LECTURE 17 PHYSICAL PROCESSES OCCURRING IN THE TISSUES OF THE BODY UNDER THE INFLUENCE OF CURRENTS AND ELECTROMAGNETIC FIELDS

LECTURE 17 PHYSICAL PROCESSES OCCURRING IN THE TISSUES OF THE BODY UNDER THE INFLUENCE OF CURRENTS AND ELECTROMAGNETIC FIELDS

1. The action of direct current.

2. Action of alternating current (LF, ZCh, UZCH). Thresholds.

3. Action of high-frequency current.

4. The action of magnetic fields.

5. The action of a constant electric field.

6. The action of an alternating electric field (UHF).

7. The action of electromagnetic waves (microwave).

8. Tasks.

Different types of biological tissues have different electrical properties. Some fabrics are dielectrics, while others are conductors. The body includes biological fluids (electrolytes) containing a large number of ions that are involved in various types of metabolic processes. For these reasons, the properties of biological tissues change significantly under the influence of currents and electromagnetic fields.

17.1. DC action

The physiological effect of direct electric current is associated with two physical processes.

First, the constant electric field causes the directed movement of ions towards the poles. The accelerating action of electrical forces is counteracted by the resistance forces that arise when ions collide with other particles. As a result, a certain average speed of movement of ions is established, which, as experience shows, is proportional to the strength of the electric field in a given place:

The proportionality coefficient b is called ion mobility.

Ion mobility numerically equal to the average speed of its movement in a given environment at a field strength of 1 V / m.

Usually an off-system unit of mobility is used - cm / hour.

The quantity mobility depends on the type of ion and the medium in which it moves. Here are the values ​​of the mobility of some ions in an aqueous medium:

Differences in the mobility of ions lead to their separation, changes in concentrations, and also to the formation of local space charges.

Second, a constant electric field has an orienting effect on dipole molecules and causes electronic polarization of molecules that do not have a dipole moment. As a result, the content of ions in the compartments of various tissues changes.

These electrokinetic processes determine the physiological response of the body to direct current.

Exposure to constant electric shock on certain areas of the human body is carried out using electrodes superimposed on the corresponding areas of the body surface.

On the electrodes, through which current is supplied to the patient, substances are released, some of which are chemically active. To prevent chemical burns of the underlying tissues, electrodes are applied through wet pads.

The physiological effect produced by direct current depends on its density and duration of action. To prevent ionic imbalance of tissues, the duration of DC procedures usually does not exceed 20-30 minutes.

All devices for conducting medical procedures with direct current have a milliammeter and a potentiometer knob on the front panel for setting the required current value.

The main physiotherapy procedures using direct current include galvanization and electrophoresis.

Galvanization- therapeutic effect on the body with a constant electric current of low voltage and low strength.

The name of the method is associated with the outdated name for direct current - "galvanic current".

When galvanizing various parts of the body, the following currents are used:

As a result of galvanization, the systems for the regulation of local blood flow are activated in the tissues. There is an expansion of the lumen of the dermal vessels and hyperemia of the skin occurs. The expansion of capillaries and an increase in the permeability of their walls occurs not only at the place of application of the electrodes, but also in deeply located tissues.

Electrophoresis- the introduction of a medicinal substance through the skin or mucous membranes using direct current.

For this, pads soaked in the drug are placed under the corresponding electrode. The drug is administered from the pole, the charge of which is possessed by its ions. Anions (iodine, heparin, bromine) are introduced through the cathode, and cations (Na, Ca, novocaine) are introduced through the anode.

Electrophoresis is a rather lengthy procedure due to low ion mobility. A concomitant effect of this procedure is galvanization.

The location of the electrodes on the patient's body and the duration of the procedure are determined by the location of the tissue to be treated.

17.2. AC action (LF, ZCh, UZCH). Thresholds

Alternating conduction current is the oscillatory motion of ions.

The effect that an alternating (sinusoidal) current has on the body depends on the frequency and amplitude of the current. In medicine, the following classification of alternating current frequencies is adopted.

