In the early 80s, the agricultural sector of the USSR demanded from the industry a multifunctional all-wheel drive truck that could equally well transport agricultural products directly from the field to warehouses, as well as from the village to processing enterprises. Moreover, the terms of reference talked about the special suitability of the truck to work in tandem with agricultural machinery - tractors, combines - directly in the field. That is, a technological vehicle with off-road and asphalt capabilities was needed. In 1982, NAMI engineers, together with specialists from the Kutaisi Automobile Plant, developed the KAZ-4540 dump truck, which was put into production two years later. The car was really new, original and - unconventionally for the Soviet industry - had a very low percentage of unification with the already produced serial equipment.
The alleged competitor of Colchis could be the British universal Bedford TM 4-4 of 1981, which was created for the armies of the NATO countries, but could do everything the same as our agricultural dump truck.
KAZ-4540
Outwardly, the cars are quite similar: the same flat, "licked" cabover cabs, round headlights in the bumper, single tires with off-road tread and high ground clearance. Visually, due to the narrow cab with a smaller glass area, the foreign car seems taller, although our virtual competitors are almost the same in height. The carrying capacity of Colchis according to the passport was 6 tons. KAZ-4540 was mainly equipped with a dump body with three-way unloading, but in small batches at the factory, and after that, in artisanal conditions, various specialized equipment was installed on its chassis. Bedford TM most often served as a carrier of special installations or a evacuator of light armored vehicles and other trucks and was capable of carrying loads weighing 6.5–8 tons (depending on the version).
There is not much to say about the interior of the compared trucks. Both in our KAZ and in the "British" dashboard, the steering wheel and door cards are made of "oak" plastic, the round large instruments are simple and informative, the control of many functions is "delegated" to unified rectangular switches, and the double cabs of both cars did not receive sleeping places - after all, they are designed for movement along local routes.
BedfordTM
The all-wheel drive KAZ-4540 was equipped with an eight-cylinder production diesel engine, the power of which was 160 hp. The power unit was not located strictly under the cab, but with a slight offset towards the body. Paired with a diesel engine, an eight-speed manual gearbox, combined with a single-stage transfer case, worked. Interestingly, to simplify maintenance, the box is not rigidly connected to the engine, but through an intermediate cardan shaft. A specially designed gearbox allows the truck to move for a long time on arable land parallel to the combine with a minimum speed of 2 km / h. Of the off-road “gadgets”, the dump truck could boast of locking the cross-axle differential of the rear axle.
An 8.2-liter turbodiesel with 206 hp was hidden under the Bedford's cab. in combination with a six-speed manual transmission, a "razdatka" was placed behind the box. The suspensions of the machines are structurally the same - on four longitudinal springs. Due to poor off-road capability and low reliability of engines, NATO abandoned British vehicles by the end of the 80s.
KAZ-4540
GAZ-4301 - Renault Midliner S100
Since the 1960s, the creators have followed two paths to the creation of medium-duty delivery trucks - either the American one, with a bonnet layout, or the European one, with a cab above the engine. In the USSR, the bonnet layout was always preferred, and the new GAZ-4301 of 1984, which replaced the GAZ-53, became the same. In the same years in Europe, Renault, together with Saviem, Volvo, DAF, and Magirus-Deutz, having created the "Club of Four", cooperated and by 1980-81 had designed a single universal model, which in the "renault" version was called the Renault Midliner S100.
The designers designed the new truck from Gorky in the same style as the more load-bearing ZIL-169: the GAZ-4301 has a square radiator grill, headlights in angular fenders and a front tapering towards the nose. The cabover Midliner also has an angular cabin, an unpainted plastic grille, but overall looks more modern, as it turned out to be similar to cabover cars from the 90s and early zero.
GAZ-4301
At first glance, the interiors of the compared cars look similar. They are related by cheap rough plastic, simple round instruments, a whole panel of lamp indicators, air vents and a roomy glove box. But a closer look shows that the foreign car offers more comfort to the driver. Its steering wheel is soft, and not made of hard plastic, the gearshift lever is located closer to the driver's workplace, the seats have armrests, there is a regular place for the radio and audio preparation. For an additional fee, Renault could be purchased with an extended sleeper cab. GAZ-4301 with a seat for the driver's rest was not mass-produced.
