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An alternating magnetic field excited by a changing current creates an electric field in the surrounding space, which in turn excites a magnetic field, etc. Mutually generating each other, these fields form a single alternating electromagnetic field - an electromagnetic wave. Having arisen in the place where there is a current-carrying wire, the electromagnetic field propagates through space at the speed of light -300,000 km/s.

Magnetotherapy

In the frequency spectrum, different places are occupied by radio waves, light, x-ray radiation and others electromagnetic radiation. They are usually characterized by continuously coupled electric and magnetic fields.

Synchrophasotrons

Currently, a magnetic field is understood as special shape matter consisting of charged particles. IN modern physics Beams of charged particles are used to penetrate deep into atoms in order to study them. The force with which a magnetic field acts on a moving charged particle is called the Lorentz force.

Flow meters - counters

The method is based on the application of Faraday's law for a conductor in a magnetic field: an emf is induced in a flow of electrically conducting fluid moving in a magnetic field, proportional to speed flow, converted by the electronic part into an electrical analog/digital signal.

Generator DC

In generator mode, the machine's armature rotates under the influence of an external torque. Between the stator poles there is a constant magnetic flux piercing anchor. The conductors of the armature winding move in a magnetic field and, therefore, an EMF is induced in them, the direction of which can be determined by the rule " right hand". In this case, on one brush a positive potential regarding the second one. If you connect a load to the generator terminals, current will flow through it.

Transformers

Transformers are widely used in transmitting electrical energy to long distances, its distribution between receivers, as well as in various rectifying, amplifying, signaling and other devices.

Energy conversion in a transformer is carried out by an alternating magnetic field. A transformer is a core made of thin steel plates insulated from one another, on which two and sometimes more windings (coils) of insulated wire are placed. Winding to which a source of electrical energy is connected AC, is called the primary winding, the remaining windings are called secondary.

If the secondary winding of a transformer has three times more turns wound than the primary winding, then the magnetic field created in the core by the primary winding, crossing the turns of the secondary winding, will create three times the voltage in it.

By using a transformer with a reverse turns ratio, you can just as easily obtain a reduced voltage.

The first law of electromagnetism describes the flow electric field:

where ε 0 is some constant (read epsilon-zero). If there are no charges inside the surface, but there are charges outside it (even very close), then it’s all the same average the normal component of E is zero, so there is no flow through the surface. To show the usefulness of this type of statement, we will prove that equation (1.6) coincides with Coulomb's law, if only we take into account that the field of an individual charge must be spherically symmetric. Let's take you around point charge sphere. Then the average normal component is exactly equal to the value of E at any point, because the field must be directed along the radius and have the same value at all points on the sphere. Our rule then states that the field on the surface of a sphere multiplied by the area of ​​the sphere (i.e., the flux flowing out of the sphere) is proportional to the charge inside it. If you increase the radius of a sphere, its area increases as the square of the radius. The product of the average normal component of the electric field by this area must still be equal to the internal charge, which means that the field must decrease as the square of the distance; This is how a field of “inverse squares” is obtained.

If we take an arbitrary curve in space and measure the circulation of the electric field along this curve, it turns out that it is in general case is not equal to zero (although this is true in a Coulomb field). Instead, the second law holds true for electricity, stating that

And finally, the formulation of laws electromagnetic field will be completed if we write two corresponding equations for the magnetic field B:

And for the surface S, limited curve WITH:

The constant c 2 that appears in equation (1.9) is the square of the speed of light. Its appearance is justified by the fact that magnetism is essentially a relativistic manifestation of electricity. And the constant ε 0 is set so that the usual units of force arise electric current.

Equations (1.6) - (1.9), as well as equation (1.1) are all the laws of electrodynamics. As you remember, Newton's laws were very simple to write, but many complex consequences followed from them, so it took a lot of time to study them all. The laws of electromagnetism are incomparably more difficult to write, and we should expect that the consequences from them will be much more complicated, and now we will have to understand them for a very long time.

We can illustrate some laws of electrodynamics with a series of simple experiments that can show us, at least qualitatively, the relationship between the electric and magnetic fields. You become familiar with the first term in equation (1.1) when combing your hair, so we won’t talk about it. The second term in equation (1.1) can be demonstrated by passing a current through a wire hanging over a magnetic bar, as shown in Fig. 1.6. When the current is turned on, the wire moves due to the force acting on it F = qvXB. When goes to the wire current, the charges inside it move, that is, they have a speed v, and they are acted upon by the magnetic field of the magnet, as a result of which the wire moves to the side.