Like direct current, alternating current has an irritating effect on body tissues. Excitation of nerve and muscle tissues by direct or alternating current (ν below 100 kHz) can cause electrical injury. Excitation processes in a rhythm not characteristic of the body disrupt normal life activity. Such disorders in the heart, respiratory muscles, and central nervous system are especially dangerous. The most dangerous are frequencies of 30-300 Hz. It should be understood that the damaging effect of alternating current is determined not by the voltage, but by the charge passing over half the period. This is due to the fact that the action of the current on the tissue is based on their polarization, the degree of which is proportional to the value of the passed charge. That's why for currents high frequency(half-period is very small) the damaging effect does not occur even with currents in tens of amperes. While a current with a frequency of 50 Hz can cause death of a person with a force of 0.1 A.

The doctor encounters currents of the LF and ZF ranges not only as a traumatic factor. They are used for electrodiagnostics and electrical stimulation of biological systems. As a rule, for these purposes, not sinusoidal, but impulse currents are used.

Current thresholds

We know (Lecture 3) that the perception of sound is characterized by two threshold values ​​- the threshold of hearing and the threshold of pain. Similar values ​​are used for alternating currents in the LF and AF ranges.

Sensible current threshold- the minimum current strength, the irritating effect of which is felt by the "average" person.

A person's response to a current is determined not only by its strength and frequency, but also by the area through which the current passes. The dependence of the threshold of perceptible current in the section "forearm - hand" for an average man is shown in Fig. 17.1 (curve 1). For frequency

Rice. 17.1. Dependence of the average value of the perceptible current threshold (1) and the non-letting current threshold (2) on the frequency

50 Hz (industrial current) this value is approximately 1 mA.

The industrial current of 3 mA causes a slight tingling sensation in the fingers touching the conductor. A current of 3-5 mA causes an irritating sensation throughout the hand. A current of 8-10 mA leads to involuntary contraction of the muscles of the hand and forearm. At a current of about 15 mA, involuntary muscle contractions acquire such force that a person is unable to unclench the hand holding the conductor.

Non-releasing current threshold - the minimum current that causes the "average" person to bend the joints so that the person cannot independently free himself from the conductor - the voltage source.

The dependence of the non-releasing current threshold for an average man is shown in Fig. 17.1 (curve 2). In children and women, thresholds are usually lower.

Exceeding the threshold of non-releasing current can be fatal for a person (paralysis of the respiratory muscles, cardiac fibrillation).

17.3. High frequency current action

At frequencies above 100 kHz, the irritating effect of alternating current ceases completely. This is primarily due to the fact that at such frequencies the gate processes of ion channels do not have time to

work and the intracellular composition does not change. The main primary effect in this case is thermal effect.(Direct current, LF and AF currents are unsuitable for heating tissues, since their use at high values ​​can lead to electrolysis and destruction).

The specific thermal power released in the tissues is determined by the formula (10.10): q = j 2 p, where ρ is the tissue resistivity, and j is the current density in it. The strength of the current, and therefore its density, depends on impedance tissue, which in turn depends on frequency (see Lecture 15). Therefore, by selecting the frequency of the current, it is possible to achieve a selective thermal effect on the tissues of the desired type.

Advantages therapeutic warming up with HF currents in front of a conventional heating pad are obvious:

Heat is released in the internal parts of the body, and does not enter through the skin;

By selecting the appropriate frequency, you can selectively influence the desired type of tissue;

The amount of heat generated can be dosed by adjusting output power generator.

Use of high-frequency currents in medicine

Heating tissues with high-frequency currents is used in the following physiotherapeutic procedures.

Diathermy- the method of electrotherapy, which consists in a local effect on the body with an alternating current of high frequency and great strength, leading to an increase in tissue temperature.

With diathermy, a current with a frequency of 1-2 MHz and a force of 1-1.5 A is used. Lead electrodes are applied to the patient's body so that the heated area is between them. The voltage value is 100-150 V. The current density is determined by the area of ​​the electrodes and the total resistance of the tissue between them. The tissues with high resistivity (skin, fat, muscles) are heated more strongly. The organs rich in blood or lymph (lungs, liver, lymph nodes) heat up less.

The disadvantage of diathermy is the unproductive release of heat in the skin layer and subcutaneous tissue.

Local darsonvalization - method of electrotherapy, which consists in local exposure to the body with a weak impulse current of high frequency and high voltage.

With darsonvalization, a current with a frequency of 100-400 kHz and a voltage of tens of kV is used. In this case, only one glass electrode filled with graphite is applied to the patient's body (Fig. 17.2).

