the 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 left hand rule follows: if the palm of the left hand is positioned so that the magnetic lines of force enter it, and 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 a finite length is found by integrating over the entire length.

For 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 strength, located perpendicular to the magnetic lines of force.

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

So far, we have looked at the macro currents flowing in conductors. However, according to Ampere's assumption, in any body there are microscopic currents due to 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 micro currents, i.e. at the same macrocurrent, the vector has different values ​​in different media.

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

For a homogeneous isotropic medium

,

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

 - 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.

Stream vector(magnetic flux) through the site dS is called a scalar equal to

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

 is the angle between vectors and.

Directional surface element,

The flow of a vector is an algebraic quantity,

if - when leaving the surface;

if - at the entrance to 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 m 2, located perpendicular to a uniform magnetic field, the induction of which is equal to 1 T.

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

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

Bio-Savart-Laplace's law and its application for calculating magnetic fields.

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

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

- Bio-Savart-Laplace law.

It follows from the Bio-Sovar-Laplace law 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 Bio-Savart-Laplace law is of practical importance, since 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 principle of superposition together with the Biot-Savart-Laplace law makes it possible to relatively easily calculate specific magnetic fields.

Let's look at some examples.

A. Magnetic field of a straight conductor with 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 in 1820 experimentally discovered 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 the system of units and is equal to 1 in SI.

C
An integral over a closed contour is called the circularization of a vector.

This formula is called circulation theorem or total current law:

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The magnetic field can be created by the current of charged particles and / or the magnetic moments of electrons in atoms (and the magnetic moments of other particles, although to a much lesser extent) (permanent magnets).

In addition, it appears in the presence of a time-varying electric field.

The main force characteristic of the magnetic field is vector of magnetic induction (vector of magnetic field induction). From a mathematical point of view, it is a vector field that defines and concretizes the physical concept of a magnetic field. Often, the vector of magnetic induction is called simply the magnetic field for brevity (although this is probably not the most strict use of the term).

Another fundamental characteristic of the magnetic field (alternative magnetic induction and closely interconnected with it, practically equal to it in physical value) is vector potential .

A magnetic field can be called a special type of matter, through which interaction is carried out between moving charged particles or bodies with a magnetic moment.

Magnetic fields are a necessary (in context) consequence of the existence of electric fields.

• 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) - by a virtual one.

Sources of magnetic field

A magnetic field is created (generated) by a current of charged particles, or a time-varying electric field, or the intrinsic magnetic moments of particles (the latter, for uniformity of the picture, can be formally reduced to electric currents).

Calculation

In simple cases, the magnetic field of a conductor with a current (including for the case of a current distributed arbitrarily over volume or space) can be found from the Biot-Savard-Laplace law or the circulation theorem (aka Ampere's law). In principle, this method is limited to the case (approximation) of magnetostatics - that is, to the case of constant (if we are talking about strict applicability) or rather slowly varying (if we are talking about an approximate application) magnetic and electric fields.

In more difficult situations is sought as a solution to Maxwell's equations.

Magnetic field manifestation

A magnetic field manifests itself in the action on the magnetic moments of particles and bodies, on moving charged particles (or conductors with current). The force acting on an electrically charged particle moving in a magnetic field is called the Lorentz force, which is always directed perpendicular to the vectors v and B... It is proportional to the particle charge q component of speed v perpendicular to the direction of the magnetic field vector B, and the magnitude of the magnetic field induction B... In the SI system of units, the Lorentz force is expressed as follows:

in the CGS system of units:

where square brackets denote the cross product.

Also (due to the action of the Lorentz force on charged particles moving along the conductor), the magnetic field acts on the conductor with current. The force acting on a conductor with current is called the Ampere force. This force consists of forces acting on individual charges moving inside the conductor.