The GAZon was equipped with a 6.2-liter GAZ-542 six-cylinder diesel engine with a capacity of 142 hp. air-cooled, which was a universal solution for a country with a large number of climatic zones. The motor was a licensed copy of the Deutz unit, and its resource before overhaul was calculated at the level of 300 thousand km. A five-speed manual gearbox was developed in-house. In order to seamlessly use the truck in tandem with agricultural vehicles at ultra-low speeds, the gear ratio of the first gear was made large. The drive was traditionally on the rear axle.
Renault Midliner S
The base for the Renault Midliner was a 5.4-liter in-line diesel "six" water-cooled with a capacity of 150 hp. A five-speed manual gearbox developed by ZF Friedrichshafen worked with her. The springs cope with the loads and bumps on the four wheels of both trucks. Despite their versatility, GAZs were more often used in rural areas, and Renault worked more in cities between warehouses and enterprises.
MAZ-5432 - Mercedes-Benz NG 80
Trunk tractors with a high level of comfort for the driver until the beginning of the 80s were absent as a class in the Soviet Union. It was not in vain that the USSR purchased foreign-made tractors for flights to Western Europe. But in 1981 the situation changed: the production of the MAZ-5432 truck tractor started in Minsk. A year earlier, Mercedes-Benz updated the family of its New Generation long-range vehicles, which received the NG 80 index.
MAZ-5432
The first-born of the fourth generation of tractors of the Minsk Automobile Plant received a radically new design - definitely at the level of foreign analogues. In general, these two machines are outwardly similar, but each has small characteristic features. So, MAZ stands out due to the marker lights and direction indicators located high on the edges of the cab. You can’t confuse a Mercedes with anything because of the wedge-shaped front panel of the cab, the shape of which is caused by efforts to improve the streamlining of the car. For the convenience of getting inside the cabin of both machines, they were equipped with wide steps and handles on the sides of the cabin. The maximum weight of a semi-trailer for MAZ was 21 tons, and for Mercedes-Benz - 15.5–16 tons, depending on the version.
Mercedes-Benz NG80
The "German", of course, offered his crew even more various "chips", ranging from air conditioning and berths with fall protection in the form of nets to electric windows. Nevertheless, MAZ was also quite cool - its high level of equipment and performance is evidenced by the fact that it became the first domestic car that passed homologation tests at a research center near the capital of France and was allowed to operate on all roads in Europe.
Mercedes-Benz NG80
The Minsk tractor was equipped with a modernized 12-cylinder YaMZ-238M2 diesel engine with a volume of 14.86 liters and a power of 280 hp. The eight-speed manual transmission designed for it, equipped with a demultiplier, made it possible to significantly reduce the appetite of the truck, so that a loaded car could travel about 1,000 km on one full tank. Several eight-cylinder diesel engines with power from 280 to 375 hp were installed on German cars. The maximum speed of the tractor from Minsk was 85 km / h, while the Merc with the most modest engine could accelerate to 110 km / h. Both cars had a power steering, pneumatic brakes with an amplifier, but in addition, a foreign car could be equipped with an anti-lock brake system for an extra charge. MAZs were equipped with spring suspension of all wheels, and on the Mercedes-Benz NG 80 it could be different: cheap versions were also equipped with good old springs, but on rich trim levels, pneumatic cylinders were installed on all wheels.
MAZ-5432
Epilogue
Concluding a series of materials on comparing domestic and foreign cars from the 80s, it should be noted that most of them have lived a successful conveyor life, and some designs, after deep modernization, are produced to this day. But for a number of Soviet automobile plants, it was this successful decade that became the swan song. After that, due to political upheavals, our auto industry began a steep downward spiral, and only the strongest came out of it.
There are two different types of magnets. Some are the so-called permanent magnets, made from "hard magnetic" materials. Their magnetic properties are not related to the use of external sources or currents. Another type includes the so-called electromagnets with a core of "soft magnetic" iron. The magnetic fields created by them are mainly due to the fact that an electric current passes through the wire of the winding covering the core.
Magnetic poles and magnetic field.
The magnetic properties of a bar magnet are most noticeable near its ends. If such a magnet is suspended from the middle part so that it can freely rotate in a horizontal plane, then it will take a position approximately corresponding to the direction from north to south. The end of the rod pointing north is called the north pole, and the opposite end is called the south pole. Opposite poles of two magnets attract each other, while like poles repel each other.