Rice. 17.2. Darsonvalization of the face (a), gums (b)

Graphite, glass and the surface of the body, to which the electrode is applied, form a capacitor C 1 (Fig. 17.3). The second electrode is located inside the body of the device. This electrode, the patient's body and the air layer between them form the capacitor C 2. Electrical diagram connection is shown in fig. 17.3. It includes two capacitors and a resistor R representing the resistance of the heated area.

Rice. 17.3. Darsonvalization wiring diagram

At a frequency of 100-400 kHz, the impedance of the circuit provides the current in the circuit I = 10-15 mA. In the air gap between the electrode E and the surface of the body, an electric discharge arises, which

stimulates physiological processes in the skin and causes destruction of the membranes of microorganisms.

High frequency currents are also used for surgical purposes.

Diathermocoagulation- moxibustion, "welding" of fabric. In this case, a current with a density of 6-10 mA / mm 2 is applied, as a result of which the tissue temperature rises and the tissue coagulates.

Diathermotomy- dissection of tissues using a blade-shaped electrode, which gives a narrow, even incision without capillary bleeding. In this case, the current density is 40 mA / mm 2.

The electrosurgical effect is accompanied by less blood loss.

17.4. The action of magnetic fields

The magnetic field exerts a forceful effect on moving charged particles (ions) and an orienting effect on particles with a magnetic moment. An alternating magnetic field creates Foucault currents in conducting tissues, which have both thermal and irritating effects. Various biological effects are associated with these physical effects. They are conventionally divided into thermal and non-thermal.

The magnetic fields used in medicine are created by permanent magnets or solenoid coils called inductors. During therapeutic procedures using a magnetic field, the patient does not have contact with live conductors. Therefore, these procedures are electrical safe.

Constant magnetic field

Permanent magnetotherapy- therapeutic use of non-thermal effects of a constant magnetic field.

Constant magnetic fields with an induction of 1-50 mT cause a rearrangement of the liquid-crystal structures of biological membranes, which significantly changes the permeability of the lipid bilayer and leads to an increase in the metabolic and enzymatic activity of cells. In the cytoplasm, such fields induce gel-sol phase transitions. The effect of a constant magnetic field on blood and

Rice. 17.4. Radiculitis belt

lymph can significantly change their viscosity and other physicochemical properties. At the same time, it should be emphasized that the physical nature of the effect of a constant magnetic field on biological objects has been poorly studied.

Currently, several types of devices are used for therapeutic purposes.

1. Magnetoelasts made from a mixture of a polymer substance with a powdered ferromagnetic filler (has many local magnetic poles). Sets of elastic magnets in a corset form the basis of all kinds of radiculitis belts (Fig. 17.4). Magnetic induction 8-16 mT.

2. Magnets are ring, plate, disc. Magnetic induction 60-130 mT.

3. Micromagnets - magnetized needles, balls, clips (for magnetopuncture). Magnetic induction 60-100 mT.

4. Plate magnets are used as bracelets worn on the patient's wrist. Magnetic induction 20-70 mT.

Variable magnetic field

The therapeutic effect of an alternating magnetic field is associated with both thermal and non-thermal effects of Foucault currents that arise in a conducting medium when the magnetic field changes.

Pulse magnetotherapy- therapeutic application of a pulsed magnetic field at a low pulse repetition rate (0.125-1000 imp / s).

Non-thermal effects are used here. Foucault currents of considerable density are capable of causing excitation of the fibers of peripheral nerves and rhythmic contractions of myofibrils of skeletal muscles, smooth muscles of blood vessels and internal organs. Eddy currents of low frequency are able to block afferent impulses from a painful focus (relief of pain syndrome).

Figure 17.5 shows the therapeutic effect of the pulsed field on the lower limb, placed inside the solenoid block. Here, a field with a frequency of 10 pulses / s and an induction of 30 mT is used.

Rice. 17.5. Location of the inductor for low-frequency magnetotherapy of the lower limb

High frequency magnetotherapy- therapeutic use of the magnetic component of a harmonic electromagnetic field of high frequency (the outdated name of this method is inductothermy).

