There are two main types of magnets: permanent and electromagnets. It is possible to determine what a permanent magnet is based on its main property. The permanent magnet gets its name from the fact that its magnetism is always "on". It generates its own magnetic field, unlike an electromagnet, which is made from wire wrapped around an iron core and requires current to flow to create a magnetic field.
History of the study of magnetic properties
Centuries ago, people discovered that some types of rocks have original features: they are attracted to iron objects. The mention of magnetite is found in ancient historical chronicles: more than two thousand years ago in European and much earlier in East Asian. At first it was assessed as a curious object.
Later, magnetite was used for navigation, finding that it tends to take a certain position when it is given the freedom to rotate. A scientific study by P. Peregrine in the 13th century showed that steel could acquire these characteristics after being rubbed with magnetite.
Magnetized objects had two poles: "north" and "south", relative to the Earth's magnetic field. As Peregrine discovered, it was not possible to isolate one of the poles by cutting a fragment of magnetite in two - each separate fragment had its own pair of poles as a result.
In accordance with today's ideas, the magnetic field of permanent magnets is the resulting orientation of electrons in a single direction. Only some types of materials interact with magnetic fields, a much smaller number of them are able to maintain a constant magnetic field.
Properties of permanent magnets
The main properties of permanent magnets and the field they create are:
- the existence of two poles;
- opposite poles attract and like poles repel (like positive and negative charges);
- magnetic force imperceptibly propagates in space and passes through objects (paper, wood);
- there is an increase in the MF intensity near the poles.
Permanent magnets support MT without external help. Materials depending on the magnetic properties are divided into the main types:
- ferromagnets - easily magnetized;
- paramagnets - magnetized with great difficulty;
- diamagnets - tend to reflect the external MF by magnetization in the opposite direction.
Important! Soft magnetic materials such as steel conduct magnetism when attached to a magnet, but this stops when it is removed. Permanent magnets are made from magnetically hard materials.
How a permanent magnet works
His work is related to atomic structure. All ferromagnets create a natural, albeit weak, magnetic field, thanks to the electrons surrounding the nuclei of atoms. These groups of atoms are able to orient in a single direction and are called magnetic domains. Each domain has two poles: north and south. When a ferromagnetic material is not magnetized, its regions are oriented in random directions, and their MFs cancel each other out.
To create permanent magnets, ferromagnets are heated at very high temperatures and subjected to a strong external magnetic field. This leads to the fact that individual magnetic domains inside the material begin to orient themselves in the direction of the external magnetic field until all the domains align, reaching the magnetic saturation point. The material is then cooled and the aligned domains are locked into position. After the removal of the external MF, magnetically hard materials will retain most of their domains, creating a permanent magnet.
Characteristics of a permanent magnet
- The magnetic force is characterized by residual magnetic induction. Designated Br. This is the force that remains after the disappearance of the external MT. Measured in tests (Tl) or gauss (Gs);
- Coercivity or resistance to demagnetization - Ns. Measured in A / m. Shows what the intensity of the external MF should be in order to demagnetize the material;
- Maximum energy - BHmax. Calculated by multiplying the residual magnetic force Br and the coercivity Hc. Measured in MGSE (megagaussersted);
- The temperature coefficient of the residual magnetic force is Тс of Br. Characterizes the dependence of Br on the temperature value;
- Tmax is the highest temperature value at which permanent magnets lose their properties with the possibility of reverse recovery;
- Tcur is the highest temperature value at which the magnetic material permanently loses its properties. This indicator is called the Curie temperature.
The individual characteristics of a magnet change with temperature. At different temperatures, different types of magnetic materials work differently.
Important! All permanent magnets lose a percentage of magnetism as the temperature rises, but at a different rate depending on their type.
Types of permanent magnets
There are five types of permanent magnets in total, each of which is made differently based on materials with different properties:
- alnico;
- ferrites;
- rare earth SmCo based on cobalt and samarium;
- neodymium;
- polymeric.
Alnico
These are permanent magnets composed primarily of a combination of aluminum, nickel, and cobalt, but may also include copper, iron, and titanium. Due to the properties of alnico magnets, they can operate at the highest temperatures while retaining their magnetism, however, they demagnetize more easily than ferrite or rare earth SmCo. They were the first mass-produced permanent magnets, replacing magnetized metals and expensive electromagnets.
