What is a magnetic field definition. Magnetic field, its properties and characteristics

Let's understand together what a magnetic field is. After all, many people live in this field all their lives and do not even think about it. Time to fix it!

A magnetic field

A magnetic field – special kind matter. It manifests itself in action on moving electric charges and bodies that have their own magnetic moment (permanent magnets).

Important: a magnetic field does not act on stationary charges! A magnetic field is also created by moving electric charges, or by a time-varying electric field, or by the magnetic moments of electrons in atoms. That is, any wire through which current flows also becomes a magnet!

A body that has its own magnetic field.

A magnet has poles called north and south. The designations "northern" and "southern" are given only for convenience (as "plus" and "minus" in electricity).

The magnetic field is represented by power magnetic lines . The lines of force are continuous and closed, and their direction always coincides with the direction of the field forces. If metal shavings are scattered around a permanent magnet, metal particles will show a clear picture of field lines. magnetic field leaving the north and entering the south pole. Graphical characteristic of the magnetic field - lines of force.

Magnetic field characteristics

The main characteristics of the magnetic field are magnetic induction, magnetic flux and magnetic permeability. But let's talk about everything in order.

Immediately, we note that all units of measurement are given in the system SI.

Magnetic induction B - vector physical quantity, which is the main power characteristic of the magnetic field. Denoted by letter B . The unit of measurement of magnetic induction - Tesla (Tl).

Magnetic induction indicates how strong a field is by determining the force with which it acts on a charge. This force is called Lorentz force.

Here q - charge, v - its speed in a magnetic field, B - induction, F is the Lorentz force with which the field acts on the charge.

F- a physical quantity equal to the product of magnetic induction by the area of ​​the contour and the cosine between the induction vector and the normal to the plane of the contour through which the flow passes. Magnetic flux is a scalar characteristic of a magnetic field.

We can say that the magnetic flux characterizes the number of magnetic induction lines penetrating a unit area. The magnetic flux is measured in Weberach (WB).

Magnetic permeability is the coefficient that determines the magnetic properties of the medium. One of the parameters on which the magnetic induction of the field depends is the magnetic permeability.

Our planet has been a huge magnet for several billion years. The induction of the Earth's magnetic field varies depending on the coordinates. At the equator, it is about 3.1 times 10 to the minus fifth power of Tesla. In addition, there are magnetic anomalies, where the value and direction of the field differ significantly from neighboring areas. One of the largest magnetic anomalies on the planet - Kursk and Brazilian magnetic anomaly.

The origin of the Earth's magnetic field is still a mystery to scientists. It is assumed that the source of the field is the liquid metal core of the Earth. The core is moving, which means that the molten iron-nickel alloy is moving, and the movement of charged particles is the electric current that generates the magnetic field. The problem is that this theory geodynamo) does not explain how the field is kept stable.

The earth is a huge magnetic dipole. The magnetic poles do not coincide with the geographic ones, although they are in close proximity. Moreover, the Earth's magnetic poles are moving. Their displacement has been recorded since 1885. For example, over the past hundred years, the magnetic pole in the Southern Hemisphere has shifted by almost 900 kilometers and is now in the Southern Ocean. The pole of the Arctic hemisphere is moving across the Arctic Ocean towards the East Siberian magnetic anomaly, the speed of its movement (according to 2004 data) was about 60 kilometers per year. Now there is an acceleration of the movement of the poles - on average, the speed is growing by 3 kilometers per year.

What is the significance of the Earth's magnetic field for us? First of all, the Earth's magnetic field protects the planet from cosmic rays and the solar wind. Charged particles from deep space do not fall directly to the ground, but are deflected by a giant magnet and move along its lines of force. Thus, all living things are protected from harmful radiation.

During the history of the Earth, there have been several inversions(changes) of magnetic poles. Pole inversion is when they change places. The last time this phenomenon occurred about 800 thousand years ago, and there were more than 400 geomagnetic reversals in the history of the Earth. Some scientists believe that, given the observed acceleration of the movement of the magnetic poles, the next pole reversal should be expected in the next couple of thousand years.

Fortunately, no reversal of poles is expected in our century. So, you can think about the pleasant and enjoy life in the good old constant field of the Earth, having considered the main properties and characteristics of the magnetic field. And so that you can do this, there are our authors, who can be entrusted with some of the educational troubles with confidence in success! and other types of work you can order at the link.

