High-frequency currents (HF) are considered to be currents for which the condition of quasi-stationarity is not satisfied, resulting in a strongly pronounced skin effect
High-frequency currents (HF) are considered to be currents for which the condition of quasi-stationarity is not met, which results in a strongly pronounced skin effect. For this reason, the current flows along the surface of the conductor without penetrating into its volume. The frequency of such currents exceeds 10,000 Hz.
To obtain currents with a frequency of more than a few tens of kilohertz, electric machine generators are used, which include a stator and a rotor. On their surfaces facing each other there are teeth, due to the mutual movement of which a pulsation occurs. magnetic field. The final frequency of the current received at the output is equal to the product of the rotor speed and the number of teeth on it.
Also, to obtain HDTV, oscillatory circuits are used, for example, an electrical circuit, which includes inductance and capacitance. To obtain HDTV frequencies of billions of hertz, installations with a hollow oscillatory circuit are used (WOF, TWT, magnetron, klystron).
If the conductor is placed in the magnetic field of a coil in which a high-frequency current flows, then large eddy currents will arise in the conductor, which will heat it. The temperature and intensity of heating can be adjusted by changing the current in the coils. Due to this property, HDTV is used in many areas of human activity: in induction furnaces, in metallurgy for surface hardening of parts, medicine, agriculture, household appliances ( microwaves, various devices for cooking), radio communications, radar, television, etc.
Examples of the use of high frequency currents
With the help of HDTV in induction furnaces, any metals can be melted. The advantage of this type of smelting lies in the possibility of smelting under conditions of complete vacuum, when contact with the atmosphere is excluded. This makes it possible to produce alloys that are pure in terms of non-metallic inclusions and unsaturated with gases (hydrogen, nitrogen).
On hardening machines with the help of HDTV, it is possible to harden steel products only in the surface layer due to the skin effect. This makes it possible to obtain parts with a hard surface that can withstand significant loads and at the same time without compromising wear resistance and ductility, since the core remains soft.
In medicine, high-frequency currents have long been used in UHF devices, where heating of any human organs is carried out by heating the dielectric. HDTV of even very high current strength is harmless to humans, since it flows exclusively in the most superficial layers of the skin. Also in medicine, electroknives based on high-frequency current are used, with the help of which blood vessels are “brewed” and tissues are cut.
DEPARTMENT OF EDUCATION AND SCIENCE OF THE KEMEROVSK REGION
State educational institution middle vocational education
Kemerovo vocational technical school
High frequency currents.
Prepared by: physics teachers
Shcherbunova Evgenia Olegovna and
Kolabina Galina Alekseevna
Kemerovo
What are high frequency currents?
Currents with a frequency above 10,000 Hz are called high frequency currents (HF). They are obtained with electronic devices.
If a conductor is placed inside a coil through which a high-frequency current flows, then eddy currents will occur in the conductor. Eddy currents heat the conductor. The heating rate and temperature can be easily adjusted by changing the current in the coil.
The most refractory metals can be melted in an induction furnace. To obtain highly pure substances, melting can be carried out in a vacuum and even without a crucible, by suspending the molten metal in a magnetic field. High heating rate is very convenient for rolling and forging metal. By selecting the shape of the coils, it is possible to solder and weld parts at the best temperature regime.
induction melting furnace
The current i flowing through the conductor creates a magnetic field B. At very high frequencies, the influence of the eddy electric field E generated by the change in the field B becomes noticeable.
The influence of the field E increases the current on the surface of the conductor and weakens it in the middle. At a sufficiently high frequency, the current flows only in the surface layer of the conductor.
The method of surface hardening of steel products was invented and proposed by the Russian scientist V.P. Vologdin. At high frequency, the induction current heats only the surface layer of the part. After rapid cooling, a non-brittle product with a hard surface is obtained.
hardening machine
See more here: Induction heating and hardening plants
The action of high-frequency currents on dielectrics
Dielectrics are acted upon by a high-frequency electric field, placing them between the plates of a capacitor. Part of the energy of the electric field is spent in this case on heating the dielectric. Heating with HDTV is especially good if the thermal conductivity of the substance is low.
High-frequency heating of dielectrics (dielectric heating) is widely used for drying and gluing wood, for the production of rubber and plastics.