As a result of the phenomenon of electromagnetic induction (as in the case of a pulsed magnetic field), Foucault eddy currents are formed in conducting tissues, heating the object. For a harmonic magnetic field, the Foucault current density is proportional to its frequency (ν). The pronounced thermal effect begins to manifest itself at frequencies of the order of 10 MHz. The amount of heat released per unit time per unit volume of the conductor is determined by the formula

Here ρ is tissue resistivity. The proportionality coefficient k depends on the geometric characteristics of the heated area.

Unlike methods of treatment with high-frequency currents, the main thermal effect in this case is on tissues with low resistivity. Therefore, tissues rich in blood vessels, for example, muscles, are heated more. Tissues such as fat are less heated.

To form an alternating magnetic field, solenoid inductors are used (Fig. 17.6).

Rice. 17.6. The scheme of exposure to an alternating magnetic field

For physiotherapeutic procedures, alternating magnetic fields with a frequency of 10-15 MHz are used. In this case, cable inductors of various shapes are used (Fig. 17.7): a - flat longitudinal loop (more often on the back); b - flat round spiral (on the body); c - cylindrical spiral (on the limbs).

As a result of the release of heat, a uniform local heating of the irradiated tissue occurs by 2-4 degrees to a depth of 8-12 cm, as well as an increase in the patient's body temperature by 0.3-0.9 degrees.

In the process of high-frequency magnetotherapy, a non-thermal effect is also manifested: eddy currents cause a change in the nature of the interaction of the intrinsic magnetic fields of charged particles in the tissue, but this mechanism is not understood in detail here.

Rice. 17.7. Ways of applying a cable inductor for various methods of high-frequency magnetotherapy:

a - flat longitudinal loop, b - flat round spiral, c - cylindrical spiral

17.5. Constant electric field action

The oldest among the currently used electrotherapy methods is franklinization- therapeutic effect of a constant electric field of high intensity.

To form an electric field, electrodes of various shapes with needles at the ends are used. In the procedures of general franklinization (Fig.17.8, a- electrostatic shower) the intensity of the electric field at the patient's head reaches 90 kV / m. The strength of the electric field inside the human body is about 10 mV / m. In the conducting tissues, weak currents arise that change the functional properties of the conducting nerve pathways and significantly limit the flow of afferent impulses to the overlying parts of the central nervous system, which leads to an increase in braking processes in the cortex and subcortical centers. As a result, the patient's blood pressure decreases, the respiratory rate decreases and its depth increases, fatigue decreases and the working capacity increases.

With local franklinization (Fig. 17.8, b), individual parts of the body are exposed to the electric field.

Rice. 17.8. General (a) and local (b) Franklinization

Rice. 17.9. Aeroionizer system A.L. Chizhevsky with a head electrode (a), electrode for general aeroionization (b)

The effect of local franklinization is enhanced by the action of an electric field on the needles inserted into biologically active points - acupuncture franklinization.

To carry out group Franklinization procedures, a high-voltage generator is used - an electro-effluvial Chizhevsky lamp(air ionizer). This system is designed to receive ionized air, in particular oxygen ions (ozone), which have a biological effect. Aeroionizer system A.L. Chizhevsky (Fig. 17.9) supplies a high constant voltage to the "electric fluvial chandelier", equipped with a large number of sharp ends - needles.

In this case, a corona discharge occurs between the electrode and the human body, ionization of air molecules occurs, and a flow of aerons and ozone (electro effluvium) is formed. The face, collar zone, and upper respiratory tract are exposed to air ions.

17.6. Action of an alternating electric field

(UHF)

The alternating electromagnetic field causes oscillatory motion ions (alternating current) and torsional vibrations dipole molecules. These processes are accompanied by the release of heat.

Impact of the UHF field on the conductor

Specific thermal power released in the conductor due to oscillatory motion ions, is determined by the formula

where E is the strength of the electric field inside the substance, ρ - the specific resistance of the substance.

This formula is unsuitable for direct calculations, since it includes the strength E of the electric field inside the substance. This value is rather difficult to calculate (see problem 1). At those frequencies that are used in medical procedures (UHF), the specific thermal power is determined by the formula

where U is the effective value of the voltage across the electrodes creating an alternating electric field, k is some geometric coefficient (see problem 2).

Impact of the UHF field on the dielectric

Leads to heat release (dielectric loss).