We still remember about the magnetic field from school, that's just what it is, "pops up" in the memories of not everyone. Let's refresh what we've been through, and maybe tell you something new, useful and interesting.

Determination of the magnetic field

A magnetic field is a force field that acts on moving electric charges (particles). Due 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 viruetsya 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).

For the first time, the designation of the magnetic field was introduced by M. Faraday in 1845, although its meaning was a little erroneous, since it was believed that both electric and magnetic effects and interactions are based on 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 fix 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, which is present in each microparticle.

Magnetic field, where can it be found?

No matter how strange it may sound, but 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 is in all objects, it only manifests itself in a lower valence.

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

Immovable 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;
• mobile charges.

A magnetic field this is the matter that arises around sources of electric current, as well as around permanent magnets. In space, the magnetic field is displayed as a combination of forces that can affect magnetized bodies. This action is explained by the presence of driving discharges at the molecular level.

The magnetic field is formed only around electric charges that are in motion. That is why the magnetic electric field are integral and together form electromagnetic field. The components of the magnetic field are interconnected and act on each other, changing their properties.

Magnetic field properties:
1. The magnetic field arises under the influence of driving charges of electric current.
2. At any point, the magnetic field is characterized by the vector physical quantity entitled magnetic induction, which is the force characteristic of the magnetic field.
3. The magnetic field can only affect magnets, conductive conductors and moving charges.
4. The magnetic field can be of constant and variable type
5. The magnetic field is measured only by special devices and cannot be perceived by the human senses.
6. The magnetic field is electrodynamic, as it is generated only during the movement of charged particles and affects only the charges that are in motion.
7. Charged particles move along a perpendicular trajectory.

The size of the magnetic field depends on the rate of change of the magnetic field. Accordingly, there are two types of magnetic field: dynamic magnetic field and gravitational magnetic field. Gravitational magnetic field arises only near elementary particles and is formed depending on the structural features of these particles.

Magnetic moment occurs when a magnetic field acts on a conductive frame. In other words, the magnetic moment is a vector that is located on the line that runs perpendicular to the frame.

The magnetic field can be represented graphically using magnetic lines of force. These lines are drawn in such a direction that the direction of the field forces coincides with the direction of the field line itself. Magnetic field lines are continuous and closed at the same time.

The direction of the magnetic field is determined using a magnetic needle. The lines of force also determine the polarity of the magnet, the end with the exit of the lines of force is the north pole, and the end with the entrance of these lines is the south pole.

It is very convenient to visually assess the magnetic field using ordinary iron filings and a piece of paper.
If we put a sheet of paper on a permanent magnet, and sprinkle sawdust on top, then the iron particles will line up according to the magnetic field lines.

The direction of the lines of force for the conductor is conveniently determined by the famous gimlet rule or rule right hand . If we wrap our arms around the conductor so that thumb looked in the direction of the current (from minus to plus), then the 4 remaining fingers will show us the direction of the magnetic field lines.

And the direction of the Lorentz force - the force with which the magnetic field acts on a charged particle or conductor with current, according to left hand rule.
If we place left hand in a magnetic field so that 4 fingers looked in the direction of the current in the conductor, and the lines of force entered the palm, then the thumb will indicate the direction of the Lorentz force, the force acting on the conductor placed in a magnetic field.

That's about it. Be sure to ask any questions in the comments.

So far, we have considered the magnetic field created by current-carrying conductors. However, a magnetic field is created and permanent magnets, in which there is no electric current, in the sense that the charged particles do not make a directed movement along the conductor. Even before the discovery of Oersted, the magnetic field of permanent magnets was tried to be explained by the presence of magnetic charges located in the body, just as electric charges create an electric field. The opposite poles of a magnet were considered to be concentrations of magnetic charges of different signs. However, the first difficulty was the impossibility of separating these poles. After cutting the bar magnet it was not possible to separate the north and south poles- it turned out two magnets, each of which had both a north and a south pole. The search for magnetic charges (“monopoles”) continues to this day, and so far without success. Ampère offered a more natural explanation. Since a coil with current creates a field similar to the field of a bar magnet, Ampère suggested that in matter, or rather in atoms, there are charged particles that make Roundabout Circulation, and thus creating circular "atomic" currents.