High frequency currents in medicine
UHF therapy is a dielectric heating of body tissues. Deadly for a person is a constant and low-frequency current in excess of a few milliamps. A high frequency current (≈ 1 MHz), even at a strength of 1 A, causes only tissue heating and is used for treatment.
Electroknife is a high-frequency apparatus widely used in medicine. It cuts tissue and "brews" blood vessels.
Other applications of high frequency currents
Grain treated with HDTV before sowing significantly increases the yield.
Induction heating of gas plasma makes it possible to obtain high temperatures.
A 2400 MHz field in a microwave oven cooks soup right in the bowl in 2-3 minutes.
The action of the mine detector is based on changing the parameters of the oscillatory circuit when the coil is brought to a metal object.
High frequency currents are also used for radio communications, television and radar.
List of sources:
1. Dmitrieva, V.F. Physics: a textbook for student educational institutions of secondary vocational education [Text] / V.F. Dmitriev. –6th edition. stereotype. - M .: Publishing Center Academy, 2005. - 280-288.
Internet resources:
Single window of access to educational resources [ Electronic resource]. - Access mode: http:// window. edu. en/ window, free. - Zagl. from the screen. - (Date of treatment: 11/11/2014).
Electronic library system "KnigaFond" [Electronic resource]. - Access mode: http://www.knigafund.ru/, for access to information. resources require authorization. - Zagl. from the screen. - (Date of treatment: 11/11/2014).
Portal of natural sciences » [Electronic resource]. - Access mode: http://e-science.ru/physics, free. - Zagl. from the screen. - (Date of treatment: 11/11/2014).
Darsonvalization is the use of high-frequency current (110 kHz) and voltage (25-30 kV) with a low current strength, modulated in a series of oscillations with a duration of 100 μs, following at a frequency of 100 Hz, for a therapeutic purpose. The current is so high voltage weakens when passing through the rarefied air of a glass electrode, forming a high-frequency corona discharge in the air layer between the body surface and the electrode wall. The mechanism of therapeutic action is determined by the passage of high-frequency current through the tissues and the impact on skin receptors and surface tissues of electrical discharges. As a result, there is an expansion of superficial blood vessels and an increase in blood flow through them, an expansion of spastically narrowed and with an increased tone of vessels, and restoration of impaired blood flow in them. This leads to the cessation of tissue ischemia and the pains caused by it, feelings of numbness, paresthesia, improvement of tissue trophism, including vascular walls.
The therapeutic use of currents of supratonal frequency (TNCH) consists in exposing the body to high-frequency alternating current (22 kHz) at a voltage of 4.5-5 kV. By appearance, the technique of performing procedures and techniques, the method is very similar to local darsonvalization. The difference lies in the fact that not a pulsed, but a continuous current of lower frequency and voltage is used and it is passed through a glass electrode filled with neon. All this determines the differences in the therapeutic effect. Due to the continuity of the current in the tissues, more heat generation occurs - patients feel heat at the site of exposure. A lower voltage eliminates the irritating effect of a spark discharge, the effects are better tolerated by patients, and therefore the method is more often used in pediatric practice.
Transformer operating modes
· idle mode. This mode is characterized by an open transformer secondary circuit, as a result of which no current flows in it. With the help of no-load experience, it is possible to determine the transformer efficiency, the transformation ratio, as well as steel losses.
· Load mode. This mode is characterized by the secondary circuit of the transformer closed on the load. This mode is the main operating mode for the transformer.
· Short circuit mode. This mode is obtained by short circuiting the secondary circuit. With it, you can determine the loss of useful power for heating the wires in the transformer circuit. This is taken into account in the equivalent circuit of a real transformer using active resistance.
28) Oscillatory circuit- an oscillator, which is electrical circuit containing connected inductor and capacitor. In such a circuit, current and voltage fluctuations can be excited.
Operating principle
Let a capacitor with capacity C be charged up to voltage . The energy stored in the capacitor is
When a capacitor is connected to an inductor, a current will flow in the circuit, which will cause an electromotive force (EMF) of self-induction in the coil, aimed at reducing the current in the circuit. The current caused by this EMF (in the absence of losses in the inductance) at the initial moment will be equal to the discharge current of the capacitor, that is, the resulting current will be equal to zero. The magnetic energy of the coil at this (initial) moment is zero.