When the wire is pushed to the left, you can expect the magnet itself to experience a push to the right. (Otherwise, the whole device could be mounted on a platform and get a reactive system in which momentum would not be conserved!) Although the force is too small to notice the movement of a magnetic rod, the movement of a more sensitive device, say a compass needle, is quite noticeable.

How does current in a wire push a magnet? The current flowing through the wire creates its own magnetic field around it, which acts on the magnet. In accordance with the last term in equation (1.9), the current should lead to circulations vector B; in our case, the field lines B are closed around the wire, as shown in Fig. 1.7. It is this field B that is responsible for the force acting on the magnet.

Equation (1.9) tells us that for a given amount of current flowing through the wire, the circulation of the field B is the same for any curve surrounding the wire. For those curves (circles, for example) that lie far from the wire, the length turns out to be greater, so the tangent component B should decrease. You can see that you would expect B to decrease linearly with distance from a long straight wire.

We said that current flowing through a wire creates a magnetic field around it and that if there is a magnetic field, then it acts with some force on the wire through which the current flows. This means that one should think that if a magnetic field is created by a current flowing in one wire, then it will act with some force on the other wire, which also carries current. This can be shown by using two freely suspended wires (Fig. 1.8). When the direction of the currents is the same, the wires attract, and when the directions are opposite, they repel.

In short, electric currents, like magnets, create magnetic fields. But then what is a magnet? Since magnetic fields are created by moving charges, could it be that the magnetic field created by a piece of iron is actually the result of currents? Apparently this is true. In our experiments, we can replace the magnetic rod with a coil of wound wire, as shown in Fig. 1.9. When current passes through the coil (as well as through the straight wire above it), exactly the same movement of the conductor is observed as before when there was a magnet instead of the coil. Everything looks as if current were continuously circulating inside a piece of iron. Indeed, the properties of magnets can be understood as a continuous current within the iron atoms. The force acting on the magnet in Fig. 1.7 is explained by the second term in equation (1.1).

Where do these currents come from? One source is the movement of electrons along atomic orbits. This is not the case with iron, but in some materials this is the origin of magnetism. In addition to rotating around the nucleus of an atom, the electron also rotates around its own axis(something similar to the rotation of the Earth); It is from this rotation that a current arises, creating a magnetic field in the iron. (We said "something like the rotation of the Earth" because in fact quantum mechanics the question is so deep that it does not fit well enough into classical concepts.) In most substances, some electrons rotate in one direction, others in the other, so that magnetism disappears, and in iron (by mysterious reason, which we will talk about later) many electrons rotate so that their axes point in one direction and this serves as a source of magnetism.

Since the fields of magnets are generated by currents, there is no need to insert additional terms into equations (1.8) and (1.9) that take into account the existence of magnets. In these equations we're talking about O everyone currents, including circular currents from rotating electrons, and the law turns out to be correct. It should also be noted that, according to equation (1.8), magnetic charges, similar to the electric charges on the right side of equation (1.6), do not exist. They were never discovered.

The first term on the right side of equation (1.9) was discovered theoretically by Maxwell; he is very important. He says change electrical fields calls magnetic phenomena. In fact, without this term the equation would lose its meaning, because without it the currents in open circuits would disappear. But in reality such currents exist; talks about this next example. Imagine a capacitor made up of two flat plates. It is charged by a current flowing into one of the plates and flowing out from the other, as shown in Fig. 1.10. Let's draw a curve around one of the wires WITH and stretch a surface over it (surface S 1) that will intersect the wire. In accordance with equation (1.9), the circulation of field B along the curve WITH is given by the magnitude of the current in the wire (multiplied by from 2). But what happens if we pull on a curve another surface S 2 in the shape of a cup, the bottom of which is located between the plates of the capacitor and does not touch the wire? No current, of course, passes through such a surface. But a simple change in the position and shape of an imaginary surface should not change the real magnetic field! The circulation of field B should remain the same. Indeed, the first term on the right side of equation (1.9) is combined with the second term in such a way that for both surfaces S 1 and S 2 the same effect occurs. For S 2 the circulation of vector B is expressed through the degree of change in the flow of vector E from one plate to another. And it turns out that the change in E is related to the current precisely in such a way that equation (1.9) turns out to be satisfied. Maxwell saw the need for this and was the first to write the complete equation.

