What are permanent magnets. Types of magnets and their use
Along with pieces of amber electrified by friction, permanent magnets were the first material evidence for ancient people. electromagnetic phenomena(lightning at the dawn of history was definitely attributed to the sphere of manifestation of non-material forces). The explanation of the nature of ferromagnetism has always occupied the inquisitive minds of scientists, however, even at present, the physical nature of the permanent magnetization of some substances, both natural and artificially created, has not yet been fully disclosed, leaving a considerable field of activity for modern and future researchers.
Traditional materials for permanent magnets
They have been actively used in industry since 1940 with the advent of alnico alloy (AlNiCo). Prior to this, permanent magnets from various grades of steel were used only in compasses and magnetos. Alnico made it possible to replace electromagnets with them and use them in devices such as motors, generators and loudspeakers.
This intrusion into our daily lives received a new impetus with the creation of ferrite magnets, and since then permanent magnets have become commonplace.
A revolution in magnetic materials began around 1970, with the creation of the samarium-cobalt family of hard magnetic materials with a hitherto unseen density of magnetic energy. Then a new generation of rare earth magnets based on neodymium, iron and boron was discovered with a much higher magnetic energy density than samarium-cobalt (SmCo) and at an expected low cost. These two families of rare earth magnets have such high energy densities that not only can they replace electromagnets, but they can be used in areas inaccessible to them. Examples are the tiny permanent magnet stepper motor in wrist watch and sound transducers in Walkman type headphones.
gradual improvement magnetic properties materials are shown in the diagram below.
neodymium permanent magnets
They represent the latest and most significant development in this field over the past decades. Their discovery was first announced almost simultaneously in late 1983 by metalworkers from Sumitomo and General Motors. They are based on the NdFeB intermetallic compound: an alloy of neodymium, iron and boron. Of these, neodymium is a rare earth element extracted from the mineral monazite.
The great interest that these permanent magnets have generated comes from the fact that for the first time a new magnetic material has been obtained that is not only stronger than the previous generation, but also more economical. It consists mainly of iron, which is much cheaper than cobalt, and neodymium, which is one of the most common rare earth materials and is more abundant on Earth than lead. The main rare earth minerals monazite and bastanesite contain five to ten times more neodymium than samarium.
Physical Mechanism of Permanent Magnetization
To explain the functioning of a permanent magnet, we must look inside it down to the atomic scale. Each atom has a set of spins of its electrons, which together form its magnetic moment. For our purposes, we can consider each atom as a small bar magnet. When a permanent magnet is demagnetized (either by heating it to a high temperature or by an external magnetic field), each atomic moment is randomly oriented (see figure below) and no regularity is observed.
When it is magnetized in a strong magnetic field, all atomic moments are oriented in the direction of the field and, as it were, interlock with each other (see figure below). This coupling makes it possible to maintain the field of a permanent magnet when the external field is removed, and also to resist demagnetization when its direction changes. The measure of the cohesive force of atomic moments is the magnitude of the coercive force of the magnet. More on this later.
In a deeper presentation of the magnetization mechanism, they do not operate with the concepts of atomic moments, but use the concept of miniature (of the order of 0.001 cm) regions inside the magnet, which initially have a constant magnetization, but are randomly oriented in the absence of an external field, so that a strict reader, if desired, can attribute the above physical the mechanism is not to the magnet as a whole. and to its separate domain.
Induction and magnetization
The atomic moments add up and form the magnetic moment of the entire permanent magnet, and its magnetization M indicates the magnitude of this moment per unit volume. The magnetic induction B shows that a permanent magnet is the result of an external magnetic force (field strength) H applied during the primary magnetization, as well as an internal magnetization M due to the orientation of the atomic (or domain) moments. Its value in general case given by the formula:
B = µ 0 (H + M),
where µ 0 is a constant.
In a permanent annular and homogeneous magnet, the field strength H inside it (in the absence of an external field) is equal to zero, since, according to the law of total current, the integral of it along any circle inside such an annular core is equal to:
H∙2πR = iw=0 , whence H=0.