Then the resulting current in the circuit will increase, and the energy from the capacitor will pass into the coil until the capacitor is completely discharged. At this point, the electrical energy of the capacitor. The magnetic energy concentrated in the coil, on the contrary, is maximum and equal to , where is the inductance of the coil,
Maximum current value.
After that, the recharging of the capacitor will begin, that is, the charge of the capacitor with a voltage of a different polarity. Recharging will take place until the magnetic energy of the coil is converted into the electrical energy of the capacitor. The capacitor, in this case, will again be charged to a voltage.
As a result, oscillations arise in the circuit, the duration of which will be inversely proportional to the energy losses in the circuit.
In general, the processes described above in a parallel oscillatory circuit are called current resonance, which means that currents flow through the inductance and capacitance, more than the current passing through the entire circuit, and these currents are greater by a certain number of times, which is called the quality factor. These large currents do not leave the limits of the circuit, since they are out of phase and compensate themselves. It is also worth noting that the resistance of a parallel oscillatory circuit tends to infinity at the resonant frequency (unlike a series oscillatory circuit, the resistance of which tends to zero at the resonant frequency), and this makes it an indispensable filter.
It is worth noting that in addition to a simple oscillatory circuit, there are also oscillatory circuits of the first, second and third kind, which take into account losses and have other features.
29) Induction alternator- Unlike other generators, the operation of an induction generator is based not on a rotating magnetic field, but on a pulsating one, in other words, the field changes not as a function of displacement, but as a function of time, which ultimately (induction of EMF) gives the same result.
Construction of induction generators involves the placement of both a constant field and coils for inducing EMF on the stator, while the rotor remains free from windings, but necessarily has a toothed shape, since all the work of the generator is based on the toothed harmonics of the rotor.
High frequency currents and their application.
High-frequency currents are such currents, the frequency of which, that is, the number of oscillations, reaches one million in one second. This type currents has found its application in mechanical engineering, where it is necessary for welding and heat treatment of surfaces of parts, and in metallurgy, where it is used for melting various metals.
The use of high frequency currents has brought such industries as mechanical engineering and metallurgy to a new level. Heat treatment of parts, carried out using high voltage currents, increases their service life, increases wear resistance, strength and hardness of the metal. Working with high frequency currents not only makes the work more efficient, but also significantly improves the quality level of the resulting products.
Maxwell's postulates
First postulate: around any alternating magnetic field there is a vortex electric field.
The direction of the vortex electric field is determined by the left screw rule if the magnetic field increases.
If the magnetic field decreases, then first the direction of the vortex electric field is determined according to the left screw rule. Then it is changed to the opposite - this will be the direction of the vortex electric field for the decreasing magnetic field.
Second postulate: around any alternating electric field there is a magnetic field.
The direction of the lines of magnetic induction is determined by the rule of the right screw, if the electric field strength increases.
If the electric field strength decreases, then first the direction of the magnetic induction lines is determined according to the right screw rule. Then it is changed to the opposite - this will be the direction of the magnetic induction lines for the decreasing electric field.
33) The Frank-Hertz Experience- an experiment that was an experimental proof of the discreteness of the internal energy of an atom. Placed in 1913 by J. Frank and G. Hertz.
The figure shows the scheme of the experiment. A potential difference V is applied to the cathode K and grid C1 of an electrovacuum tube filled with Hg (mercury) vapor, accelerating electrons, and the dependence of current I on V is removed. A retarding potential difference is applied to grid C2 and anode A. Electrons accelerated in region I experience collisions with Hg atoms in region II. If the energy of the electrons after the collision is sufficient to overcome the retarding potential in region III, then they will fall on the anode. Consequently, the readings of the galvanometer G depend on the energy loss by the electrons upon impact.
In the experiment, a monotonic increase in I was observed with an increase in the accelerating potential up to 4.9 V, that is, electrons with energy E< 4,9 эВ испытывали упругие соударения с атомами Hg и внутренняя энергия атомов не менялась. При значении V = 4,9 В (и кратных ему значениях 9,8 В, 14,7 В) появлялись резкие спады тока. Это определённым образом указывало на то, что при этих значениях V соударения электронов с атомами носят неупругий характер, то есть энергия электронов достаточна для возбуждения атомов Hg. При кратных 4,9 эв значениях энергии электроны могут испытывать неупругие столкновения несколько раз.