The amount of heat released depends on the angle δ, by which the vibrations of the molecules lag behind the fluctuations in the field strength. Injection δ called dielectric loss angle.

Specific thermal power released due to dielectric losses is determined by the ratio

Here ε - dielectric constant of the substance; E is the effective value of the field strength in the dielectric.

The magnitude of the dielectric loss tangent is determined by the nature of the dielectric and depends on the frequency. In the regions α-, β-, γ -dispersion (see section 15.6), this value undergoes sharp changes.

Application of an alternating electromagnetic field in medicine

One of the common methods of high-frequency therapy is exposure to a high-frequency UHF electric field.

Ultra-high-frequency (UHF) therapy- therapeutic use of the electrical component of the alternating electromagnetic field of ultrahigh frequency.

To carry out the treatment procedure, the area of ​​the body that is affected is placed between two electrodes, which are remote plates of the capacitor included in the electrical circuit of the UHF apparatus. A generated alternating voltage is applied to these plates, and an alternating electric field arises between them, which has a therapeutic effect (Fig. 17.10).

The methods of applying the electrodes are shown in Fig. 17.11

Heating of organs and tissues under the influence of the UHF electric field causes persistent, prolonged and deep tissue hyperemia in the affected area. Capillaries expand especially strongly, the diameter of which increases several times. Under the influence of the UHF field, regional lymphodynamics is also significantly accelerated, the permeability of the endothelium and other tissue barriers increases.

UHF therapy devices use frequencies of 40 and 27 MHz. The latter frequency is international. It corresponds to a wavelength of 11 m.

Rice. 17.10. UHF field exposure scheme

Rice. 17.11. Methods for applying electrodes:

a- transverse, b-longitudinal, v - tangential

17.7. The action of electromagnetic waves (microwave)

At the frequencies used by UHF therapy, the dielectric tissues of the body are heated more intensely than the conductive ones. With an increase in the frequency of the electromagnetic field, this order changes: more heat is released in organs and tissues rich in water (blood, lymph, muscle tissue, parenchymal organs). This is due to a decrease in the dielectric loss tangent with increasing frequency.

For a therapeutic effect on conducting tissues, waves of the decimeter and centimeter ranges (microwave therapy) are used. The impact is carried out by irradiating the surface of the corresponding area of ​​the body with a directed stream of waves, which is formed using a special emitter called a waveguide.

The mechanisms of heat release during microwave and UHF therapy are the same. The only difference is the structures that are predominantly influenced. Specific thermal power released in tissues is calculated by the formula

where I is the intensity of the wave, and k is a certain coefficient depending on the properties of the tissue.

Decimetric therapy (DCV therapy)- therapeutic use of electromagnetic waves in the decimeter range (frequency - 460 MHz, wavelength - 65.2 cm). Under the influence of this factor, orientation vibrations of dipole molecules appear in the tissues of the body. bound water, as well as side groups proteins and glycolipids plasmolemma. These fluctuations occur in a viscous cytosol medium and are accompanied by the release of heat.

Microwave (centimeter) therapy - therapeutic use of electromagnetic waves in the centimeter range (frequency - 2375 MHz, wavelength - 12.6 cm). There are no fundamental differences in the primary action of decimeter and centimeter waves. At the same time, a significant decrease in the wavelength leads to an increase in the specific gravity of relaxation vibrations of free unstructured water molecules, side chains of phospholipids and amino acids.

Microwave therapy procedures are carried out according to two main methods.

Distant technique- irradiation with electromagnetic waves is carried out remotely, while the distance between the emitter and the biological object does not exceed 5 cm. In this case, the wave energy will be reflected from the surface (in some cases, up to 70-80%).

Contact technique- the wave emitter is placed directly on the patient's body or inserted inside.

With any method of treatment, it is necessary to strictly dose the effect according to the output power generated by the emitter.

The depth of penetration of electromagnetic waves into biological tissues depends on the ability of these tissues to absorb wave energy. Centimeter waves penetrate into muscles, skin to a depth of 2 cm, into adipose tissue, bones - about 10 cm. Decimeter waves penetrate to a depth of 2 times greater.

Comparison of the effects of low-frequency and high-frequency fields (currents) is presented in the table below.