Interaction of two magnets

One of the most common manifestations of a magnetic field in everyday life is the interaction of two magnets: the same poles repel, the opposite ones attract. It seems tempting to describe the interaction between magnets as the interaction between two monopoles, and from a formal point of view, this idea is quite realizable and is often very convenient, and therefore practically useful (in calculations); However, detailed analysis shows that this is actually not a completely correct description of the phenomenon (the most obvious question that cannot be explained in the framework of such a model is the question of why monopoles can never be separated, that is, why does an experiment show that no isolated the body does not actually have a magnetic charge; in addition, the weakness of the model is that it is inapplicable to the magnetic field created by a macroscopic current, which means that if you do not consider it as a purely formal technique, it only leads to the complication of the theory in a fundamental sense).

It would be more correct to say that a magnetic dipole placed in an inhomogeneous field is acted upon by a force that tends to rotate it so that the magnetic moment of the dipole is aligned with the magnetic field. But no magnet experiences the action of a (total) force from a uniform magnetic field. Force acting on a magnetic dipole with a magnetic moment m expressed by the formula:

The force acting on a magnet (which is not a single point dipole) from the side of an inhomogeneous magnetic field can be determined by summing all the forces (determined by this formula) acting on the elementary dipoles that make up the magnet.

However, an approach is possible that reduces the interaction of magnets to the Ampere force, and the formula itself above for the force acting on a magnetic dipole can also be obtained based on the Ampere force.

The phenomenon of electromagnetic induction

Vector field H measured in amperes per meter (A / m) in SI and in oersteds in CGS. Oersteds and Gauss are identical quantities, their separation is purely terminological.

Magnetic field energy

The increment in the energy density of the magnetic field is equal to:

H- magnetic field strength, B- magnetic induction

In the linear tensor approximation, the magnetic permeability is a tensor (we denote it) and multiplication of a vector by it is tensor (matrix) multiplication:

or in components.

The energy density in this approximation is equal to:

- the components of the magnetic permeability tensor, - the tensor represented by the matrix inverse to the matrix of the magnetic permeability tensor, - the magnetic constant

When choosing the coordinate axes coinciding with the principal axes of the magnetic permeability tensor, the formulas in the components are simplified:

- the diagonal components of the magnetic permeability tensor in its own axes (the rest of the components in these special coordinates - and only in them! - are equal to zero).

In an isotropic linear magnet:

- relative magnetic permeability

In a vacuum and:

The energy of the magnetic field in the inductor can be found by the formula:

Ф - magnetic flux, I - current, L - inductance of a coil or coil with current.

Magnetic properties of substances

From a fundamental point of view, as indicated above, a magnetic field can be created (and therefore - in the context of this paragraph - and weakened or strengthened) by an alternating electric field, electric currents in the form of streams of charged particles or magnetic moments of particles.

The specific microscopic structure and properties of various substances (as well as their mixtures, alloys, states of aggregation, crystalline modifications, etc.) lead to the fact that at the macroscopic level they can behave quite diversely under the influence of an external magnetic field (in particular, weakening or increasing it to varying degrees).

In this regard, substances (and media in general) with regard to their magnetic properties are divided into the following main groups:

• Antiferromagnets are substances in which the antiferromagnetic order of the magnetic moments of atoms or ions has been established: the magnetic moments of substances are oppositely directed and equal in strength.
• Diamagnets are substances that are magnetized against the direction of an external magnetic field.
• Paramagnets are substances that are magnetized in an external magnetic field in the direction of an external magnetic field.
• Ferromagnets are substances in which a long-range ferromagnetic order of magnetic moments is established below a certain critical temperature (Curie point)
• Ferrimagnets are materials in which the magnetic moments of a substance are directed oppositely and are not equal in strength.
• The groups of substances listed above mainly include ordinary solid or (to some) liquid substances, as well as gases. The interaction with the magnetic field of superconductors and plasma is significantly different.