This idea was in good agreement with Rutherford's later proposed model of the atom. It is also clear why matter in the ordinary state practically does not exhibit magnetic properties. In order for the fields of different "coils" to add up, they must be arranged as shown in the figure so that their fields are oriented in the same direction. But by force thermal motion, their directions are randomly oriented with respect to each other in all directions. And since the magnetic fields are added according to the vector law, the total field is equal to zero. This is true for most metals and other substances. Ordering atomic currents is possible only in certain metals, called ferromagnets. It is in them that the magnetic properties are manifested very noticeably. Many metals, such as copper and aluminum, do not show noticeable magnetic properties, for example, cannot be magnetized. Most famous example ferromagnet - iron. There are rather large areas in it compared to the size of an atom (10 -6 -10 -4 cm) - domains, in which the atomic currents are already strictly ordered. The regions themselves are randomly located in relation to each other - the metal is not magnetized. By placing it in a magnetic field, we can transfer the domains to an ordered state - to magnetize the metal, and, by removing the external field, we will retain its magnetization. In the process of magnetization, the domains with the orientation of atomic currents along the external field grow, while the others decrease. We have seen that a coil with a current in a magnetic field is rotated by Ampère's force so that its magnetic field is established along the external field. This is the equilibrium position of the coil, which he seeks to occupy. After the external field is turned off, the orientation of the atomic currents is preserved. Some grades of steel retain their magnetization very stable - they can be used to make permanent magnets. Other grades are easily remagnetized, they are suitable for the production of electromagnets. If a ferromagnetic rod is placed in a solenoid, then the field created in it will increase by 10-20 thousand times.

Thus, a magnetic field is always created electric shock , or flowing through the conductor, when the charges move over distances many times greater than atomic (such currents are called macroscopic), or microscopic(atomic) currents.

Earth's magnetic field. One of the first observations of the magnetic field and its use for applied purposes was the detection of the Earth's magnetic field. AT ancient China a magnetic needle (bar magnet) was used to determine the direction to the north, which is also done in modern compasses. Obviously, in the inner part of the Earth there are some currents, which lead to the appearance of a small (about 10 -4 T) magnetic field. If we assume that it is associated with the rotation of the Earth, there are circular currents inside it around its axis, and the corresponding magnetic field (like the field of a coil) should be oriented inside the Earth along the axis of its rotation. The lines of induction should look like the one shown in the picture.

It can be seen that the north magnetic pole of the Earth is located near its south geographic pole. The lines of induction close in outer space, and near the surface of the earth they are oriented along geographic meridians. It is along them in the north direction that the northern end of the magnetic needle is set. Another important phenomenon is connected with the Earth's magnetic field. From space to the earth's atmosphere comes a large number of elementary particles, some are charged. The magnetic field acts as a barrier for them to enter the lower atmosphere, where they can be dangerous. Considering the motion of a charged particle in a magnetic field under the action of the Lorentz force, we saw that it begins to move along a helical line along the line of magnetic field induction. This is what happens to charged particles in upper layers atmosphere. Moving along the lines, they "leave" to the poles, and enter the atmosphere near the geographic poles. When they interact with molecules, a glow occurs (the emission of light by atoms), which creates the northern lights. They are not observed in non-polar latitudes.

Tangent measuring instruments. To measure the magnitude of the induction of an unknown magnetic field (for example, the Earth), it is reasonable to propose a way to compare this field with some known one. For example, with a long forward current field. Tangent method provides a way to compare. Suppose we want to measure the horizontal component of the Earth's magnetic field at some point. Let's place a long vertical wire next to it so that its middle is close to this point, and the length is much greater than the distance to it (figure, top view).

If the current does not flow in the wire, then the magnetic needle at the observation point will be established along the Earth's field (in the figure - up, along the east). We will increase the current in the wire. The arrow starts to deviate to the left. Since the current field V T appears, directed horizontally in the figure. The full field is directed along the diagonal of the rectangle, as required by the rule of adding the vectors B and B T. When the current reaches a certain value I 0 , the angle formed by the arrow will become 45 0 . This means that the equality В З \u003d В Т was fulfilled. But the field В Т is known to us. By measuring x and I 0 with an ammeter, you can calculate V T, and therefore V Z. The method is called tangent because the condition is met.

Sources permanent magnetic fields (PMF) workplaces are permanent magnets, electromagnets, high-current systems direct current(DC transmission lines, electrolyte baths, etc.).

Permanent magnets and electromagnets are widely used in instrumentation, magnetic washers for cranes, magnetic separators, magnetic water treatment devices, magnetohydrodynamic generators (MHD), nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), as well as in physiotherapy practice.