Therefore, the magnetization in a ring magnet is:
In an open magnet, for example, in the same annular, but with an air gap of width l zaz in a core of length l ser, in the absence of an external field and the same induction B inside the core and in the gap, according to the law of total current, we obtain:
H ser l ser + (1/ µ 0)Bl zas = iw=0.
Since B \u003d µ 0 (H ser + M ser), then, substituting its expression into the previous one, we get:
H ser (l ser + l zas) + M ser l zas \u003d 0,
H ser \u003d ─ M ser l zas (l ser + l zas).
In the air gap:
H zaz \u003d B / µ 0,
moreover, B is determined by the given M ser and the found H ser.
Magnetization curve
Starting from the non-magnetized state, when H increases from zero, due to the orientation of all atomic moments in the direction of the external field, M and B rapidly increase, changing along the “a” section of the main magnetization curve (see figure below).
When all the atomic moments are aligned, M comes to its saturation value, and a further increase in B is due solely to the applied field (section b of the main curve in the figure below). When the external field decreases to zero, the induction B decreases not along the original path, but along the “c” section due to the coupling of atomic moments, which tends to keep them in the same direction. The magnetization curve begins to describe the so-called hysteresis loop. When H (external field) approaches zero, then the induction approaches a residual value determined only by atomic moments:
B r = μ 0 (0 + M r).
After the direction of H changes, H and M act in opposite directions, and B decreases (section of the curve "d" in Fig.). The value of the field at which B decreases to zero is called the coercive force of the magnet B H C . When the magnitude of the applied field is large enough to break the cohesion of the atomic moments, they orient themselves in the new direction of the field, and the direction of M is reversed. The value of the field at which this happens is called the internal coercive force of the permanent magnet M H C . So there are two different but related coercive forces associated with a permanent magnet.
The figure below shows the basic demagnetization curves of various materials for permanent magnets.
It can be seen from it that it is NdFeB magnets that have the highest residual induction Br and coercive force (both total and internal, i.e., determined without taking into account the strength H, only from the magnetization M).
Surface (ampere) currents
The magnetic fields of permanent magnets can be considered as the fields of some of the currents associated with them, flowing along their surfaces. These currents are called ampere currents. In the usual sense of the word, there are no currents inside permanent magnets. However, comparing the magnetic fields of permanent magnets and the fields of currents in coils, the French physicist Ampere suggested that the magnetization of a substance can be explained by the flow of microscopic currents that form microscopic closed loops. And indeed, after all, the analogy between the field of a solenoid and a long cylindrical magnet is almost complete: there is a north and south pole of a permanent magnet and the same poles for a solenoid, and the pictures lines of force their fields are also very similar (see picture below).
Are there currents inside a magnet?
Imagine that the entire volume of some rod permanent magnet (with an arbitrary cross-sectional shape) is filled with microscopic Ampere currents. A cross section of a magnet with such currents is shown in the figure below.
Each of them has a magnetic moment. With the same orientation of them in the direction of the external field, they form a resulting magnetic moment that is different from zero. He defines existence. magnetic field in the apparent absence of an ordered movement of charges, in the absence of current through any section of the magnet. It is also easy to understand that inside it the currents of adjacent (contacting) circuits are compensated. Only the currents on the surface of the body, which form the surface current of the permanent magnet, turn out to be uncompensated. Its density turns out to be equal to the magnetization M.
How to get rid of moving contacts
The problem of creating a non-contact synchronous machine is known. Its traditional design with electromagnetic excitation from the poles of the rotor with coils involves the supply of current to them through moving contacts - contact rings with brushes. The disadvantages of such a technical solution are well known: these are maintenance difficulties, low reliability, and large losses in moving contacts, especially when it comes to powerful turbo and hydro generators, in the excitation circuits of which considerable electrical power is consumed.
If you make such a permanent magnet generator, then the contact problem immediately goes away. True, there is a problem of reliable fastening of magnets on a rotating rotor. This is where the experience gained in tractor construction can come in handy. There has long been used an inductor generator with permanent magnets located in the grooves of the rotor, filled with a low-melting alloy.