34) The invention of radio communication- one of the most outstanding achievements of human thought and scientific and technological progress. The need to improve the means of communication, in particular the establishment of communication without wires, was especially acute at the end of the 19th century, when the widespread introduction of electric energy into industry, agriculture, communications, transport (primarily maritime), etc.
The history of science and technology confirms that all outstanding discoveries and inventions were, firstly, historically determined, and secondly, the result of the creative efforts of scientists and engineers from different countries.
Radiotelegraph communication - telecommunication, in which discrete messages are transmitted by means of radio waves - alphabetic, numeric and sign. At the transmitting station, electrical oscillations modulated by a telegraph message enter the radiotelegraph communication line and from it to the receiving station. After detection and amplification, the telegraph message is received by ear or recorded by a receiving direct-printing telegraph apparatus.
35) Radio telephone communications- telecommunications, in which telephone (voice) messages are transmitted by means of radio waves. Information enters the radiotelephone line through a microphone, and from it - usually through a telephone. The microphone and telephone are connected directly to the radio stations or telephone lines are connected to them.
Amplitude modulation - a type of modulation in which the variable parameter of the carrier signal is its amplitude
Amplitude modulator - a device is called, the envelope of the high-frequency signal at the output of which is proportional to the low-frequency modulating oscillation. Consider the case of the simplest harmonic modulating oscillation:,
At the input of the modulator, the signal is:
where the amplitude modulation depth M should be proportional to the amplitude .
As a result of the impact of the input signal on a nonlinear element with a piecewise linear approximation, harmonics and combination components appear in the current of the latter input signals, namely components with frequencies: Components with frequencies 
and form the desired amplitude-modulated oscillation. It must be separated by a bandpass filter with a center frequency equal to the carrier and a bandwidth sufficient to separate components with frequencies .
36) Detection - Converting an electromagnetic waveform to produce a voltage or current whose magnitude is determined by the parameters of the waveform, in order to extract the information contained in the changes in these parameters
The device and operation of the simplest detector receivers - the simplest, most basic type of radio receiver. It consists of an oscillatory circuit to which the antenna and ground are connected, and a diode (in the earlier version, crystalline) detector that demodulates the amplitude-modulated signal. Signal audio frequency from the output of the detector, as a rule, is reproduced by high-impedance headphones.
Even to receive powerful radio stations, a detector receiver requires the longest possible and highly suspended antenna (preferably tens of meters), as well as proper grounding. A few important advantages of a detector receiver are that it does not require a power source, is very cheap and can be assembled from improvised means. By connecting any external low-frequency amplifier to the output of the receiver, you can get a receiver direct amplification with much better parameters. Due to these advantages, detector receivers were widely used not only in the early decades of broadcasting.
37) Propagation of radio waves - the phenomenon of energy transfer of electromagnetic oscillations in the radio frequency range (see radio emission). Various aspects of this phenomenon are studied by various technical disciplines, which are sections of radio engineering. The most general questions and problems are considered by radiophysics. The propagation of radio waves in special technical objects such as cables, antenna waveguides, is considered by specialists in applied electrodynamics, or specialists in antenna and feeder technology. Technical discipline propagation of radio waves considers only those tasks of radio emission that are associated with the propagation of radio waves in natural environments, that is, the influence of the atmosphere and near-Earth space on the radio waves of the Earth's surface, the propagation of radio waves in natural reservoirs, as well as in man-made landscapes
Types of radio waves 
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Properties of radio waves - The propagation of radio waves in the earth's space depends on the properties of the earth's surface and the properties of the atmosphere. The conditions for the propagation of radio waves along the earth's surface largely depend on the terrain, the electrical parameters of the earth's surface, and the wavelength. Like other waves, radio waves are characterized by diffraction, i.e. obstacle avoidance phenomenon. Diffraction is most pronounced when the geometric dimensions of the obstacles are commensurate with the wavelength. Radio waves that propagate near the surface of the earth and, partially due to diffraction, envelope the bulge of the globe are called terrestrial, or surface radio waves.
