Magnetic field and its characteristics. When an electric current passes through a conductor, a a magnetic field. A magnetic field represents one of the types of matter. It has energy, which manifests itself in the form of electromagnetic forces acting on individual moving electric charges (electrons and ions) and on their flows, i.e. electric current. Under the influence of electromagnetic forces, moving charged particles deviate from their original path in a direction perpendicular to the field (Fig. 34). The magnetic field is formed only around moving electric charges, and its action also extends only to moving charges. Magnetic and electric fields inseparable and form together a single electromagnetic field. Any change electric field leads to the appearance of a magnetic field and, conversely, any change in the magnetic field is accompanied by the appearance of an electric field. Electromagnetic field propagates at the speed of light, i.e. 300,000 km/s.
Graphic representation of the magnetic field. Graphically, the magnetic field is represented by magnetic lines of force, which are drawn so that the direction of the field line at each point of the field coincides with the direction of the field forces; magnetic field lines are always continuous and closed. The direction of the magnetic field at each point can be determined using a magnetic needle. The north pole of the arrow is always set in the direction of the field forces. The end of a permanent magnet, from which the field lines emerge (Fig. 35, a), is considered to be the north pole, and the opposite end, into which the field lines enter, is the south pole (the field lines passing inside the magnet are not shown). The distribution of field lines between the poles of a flat magnet can be detected using steel filings sprinkled on a sheet of paper placed on the poles (Fig. 35, b). The magnetic field in the air gap between two parallel opposite poles of a permanent magnet is characterized by a uniform distribution of magnetic force lines (Fig. 36) (field lines passing inside the magnet are not shown).
Rice. 37. Magnetic flux penetrating the coil when its positions are perpendicular (a) and inclined (b) relative to the direction of the magnetic lines of force.
For a more visual representation of the magnetic field, the field lines are placed less frequently or denser. In those places where the magnetic field is stronger, the field lines are located closer to each other, and in places where it is weaker, they are further apart. The lines of force do not intersect anywhere.
In many cases, it is convenient to consider magnetic lines of force as some elastic stretched threads that tend to contract and also repel each other (have mutual lateral thrust). This mechanical concept of lines of force makes it possible to clearly explain the emergence of electromagnetic forces during the interaction of a magnetic field and a conductor with current, as well as two magnetic fields.
The main characteristics of a magnetic field are magnetic induction, magnetic flux, magnetic permeability and magnetic field strength.
Magnetic induction and magnetic flux. The intensity of the magnetic field, i.e. its ability to produce work, is determined by a quantity called magnetic induction. The stronger the magnetic field created by a permanent magnet or electromagnet, the greater the induction it has. Magnetic induction B can be characterized by the density of magnetic field lines, i.e., the number of field lines passing through an area of 1 m 2 or 1 cm 2 located perpendicular to the magnetic field. There are homogeneous and inhomogeneous magnetic fields. In a uniform magnetic field, the magnetic induction at each point in the field has the same value and direction. The field in the air gap between the opposite poles of a magnet or electromagnet (see Fig. 36) can be considered homogeneous at some distance from its edges. Magnetic flux Ф passing through any surface is determined by the total number of magnetic lines of force penetrating this surface, for example coil 1 (Fig. 37, a), therefore, in a uniform magnetic field
F = BS (40)
where S is the cross-sectional area of the surface through which the magnetic field lines pass. It follows that in such a field the magnetic induction is equal to the flux divided by the cross-sectional area S:
B = F/S (41)
If any surface is located obliquely with respect to the direction of the magnetic field lines (Fig. 37, b), then the flux penetrating it will be less than if it is perpendicular to its position, i.e. Ф 2 will be less than Ф 1 .
In the SI system of units, magnetic flux is measured in webers (Wb), this unit has the dimension V*s (volt-second). Magnetic induction in SI units is measured in teslas (T); 1 T = 1 Wb/m2.
Magnetic permeability. Magnetic induction depends not only on the strength of the current passing through a straight conductor or coil, but also on the properties of the medium in which the magnetic field is created. The quantity characterizing the magnetic properties of a medium is absolute magnetic permeability? A. Its unit of measurement is henry per meter (1 H/m = 1 Ohm*s/m).