Permanent magnet motor
In recent decades, brushless DC motors have become widespread. Such a unit is actually an electric motor and an electronic switch of its armature winding, which acts as a collector. The electric motor is a synchronous motor with permanent magnets located on the rotor, as in Fig. above, with a fixed armature winding on the stator. The electronic switch circuitry is an inverter constant voltage(or current) of the supply network.
The main advantage of such an engine is its contactlessness. Its specific element is a photo-, induction or Hall rotor position sensor that controls the operation of the inverter.
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 external sources or currents. Another type includes the so-called electromagnets with a core of "soft magnetic" iron. The magnetic fields they create are mainly due to the fact that the winding wire surrounding the core passes through electricity.
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 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 go out into the surrounding space from the magnet at its north pole, 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. Full number 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 battery is the source of energy direct current. 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 very a large number 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. Kapitza (1894–1984) at Cambridge and at the Institute physical problems Academy of Sciences of the USSR and F. Bitter (1902–1967) at the 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 an electric power of 15 kW 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, due to which not only magnetic losses are reduced, 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. Especially high values 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 ultimately reduced to electrical, 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 magnetic substance is a tiny magnet, or magnetic dipole, so that the full magnetization of a substance is achieved when all the 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 Ampère, 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 separate atomic magnet is equal to the product " magnetic charge» poles by 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 defect crystal lattice which stops her. 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 compelling experimental confirmation 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 it is excited induction current. 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 into reverse 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 turn the spin moments in the direction own axis 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 refer 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 roundabout 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 this material. In the first case, the resonance is called magnetic, and in the second - cyclotron (due to the similarity with cyclic movement 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 is not directly observable because all the atoms in the sample precess in 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 is a value 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 certain frequency, equal to the precession frequency of all 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. Calculation of the field created by contours different 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 few Above zero, and for diamagnetic ones - 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 when different temperatures may 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 them 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.
At home, at work, in your own car or in public transport we are surrounded by various types of magnets. They power motors, sensors, microphones, and many other common things. At the same time, in each area, devices that are different in their characteristics and features are used. In general, these types of magnets are distinguished:
What are magnets
Electromagnets. The design of such products consists of an iron core, on which coils of wire are wound. By applying an electric current with different parameters of magnitude and direction, it is possible to obtain magnetic fields necessary strength and polarity.
The name of this group of magnets is an abbreviation of the names of its components: aluminum, nickel and cobalt. The main advantage of alnico alloy is the unsurpassed temperature stability of the material. Other types of magnets cannot boast of being able to be used at temperatures up to +550 ⁰ C. At the same time, this lightweight material is characterized by a weak coercive force. This means that it can be completely demagnetized when exposed to a strong external magnetic field. At the same time, due to its affordable price, alnico is an indispensable solution in many scientific and industrial sectors.
Modern magnetic products
So, we figured out the alloys. Now let's move on to what magnets are and what application they can find in everyday life. In fact, there is a huge variety of options for such products:
3) office magnets. For presentations and meetings, magnetic boards are used, which allow you to present any information visually and in detail. They are also extremely useful in school classrooms and university classrooms.
Neodymium and ferrite magnets
Many metals have magnetic qualities, which allows them to be used in many areas of industry and in everyday life. Until recently, ferrite magnets were widely used, but now they are increasingly being replaced by magnets made from an alloy of the rare earth metal neodymium, iron and boron. The latter are gaining more and more popularity. Which magnet is better - ferrite or neodymium, let's try to figure it out in this article.
Neodymium magnet
Many of us have heard of neodymium magnets. What it is? The unique qualities of the magnet are due to the presence of neodymium in the alloy - chemical element from the group of lanthanides of the periodic table. In addition to the main component, the composition of the neodymium magnet includes iron and boron, or cobalt and yttrium. A neodymium magnet is made by heating a powdered mass of active ingredients. The most distinguishing characteristic neodymium magnet - its power at a fairly small size. Such a magnet has an adhesive force that is 10 or more times greater than that of ferrite magnets.
In order for the neodymium magnet to last as long as possible, a special composition of nickel is applied to its surface. If the magnet is planned to be used in aggressive or high-temperature environments, then it is recommended to choose a zinc coating.