If a bar of unmagnetized iron is brought near one of the poles of a magnet, the latter will temporarily become magnetized. In this case, the pole of the magnetized bar closest to the pole of the magnet will be opposite in name, and the far one will be of the same name. The attraction between the pole of the magnet and the opposite pole induced by it in the bar explains the action of the magnet. Some materials (such as steel) themselves become weak permanent magnets after being near a permanent magnet or electromagnet. A steel rod can be magnetized by simply passing the end of a permanent magnet across its end.
So, the magnet attracts other magnets and objects made of magnetic materials without being in contact with them. Such an action at a distance is explained by the existence of a magnetic field in the space around the magnet. Some idea of the intensity and direction of this magnetic field can be obtained by pouring iron filings on a sheet of cardboard or glass placed on a magnet. The sawdust will line up in chains in the direction of the field, and the density of the sawdust lines will correspond to the intensity of this field. (They are thickest at the ends of the magnet, where the intensity of the magnetic field is greatest.)
M. Faraday (1791–1867) introduced the concept of closed induction lines for magnets. The lines of induction exit the magnet at its north pole into the surrounding space, enter the magnet at the south pole, and pass inside the material of the magnet from the south pole back to the north, forming a closed loop. The total number of lines of induction coming out of a magnet is called magnetic flux. Magnetic flux density, or magnetic induction ( AT) is equal to the number of lines of induction passing along the normal through an elementary area of unit size.
Magnetic induction determines the force with which a magnetic field acts on a current-carrying conductor located in it. If the conductor carrying the current I, is located perpendicular to the lines of induction, then according to Ampère's law, the force F, acting on the conductor, is perpendicular to both the field and the conductor and is proportional to the magnetic induction, the current strength and the length of the conductor. Thus, for magnetic induction B you can write an expression
where F is the force in newtons, I- current in amperes, l- length in meters. The unit of measurement for magnetic induction is tesla (T).
Galvanometer.
A galvanometer is a sensitive device for measuring weak currents. The galvanometer uses the torque generated by the interaction of a horseshoe-shaped permanent magnet with a small current-carrying coil (weak electromagnet) suspended in the gap between the poles of the magnet. The torque, and hence the deflection of the coil, is proportional to the current and the total magnetic induction in the air gap, so that the scale of the instrument is almost linear with small deflections of the coil.
Magnetizing force and magnetic field strength.
Next, one more quantity should be introduced that characterizes the magnetic effect of the electric current. Let us assume that the current passes through the wire of a long coil, inside of which the magnetizable material is located. The magnetizing force is the product of the electric current in the coil and the number of its turns (this force is measured in amperes, since the number of turns is a dimensionless quantity). Magnetic field strength H equal to the magnetizing force per unit length of the coil. Thus, the value H 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 H:
where m 0 - so-called. magnetic constant having a universal value of 4 p Ch 10 –7 H/m. In many materials, the value B approximately proportional H. However, in ferromagnetic materials, the ratio between B and H somewhat more complicated (which will be discussed below).
On fig. 1 shows a simple electromagnet designed to capture loads. The energy source is a DC battery. The figure also shows the lines of force of the field of an electromagnet, which can be detected by the usual method of iron filings.
Large electromagnets with iron cores and a very large number of ampere-turns, operating in continuous mode, have a large magnetizing force. They create a magnetic induction up to 6 T in the gap between the poles; this induction is limited only by mechanical stresses, heating of the coils and magnetic saturation of the core. A number of giant electromagnets (without a core) with water cooling, as well as installations for creating pulsed magnetic fields, were designed by P.L. Massachusetts Institute of Technology. On such magnets it was possible to achieve induction up to 50 T. A relatively small electromagnet, producing fields up to 6.2 T, consuming 15 kW of electrical power and cooled by liquid hydrogen, was developed at the Losalamos National Laboratory. Similar fields are obtained at cryogenic temperatures.
Magnetic permeability and its role in magnetism.