Application of radio waves- For the transmission of various data, signals and other information through the source and receiver of radio waves. For example cellular its different standards work at different radio frequencies, also WI-FI, ethernet radio and many others.
38) Short story development of views on the nature of light - In the second half of the 17th century, the foundations of physical optics were laid. F. Grimaldi discovers the phenomenon of light diffraction (light bending around obstacles, i.e. its deviation from rectilinear propagation) and suggests the wave nature of light. In the "Treatise on Light" published in 1690 by H. Huygens, the principle was formed, according to which each point of space, which it reached in this moment a propagating wave becomes a source of elementary spherical waves, and on its basis he derived the laws of reflection and refraction of light. Huygens established the phenomenon of light polarization - a phenomenon that occurs with a beam of light during its reflection, refraction (especially with double refraction) and consists in the fact that the oscillatory motion at all points of the beam occurs only in one plane passing through the direction of the beam, while in In an unpolarized beam, oscillations occur in all directions, perpendicular to the beam. Huygens, having developed Grimaldi's idea that light propagates not only in a straight line with refraction and reflection, but also with splitting (diffraction), gave an explanation for all known optical phenomena. He claims that light waves propagate in the ether, which is a subtle matter penetrating all bodies.
39) Speed of light in vacuum - the absolute value of the speed of propagation of electromagnetic waves in vacuum. In physics, it is traditionally denoted by the Latin letter " c» (pronounced as [tse]). The speed of light in vacuum is a fundamental constant, independent of the choice of inertial reference frame (ISR). It refers to the fundamental physical constants that characterize not just individual bodies or fields, but the properties of space-time as a whole. According to modern concepts, the speed of light in vacuum is the limiting speed of particles and propagation of interactions.
The speed of light in a transparent medium is the speed at which light travels in a medium other than vacuum. In a medium with dispersion, phase and group velocity are distinguished.
The phase velocity relates the frequency and wavelength of monochromatic light in a medium (λ = c/ν). This speed is usually (but not necessarily) less c. The ratio of the phase speed of light in vacuum to the speed of light in a medium is called the refractive index of the medium. The group speed of light in an equilibrium medium is always less c. However, in nonequilibrium media it can exceed c. In this case, however, the leading edge of the pulse still moves at a speed not exceeding the speed of light in vacuum. As a result, superluminal transmission of information remains impossible.
40) Light interference- redistribution of light intensity as a result of superposition (superposition) of several light waves. This phenomenon is accompanied by intensity maxima and minima alternating in space. Its distribution is called the interference pattern.
Newton's rings
Another method for obtaining a stable interference pattern for light is the use of air gaps, based on the same difference in the path of two parts of the wave: one - immediately reflected from the inner surface of the lens and the other - passed through the air gap under it and only then reflected. It can be obtained by placing a plano-convex lens on a glass plate with the convex side down. When the lens is illuminated from above with monochromatic light, a dark spot is formed in the place of sufficiently dense contact between the lens and the plate, surrounded by alternating dark and light concentric rings of different intensity. Dark rings correspond to interference minima, and light rings correspond to maxima, both dark and light rings are isolines of equal thickness of the air layer. By measuring the radius of a light or dark ring and determining its serial number from the center, one can determine the wavelength of monochromatic light. The steeper the surface of the lens, especially closer to the edges, the smaller the distance between adjacent light or dark rings.
41) Laws of reflection:
1. Rays incident, reflected and perpendicular to the boundary of two media at the point of incidence of the beam lie in the same plane.
2. The angle of reflection is equal to the angle of incidence:
42) Laws of refraction
The lower the speed of light in a medium, the more optically dense it is considered. A medium with a higher absolute refractive index is said to be optically denser.
If light passes from an optically less dense medium to an optically denser one (for example, from air to water or glass), then the angle of incidence is greater than the angle of refraction.
Conversely, if light passes from water or glass into air, then it is refracted from a perpendicular: the angle of incidence is less than the angle of refraction
Dip the stick into the pond. The water level should rise. But this increase is so insignificant that it is difficult to detect it. And if you alternately immerse the stick in the water and pull it out, then waves will run through the water. They are noticeable at a considerable distance from the place of origin. This mechanical movement of water can be compared to electromagnetic phenomena. Around the conductor with direct current there is a constant electromagnetic field. It is difficult to detect it away from a current-carrying conductor.