Using the device shown in FIG. 1.6, another law of electromagnetism can be demonstrated. Let's disconnect the ends of the hanging wire from the battery and connect them to a galvanometer - a device that records the passage of current through the wire. Stands only in the field of a magnet swing wire, and current will immediately flow through it. This is a new consequence of equation (1.1): the electrons in the wire will feel the action of the force F=qv X B. Their speed is now directed to the side, because they are deflected along with the wire. This v, together with the vertically directed field B of the magnet, results in a force acting on the electrons along wires and the electrons are sent to the galvanometer.

Suppose, however, that we left the wire alone and began to move the magnet. We feel that there should be no difference, because relative motion the same thing, and indeed current flows through the galvanometer. But how does a magnetic field act on charges at rest? In accordance with equation (1.1), an electric field should arise. A moving magnet must create an electric field. The question of how this happens is answered quantitatively by equation (1.7). This equation describes a set of almost very important phenomena happening in electric generators and transformers.

Most remarkable consequence of our equations is that by combining equations (1.7) and (1.9), we can understand why electromagnetic phenomena apply to long distances. The reason for this, roughly speaking, is something like this: suppose that somewhere there is a magnetic field that increases in magnitude, say, because a current is suddenly passed through a wire. Then from equation (1.7) it follows that circulation of the electric field should arise. When the electric field begins to gradually increase for circulation to occur, then, according to equation (1.9), magnetic circulation should also arise. But increasing this magnetic field will create a new circulation of the electric field, etc. In this way, the fields propagate through space without the need for charges or currents anywhere other than the source of the fields. This is the way we we see each other! All this is hidden in the electromagnetic field equations.

Equation heat balance the thermistor has the form

I2 R =ξ (Qп – Qс ) ·S,

where ξ is the heat transfer coefficient, depending on the speed of the medium; Qп and Qс - respectively, the temperature of the thermistor; (converter) and environment;

S is the surface area of ​​the thermistor.

If the thermistor has the shape of a cylinder and is located across the flow so that the angle between the axis of the cylinder and the flow velocity vector is 90°, then the heat transfer coefficients for gases and liquids are determined by the formulas

where V and υ are the speed and thermal conductivity of the medium, respectively, d is the diameter of the thermistor;

c and n are coefficients depending on the Reynolds number Re = Vd/υ;

P r = υ d - Prandtl number, depending on kinematic viscosity and

thermal conductivity of the medium.

Such a converter (thermistor) is usually included in a bridge measuring circuit. Using the above expressions, the speed V can be measured.

5.2. Use of the laws of electromagnetism in measuring technology

The electroscope, a device for detecting electric charges, is based on the phenomenon of electric repulsion of charged bodies. An electroscope consists of a metal rod to which

a thin aluminum or paper piece of paper is hung. The rod is reinforced with an ebonite or amber stopper inside a glass jar, which protects the leaf from air movement.

An electrometer is an electroscope with a metal body. If you connect the body of this device to the ground, and then touch its rod with some charged body, then part of the charge will transfer to the rod and the leaves of the electrometer will diverge to a certain angle. Such a device measures the potential difference between a conductor and ground.

An oscilloscope is a device designed to observe, record and measure the parameters of the signal under study, usually time-dependent voltage. Light beam oscilloscopes use electromechanical deviation light beam under the influence of the test voltage.

Cathode ray oscilloscopes (CRO) are built on the basis of cathode ray tubes. The deflection of the electron beam is carried out directly by an electrical signal.

The main unit of the ELO is a cathode ray tube (CRT), which is a glass evacuated flask (Fig. 10), inside of which there is an oxide cathode 1 with a heater 2, a modulator 3, anodes 4 and a system of deflecting plates 5 and 6. Part of the CRT, including which includes a cathode, modulator and anodes, is called an electron gun.

Rice. 10 Cathode ray tube

If voltage is applied to the deflection plates, the electron beam will deflect as shown in Fig. 11.

The test voltage Uy is usually applied to the vertically deflecting plates, and the unfolding voltage (in in this case linearly varying periodic with period Tr).

Rice. 11. Receiving an image on a CRT screen

Magnetoelectric system devices (ammeters, voltmeters and ohmmeters) are suitable for use in direct current circuits, and when using detectors - also for alternating current purposes. Operating principle of the measuring mechanism magnetoelectric The system uses the effect of interaction between the field of a permanent magnet and a coil (frame) through which current flows. In Fig. Figure 12 shows a typical design (moving coil).