Magnetic permeability m is a value that characterizes the magnetic properties of the material. Ferromagnetic metals Fe, Ni, Co and their alloys have very high maximum permeabilities - from 5000 (for Fe) to 800,000 (for supermalloy). In such materials at relatively low field strengths H large inductions occur B, but the relationship between these quantities is, generally speaking, non-linear due to saturation and hysteresis phenomena, which are discussed below. Ferromagnetic materials are strongly attracted by magnets. They lose their magnetic properties at temperatures above the Curie point (770°C for Fe, 358°C for Ni, 1120°C for Co) and behave like paramagnets, for which induction B up to very high tension values H is proportional to it - exactly the same as it takes place in a vacuum. Many elements and compounds are paramagnetic at all temperatures. Paramagnetic substances are characterized by being magnetized in an external magnetic field; if this field is turned off, the paramagnets return to the non-magnetized state. The magnetization in ferromagnets is preserved even after the external field is turned off.
On fig. 2 shows a typical hysteresis loop for a magnetically hard (high loss) ferromagnetic material. It characterizes the ambiguous dependence of the magnetization of a magnetically ordered material on the strength of the magnetizing field. With an increase in the magnetic field strength from the initial (zero) point ( 1 ) magnetization goes along the dashed line 1 –2 , and the value m changes significantly as the magnetization of the sample increases. At the point 2 saturation is reached, i.e. with a further increase in the intensity, the magnetization no longer increases. If we now gradually decrease the value H to zero, then the curve B(H) no longer follows the same path, but passes through the point 3 , revealing, as it were, the "memory" of the material about the "past history", hence the name "hysteresis". Obviously, in this case, some residual magnetization is retained (the segment 1 –3 ). After changing the direction of the magnetizing field to the opposite, the curve AT (H) passes the point 4 , and the segment ( 1 )–(4 ) corresponds to the coercive force that prevents demagnetization. Further growth of values (- H) leads the hysteresis curve to the third quadrant - the section 4 –5 . The subsequent decrease in the value (- H) to zero and then increasing positive values H will close the hysteresis loop through the points 6 , 7 and 2 .
Magnetically hard materials are characterized by a wide hysteresis loop covering a significant area on the diagram and therefore corresponding to large values of residual magnetization (magnetic induction) and coercive force. A narrow hysteresis loop (Fig. 3) is characteristic of soft magnetic materials such as mild steel and special alloys with high magnetic permeability. Such alloys were created in order to reduce energy losses due to hysteresis. Most of these special alloys, like ferrites, have a high electrical resistance, which reduces not only magnetic losses, but also electrical losses due to eddy currents.
Magnetic materials with high permeability are produced by annealing carried out at a temperature of about 1000 ° C, followed by tempering (gradual cooling) to room temperature. In this case, preliminary mechanical and thermal treatment, as well as the absence of impurities in the sample, are very significant. For transformer cores at the beginning of the 20th century. silicon steels were developed, the value m which increased with increasing silicon content. Between 1915 and 1920, permalloys (alloys of Ni with Fe) appeared with their characteristic narrow and almost rectangular hysteresis loop. Particularly high values of magnetic permeability m for small values H hypernic (50% Ni, 50% Fe) and mu-metal (75% Ni, 18% Fe, 5% Cu, 2% Cr) alloys differ, while in perminvar (45% Ni, 30% Fe, 25% Co ) value m practically constant over a wide range of field strength changes. Among modern magnetic materials, we should mention supermalloy, an alloy with the highest magnetic permeability (it contains 79% Ni, 15% Fe, and 5% Mo).
Theories of magnetism.
For the first time, the idea that magnetic phenomena are ultimately reduced to electrical ones arose from Ampère in 1825, when he expressed the idea of closed internal microcurrents circulating in each atom of a magnet. However, without any experimental confirmation of the presence of such currents in matter (the electron was discovered by J. Thomson only in 1897, and the description of the structure of the atom was given by Rutherford and Bohr in 1913), this theory “faded”. In 1852, W. Weber suggested that each atom of a magnetic substance is a tiny magnet, or a magnetic dipole, so that the complete magnetization of a substance is achieved when all individual atomic magnets are lined up in a certain order (Fig. 4, b). Weber believed that molecular or atomic "friction" helps these elementary magnets to maintain their ordering despite the perturbing influence of thermal vibrations. His theory was able to explain the magnetization of bodies upon contact with a magnet, as well as their demagnetization upon impact or heating; finally, the “multiplication” of magnets was also explained when a magnetized needle or magnetic rod was cut into pieces. And yet this theory did not explain either the origin of the elementary magnets themselves, or the phenomena of saturation and hysteresis. Weber's theory was improved in 1890 by J. Ewing, who replaced his 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.