In a medium with greater magnetic permeability, an electric current of a certain strength creates a magnetic field with greater induction. It has been established that the magnetic permeability of air and all substances, with the exception of ferromagnetic materials (see § 18), has approximately the same value as the magnetic permeability of vacuum. The absolute magnetic permeability of a vacuum is called the magnetic constant, ? o = 4?*10 -7 H/m. The magnetic permeability of ferromagnetic materials is thousands and even tens of thousands of times greater than the magnetic permeability of non-ferromagnetic substances. Magnetic permeability ratio? and any substance to the magnetic permeability of vacuum? o is called relative magnetic permeability:
? = ? A /? O (42)
Magnetic field strength. The intensity And does not depend on the magnetic properties of the medium, but takes into account the influence of the current strength and the shape of the conductors on the intensity of the magnetic field at a given point in space. Magnetic induction and tension are related by the relation
H = B/? a = B/(?? o) (43)
Consequently, in a medium with constant magnetic permeability, the magnetic field induction is proportional to its strength.
Magnetic field strength is measured in amperes per meter (A/m) or amperes per centimeter (A/cm).
Good day, today you will find out what is a magnetic field and where it comes from.
Every person on the planet has held at least once magnet in hand. Starting from souvenir refrigerator magnets, or working magnets for collecting iron pollen and much more. As a child, it was a funny toy that stuck to ferrous metal, but not to other metals. So what is the secret of the magnet and its magnetic field.
What is a magnetic field
At what point does a magnet begin to attract? Around each magnet there is a magnetic field, entering which objects begin to be attracted to it. The size of such a field can vary depending on the size of the magnet and its own properties.
Wikipedia term:
Magnetic field is a force field acting on moving electric charges and on bodies with a magnetic moment, regardless of the state of their motion, the magnetic component of the electromagnetic field.
Where does the magnetic field come from?
A magnetic field can be created by the current of charged particles or the magnetic moments of electrons in atoms, as well as the magnetic moments of other particles, although to a noticeably lesser extent.
Manifestation of magnetic field
The magnetic field manifests itself in the effect on the magnetic moments of particles and bodies, on moving charged particles or conductors with. The force acting on an electrically charged particle moving in a magnetic field is called the Lorentz force, which is always directed perpendicular to the vectors v and B. It is proportional to the charge of the particle q, the velocity component v perpendicular to the direction of the magnetic field vector B, and the magnitude of the magnetic field induction B.
What objects have a magnetic field
We often don't think about it, but many (if not all) objects around us are magnets. We are accustomed to the fact that a magnet is a pebble with a pronounced force of attraction towards itself, but in fact, almost everything has a force of attraction, it’s just much lower. Let’s take our planet, for example - we don’t fly into space, although we don’t hold onto the surface with anything. The Earth's field is much weaker than the field of a pebble magnet, so it holds us only due to its enormous size - if you have ever seen how people walk on the Moon (the diameter of which is four times smaller), you will clearly understand what we are talking about . The Earth's gravity is based largely on the metallic components of its crust and core - they have a powerful magnetic field. You may have heard that near large deposits of iron ore, compasses no longer point correctly to the north - this is because the principle of the compass is based on the interaction of magnetic fields, and the iron ore attracts its needle.
When connecting two parallel conductors to electrical current, they will attract or repel, depending on the direction (polarity) of the connected current. This is explained by the phenomenon of the emergence of a special kind of matter around these conductors. This matter is called a magnetic field (MF). Magnetic force is the force with which conductors act on each other.
The theory of magnetism arose in ancient times, in the ancient civilization of Asia. In the mountains of Magnesia they found a special rock, pieces of which could be attracted to each other. Based on the name of the place, this rock was called “magnetic”. A bar magnet contains two poles. Its magnetic properties are especially pronounced at the poles.
A magnet hanging on a thread will show the sides of the horizon with its poles. Its poles will be turned north and south. The compass device operates on this principle. Opposite poles of two magnets attract, and like poles repel.