Toki Foucault

Foucault currents (eddy currents) are closed electric currents in a massive conductor that occur when the magnetic flux penetrating it changes. They are induction currents generated in a conducting body either as a result of a change in time of the magnetic field in which it is located, or as a result of the movement of a body in a magnetic field, leading to a change in the magnetic flux through the body or any part of it. According to Lenz's rule, the magnetic field of Foucault currents is directed so as to counteract the change in magnetic flux that induces these currents.

The history of the development of ideas about the magnetic field

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

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

Expanding on these experiments, Ampere published his own successful model of magnetism in 1825. In it, he showed the equivalence electric current in magnets, and instead of magnetic charge dipoles of the Poisson model, proposed the idea that magnetism is associated with constantly flowing current loops. This idea explained why a magnetic charge could not be isolated. In addition, Ampere deduced a law named after him, which, like the Bio-Savard-Laplace law, correctly described the magnetic field created by direct current, and also introduced the theorem on the circulation of the magnetic field. Also in this work, Ampere coined the term "electrodynamics" to describe the relationship between electricity and magnetism.

Although the magnetic field strength of a moving electric charge implied in Ampere's law was not explicitly stated, in 1892 Hendrik Lorentz derived it from Maxwell's equations. In this case, the classical theory of electrodynamics was basically completed.

The twentieth century expanded the views on electrodynamics, thanks to the emergence of the theory of relativity and quantum mechanics. Albert Einstein, in his 1905 article, where his theory of relativity was substantiated, showed that electric and magnetic fields are part of the same phenomenon, considered in different frames of reference. (See Moving Magnet and the Conductor Problem — The Thought Experiment That Ultimately Aid Einstein in Developing Special Relativity.) Finally, quantum mechanics was combined with electrodynamics to form quantum electrodynamics (QED).

see also

• Magnetic film visualizer

Notes (edit)

1. TSB. 1973, "Soviet Encyclopedia".
2. In special cases, a magnetic field can exist in the absence of an electric field, but generally speaking, the magnetic field is deeply interconnected with the electric one both dynamically (mutual generation of alternating electric and magnetic fields from each other) and in the sense that when passing to a new frame of reference, the magnetic and the electric field are expressed through each other, that is, generally speaking, they cannot be unconditionally separated.
3. Yavorskiy B.M., Detlaf A.A. Physics Handbook: 2nd ed., Rev. - M .: Nauka, Main edition of physical and mathematical literature, 1985, - 512 p.
4. In SI, magnetic induction is measured in teslas (T), in the CGS system in gauss.
5. They exactly coincide in the CGS system of units, in SI - they differ in a constant coefficient, which, of course, does not change the fact of their practical physical identity.
6. The most important and surface difference here is that the force acting on a moving particle (or on a magnetic dipole) is calculated exactly through and not through. Any other physically correct and meaningful measurement method will also make it possible to measure, although for a formal calculation it sometimes turns out to be more convenient - what, in fact, is the point of introducing this auxiliary quantity (otherwise it would be possible to do without it at all, using only
7. However, it should be well understood that a number of fundamental properties of this "matter" are fundamentally different from the properties of the usual type of "matter", which could be designated by the term "substance".
8. See Ampere's Theorem.
9. For a homogeneous field, this expression gives zero force, since all derivatives are equal to zero B by coordinates.
10. Sivukhin D.V. General course of physics. - Ed. 4th, stereotypical. - M .: Fizmatlit; Publishing house of MIPT, 2004. - T. III. Electricity. - 656 p. - ISBN 5-9221-0227-3; ISBN 5-89155-086-5.

Earth's magnetic field

A magnetic field is a force field that acts on moving electric charges and on bodies with a magnetic moment, regardless of the state of their motion.

Sources of a macroscopic magnetic field are magnetized bodies, conductors with current, and moving electrically charged bodies. The nature of these sources is the same: the magnetic field arises as a result of the movement of charged microparticles (electrons, protons, ions), as well as due to the presence of an intrinsic (spin) magnetic moment in the microparticles.