The approach to the problem, once proposed by Ampere, received a second life in 1905, when 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, randomly oriented when the external field is absent, but acquiring an ordered orientation after its application. In this case, the approximation to complete ordering corresponds to saturation of the magnetization. In addition, Langevin introduced the concept of a magnetic moment, which for a single atomic magnet is equal to the product of the "magnetic charge" of the pole and the distance between the poles. Thus, the weak magnetism of paramagnetic materials is due to the total magnetic moment created by uncompensated electron currents.
In 1907, P. Weiss introduced the concept of "domain", which became an important contribution to the modern theory of magnetism. Weiss imagined domains as small "colonies" of atoms, within which the magnetic moments of all atoms, for some reason, are forced to maintain the same orientation, so that each domain is magnetized to saturation. A separate domain can have linear dimensions of the order of 0.01 mm and, accordingly, a volume of the order of 10–6 mm 3 . The domains are separated by the so-called Bloch walls, the thickness of which does not exceed 1000 atomic dimensions. The “wall” and two oppositely oriented domains are shown schematically in Fig. 5. Such walls are "transition layers" in which the direction of the domain magnetization changes.
In the general case, three sections can be distinguished on the initial magnetization curve (Fig. 6). In the initial section, the wall, under the action of an external field, moves through the thickness of the substance until it encounters a crystal lattice defect, which stops it. By increasing the field strength, the wall can be forced to move further through the middle section between the dashed lines. If after that the field strength is again reduced to zero, then the walls will no longer return to their original position, so that the sample will remain partially magnetized. This explains the hysteresis of the magnet. At the end of the curve, the process ends with the saturation of the sample magnetization due to the ordering of the magnetization within the last disordered domains. This process is almost completely reversible. Magnetic hardness is exhibited by those materials in which the atomic lattice contains many defects that prevent the movement of interdomain walls. This can be achieved by mechanical and thermal processing, for example by compressing and then sintering the powdered material. In alnico alloys and their analogues, the same result is achieved by fusing metals into a complex structure.
In addition to paramagnetic and ferromagnetic materials, there are materials with so-called antiferromagnetic and ferrimagnetic properties. The difference between these types of magnetism is illustrated in Fig. 7. Based on the concept of domains, paramagnetism can be considered as a phenomenon due to the presence in the material of small groups of magnetic dipoles, in which individual dipoles interact very weakly with each other (or do not interact at all) and therefore, in the absence of an external field, they take only random orientations ( Fig. 7, a). In ferromagnetic materials, within each domain, there is a strong interaction between individual dipoles, leading to their ordered parallel alignment (Fig. 7, b). In antiferromagnetic materials, on the contrary, the interaction between individual dipoles leads to their antiparallel ordered alignment, so that the total magnetic moment of each domain is zero (Fig. 7, in). Finally, in ferrimagnetic materials (for example, ferrites) there is both parallel and antiparallel ordering (Fig. 7, G), resulting in weak magnetism.
There are two convincing experimental confirmations of the existence of domains. The first of them is the so-called Barkhausen effect, the second is the powder figure method. In 1919, G. Barkhausen established that when an external field is applied to a sample of a ferromagnetic material, its magnetization changes in small discrete portions. From the point of view of the domain theory, this is nothing more than a jump-like advancement of the interdomain wall, which encounters individual defects that hold it back on its way. This effect is usually detected using a coil in which a ferromagnetic rod or wire is placed. If a strong magnet is alternately brought to the sample and removed from it, the sample will be magnetized and remagnetized. Jump-like changes in the magnetization of the sample change the magnetic flux through the coil, and an induction current is excited in it. The voltage that arises in this case in the coil is amplified and fed to the input of a pair of acoustic headphones. Clicks perceived through the headphones indicate an abrupt change in magnetization.
To reveal the domain structure of a magnet by the method of powder figures, a drop of a colloidal suspension of a ferromagnetic powder (usually Fe 3 O 4) 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. Such a structure can be studied under a microscope. A method has also been proposed based on the passage of polarized light through a transparent ferromagnetic material.
Weiss's original theory of magnetism in its main features has retained its significance to the present day, however, having received an updated interpretation based on the concept of uncompensated electron spins as a factor determining atomic magnetism. The hypothesis of the existence of an intrinsic moment of an electron was put forward in 1926 by S. Goudsmit and J. Uhlenbeck, and at present it is electrons as spin carriers that are considered as “elementary magnets”.