But if an alternating electric current is passed through the conductor, then the electromagnetic forces around the conductor will change all the time, i.e., the electromagnetic field around it will wave. Electromagnetic waves run from an alternating current conductor.
The distance between the two closest wave crests on a pond is the wavelength. It is denoted by the Greek letter λ (lambda). The time during which any part of the undulating surface of the water rises, falls, and again returns to its initial position is the period of oscillation T. The reciprocal is called the oscillation frequency and is denoted by the letter f. The oscillation frequency is measured in periods per second. The unit of measurement of the frequency of oscillations, corresponding to one period per second, is named hertz (Hz) - in honor of Heinrich Rudolf Hertz (1857 - 1894), the famous researcher of oscillations and waves (1 thousand hertz \u003d 1 kilohertz, 1 million hertz \u003d 1 megahertz) .
Wave speed ( With) is the distance over which the waves propagate in one second. During one period T, the wave motion has time to propagate just for the length of one wave X. The following relations are valid for the wave motion:
with T = λ; c / f = λ
These relationships between oscillation frequency, wavelength and wave speed are true not only for waves on water, but also for any oscillations and waves.
It is necessary to immediately emphasize one property of electromagnetic oscillations. When they propagate in empty space, whatever their frequency, whatever their wavelength, their speed of propagation is always the same -300,000 km/sec. Visible light is one of the types of electromagnetic oscillations (with a wavelength of 0.4 to 0.7 millimicrons and a frequency of 10 14 - 10 15 Hz). The propagation speed of electromagnetic waves is the speed of light (3 10 10 cm/sec).
In air and in other gases, the propagation velocity of electromagnetic oscillations is only slightly less than in a vacuum. And in various liquid and solid media, it can be several times less than in a vacuum; in addition, here it depends on the oscillation frequency.
| The smallest and largest There are many units of energy: erg, joule, calorie, etc. The smallest of them is the electron volt: an electron accelerated in an electric field between points with a potential difference of 1 V will have an energy of 1 electron volt. The largest unit of energy was recently proposed by the Indian scientist Homi Baba for calculating the world's energy reserves. Its unit is equal to the thermal energy that is released during the combustion of 33 billion tons of coal. The scientist took this amount of coal because over the past 20 years, during which a lot of coal was mined and burned, exactly 33 billion tons were extracted from the earth's bowels. |
Radiation and Emitters
We live in the world of electromagnetic oscillations. Both sunlight, and mysterious streams of cosmic rays falling on the Earth from interstellar spaces, and heat emitted by a hot furnace, and electric current circulating in power networks - all these are electromagnetic vibrations. All of them propagate in the form of waves, in the form of rays.
Every object, every body that generates waves is called a radiator. The stick used to chat in the pond is a water wave emitter. Water resists its movement. To move the stick, you need to expend power. This power transmitted to the water is numerically equal to the product of the square of the speed of the stick and the resistance to movement. Part of this power is converted into heat - it goes to heat the water, and partly goes to the formation of waves.
It can be said that impedance, experienced by the stick, is the sum of two resistances: one of them is the resistance to heat generation, and the other is the resistance to wave formation - radiation resistance, as it is commonly called.
The same laws apply to electromagnetic phenomena. The power that an electric current consumes in a conductor is equal to the product of the resistance of the conductor and the square of the current in it. If you take the current in amps and the resistance in ohms, then the power will be in watts.
In the electrical resistance of any conductor (as in the mechanical resistance of water to the movement of a stick), two components can be distinguished: resistance to heat generation - ohmic resistance and radiation resistance - resistance caused by the formation of electromagnetic waves around the conductor that carry energy with them.
Take, for example, an electric hot plate, for which the ohmic resistance is 20 ohms, and the current is 5 A. The power converted into heat in this tile will be 500 watts (0.5 kW). To calculate the power of the waves running from the emitter, it is necessary to multiply the square of the current in the conductor by the radiation resistance of this conductor.
The radiation resistance is in a complex dependence on the shape of the conductor, on its dimensions, on the length of the emitted electromagnetic wave. But for a single rectilinear conductor, at all points of which there is a current of the same direction and the same strength, the radiation resistance (in ohms) is expressed by a relatively simple formula:
R izl \u003d 3200 (l / λ) 2
Here l is the length of the conductor, and λ - the length of the electromagnetic wave (this formula is valid for l significantly smaller than λ ).