Scientists have discovered that a magnetized needle located near a conductor is deflected when an electric current passes through it. This indicates that an MP is formed around it.
The magnetic field affects:
Moving electric charges.
Substances called ferromagnets: iron, cast iron, their alloys.
Permanent magnets are bodies that have a common magnetic moment of charged particles (electrons).
1 - South pole of the magnet
2 - North pole of the magnet
3 - MP using the example of metal filings
4 - Magnetic field direction
Lines of force appear when a permanent magnet approaches a paper sheet on which a layer of iron filings is poured. The figure clearly shows the locations of the poles with oriented lines of force.
Magnetic field sources
- Electric field changing over time.
- Mobile charges.
- Permanent magnets.
We have been familiar with permanent magnets since childhood. They were used as toys that attracted various metal parts. They were attached to the refrigerator, they were built into various toys.
Electric charges that are in motion most often have more magnetic energy compared to permanent magnets.
Properties
- The main distinguishing feature and property of the magnetic field is relativity. If you leave a charged body motionless in a certain frame of reference, and place a magnetic needle nearby, then it will point to the north, and at the same time will not “feel” an extraneous field, except for the field of the earth. And if you start moving a charged body near the arrow, then an MP will appear around the body. As a result, it becomes clear that the MF is formed only when a certain charge moves.
- A magnetic field can influence and influence electric current. It can be detected by monitoring the movement of charged electrons. In a magnetic field, particles with a charge will be deflected, conductors with flowing current will move. The frame with the current supply connected will begin to rotate, and the magnetized materials will move a certain distance. The compass needle is most often colored blue. It is a strip of magnetized steel. The compass always points north, since the Earth has a magnetic field. The entire planet is like a big magnet with its own poles.
The magnetic field is not perceived by human organs and can only be detected by special devices and sensors. It comes in variable and permanent types. The alternating field is usually created by special inductors that operate on alternating current. A constant field is formed by a constant electric field.
Rules
Let's consider the basic rules for depicting the magnetic field for various conductors.
Gimlet rule
The line of force is depicted in a plane, which is located at an angle of 90 0 to the path of current flow so that at each point the force is directed tangentially to the line.
To determine the direction of magnetic forces, you need to remember the rule of a gimlet with a right-hand thread.
The gimlet must be positioned along the same axis with the current vector, the handle must be rotated so that the gimlet moves in the direction of its direction. In this case, the orientation of the lines is determined by rotating the gimlet handle.
Ring gimlet rule
The translational movement of the gimlet in a conductor made in the form of a ring shows how the induction is oriented; the rotation coincides with the flow of current.
The lines of force have their continuation inside the magnet and cannot be open.
The magnetic field of different sources is added to each other. In doing so, they create a common field.
Magnets with the same poles repel, and magnets with different poles attract. The value of the interaction strength depends on the distance between them. As the poles approach, the force increases.
Magnetic field parameters
- Flow coupling ( Ψ ).
- Magnetic induction vector ( IN).
- Magnetic flux ( F).
The intensity of the magnetic field is calculated by the size of the magnetic induction vector, which depends on the force F, and is formed by the current I along a conductor having a length l: B = F / (I * l).
Magnetic induction is measured in Tesla (T), in honor of the scientist who studied the phenomena of magnetism and worked on their calculation methods. 1 T is equal to the magnetic flux induction force 1 N at length 1m straight conductor at an angle 90 0 to the direction of the field, with a flowing current of one ampere:
1 T = 1 x H / (A x m).
Left hand rule
The rule finds the direction of the magnetic induction vector.
If the palm of the left hand is placed in the field so that the magnetic field lines enter the palm from the north pole at 90 0, and 4 fingers are placed along the current flow, the thumb will show the direction of the magnetic force.
If the conductor is at a different angle, then the force will directly depend on the current and the projection of the conductor onto the plane at a right angle.
The force does not depend on the type of conductor material and its cross-section. If there is no conductor, and the charges move in a different medium, then the force will not change.
When the magnetic field vector is directed in one direction of one magnitude, the field is called uniform. Different environments affect the size of the induction vector.