Application:
- electric motors;
- heat treatment;
- bearings;
- aerospace vehicles;
- military equipment;
- high-temperature loading and unloading equipment;
- microphones.
Ferrites
For the manufacture of ferrite magnets, also known as ceramic, strontium carbonate and iron oxide are used in a ratio of 10/90. Both materials are abundant and economically available.
Due to low production costs, resistance to heat (up to 250°C) and corrosion, ferrite magnets are one of the most popular for everyday use. They have greater internal coercivity than alnico, but less magnetic force than neodymium counterparts.
Application:
- sound speakers;
- security systems;
- large plate magnets to remove iron contamination from process lines;
- electric motors and generators;
- medical instruments;
- lifting magnets;
- marine search magnets;
- devices based on the operation of eddy currents;
- switches and relays;
- brakes.
SmCo Rare Earth Magnets
Cobalt and samarium magnets operate over a wide temperature range, have high temperature coefficients and high corrosion resistance. This type retains its magnetic properties even at temperatures below absolute zero, making them popular for use in cryogenic applications.
Application:
- turbotechnics;
- pump couplings;
- wet environments;
- high temperature devices;
- miniature electric racing cars;
- electronic devices for operation in critical conditions.
Neodymium magnets
The strongest existing magnets, consisting of an alloy of neodymium, iron and boron. Due to their enormous strength, even miniature magnets are effective. This provides versatility of use. Each person is constantly next to one of the neodymium magnets. They are, for example, in a smartphone. The manufacture of electric motors, medical equipment, radio electronics rely on heavy-duty neodymium magnets. Due to their super strength, huge magnetic force and resistance to demagnetization, samples up to 1 mm can be produced.
Application:
- hard disks;
- sound-reproducing devices - microphones, acoustic sensors, headphones, loudspeakers;
- prostheses;
- magnetic coupling pumps;
- door closers;
- engines and generators;
- locks on jewelry;
- MRI scanners;
- magnetotherapy;
- ABS sensors in cars;
- lifting equipment;
- magnetic separators;
- reed switches, etc.
Flexible magnets contain magnetic particles inside a polymer binder. They are used for unique devices where it is impossible to install solid analogues.
Application:
- display advertising - quick fixation and quick removal at exhibitions and events;
- vehicle signs, educational school panels, company logos;
- toys, puzzles and games;
- masking surfaces for painting;
- calendars and magnetic bookmarks;
- window and door seals.
Most permanent magnets are brittle and should not be used as structural elements. They are made in standard forms: rings, rods, discs, and individual: trapezoids, arcs, etc. Due to the high iron content, neodymium magnets are susceptible to corrosion, therefore they are coated on top with nickel, stainless steel, teflon, titanium, rubber and other materials.
Video
a) General information. To create a constant magnetic field in a number of electrical devices, permanent magnets are used, which are made of magnetically hard materials with a wide hysteresis loop (Fig. 5.6).
The work of a permanent magnet occurs in the area from H=0 before H \u003d - H s. This part of the loop is called the demagnetization curve.
Consider the basic relationships in a permanent magnet, which has the shape of a toroid with one small gap b(fig.5.6). Owing to the shape of a toroid and a small gap, stray fluxes in such a magnet can be neglected. If the gap is small, then the magnetic field in it can be considered uniform.
Fig.5.6. Permanent Magnet Demagnetization Curve
If buckling is neglected, then the induction in the gap AT & and inside the magnet AT are the same.
Based on the total current law in closed-loop integration 1231 rice. we get:
Fig.5.7. Permanent magnet shaped like a toroid
Thus, the field strength in the gap is directed opposite to the field strength in the magnet body. For a DC electromagnet having a similar shape of the magnetic circuit, without taking into account saturation, you can write:.
Comparing it can be seen that in the case of a permanent magnet n. c, which creates a flow in the working gap, is the product of the tension in the magnet body and its length with the opposite sign - Hl.
Taking advantage of the fact that
, (5.29)
, (5.30)
where S- the area of the pole; - conductivity of the air gap.