Rice. 12. Typical moving coil design Permanent magnet 1, magnetic circuit with pole pieces 2 and

fixed core 3 make up the magnetic system of the mechanism. A strong, uniform radial magnetic field is created in the gap between the pole pieces and the core, in which there is a movable rectangular coil (frame) 4, wound with copper or aluminum wire on a frame. The coil is fixed between the axle shafts 5 and 6. Spiral springs 7 and 8 are designed to create a counteracting torque and, at the same time, to supply the measured current.

The frame is rigidly connected to the arrow 9. To balance the moving part, there are movable weights on the antennae 10.

Conversion equation:

α = I(BnS / W),

where B is the magnetic induction in the gap;

α - angle of rotation of the moving part; S – frame area;

n – number of coil turns;

W – specific counteracting moment. 51

Devices of electromagnetic, electrodynamic, ferrodynamic and electrostatic systems Widely used as standard electromechanical ammeters, voltmeters, wattmeters and frequency meters.

The operating principle of electrodynamic devices is based on the interaction of the magnetic fields of two coils through which current flows.

The structure of such a measuring mechanism is shown in Fig. 13.

Rice. 13. Electromechanical converter of the electrodynamic system

Inside the fixed coil 1, a moving coil 2 can rotate, current to which is supplied through springs.

The rotation of the coil is carried out by a torque caused by the interaction of the magnetic fields of coils 1 and 2. The counteracting torque is created by special springs (not shown in Fig. 13).

The transformation equation of this mechanism is:

α = W 1 ∂ ∂ M α I 1 I 2 ,

where W is the specific counteracting moment;

α - angle of rotation of the moving part; M is the mutual inductance of the coils.

This mechanism can be used to measure constants

and alternating currents, voltages and power.

Ferrodynamic measuring mechanisms are essentially

are a type of electrodynamic devices, from which they differ only in design, since the coil has a soft magnetic core (magnetic core), between the strips of which a moving coil is placed. The presence of the core significantly increases the magnetic field of the stationary coil, and therefore the sensitivity.

In electrostatic devices The principle of interaction between electrically charged conductors is implemented.

One of the common designs of a detailed measuring mechanism is shown in Fig. 14.

Fig. 14. Converter electrostatic system Movable aluminum plate 1, fixed together with the arrow

on axis 3, can move, interacting with two electrically connected fixed plates 2. Input terminals (not shown), to which the measured voltage is supplied, are connected to the movable and fixed plates.

Under the influence of electrostatic forces, the movable plate is pulled into the space between the fixed plates. Movement

stops when the counteracting moment of the twisted plate becomes equal to the torque.

The transformation equation of such a mechanism has the form

α = 2 1 W ∂ d C α U 2 ,

where U is the measured voltage;

W – specific counteracting moment; C is the capacitance between the plates.

Similar converters are used to develop voltmeters of direct and alternating currents.

Operating principle of the devices electromagnetic system is based on the interaction of a magnetic field created by a current in a stationary coil with a moving ferromagnetic core. One of the most common designs is shown in Fig. 15.

Rice. 15. Electromagnetic system converter:

I – coil, 2 – core, 3 – spiral spring creating a counteracting moment, 4 – air damper

Under the influence of a magnetic field, the core is pulled inward

There are four fundamental forces physics, and one of them is called electromagnetism. Conventional magnets have limited use. An electromagnet is a device that creates an electric current during the passage. Since electricity can be turned on and off, so can an electromagnet. It can even be weakened or strengthened by decreasing or increasing the current. Electromagnets find their application in various everyday electrical appliances, including different areas industries, from conventional switches to spacecraft propulsion systems.

What is an electromagnet?

An electromagnet can be considered as a temporary magnet that functions with the flow of electricity and its polarity can be easily changed by changing Also the strength of an electromagnet can be changed by changing the amount of current flowing through it.

The scope of application of electromagnetism is unusually wide. For example, magnetic switches are preferred because they are less susceptible to temperature changes and are able to maintain rated current without nuisance tripping.

Electromagnets and their applications

Here are some of the examples where they are used:

• Motors and generators. Thanks to electromagnets, it has become possible to produce electric motors and generators that operate on the principle electromagnetic induction. This phenomenon was discovered by scientist Michael Faraday. He proved that electric current creates a magnetic field. Generator uses external force wind, moving water or steam rotates a shaft, which causes a set of magnets to move around a coiled wire to create an electric current. Thus, electromagnets convert other types of energy into electrical energy.
• Practice industrial use. Only materials made from iron, nickel, cobalt or their alloys, as well as some natural minerals, react to a magnetic field. Where are electromagnets used? One of the areas of practical application is metal sorting. Since the mentioned elements are used in production, iron-containing alloys are effectively sorted using an electromagnet.
• Where are electromagnets used? They can also be used to lift and move massive objects, for example, cars before disposal. They are also used in transportation. Trains in Asia and Europe use electromagnets to transport cars. This helps them move at phenomenal speeds.