Magnetic flux
Magnetic induction passing through a certain area S and limited by this area is a magnetic flux.
If the area is inclined at a certain angle α to the induction line, the magnetic flux is reduced by the size of the cosine of this angle. Its greatest value is formed when the area is at right angles to the magnetic induction:
F = B * S.
Magnetic flux is measured in a unit such as "weber", which is equal to the flow of induction of magnitude 1 T by area in 1 m2.
Flux linkage
This concept is used to create a general value of magnetic flux, which is created from a certain number of conductors located between the magnetic poles.
In the case where the same current I flows through a winding with a number of turns n, the total magnetic flux formed by all turns is the flux linkage.
Flux linkage Ψ measured in Webers, and equals: Ψ = n * Ф.
Magnetic properties
Magnetic permeability determines how much the magnetic field in a certain medium is lower or higher than the field induction in a vacuum. A substance is called magnetized if it produces its own magnetic field. When a substance is placed in a magnetic field, it becomes magnetized.
Scientists have determined the reason why bodies acquire magnetic properties. According to scientists' hypothesis, there are microscopic electric currents inside substances. An electron has its own magnetic moment, which is of a quantum nature, and moves along a certain orbit in atoms. It is these small currents that determine magnetic properties.
If the currents move randomly, then the magnetic fields caused by them are self-compensating. The external field makes the currents ordered, so a magnetic field is formed. This is the magnetization of the substance.
Various substances can be divided according to the properties of their interaction with magnetic fields.
They are divided into groups:
Paramagnets– substances that have magnetization properties in the direction of an external field and have a low potential for magnetism. They have a positive field strength. Such substances include ferric chloride, manganese, platinum, etc.
Ferrimagnets– substances with magnetic moments unbalanced in direction and value. They are characterized by the presence of uncompensated antiferromagnetism. Field strength and temperature affect their magnetic susceptibility (various oxides).
Ferromagnets– substances with increased positive susceptibility, depending on tension and temperature (crystals of cobalt, nickel, etc.).
Diamagnets– have the property of magnetization in the opposite direction of the external field, that is, a negative value of magnetic susceptibility, independent of voltage. In the absence of a field, this substance will not have magnetic properties. These substances include: silver, bismuth, nitrogen, zinc, hydrogen and other substances.
Antiferromagnets
– have a balanced magnetic moment, resulting in a low degree of magnetization of the substance. When heated, a phase transition of the substance occurs, during which paramagnetic properties appear. When the temperature drops below a certain limit, such properties will not appear (chromium, manganese).
The magnets considered are also classified into two more categories:
Soft magnetic materials
. They have low coercivity. In low-power magnetic fields they can become saturated. During the magnetization reversal process, they experience minor losses. As a result, such materials are used for the production of cores of electrical devices operating on alternating voltage (, generator,).
Hard magnetic materials. They have an increased coercive force. To remagnetize them, a strong magnetic field is required. Such materials are used in the production of permanent magnets.
The magnetic properties of various substances find their use in engineering projects and inventions.
Magnetic circuits
A combination of several magnetic substances is called a magnetic circuit. They are similar and are determined by similar laws of mathematics.
Electrical devices, inductances, etc. operate on the basis of magnetic circuits. In a functioning electromagnet, the flux flows through a magnetic circuit made of ferromagnetic material and air, which is not ferromagnetic. The combination of these components is a magnetic circuit. Many electrical devices contain magnetic circuits in their design.
According to modern ideas, it was formed approximately 4.5 billion years ago, and from that moment our planet has been surrounded by a magnetic field. Everything on Earth, including people, animals and plants, is affected by it.
The magnetic field extends to an altitude of about 100,000 km (Fig. 1). It deflects or captures solar wind particles that are harmful to all living organisms. These charged particles form the Earth's radiation belt, and the entire region of near-Earth space in which they are located is called magnetosphere(Fig. 2). On the side of the Earth illuminated by the Sun, the magnetosphere is limited by a spherical surface with a radius of approximately 10-15 Earth radii, and on the opposite side it is extended like a comet's tail over a distance of up to several thousand Earth radii, forming a geomagnetic tail. The magnetosphere is separated from the interplanetary field by a transition region.