The equation is the equation of a straight line passing through the origin in the second quadrant at an angle a to the axis H. Given the scale of induction t in and tension t n angle a is defined by the equality
Since the induction and strength of the magnetic field in the body of a permanent magnet are connected by a demagnetization curve, the intersection of this straight line with the demagnetization curve (point BUT in Fig.5.6) and determines the state of the core at a given gap.
With a closed circuit and
With growth b conductivity of the working gap and tga decrease, the induction in the working gap decreases, and the field strength inside the magnet increases.
One of the important characteristics of a permanent magnet is the energy of the magnetic field in the working gap W t . Considering that the field in the gap is uniform,
Substituting value H we get:
, (5.35)
where V M is the volume of the magnet body.
Thus, the energy in the working gap is equal to the energy inside the magnet.
Product dependency B(-H) in the induction function is shown in Fig.5.6. Obviously, for point C, where B(-H) reaches its maximum value, the energy in the air gap also reaches its maximum value, and from the point of view of using a permanent magnet, this point is optimal. It can be shown that the point C corresponding to the maximum of the product is the point of intersection with the beam demagnetization curve OK, through a point with coordinates and .
Let us consider in more detail the influence of the gap b by the amount of induction AT(fig.5.6). If the magnetization of the magnet was carried out with a gap b, then after the removal of the external field in the body of the magnet, an induction will be established corresponding to the point BUT. The position of this point is determined by the gap b.
Decrease the gap to the value , then
. (5.36)
With a decrease in the gap, the induction in the magnet body increases, however, the process of changing the induction does not follow the demagnetization curve, but along the branch of a private hysteresis loop AMD. Induction AT 1 is determined by the point of intersection of this branch with a ray drawn at an angle to the axis - H(dot D).
If we increase the gap again to the value b, then the induction will drop to the value AT, and dependence B (H) will be determined by the branch DNA private hysteresis loop. Usually partial hysteresis loop AMDNA narrow enough and replaced by a straight AD, which is called the return line. The slope to the horizontal axis (+ H) of this line is called the return coefficient:
. (5.37)
The demagnetization characteristic of a material is usually not given in full, but only the saturation induction values are given. B s , residual induction In g, coercive force N s. To calculate a magnet, it is necessary to know the entire demagnetization curve, which for most magnetically hard materials is well approximated by the formula
The demagnetization curve given by (5.30) can be easily plotted graphically if one knows B s , B r .
b) Determination of the flow in the working gap for a given magnetic circuit. In a real system with a permanent magnet, the flow in the working gap differs from the flow in the neutral section (in the middle of the magnet) due to the presence of scattering and buckling flows (Fig.).
The flow in the neutral section is equal to:
, (5.39)
where is the flow in the neutral section;
Bulging flow at the poles;
Flux scattering;
workflow.
The scattering coefficient o is determined by the equality
If we accept that flows 
created by the same magnetic potential difference, then
. (5.41)
We find the induction in the neutral section by defining:
,
and using the demagnetization curve Fig.5.6. The induction in the working gap is equal to:
since the flow in the working gap is several times less than the flow in the neutral section.
Very often, the magnetization of the system occurs in an unassembled state, when the conductivity of the working gap is reduced due to the absence of parts made of ferromagnetic material. In this case, the calculation is carried out using a direct return. If the leakage fluxes are significant, then the calculation is recommended to be carried out by sections, as well as in the case of an electromagnet.
Stray fluxes in permanent magnets play a much greater role than in electromagnets. The fact is that the magnetic permeability of hard magnetic materials is much lower than that of soft magnetic materials, from which systems for electromagnets are made. Stray fluxes cause a significant drop in the magnetic potential along the permanent magnet and reduce n. c, and hence the flow in the working gap.
The dissipation coefficient of the completed systems varies over a fairly wide range. The calculation of the scattering coefficient and scattering fluxes is associated with great difficulties. Therefore, when developing a new design, it is recommended to determine the value of the scattering coefficient on a special model in which the permanent magnet is replaced by an electromagnet. The magnetizing winding is chosen so as to obtain the necessary flux in the working gap.