An alternating magnetic field also occurs when the electric field changes over time. In turn, when the magnetic field changes with time, an electric field arises. A complete description of the electric and magnetic fields in their relationship is given by Maxwell's equations. To characterize the magnetic field, the concept of field lines of force (lines of magnetic induction) is often introduced.

Various types of magnetometers are used to measure the characteristics of the magnetic field and the magnetic properties of substances. The unit of magnetic field induction in the CGS system is Gauss (G), in the International System of Units (SI) - Tesla (T), 1 T = 104 G. The intensity is measured, respectively, in oersteds (E) and amperes per meter (A / m, 1 A / m = 0.01256 Oe; the energy of the magnetic field - in Erg / cm 2 or J / m 2, 1 J / m 2 = 10 erg / cm 2.

The compass reacts
on the earth's magnetic field

Magnetic fields in nature are extremely diverse both in their scale and in the effects they cause. The Earth's magnetic field, which forms the Earth's magnetosphere, extends to a distance of 70-80 thousand km towards the Sun and many millions of km in the opposite direction. At the Earth's surface, the magnetic field is on average 50 μT, at the boundary of the magnetosphere ~ 10 -3 G. The geomagnetic field screens the Earth's surface and biosphere from the flow of charged particles of the solar wind and partially cosmic rays. The influence of the geomagnetic field itself on the vital activity of organisms is studied by magnetobiology. In near-earth space, the magnetic field forms a magnetic trap for high-energy charged particles - the Earth's radiation belt. Particles contained in the radiation belt pose a significant hazard when flying into space. The origin of the Earth's magnetic field is associated with the convective motions of conducting liquid matter in the earth's core.

Direct measurements using spacecraft have shown that the space bodies closest to the Earth - the Moon, the planets Venus and Mars - do not have their own magnetic field, similar to that of the Earth. From other planets Solar system only Jupiter and, apparently, Saturn have their own magnetic fields, sufficient to create planetary magnetic traps. Jupiter found magnetic fields up to 10 G and a number of characteristic phenomena (magnetic storms, synchrotron radio emission, and others), indicating a significant role of the magnetic field in planetary processes.

© Photo: http://www.tesis.lebedev.ru
Photo of the Sun
in a narrow spectrum

The interplanetary magnetic field is mainly the field of the solar wind (the continuously expanding plasma of the solar corona). Near the Earth's orbit, the interplanetary field is ~ 10 -4 -10 -5 G. The regularity of the interplanetary magnetic field can be disrupted due to the development different types plasma instability, the passage of shock waves and the propagation of streams of fast particles generated by solar flares.

In all processes on the Sun - flares, the appearance of spots and prominences, the birth of solar cosmic rays, the magnetic field plays crucial role... Measurements based on the Zeeman effect have shown that the magnetic field of sunspots reaches several thousand G, prominences are held by fields of ~ 10-100 G (with an average value of the total magnetic field of the Sun of ~ 1 G).

Magnetic storms

Magnetic storms are strong perturbations of the Earth's magnetic field, sharply disrupting the smooth daily course of the elements of terrestrial magnetism. Magnetic storms last from several hours to several days and are observed simultaneously all over the Earth.

As a rule, magnetic storms consist of preliminary, initial and main phases, as well as a recovery phase. In the preliminary phase, slight changes in the geomagnetic field are observed (mainly at high latitudes), as well as the excitation of characteristic short-period field oscillations. The initial phase is characterized by a sudden change in the individual components of the field throughout the Earth, and the main phase is characterized by large field fluctuations and a strong decrease in the horizontal component. During the recovery phase of the magnetic storm, the field returns to its normal value.