17.8. Tasks

1. Derive a formula for calculating the specific thermal power in a conductor, which is placed in an alternating electric field. Consider the following model: an electric field is created by two plates of area S, connected to the poles of a high-frequency generator with an effective voltage U and an angular frequency ω. Distance between plates l<< размеров пластин. Между пластинами помещен проводник с удельным сопротивлением ρ толщиной h, форма и размеры которого совпадают с формой и размерами пластин. Проводник расположен симметрично пластинам.

Solution

In the applied literature, to calculate the specific thermal power, the formula is given: q = E 2 / p, where E is the strength of the electric field inside the conductor. This formula, being physically correct, is not only unsuitable for calculations, but also gives rise to serious delusions. For example, this formula does not contain the frequency ω, and one gets the impression that q is also independent of frequency. Further, the resistivity ρ is in the denominator, although in fact, at the frequencies of UHF therapy, it should be in the numerator.

The reason for such discrepancies is that the intensity E included in this formula is not given by size. The preset values ​​are: voltage U, distance between electrodes l, the thickness of the conductor h and its resistivity ρ. The strength of the electric field inside the conductor depends on them in a rather complex way. We will obtain the correct formula for calculating the specific thermal power.

The figure shows the electrical circuit and the calculation of the impedance (C 0 - air capacitor). The effective value of the current in the circuit and the released thermal power are equal:

Let us show that this formula coincides with the formula q = E 2 / p. Indeed, the voltage drop across the conductor and the field strength in it are respectively equal:

On the low frequencies, when the capacitive resistance is significantly greater than the active resistance, the following approximation is obtained:

2. Determine which formula should be used to calculate the specific thermal power of the conduction current released in muscle tissue during UHF heating of muscle tissue. Use the results of the previous task with the following values:

ν = 40 MHz, l= 15 cm, h = 10 cm, ρ = 1.5 Ohm-m.

3. Obtain a formula for calculating the specific thermal power released in a dielectric, if in Problem 1 we replace a conducting plate with a dielectric with permittivity ε.

Having performed the obvious calculations, we find

4. What capacity should the therapeutic circuit of devices for UHF therapy and inductothermy have if their resonant frequencies and inductances are equal, respectively:

5. In microwave therapy, electromagnetic waves are used in the decimeter range λ 1 = 65 cm and the centimeter range λ 2 = 12.6 cm. Determine the appropriate frequencies.

Answer:ν 1 = 460 MHz; ν 2 = 2375 MHz.

6. The therapeutic circuit of the UHF apparatus operating at a frequency of 40.68 MHz consists of a 0.17 μH inductor and a variable capacitor C n = 10-80 pF, shunted with a capacitor C 0 = 48 pF. At what capacity of the variable capacitor will the therapy circuit be tuned to resonance with the anode circuit?

Transformer operating modes

· Idling mode. This mode is characterized by an open secondary circuit of the transformer, as a result of which no current flows in it. With the help of the no-load experiment, it is possible to determine the efficiency of the transformer, the transformation ratio, as well as the losses in steel.

· 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 its help, it is possible to 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 an active resistance.

28) Oscillatory circuit- an oscillator, which is an electrical circuit containing a connected inductor and a capacitor. In such a circuit, current and voltage fluctuations can be excited.

Operating principle

Let a capacitor of capacitance 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 inductance) at the initial moment will be equal to the discharge current of the capacitor, that is, the resulting current will be 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 be transferred to the coil until the capacitor is completely discharged. At this moment, the electrical energy of the capacitor. The magnetic energy concentrated in the coil, on the contrary, is maximum and is equal to, where is the inductance of the coil,

Maximum current value.

After that, the capacitor will begin to recharge, that is, the capacitor will be charged 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 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 resonance currents, which means that currents flow through the inductance and capacitance that are greater 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 circuit limits, since they are antiphase and compensate for themselves. It is also worth noting that the resistance of the parallel oscillatory circuit at the resonant frequency tends to infinity (in contrast to the serial oscillatory circuit, the resistance of which at the resonant frequency tends to zero), and this makes it an irreplaceable 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 (EMF guidance) gives the same result.

Induction generator design assumes the placement of both a constant field and coils for induction of the EMF on the stator, while the rotor remains free of windings, but necessarily has a toothed shape, since all the work of the generator is based on the tooth harmonics of the rotor.