Fig.5.8. Magnetic circuit with a permanent magnet and leakage and buckling fluxes
c) Determining the dimensions of the magnet according to the required induction in the working gap. This task is even more difficult than determining the flow with known dimensions. When choosing the dimensions of a magnetic circuit, one usually strives to ensure that the induction At 0 and tension H 0 in the neutral section corresponded to the maximum value of the product N 0 V 0 . In this case, the volume of the magnet will be minimal. The following recommendations are given for the choice of materials. If it is required to obtain a large value of induction at large gaps, then the most suitable material is magnico. If it is necessary to create small inductions with a large gap, then alnisi can be recommended. With small working gaps and a large value of induction, it is advisable to use an alni.
The cross section of the magnet is selected from the following considerations. The induction in the neutral section is chosen equal to At 0 . Then the flow in the neutral section
,
where is the cross section of the magnet
.
Induction values in the working gap In r and the area of the pole are given values. The most difficult is to determine the value of the coefficient scattering. Its value depends on the design and induction in the core. If the cross section of the magnet turned out to be large, then several magnets connected in parallel are used. The length of the magnet is determined from the condition for creating the necessary NS. in the working gap with tension in the body of the magnet H 0:
where b p - the value of the working gap.
After choosing the main dimensions and designing the magnet, a verification calculation is carried out according to the method described earlier.
d) Stabilization of the characteristics of the magnet. During the operation of the magnet, a decrease in the flow in the working gap of the system is observed - the aging of the magnet. There are structural, mechanical and magnetic aging.
Structural aging occurs due to the fact that after hardening of the material, internal stresses arise in it, the material acquires an inhomogeneous structure. In the process of work, the material becomes more homogeneous, internal stresses disappear. In this case, the residual induction In t and coercive force N s decrease. To combat structural aging, the material is subjected to heat treatment in the form of tempering. In this case, internal stresses in the material disappear. Its characteristics become more stable. Aluminum-nickel alloys (alni, etc.) do not require structural stabilization.
Mechanical aging occurs with shock and vibration of the magnet. In order to make the magnet insensitive to mechanical influences, it is subjected to artificial aging. The magnet specimens are subjected to such shocks and vibrations as are encountered in operation before installation in the apparatus.
Magnetic aging is a change in the properties of a material under the action of external magnetic fields. A positive external field increases the induction along the return line, and a negative one reduces it along the demagnetization curve. In order to make the magnet more stable, it is subjected to a demagnetizing field, after which the magnet operates on a return line. Due to the lower steepness of the return line, the influence of external fields is reduced. When calculating magnetic systems with permanent magnets, it must be taken into account that in the process of stabilization, the magnetic flux decreases by 10-15%.
Switching magnetic flux systems are based on magnetic flux switching relative to detachable coils.
The essence of the CE devices considered on the Internet is that there is a magnet for which we pay once, and there is a magnetic field of the magnet for which no one pays money.
The question is that it is necessary to create such conditions in transformers with switching magnetic fluxes under which the magnet field becomes controllable and we direct it. interrupt. redirect like this. so that the energy for switching is minimal or cost-free
In order to consider options for these systems, I decided to study and bring my thoughts on fresh ideas.
To begin with, I wanted to look at what magnetic properties a ferromagnetic material has, etc. Magnetic materials have a coercive force.
Accordingly, the coercive force obtained from the cycle, or from the cycle, is considered. are designated respectively
The coercive force is always greater. This fact is explained by the fact that in the right half-plane of the hysteresis graph, the value is greater than by the value:
In the left half-plane, on the contrary, it is less than , by the value . Accordingly, in the first case, the curves will be located above the curves, and in the second, below. This makes the hysteresis cycle narrower than the cycle.
Coercive force
Coercive force - (from lat. coercitio - holding), the value of the magnetic field strength necessary for the complete demagnetization of a ferro- or ferrimagnetic substance. It is measured in Ampere/meter (in the SI system). According to the magnitude of the coercive force, the following magnetic materials are distinguished
Soft magnetic materials are materials with a low coercive force that are magnetized to saturation and remagnetized in relatively weak magnetic fields of about 8–800 A/m. After magnetization reversal, they do not externally exhibit magnetic properties, since they consist of randomly oriented regions magnetized to saturation. An example would be various steels. The more coercive force a magnet has, the more resistant it is to demagnetizing factors. Hard magnetic materials are materials with a high coercive force that are magnetized to saturation and remagnetized in relatively strong magnetic fields with a strength of thousands and tens of thousands of a/m. After magnetization, magnetically hard materials remain permanent magnets due to high values of coercive force and magnetic induction. Examples are rare earth magnets NdFeB and SmCo, barium and strontium hard magnetic ferrites.