Earth's magnetic poles
The axis of the earth's magnet is inclined relative to the earth's rotation axis by 12°. It is located approximately 400 km away from the center of the Earth. The points at which this axis intersects the surface of the planet are magnetic poles. The Earth's magnetic poles do not coincide with the true geographic poles. Currently, the coordinates of the magnetic poles are as follows: north - 77° north latitude. and 102°W; southern - (65° S and 139° E).
Rice. 1. The structure of the Earth’s magnetic field
Rice. 2. Structure of the magnetosphere
Lines of force running from one magnetic pole to another are called magnetic meridians. An angle is formed between the magnetic and geographic meridians, called magnetic declination. Every place on Earth has its own declination angle. In the Moscow region the declination angle is 7° to the east, and in Yakutsk it is about 17° to the west. This means that the northern end of the compass needle in Moscow deviates by T to the right of the geographic meridian passing through Moscow, and in Yakutsk - by 17° to the left of the corresponding meridian.
A freely suspended magnetic needle is located horizontally only on the line of the magnetic equator, which does not coincide with the geographical one. If you move north of the magnetic equator, the northern end of the needle will gradually descend. The angle formed by a magnetic needle and a horizontal plane is called magnetic inclination. At the North and South magnetic poles, the magnetic inclination is greatest. It is equal to 90°. At the North Magnetic Pole, a freely suspended magnetic needle will be installed vertically with its northern end down, and at the South Magnetic Pole its southern end will go down. Thus, the magnetic needle shows the direction of the magnetic field lines above the earth's surface.
Over time, the position of the magnetic poles relative to the earth's surface changes.
The magnetic pole was discovered by explorer James C. Ross in 1831, hundreds of kilometers from its current location. On average, it moves 15 km in one year. In recent years, the speed of movement of the magnetic poles has increased sharply. For example, the North Magnetic Pole is currently moving at a speed of about 40 km per year.
The reversal of the Earth's magnetic poles is called magnetic field inversion.
Throughout the geological history of our planet, the Earth's magnetic field has changed its polarity more than 100 times.
The magnetic field is characterized by intensity. In some places on Earth, magnetic field lines deviate from the normal field, forming anomalies. For example, in the area of the Kursk Magnetic Anomaly (KMA), the field strength is four times higher than normal.
There are daily variations in the Earth's magnetic field. The reason for these changes in the Earth's magnetic field is electric currents flowing in the atmosphere at high altitudes. They are caused by solar radiation. Under the influence of the solar wind, the Earth's magnetic field is distorted and acquires a “trail” in the direction from the Sun, which extends for hundreds of thousands of kilometers. The main cause of the solar wind, as we already know, is the enormous ejections of matter from the solar corona. As they move towards the Earth, they turn into magnetic clouds and lead to strong, sometimes extreme disturbances on the Earth. Particularly strong disturbances of the Earth's magnetic field - magnetic storms. Some magnetic storms begin suddenly and almost simultaneously across the entire Earth, while others develop gradually. They can last for several hours or even days. Magnetic storms often occur 1-2 days after a solar flare due to the Earth passing through a stream of particles ejected by the Sun. Based on the delay time, the speed of such a corpuscular flow is estimated at several million km/h.
During strong magnetic storms, the normal operation of the telegraph, telephone and radio is disrupted.
Magnetic storms are often observed at latitude 66-67° (in the aurora zone) and occur simultaneously with auroras.
The structure of the Earth's magnetic field varies depending on the latitude of the area. The permeability of the magnetic field increases towards the poles. Over the polar regions, the magnetic field lines are more or less perpendicular to the earth's surface and have a funnel-shaped configuration. Through them, part of the solar wind from the dayside penetrates into the magnetosphere and then into the upper atmosphere. During magnetic storms, particles from the tail of the magnetosphere rush here, reaching the boundaries of the upper atmosphere in the high latitudes of the Northern and Southern Hemispheres. It is these charged particles that cause the auroras here.