Electromagnets in everyday life

Electromagnets are often used to store information, as many materials are capable of absorbing a magnetic field, which can then be read to retrieve information. They find application in almost any modern device.

Where are electromagnets used? In everyday life they are used in a number of household appliances. One of useful characteristics An electromagnet is capable of changing when changing the strength and direction of the current flowing through the coils or windings around it. Speakers, loudspeakers and tape recorders are devices in which this effect is realized. Some electromagnets can be very strong, and their strength can be adjusted.

Where are electromagnets used in life? The simplest examples are electromagnetic locks. An electromagnetic lock is used for the door, creating a strong field. As long as current passes through the electromagnet, the door remains closed. Televisions, computers, cars, elevators and copying machines - this is where electromagnets are used, and this is not a complete list.

Electromagnetic forces

The strength of the electromagnetic field can be adjusted by changing the electric current passing through the wires wrapped around the magnet. If the direction of the electric current is reversed, the polarity of the magnetic field also reverses. This effect is used to create fields in a computer's magnetic tape or hard drive for storing information, as well as in speaker speakers in radios, televisions, and stereo systems.

Magnetism and electricity

The dictionary definitions of electricity and magnetism are different, although they are manifestations of the same force. When electric charges moving, they create a magnetic field. Its change, in turn, leads to the generation of electric current.

Inventors use electromagnetic forces to create electric motors, generators, toys, consumer electronics and many other invaluable devices that are impossible to imagine without. daily life modern man. Electromagnets are inextricably linked with electricity; they simply cannot work without external source nutrition.

Application of lifting and large-scale electromagnets

Electric motors and generators are vital in modern world. Motor accepts electrical energy and uses a magnet to convert electrical energy into kinetic energy. A generator, on the other hand, converts motion using magnets to generate electricity. When moving large metal objects, lifting electromagnets are used. They are also necessary when sorting scrap metal, to separate cast iron and other ferrous metals from non-ferrous ones.

A real miracle of technology is a Japanese levitating train capable of reaching speeds of up to 320 kilometers per hour. It uses electromagnets to help it float in the air and move incredibly fast. Naval forces The US is conducting high-tech experiments with a futuristic electromagnetic rail gun. She can direct her projectiles over considerable distances with great speed. The shells have enormous kinetic energy, therefore they can hit targets without the use of explosives.

The concept of electromagnetic induction

When studying electricity and magnetism, an important concept is when a flow of electricity occurs in a conductor in the presence of a changing magnetic field. The use of electromagnets with their induction principles are actively used in electric motors, generators and transformers.

Where can electromagnets be used in medicine?

Magnetic resonance imaging (MRI) scanners also operate using electromagnets. This is specialized medical method for examination internal organs people who are not available for direct examination. Along with the main one, additional gradient magnets are used.

Where are electromagnets used? They are present in all types of electrical devices, including hard drives, speakers, engines, generators. Electromagnets are used everywhere and, despite their invisibility, occupy important place in the life of a modern person.

Rutherford was confused. He succeeded brilliantly in revealing internal structure atom, however, having done this, the scientist revealed largest conflict V physical science. The gold foil experiment demonstrated that the atom is a tiny "planetary" system. However, the theory of electromagnetism predicted that such a system was categorically unstable - it would not last even “the blink of an eye.” It was a paradoxical situation, and finding a way out of it seemed almost impossible. However, one person - a young Danish physicist - succeeded.

Niels Bohr (1885–1962) came to England in 1911 after receiving his doctorate in Copenhagen, and since then worked under the direction first of J. J. Thomson and then of Rutherford. He understood that planetary model Rutherford's atom, supported by serious experimental data, is quite convincing. But at the same time, he understood that the laws of electromagnetism, which gave the world electric motors and dynamos, were no less convincing. Bohr's revolutionary solution to the atomic paradox was both simple and daring. In 1913, Bohr announced that the laws of electromagnetism simply did not apply inside atoms. Electrons rotating around the nucleus do not emit electromagnetic waves and therefore do not fall in a spiral onto the core. In short, the known laws of physics do not apply to the field of ultra-small objects.