Influence of the solar wind
to the Earth's magnetosphere

Magnetic storms are caused by streams of solar plasma from the active regions of the Sun, superimposed on the calm solar wind. Therefore, magnetic storms are more often observed near the maxima of the 11-year cycle solar activity... Reaching the Earth, the solar plasma streams increase the compression of the magnetosphere, causing the initial phase of the magnetic storm, and partially penetrate into the Earth's magnetosphere. The penetration of high-energy particles into the Earth's upper atmosphere and their effect on the magnetosphere lead to the generation and amplification of electric currents in it, reaching the highest intensity in the polar regions of the ionosphere, which is associated with the presence of a high-latitude zone of magnetic activity. Changes in magnetospheric-ionospheric current systems manifest themselves on the Earth's surface in the form of irregular magnetic disturbances.

In the phenomena of the microworld, the role of the magnetic field is just as important as on the cosmic scale. This is explained by the existence of all particles - structural elements of matter (electrons, protons, neutrons), a magnetic moment, as well as the action of a magnetic field on moving electric charges.

Application of magnetic fields in science and technology. Magnetic fields are usually subdivided into weak (up to 500 Gs), medium (500 G - 40 kG), strong (40 kG - 1 MG) and superstrong (over 1 MG). Almost all electrical engineering, radio engineering and electronics are based on the use of weak and medium magnetic fields. Weak and medium magnetic fields are obtained using permanent magnets, electromagnets, uncooled solenoids, superconducting magnets.

Sources of magnetic field

All sources of magnetic fields can be divided into artificial and natural. The main natural sources of the magnetic field are the own magnetic field of the planet Earth and the solar wind. Artificial sources include all the electromagnetic fields that are so abundant in our modern world, and our homes in particular. More details about, and read on ours.

Electric vehicles are a powerful source of magnetic fields in the range from 0 to 1000 Hz. Rail transport uses alternating current. City transport is permanent. The maximum values ​​of the magnetic field induction in suburban electric transport reach 75 μT, the average values ​​are about 20 μT. The average values ​​for DC-driven vehicles are fixed at 29 μT. In trams, where the return wire is rails, the magnetic fields compensate each other at a much greater distance than in the trolleybus wires, and inside the trolleybus, the fluctuations of the magnetic field are small even during acceleration. But the biggest fluctuations in the magnetic field are in the subway. When the train departs, the magnetic field on the platform is 50-100 μT and more, exceeding the geomagnetic field. Even when the train disappeared long ago in the tunnel, the magnetic field does not return to its previous value. Only after the train has passed the next point of connection to the contact rail, the magnetic field will return to the old value. True, sometimes it does not have time: the next train is already approaching the platform and when it brakes, the magnetic field changes again. In the carriage itself, the magnetic field is even stronger - 150-200 μT, that is, ten times more than in a conventional electric train.

The values ​​of the induction of magnetic fields most often encountered in our daily life are shown in the diagram below. Looking at this diagram, it becomes clear that we are exposed to magnetic fields all the time and everywhere. According to some scientists, magnetic fields with an induction of more than 0.2 μT are considered harmful. Naturally, certain precautions should be taken to protect ourselves from the harmful effects of the fields around us. Just by following a few simple rules, you can significantly reduce the effect of magnetic fields on your body.

The current SanPiN 2.1.2.2801-10 "Changes and additions No. 1 to SanPiN 2.1.2.2645-10" Sanitary and epidemiological requirements for living conditions in residential buildings and premises "says the following:" The maximum permissible level of weakening of the geomagnetic field in the premises of residential buildings is established equal to 1.5 ". Also, the maximum permissible values ​​\ u200b \ u200bof the intensity and intensity of the magnetic field with a frequency of 50 Hz have been established:

• in residential premises - 5 μT or 4 A / m;
• in non-residential premises of residential buildings, in residential areas, including on the territory of garden plots - 10 μT or 8 A / m.

Based on the specified standards, everyone can calculate how many electrical appliances can be turned on and standby in each specific room, or, on the basis of which recommendations will be issued for normalizing the living space.

Related Videos

A short scientific film about the Earth's magnetic field

References

1. Great Soviet Encyclopedia.