So, magnetic storms and daily changes in the magnetic field are explained, as we have already found out, by solar radiation. But what is the main reason that creates the permanent magnetism of the Earth? Theoretically, it was possible to prove that 99% of the Earth’s magnetic field is caused by sources hidden inside the planet. The main magnetic field is caused by sources located in the depths of the Earth. They can be roughly divided into two groups. The main part of them is associated with processes in the earth's core, where, due to continuous and regular movements of electrically conductive matter, a system of electric currents is created. The other is due to the fact that the rocks of the earth’s crust, when magnetized by the main electric field (the field of the core), create their own magnetic field, which is summed with the magnetic field of the core.
In addition to the magnetic field around the Earth, there are other fields: a) gravitational; b) electric; c) thermal.
Gravitational field The earth is called the gravity field. It is directed along a plumb line perpendicular to the surface of the geoid. If the Earth had the shape of an ellipsoid of revolution and masses were evenly distributed in it, then it would have a normal gravitational field. The difference between the intensity of the real gravitational field and the theoretical one is a gravity anomaly. Different material composition and density of rocks cause these anomalies. But other reasons are also possible. They can be explained by the following process - the equilibrium of the solid and relatively light earth's crust on the heavier upper mantle, where the pressure of the overlying layers is equalized. These currents cause tectonic deformations, the movement of lithospheric plates and thereby create the macrorelief of the Earth. Gravity holds the atmosphere, hydrosphere, people, animals on Earth. Gravity must be taken into account when studying processes in the geographic envelope. The term " geotropism" are the growth movements of plant organs, which, under the influence of the force of gravity, always ensure the vertical direction of growth of the primary root perpendicular to the surface of the Earth. Gravity biology uses plants as experimental subjects.
If gravity is not taken into account, it is impossible to calculate the initial data for launching rockets and spacecraft, to carry out gravimetric exploration of ore deposits, and, finally, the further development of astronomy, physics and other sciences is impossible.
Magnetic field is called a special, different from substance, type of matter through which the action of a magnet is transmitted to other bodies.
A magnetic field occurs in the space surrounding moving electric charges and permanent magnets. It only affects moving charges. Under the influence of electromagnetic forces, moving charged particles are deflected
From its original path in a direction perpendicular to the field.
Magnetic and electric fields are inseparable and together form a single electromagnetic field. Any change electric field leads to the appearance of a magnetic field, and, conversely, any change in the magnetic field is accompanied by the appearance of an electric field. The electromagnetic field propagates at the speed of light, i.e. 300,000 km/s.
The effect of permanent magnets and electromagnets on ferromagnetic bodies, the existence and inextricable unity of the poles of magnets and their interaction (opposite poles attract, like poles repel) are well known. Similarly
with the magnetic poles of the Earth, the poles of magnets are called northern and southern.
The magnetic field is clearly depicted by magnetic lines of force, which determine the direction of the magnetic field in space (Fig..1). These lines have neither beginning nor end, i.e. are closed.
The magnetic field lines of a straight conductor are concentric circles surrounding the wire. The stronger the current, the stronger the magnetic field around the wire. As you move away from the current-carrying wire, the magnetic field weakens.
In the space surrounding a magnet or electromagnet, the direction from North Pole to South Pole. The more intense the magnetic field, the higher the density of field lines.
The direction of the magnetic field lines is determined gimlet rule:.
Rice. 1. Magnetic field of magnets:
a - straight; b - horseshoe-shaped
Rice. 2. Magnetic field:
a - straight wire; b - inductive coil
If you screw in the screw in the direction of the current, then the magnetic field lines will be directed along the direction of the screw (Fig. 2 a)
To obtain a stronger magnetic field, inductive coils with wire windings are used. In this case, the magnetic fields of the individual turns of the inductive coil add up and their lines of force merge into a common magnetic flux.
Magnetic lines of force come out of the inductive coil
at the end where the current is directed counterclockwise, i.e. this end is the north magnetic pole (Fig. 2, b).
When the direction of the current in the inductive coil changes, the direction of the magnetic field will also change.