To clarify this concept, consider (Fig. 8) a free atom of iron, a typical ferromagnetic material. Its two shells ( K and L), closest to the nucleus, are filled with electrons, with two on the first of them, and eight on the second. AT K-shell, the spin of one of the electrons is positive, and the other is negative. AT 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 spins of the electrons within the same shell cancel out completely, so that the total magnetic moment is zero. AT M-shell, the situation is different, because of the six electrons in the third subshell, five electrons have spins directed in one direction, and only the sixth - in the other. As a result, four uncompensated spins remain, which determines the magnetic properties of the iron atom. (In the outer N-shell has only two valence electrons, which do not contribute to the magnetism of the iron atom.) The magnetism of other ferromagnets, such as nickel and cobalt, is explained in a similar way. Since neighboring atoms in an iron sample strongly interact with each other, and their electrons are partially collectivized, this explanation should be considered only as an illustrative, but very simplified scheme of the real situation.
The theory of atomic magnetism, based on the electron spin, is supported by two interesting gyromagnetic experiments, one of which was carried out by A. Einstein and W. de Haas, and the other by S. Barnett. In the first of these experiments, a cylinder of ferromagnetic material was suspended as shown in Fig. 9. If a current is passed through the winding wire, then the cylinder rotates around its axis. When the direction of the current (and hence the magnetic field) changes, it turns in the opposite direction. In both cases, the rotation of the cylinder is due to the ordering of the electron spins. In Barnett's experiment, on the contrary, a suspended cylinder, sharply brought into a state of rotation, is magnetized in the absence of a magnetic field. This effect is explained by the fact that during the rotation of the magnet a gyroscopic moment is created, which tends to rotate the spin moments in the direction of its own axis of rotation.
For a more complete explanation of the nature and origin of short-range forces that order neighboring atomic magnets and counteract the disordering effect of thermal motion, one should turn to quantum mechanics. A quantum mechanical explanation of the nature of these forces was proposed in 1928 by W. Heisenberg, who postulated the existence of exchange interactions between neighboring atoms. Later, G. Bethe and J. Slater showed that the exchange forces increase significantly with decreasing distance between atoms, but after reaching a certain minimum interatomic distance, they drop to zero.
MAGNETIC PROPERTIES OF SUBSTANCE
One of the first extensive and systematic studies of the magnetic properties of matter was undertaken by P. Curie. He found that according to their magnetic properties, all substances can be divided into three classes. The first includes substances with pronounced magnetic properties, similar to those of iron. Such substances are called ferromagnetic; their magnetic field is noticeable at considerable distances ( cm. higher). Substances called paramagnetic fall into the second class; their magnetic properties are generally similar to those of ferromagnetic materials, but much weaker. For example, the force of attraction to the poles of a powerful electromagnet can pull an iron hammer out of your hands, and in order to detect the attraction of a paramagnetic substance to the same magnet, as a rule, very sensitive analytical balances are needed. The last, third class includes the so-called diamagnetic substances. They are repelled by an electromagnet, i.e. the force acting on diamagnets is directed opposite to that acting on ferro- and paramagnets.
Measurement of magnetic properties.
In the study of magnetic properties, measurements of two types are most important. The first of them is the measurement of the force acting on the sample near the magnet; this is how the magnetization of the sample is determined. The second includes measurements of "resonant" frequencies associated with the magnetization of matter. Atoms are tiny "gyroscopes" and in a magnetic field precess (like a normal spinning top under the influence of a torque created by gravity) at a frequency that can be measured. In addition, a force acts on free charged particles moving at right angles to the lines of magnetic induction, as well as on the electron current in a conductor. It causes the particle to move in a circular orbit, the radius of which is given by
R = mv/eB,
where m is the mass of the particle, v- her speed e is its charge, and B is the magnetic induction of the field. The frequency of such a circular motion is equal to
where f measured in hertz e- in pendants, m- in kilograms, B- in Tesla. This frequency characterizes the movement of charged particles in a substance in a magnetic field. Both types of motion (precession and motion in circular orbits) can be excited by alternating fields with resonant frequencies equal to the "natural" frequencies characteristic of a given material. In the first case, the resonance is called magnetic, and in the second, cyclotron (in view of the similarity with the cyclic motion of a subatomic particle in a cyclotron).