With an increase in the mass of the particle, the radius of curvature of the trajectory increases, and according to Newton's first law, its inertia increases.
With an increase in magnetic induction, the radius of curvature of the trajectory decreases, i.e. the centripetal acceleration of the particle increases. Consequently, under the action of the same force, the change in particle velocity will be smaller, and the radius of curvature of the trajectory will be larger.
With an increase in the charge of the particle, the Lorentz force (magnetic component) increases, therefore, the centripetal acceleration also increases.
When the speed of the particle changes, the radius of curvature of its trajectory changes, the centripetal acceleration changes, which follows from the laws of mechanics.
If a particle flies into a uniform magnetic field by induction AT at an angle other than 90°, then the horizontal component of the velocity does not change, and the vertical component acquires centripetal acceleration under the action of the Lorentz force, and the particle will describe a circle in a plane perpendicular to the vector of magnetic induction and velocity. Due to the simultaneous movement along the direction of the induction vector, the particle describes a helix, and will return to the original horizontal at regular intervals, i.e. cross it at equal distances.
The retarding interaction of magnetic fields is caused by Foucault currents
As soon as the circuit in the inductor is closed, two oppositely directed flows begin to act around the conductor. According to Lenz's law, the positive charges of the electrogas (ether) begin their helical movement, setting in motion the atoms, according to which the electrical connection is established. From here it is mono to explain existence of magnetic action and counteraction.
By this I explain the inhibition of the exciting magnetic field and its counteraction in a closed circuit, the braking effect in the electric generator (mechanical braking or resistance to the rotor of the electric generator to the mechanically applied force and the opposition (braking) of the Foucault current to a falling neodymium magnet falling in a copper tube.
A little about magnetic motors
The principle of switching magnetic fluxes is also applied here.
But it's easier to go to the drawings.
How should this system work?
The middle coil is removable and operates on a relatively wide pulse length, which is created by the passage of magnetic fluxes from the magnets shown in the diagram.
The pulse length is determined by the inductance of the coil and the load resistance.
As soon as the time runs out and the core becomes magnetized, it is necessary to interrupt, demagnetize or remagnetize the core itself. to continue working with the load.
What is a permanent magnet? A permanent magnet is a body capable of maintaining magnetization for a long time. As a result of multiple studies, numerous experiments, we can say that only three substances on Earth can be permanent magnets (Fig. 1).
Rice. 1. Permanent magnets. ()
Only these three substances and their alloys can be permanent magnets, only they can be magnetized and maintain such a state for a long time.
Permanent magnets have been used for a very long time, and first of all, these are spatial orientation devices - the first compass was invented in China in order to navigate in the desert. Today, no one argues about magnetic needles, permanent magnets, they are used everywhere in telephones and radio transmitters and simply in various electrical products. They can be different: there are bar magnets (Fig. 2)
Rice. 2. Bar magnet ()
And there are magnets that are called arcuate or horseshoe (Fig. 3)
Rice. 3. Arcuate magnet ()
The study of permanent magnets is associated exclusively with their interaction. The magnetic field can be created by electric current and a permanent magnet, so the first thing that was done was research with magnetic needles. If you bring the magnet to the arrow, then we will see the interaction - the same poles will repel, and the opposite ones will attract. This interaction is observed with all magnets.
Let's place small magnetic arrows along the bar magnet (Fig. 4), the south pole will interact with the north, and the north will attract the south. The magnetic needles will be placed along the magnetic field line. It is generally accepted that the magnetic lines are directed outside the permanent magnet from the north pole to the south, and inside the magnet from the south pole to the north. Thus, the magnetic lines are closed in the same way as the electric current, these are concentric circles, they are closed inside the magnet itself. It turns out that outside the magnet the magnetic field is directed from north to south, and inside the magnet from south to north.