High frequency currents and their application.

High-frequency currents are those currents, the frequency of which, that is, the number of oscillations, reaches one million per second. This type of currents has found its application in mechanical engineering, where it is necessary for welding and heat treatment of the surfaces of parts, and in metallurgy, where it is used to melt 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 by the rule of the left screw. Then it is changed to the opposite - this will be the direction of the vortex electric field for the decreasing magnetic field.

Second postulate: there is a magnetic field around any alternating electric field.

The direction of the lines of magnetic induction is determined by the rule of the right screw if the electric field increases.

If the electric field strength decreases, then first the direction of the magnetic induction lines is determined according to the rule of the right screw. Then it is changed to the opposite - this will be the direction of the lines of magnetic induction for a decreasing electric field.

33) The Frank - Hertz Experience- an experience that was an experimental proof of the discreteness of the internal energy of the atom. Staged in 1913 by J. Frank and G. Hertz.

The figure shows a diagram of the experiment. A potential difference V is applied to the cathode K and grid C1 of the vacuum tube filled with Hg (mercury) vapor, accelerating the electrons, and the dependence of the current I on V is removed. A decelerating potential difference is applied to the grid C2 and the anode A. Electrons accelerated in region I collide with Hg atoms in region II. If the energy of the electrons after collision is sufficient to overcome the decelerating potential in region III, then they will reach the anode. Consequently, the readings of the galvanometer G depend on the loss of energy by 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 communication facilities, in particular to establish communication without wires, became especially acute at the end of the 19th century, when the widespread introduction of electric energy into industry, agriculture, communications, transport (primarily sea), etc. began.
The history of science and technology confirms that all outstanding discoveries and inventions were, firstly, historically conditioned, and secondly, the result of the creative efforts of scientists and engineers from different countries.

Radiotelegraph communication - telecommunications, in which discrete messages are transmitted by means of radio waves - alphabetic, numeric and signed. At the transmitting station, electrical oscillations modulated by telegraph communication are fed into 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) Radiotelephone communication telecommunications, in which telephone (voice) messages are transmitted via radio waves. Information enters the radiotelephone line through a microphone, and from there - usually through a telephone. Microphone and telephone are connected to radio stations directly 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 - is called a device, 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 acts:

where the amplitude modulation depth M should be proportional to the amplitude.

As a result of the influence of the input signal on a nonlinear element with a piecewise linear approximation in the current of the latter, harmonics and combination components of the input signals appear, namely, components with frequencies: Components with frequencies and form the required amplitude-modulated oscillation. It should be allocated with a bandpass filter with a center frequency equal to the carrier and a bandwidth sufficient to isolate the frequency components.

36) Detection - Conversion of an electromagnetic wave to obtain a voltage or current, the magnitude of which is determined by the parameters of the oscillation, in order to extract information contained in the changes in these parameters

The structure and operation of the simplest detector receivers - the simplest, most basic, type of radio. It consists of an oscillatory circuit, to which the antenna and ground are connected, and a diode (in the earlier version of the crystal) detector, which demodulates the amplitude-modulated signal. The audio signal from the detector output is usually reproduced by high impedance headphones.

Even for the reception of powerful radio stations, the detector receiver requires as long and highly suspended antenna as possible (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 direct amplification receiver with much better parameters. Due to these advantages, detector receivers were widely used not only in the first decades of radio broadcasting.

37) Radio wave propagation - the phenomenon of the transfer of energy of electromagnetic waves in the radio frequency range (see. Radio emission). Different aspects of this phenomenon are studied by various technical disciplines that are branches of radio engineering. The most general questions and tasks 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 radio propagation considers only those tasks of radio emission that are associated with the propagation of radio waves in natural environments, that is, the effect on radio waves of the Earth's surface of the atmosphere and near-earth space, the propagation of radio waves in natural reservoirs, as well as in man-made landscapes

Types of radio waves -

Radio wave properties - 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, electrical parameters of the earth's surface and wavelength. Like other waves, radio waves are characterized by diffraction, i.e. the phenomenon of avoiding obstacles. Diffraction is most pronounced when the geometric dimensions of the obstacles are commensurate with the wavelength. Radio waves propagating near the surface of the earth and partly due to diffraction enveloping the bulge of the earth are called terrestrial, or surface radio waves.