With approximate estimates, this formula can be applied to any electrical structures, any machines and devices, for example, for a heating plate, in which the wire is not straight, but coiled into a spiral laid in a zigzag. But as l in the formula for radiation resistance, it is necessary to substitute not the full length of the conductor, but one of the given dimensions of the structure under consideration. For heating tiles l approximately equal to the diameter of the tile.
generated at central power plants alternating current with a frequency of 50 Hz. This current corresponds to an electromagnetic wave with a length of 6 thousand km. Not only electric stoves, but also the largest electrical machines and apparatuses, and even long-distance power lines, have dimensions l many times smaller than the length of this electromagnetic wave. The radiation resistance of the largest electrical machines and devices for a current with a frequency of 50 Hz is measured in negligible fractions of an ohm. Even at currents of thousands of amperes, less than one watt is radiated.
Therefore, in practice, when using industrial current with a frequency of 50 Hz, it is not necessary to take into account its wave properties. The energy of this current is firmly “tied” to the wires. To connect a consumer (lamps, stoves, motors, etc.), direct contact with current-carrying wires is necessary.
With an increase in the frequency of the current, the length of the electromagnetic wave decreases. For example, for a current with a frequency of 50 MHz, it is 3 m. With such a wave, even a small conductor can have significant radiation resistance and, at relatively small currents, radiate significant amounts of energy.
According to refined calculations, a half-wave conductor (l=λ/2) has radiation resistance R izd. about 73 ohms. With a current of, say, 10 a, the radiated power will be 7.3 kW. A conductor capable of radiating electromagnetic energy is called an antenna. This term was borrowed by electricians at the end of the last century from entomology - an antenna is called an antenna-tentacle in insects.
At the origins of radio engineering
Electromagnetic oscillations that occur at a frequency of a million billion hertz, our vision feels like light. A thousand times slower vibrations can be felt by the skin as heat rays.
Electromagnetic oscillations, the frequency of which ranges from a few kilohertz to thousands of megahertz, are not perceived by the senses, but they are of great importance in our lives. These vibrations are able to propagate, like light and heat, in the form of rays. In Latin, the word for "ray" is "radius". From this root the word "radio waves" is formed. These are oscillations generated by high frequency currents. Their main, most important application is wireless telegraph and telephone communications. For the first time in the world, wireless transmission of signals by radio waves was practically carried out by the Russian scientist Alexander Stepanovich Popov. On May 7 (April 25), 1895, at a meeting of the physical department of the Russian Physical and Chemical Society, he demonstrated the reception of radio waves.
Nowadays, with the help of radio, you can establish a wireless connection between any point on the globe. New branches of high-frequency technology arose - radar, television. Radio engineering began to be used in various industries.
It is correct to start the review of high-frequency technology with methods for obtaining high-frequency alternating currents.
The oldest and simplest way to produce high frequency electromagnetic oscillations is to discharge a capacitor through a spark. The first radio transmitters of A. S. Popov had spark generators with such simple spark gaps in the form of two balls separated by an air gap.
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| Machine high frequency current generator. |
At the beginning of our century, improved spark gaps appeared, which gave high-frequency oscillations with a power of up to 100 kW. But they had a great loss of energy. Currently, there are more advanced sources of high frequency currents (HF).
To obtain currents with a frequency of up to several kilohertz, machine generators are usually used. Such a generator consists of two main parts - a fixed stator and a rotating rotor. The surfaces of the rotor and stator facing each other are toothed. When the rotor rotates, the mutual movement of these teeth causes a pulsation of the magnetic flux. In the working winding of the generator, laid on the stator, there is a variable electromotive force (emf). The frequency of the current is equal to the product of the number of rotor teeth and the number of revolutions per second. For example, with 50 teeth on the rotor and its rotation speed of 50 rpm, a current-frequency of 2500 Hz is obtained.
At present, HDTV machine generators with a power of up to several hundred kilowatts are being produced. They give frequencies from a few hundred hertz to 10 kHz.
One of the most common modern ways receiving HDTV is an application oscillatory circuits connected to electrically controlled valves.
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