Rice. 4. Lines of the magnetic field of a bar magnet ()
In order to observe the shape of the magnetic field of a bar magnet, the shape of the magnetic field of an arcuate magnet, we will use the following devices or details. Take a transparent plate, iron filings and conduct an experiment. Let's sprinkle iron filings on the plate located on the bar magnet (Fig. 5):
Rice. 5. The shape of the magnetic field of the bar magnet ()
We see that the lines of the magnetic field come out of the north pole and enter the south pole, by the density of the lines we can judge the poles of the magnet, where the lines are thicker - there are the poles of the magnet (Fig. 6).
Rice. 6. The shape of the magnetic field of the arc-shaped magnet ()
We will carry out a similar experiment with an arcuate magnet. We see that the magnetic lines start at the north and end at the south pole all over the magnet.
We already know that the magnetic field is formed only around magnets and electric currents. How can we determine the Earth's magnetic field? Any arrow, any compass in the Earth's magnetic field is strictly oriented. Since the magnetic needle is strictly oriented in space, therefore, a magnetic field acts on it, and this is the magnetic field of the Earth. It can be concluded that our Earth is a large magnet (Fig. 7) and, accordingly, this magnet creates a rather powerful magnetic field in space. When we look at a magnetic compass needle, we know that the red arrow points south and the blue one points north. How are the Earth's magnetic poles located? In this case, it is necessary to remember that the south magnetic pole is located at the geographic north pole of the Earth and the north magnetic pole of the Earth is located at the geographic south pole. If we consider the Earth as a body in space, then we can say that when we go north along the compass, we will come to the south magnetic pole, and when we go south, we will get to the north magnetic pole. At the equator, the compass needle will be located almost horizontally relative to the surface of the Earth, and the closer we are to the poles, the more vertical the arrow will be. The Earth's magnetic field could change, there were times when the poles changed relative to each other, that is, the south was where the north was, and vice versa. According to scientists, this was a harbinger of great catastrophes on Earth. This has not been observed for the last several tens of millennia.
Rice. 7. Earth's magnetic field ()
The magnetic and geographic poles do not match. There is also a magnetic field inside the Earth itself, and, like in a permanent magnet, it is directed from the south magnetic pole to the north.
Where does the magnetic field in permanent magnets come from? The answer to this question was given by the French scientist Andre-Marie Ampère. He expressed the idea that the magnetic field of permanent magnets is explained by elementary, simple currents flowing inside permanent magnets. These simplest elementary currents amplify each other in a certain way and create a magnetic field. A negatively charged particle - an electron - moves around the nucleus of an atom, this movement can be considered directed, and, accordingly, a magnetic field is created around such a moving charge. Inside any body, the number of atoms and electrons is simply huge, respectively, all these elementary currents take an ordered direction, and we get a fairly significant magnetic field. We can say the same about the Earth, that is, the Earth's magnetic field is very similar to the magnetic field of a permanent magnet. And a permanent magnet is a rather bright characteristic of any manifestation of a magnetic field.
In addition to the existence of magnetic storms, there are also magnetic anomalies. They are related to the solar magnetic field. When sufficiently powerful explosions or ejections occur on the Sun, they do not occur without the help of the manifestation of the Sun's magnetic field. This echo reaches the Earth and affects its magnetic field, as a result, we observe magnetic storms. Magnetic anomalies are associated with iron ore deposits in the Earth, huge deposits are magnetized by the Earth's magnetic field for a long time, and all bodies around will experience a magnetic field from this anomaly, the compass needles will show the wrong direction.
In the next lesson, we will consider other phenomena associated with magnetic actions.
Bibliography
- Gendenstein L.E., Kaidalov A.B., Kozhevnikov V.B. Physics 8 / Ed. Orlova V.A., Roizena I.I. - M.: Mnemosyne.
- Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
- Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.
- Class-fizika.narod.ru ().
- Class-fizika.narod.ru ().
- Files.school-collection.edu.ru ().
Homework
- Which end of the compass needle is attracted to the north pole of the earth?
- In what place of the Earth you cannot trust the magnetic needle?
- What does the density of lines on a magnet indicate?