Speaking about the magnetic properties of atoms, it is necessary to pay special attention to their angular momentum. The magnetic field acts on a rotating atomic dipole, trying to rotate it and set it parallel to the field. Instead, the atom begins to precess around the direction of the field (Fig. 10) with a frequency depending on the dipole moment and the strength of the applied field.
The precession of atoms cannot be directly observed, since all the atoms of the sample precess in a different phase. If, however, a small alternating field directed perpendicular to the constant ordering field is applied, then a certain phase relationship is established between the precessing atoms, and their total magnetic moment begins to precess with a frequency equal to the frequency of the precession of individual magnetic moments. The angular velocity of precession is of great importance. As a rule, this value is of the order of 10 10 Hz/T for the magnetization associated with electrons, and of the order of 10 7 Hz/T for the magnetization associated with positive charges in the nuclei of atoms.
A schematic diagram of the installation for observing nuclear magnetic resonance (NMR) is shown in fig. 11. The substance under study is introduced into a uniform constant field between the poles. If an RF field is then excited with a small coil around the test tube, resonance can be achieved at a certain frequency, equal to the precession frequency of all the nuclear "gyroscopes" of the sample. Measurements are similar to tuning a radio receiver to the frequency of a particular station.
Magnetic resonance methods make it possible to study not only the magnetic properties of specific atoms and nuclei, but also the properties of their environment. The point is that magnetic fields in solids and molecules are inhomogeneous, since they are distorted by atomic charges, and the details of the course of the experimental resonance curve are determined by the local field in the region where the precessing nucleus is located. This makes it possible to study the features of the structure of a particular sample by resonance methods.
Calculation of magnetic properties.
The magnetic induction of the Earth's field is 0.5×10 -4 T, while the field between the poles of a strong electromagnet is of the order of 2 T or more.
The magnetic field created by any configuration of currents can be calculated using the Biot-Savart-Laplace formula for the magnetic induction of the field created by the current element. The calculation of the field created by contours of various shapes and cylindrical coils is in many cases very complicated. Below are formulas for a number of simple cases. Magnetic induction (in teslas) of the field created by a long straight wire with current I
The field of a magnetized iron rod is similar to the external field of a long solenoid with the number of ampere turns per unit length corresponding to the current in the atoms on the surface of the magnetized rod, since the currents inside the rod cancel each other out (Fig. 12). By the name of Ampere, such a surface current is called Ampère. Magnetic field strength H a, created by the Ampere current, is equal to the magnetic moment of the unit volume of the rod M.
If an iron rod is inserted into the solenoid, then in addition to the fact that the solenoid current creates a magnetic field H, the ordering of atomic dipoles in the magnetized material of the rod creates magnetization M. In this case, the total magnetic flux is determined by the sum of the real and ampere currents, so that B = m 0(H + H a), or B = m 0(H+M). Attitude M/H called magnetic susceptibility and is denoted by the Greek letter c; c is a dimensionless quantity characterizing the ability of a material to be magnetized in a magnetic field.
Value B/H, which characterizes the magnetic properties of the material, is called the magnetic permeability and is denoted by m a, and m a = m 0m, where m a is absolute, and m- relative permeability,
In ferromagnetic substances, the value c can have very large values - up to 10 4 ё 10 6 . Value c paramagnetic materials have a little more than zero, and diamagnetic materials have a little less. Only in vacuum and in very weak fields are the quantities c and m are constant and do not depend on the external field. Dependency induction B from H is usually non-linear, and its graphs, the so-called. magnetization curves for different materials and even at different temperatures can differ significantly (examples of such curves are shown in Figs. 2 and 3).
The magnetic properties of matter are very complex, and a thorough understanding of their structure requires a thorough analysis of the structure of atoms, their interactions in molecules, their collisions in gases, and their mutual influence in solids and liquids; the magnetic properties of liquids are still the least studied.
The Earth has a magnetic field, the reasons for the existence of which have not been established. A magnetic field has two magnetic poles and a magnetic axis. The position of the magnetic poles does not coincide with the position of the geographic ones. The magnetic poles are located in the Northern and Southern hemispheres asymmetrically relative to each other. In this regard, the line connecting them - the magnetic axis of the Earth forms an angle of up to 11 ° with the axis of its rotation.