Main physical parameters characterizing the PMP are field strength (N), magnetic flux (F) and magnetic induction (V). In the SI system, the unit of measurement of the magnetic field strength is ampere per meter (A/m), magnetic flux - Weber (Wb ), magnetic flux density (magnetic induction) - tesla (Tl ).

Changes in the state of health of persons working with PMF sources were revealed. Most often, these changes manifest themselves in the form of vegetative dystonia, asthenovegetative and peripheral vasovegetative syndromes, or a combination thereof.

According to the standard in force in our country (“Maximum acceptable levels exposure to constant magnetic fields when working with magnetic devices and magnetic materials ”No. 1742-77), the PMF intensity at workplaces should not exceed 8 kA / m (10 mT). Permissible levels of PMF recommended by the International Committee on Non-Ionizing Radiation (1991) are differentiated by the contingent, the place of exposure and the time of work. For professionals: 0.2 Tl - when exposed to a full working day (8 hours); 2 Tl - with a short-term effect on the body; 5 Tl - with a short-term impact on the hands. For the population, the level of continuous exposure to PMF should not exceed 0.01 T.

RF EMP sources are widely used in the most various industries National economy. They are used to transmit information at a distance (broadcasting, radiotelephone communications, television, radar, etc.). In industry, electromagnetic radiation of the radio wave range is used for induction and dielectric heating of materials (hardening, melting, soldering, welding, metal deposition, heating of internal metal parts electrovacuum devices in the process of pumping, drying wood, heating plastics, gluing plastic compounds, heat treatment food products and etc.). EMR is widely used in scientific research(radiospectroscopy, radio astronomy) and medicine (physiotherapy, surgery, oncology). In a number of cases, EMR occur as a side unused factor, for example, near overhead power lines (OL), transformer substations, electrical appliances, including household purpose. The main sources of EMF RF radiation in environment serve as antenna systems of radar stations (RLS), radio and television and radio stations, including mobile radio systems and overhead power lines.

The human and animal body is very sensitive to the effects of RF EMF.

Critical organs and systems include: central nervous system, eyes, gonads, and according to some authors, the hematopoietic system. The biological effect of these radiations depends on the wavelength (or radiation frequency), generation mode (continuous, pulsed) and conditions of exposure to the body (constant, intermittent; general, local; intensity; duration). It is noted that biological activity decreases with increasing wavelength (or decreasing frequency) of radiation. The most active are centi-, deci-, and meter-wave bands. Injuries caused by RF EMR can be acute or chronic. Acute ones arise under the action of significant thermal radiation intensities. They are extremely rare - in case of accidents or gross violations of safety regulations at the radar. For professional conditions more characteristic are chronic lesions, which are detected, as a rule, after several years of work with microwave EMR sources.

Main normative documents that regulate the permissible levels of exposure to RF EMR are: GOST 12.1.006 - 84 “SSBT. Electromagnetic fields of radio frequencies.

Permissible levels "and SanPiN 2.2.4 / 2.1.8.055-96" electromagnetic radiation radio frequency band". They normalize the energy exposure (EE) for electric (E) and magnetic (H) fields, as well as the energy flux density (PEF) for a working day (Table 5.11).

Table 5.11.

Maximum allowable levels (MPL) per working day for employees

With EMI RF

Parameter	Frequency bands, MHz
Name	unit of measurement	0,003-3	3-30	30-300	300-300000
EE E	(W/m) 2 *h				-
uh n	(A/m) 2 *h		-	-	-
ppe	(μW / cm 2) * h	-	-	-

For the entire population under continuous exposure, the following maximum strength levels are established electric field, V/m:

Frequency range MHz

0,03-0,30........................................................... 25

0,3-3,0.............................................................. 15

3-30.................................................................. 10

30-300............................................................... 3*

300-300000...................................................... 10

* Except for TV stations, the remote controls for which are differentiated according to

depending on the frequency from 2.5 to 5 V/m.

The number of devices operating in the radio frequency range includes video displays of personal computer terminals. Nowadays personal computers(PC) are widely used in production, in scientific research, in medical institutions, at home, in universities, schools and even kindergartens. When used in the production of PCs, depending on technological tasks, they can affect the human body for a long time (within a working day). AT living conditions PC usage time is generally out of control.

For PC video display terminals (VDT), the following EMI remote controls are installed (SanPiN 2.2.2.542-96 “Hygienic requirements for video display terminals, personal electronic computers and organization of work”) - table. 5.12.

Table 5.12. Maximum allowable levels of EMP generated by VDT