Neodymium magnets are widely used:
As a vise or clamp - neodymium power ensures uniform clamping of the material placed between the magnets.
For entertainment - both children and adults are equally interested in watching the tricks set with the help of this magnet.
To search for objects made of steel and iron.
For magnetizing metal objects. The things that a neodymium magnet magnetizes include screwdrivers, needles, knives and other products.
For reliable fastening on the surface of various objects.
Types of neodymium magnets
Neodymium magnets are available in various configurations and weights. Even a small magnet, 25 * 5 mm in size, can withstand a weight of up to nine kilograms and, if handled carelessly, can damage the skin. And when using magnets of a larger mass, it is all the more necessary to observe certain safety measures in order to exclude possible injuries.
Ferrite magnet - what is it
The most common among the usual ones are ferrite magnets, which are an alloy of iron oxide with oxides of other metals. Simple magnets are most often made in the form of a horseshoe. Among the main characteristics of ferromagnets are:
Good temperature resistance.
High magnetic permeability.
Low cost.
Ferrite magnets are usually marked with pole markings in red and blue.
Comparison of magnets
So what is the difference between a neodymium magnet and a regular magnet, and how can these differences be visually determined? Neodymium magnets have become very popular not so long ago (their production technologies are only about 30 years old), but they are already used in almost all spheres of life. As already mentioned, the most important difference between a neodymium magnet and a conventional one is its adhesion strength and the main magnetic characteristics: magnetic energy, remanent magnetic induction and coercive force. The values of these characteristics are many times higher than those of ferromagnets. The easiest way to determine the type of magnet is to try removing it from an iron surface. If it is easily separated, then it is a ferromagnet, but if it is possible to remove the magnet only after applying certain efforts, then we have a neodymium magnet. In addition to this feature, magnets differ in a number of ways.
Life time
If ferromagnets serve for about 10 years at correct use and then completely demagnetized, the service life of a neodymium magnet is practically unlimited. Behind human age The strength of neodymium magnets is lost by only 1%.
Force of gravity
The attractive force of a neodymium magnet with the same dimensions is about 10 times higher than the force of a ferromagnet. Therefore, a small but very powerful magnet can be used in computers and acoustic systems, as well as for making various souvenirs and jewelry.
The form
Ferro magnets are mainly produced in the form of a horseshoe with red and blue legs showing the negative and positive poles. The horseshoe shape allows you to close the magnetic field lines to increase the service life of the ferromagnet. Neodymium magnets are produced in a variety of shapes and configurations - a parallelepiped, a ring, a disk, and others. On their surface, you can place several poles, that is, make them "multipolar".
Price
A neodymium magnet is more expensive than a ferrite one, which is justified by its characteristics and service life. Having bought a neodymium magnet, you get an almost “eternal” magnet, at least its qualities will hardly change during your life.
Advantages And Applications Of Neodymium Magnet
Thus, a neodymium magnet, despite more high price, has undeniable advantages over conventional ferrite. Increased power, long service life, various shape manufacturing provided the neodymium-iron-boron alloy magnet with high demand among consumers.
Why you need a neodymium magnet
What does a neodymium magnet mean for a modern person in Everyday life? In addition to the above uses, the popular material is used for:
Cleaning aquariums and other containers, as well as engine and transmission oils used in automotive equipment.
Accurate alignment of metal surfaces.
Degaussing discs, films and for many other actions.
Of course, all the characteristics of neodymium magnets listed in the article matter only when purchasing high-quality materials. Everyone who separately bought neodymium in the World of Magnets knows that the online store provides all the necessary guarantees and quality certificates, and also provides each buyer with competent advice.
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 is a special kind of matter. It manifests itself in the 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 magnetic moments 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 force 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, the metal particles will show a clear picture of the magnetic field lines emerging from 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- physical quantity, equal to the product magnetic induction on 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- scalar characteristic of the 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 southern hemisphere moved 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 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) 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, to whom you can entrust part of the educational troubles with confidence in success! and other types of work you can order at the link.
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