Earth's magnetism is characterized by magnetic intensity, declination and inclination. Magnetic strength is measured in oersteds.
Magnetic declination is the angle of deviation of the magnetic needle from the geographic meridian at a given location. Since the magnetic needle indicates the direction of the magnetic meridian, the magnetic declination will correspond to the angle between the magnetic and geographic meridians. Declension can be east or west. Lines connecting identical declinations on a map are called isogons. The declination isogon equal to zero is called the zero magnetic meridian. The isogons radiate from the magnetic pole in the southern hemisphere and converge at the magnetic pole in the northern hemisphere.
Magnetic inclination is the angle of inclination of the magnetic needle to the horizon. Lines connecting points of equal inclination are called isoclines. The zero isocline is called the magnetic equator. Isoclines, like parallels, stretch in the latitudinal direction and vary from 0 to 90°.
The smooth course of isogones and isoclines in some places of the earth's surface is quite sharply disturbed, which is associated with the existence of magnetic anomalies. Large accumulations of iron ores can serve as sources of such anomalies. The largest magnetic anomaly is Kursk. Magnetic anomalies can also be caused by breaks in the earth's crust - faults, reverse faults, as a result of which rocks with different magnetic characteristics come into contact, etc. Magnetic anomalies are widely used to search for mineral deposits and study the structure of the subsoil.
The values of magnetic intensities, declinations and inclinations experience daily and secular fluctuations (variations).
Diurnal variations are caused by solar and lunar perturbations of the ionosphere and are more pronounced in summer than in winter, and more during the day than at night. Much more intense
century variations. It is believed that they are due to changes occurring in the upper layers of the earth's core. Secular variations in different geographical points are different.
Sudden, lasting several days, magnetic fluctuations (magnetic storms) are associated with solar activity and are most intense at high latitudes.
§ 4. Heat of the Earth
The Earth receives heat from two sources: from the Sun and from its own bowels. The thermal state of the Earth's surface almost entirely depends on its heating by the Sun. However, under the influence of many factors, there is a redistribution of solar heat that has fallen on the Earth's surface. Different points on the earth's surface receive an unequal amount of heat due to the inclined position of the Earth's axis of rotation relative to the plane of the ecliptic.
To compare temperature conditions, the concepts of average daily, average monthly and average annual temperatures in certain parts of the Earth's surface are introduced.
The highest temperature fluctuations are experienced by the upper layer of the Earth. Deeper from the surface, daily, monthly and annual temperature fluctuations gradually decrease. The thickness of the earth's crust, within which rocks are affected by solar heat, is called the heliothermal zone. The depth of this zone varies from a few meters to 30 m.
Under the solar thermal zone there is a belt of constant temperature, where seasonal temperature fluctuations do not affect. In the Moscow area, it is located at a depth of 20 m.
Below the belt of constant temperature is the geothermal zone. In this zone, the temperature rises with depth due to the internal heat of the Earth - by an average of 1 ° C for every 33 m. This depth interval is called the “geothermal step”. The increase in temperature when deepening into the Earth by 100 m is called the geothermal gradient. The values of the geothermal step and gradient are inversely proportional and different for different regions of the Earth. Their product is a constant value and equals 100. If, for example, the step is 25 m, then the gradient is 4 °C.
Differences in the values of the geothermal step can be due to different radioactivity and thermal conductivity of rocks, hydrochemical processes in the bowels, the nature of the occurrence of rocks, the temperature of groundwater, and remoteness from oceans and seas.
The value of the geothermal step varies over a wide range. In the area of Pyatigorsk it is 1.5 m, Leningrad - 19.6 m, Moscow - 38.4 m, in Karelia - more than 100 m, in the region of the Volga region and Bashkiria - 50 m, etc. 14
The main source of internal heat of the Earth is the radioactive decay of substances concentrated mainly in the earth's crust. It is assumed that the heat in it increases in accordance with the geothermal step to a depth of 15-20 km. Deeper there is a sharp increase in the value of the geothermal step. Experts believe that the temperature in the center of the Earth does not exceed 4000 ° C. If the value of the geothermal step remained the same to the center of the Earth, then the temperature at a depth of 900 km would be 27,000 °C, and in the center of the Earth it would reach approximately 193,000 °C.