Application of radio waves- For transmission of various data, signals and other information through the source and receiver of radio waves. For example, cellular communication, its different standards, work at different radio frequencies, also WI-FI, radio ethernet, and many others.

38) a brief history of the 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 makes an assumption about the wave nature of light. In the Treatise on Light, published in 1690, H. Huygens formed the principle according to which every point in space, which the propagating wave has reached at a given moment, 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 polarization of light - a phenomenon that occurs with a beam of light when it is reflected, refracted (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, vibrations occur in all directions, perpendicular to the beam. Huygens, having developed Grimaldi's idea that light propagates not only rectilinearly with refraction and reflection, but also with splitting (diffraction), gave an explanation for all known optical phenomena. He claims that light waves propagate in ether, which is a subtle matter permeating all bodies.

39) Light speed in vacuum - the absolute value of the speed of propagation of electromagnetic waves in a vacuum. In physics, it is traditionally denoted by the Latin letter " c"(Pronounced as [tse]). The speed of light in a vacuum is a fundamental constant that does not depend on the choice of an inertial reference frame (IFR). 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 a vacuum is the limiting speed of movement of particles and the propagation of interactions.

The speed of light in a transparent medium- the speed with which light travels in a medium other than vacuum. In a dispersive medium, phase and group velocities 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 a 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 that does not exceed the speed of light in vacuum. As a result, superluminal information transmission 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 of obtaining a stable interference pattern for light is the use of air gaps, based on the same path difference between the two parts of the wave: one - immediately reflected from the inner surface of the lens and the other - passing 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 convexity downward. When the lens is illuminated from above with monochromatic light, a dark spot is formed in the place of a fairly tight contact between the lens and the plate, surrounded by alternating dark and light concentric rings of different intensities. Dark rings correspond to interference minima, and light rings - to maxima, while dark and light rings are contour lines of equal thickness of the air gap. By measuring the radius of a light or dark ring and determining its serial number from the center, you 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, raised to the boundary of two media at the point of incidence of the ray, lie in the same plane.

2.The angle of reflection is equal to the angle of incidence:

42) Refraction laws

The lower the speed of light in a medium, the more optically dense it is considered. A medium with a high absolute refractive index is called 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 the perpendicular: the angle of incidence is less than the angle of refraction

High frequency currents (HFC) are considered to be currents for which the condition of quasi-stationarity is not satisfied, which results in a strongly pronounced skin effect

High frequency currents (HFC) are considered to be currents for which the quasi-stationarity condition is not satisfied, 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 several 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 there is a pulsation of the magnetic field. The final frequency of the current received at the output is equal to the product of the rotor speed by the number of teeth on it.

Also, to obtain HDTV, oscillatory circuits are used, for example, an electric circuit, which includes inductance and capacitance. To obtain HDTV frequencies in billions of hertz, installations with a hollow oscillatory circuit (BWO, TWT, magnetron, klystron) are used.

If a 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 up. 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, in household appliances (microwave ovens, various cooking devices), radio communications, radar, television, etc.

Examples of using high frequency currents

With the help of HFC in induction furnaces, you can melt any metal. The advantage of this type of smelting lies in the possibility of smelting under full vacuum conditions, 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 HFC, 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 reducing wear resistance and ductility, since the core remains soft.

In medicine, high-frequency currents have long been used in UHF devices, where heating of a dielectric is used to heat any human organs. HFCs of even very high amperage are harmless to humans, since they flow exclusively in the most superficial layers of the skin. Also in medicine, high frequency electric knives are used, with the help of which blood vessels are "welded" and tissues are cut.

Electric currents are widely used in physiotherapy. At the same time, changes in their parameters can diametrically affect the mechanisms of action and the observed effects on the body.

High-frequency currents in physiotherapy

Medical currents are classified as low, medium, and high. High-frequency current is detected at a frequency of more than 100,000 hertz.

High-frequency currents are generated by special equipment and are applied without direct contact with the patient. An exception is the method of local darsonvalization, which uses high-frequency currents through special electrodes on the body.

Many physiological effects of HF currents are based on the formation of endogenous heat in tissues. High-frequency currents cause small vibrations at the molecular level, resulting in the release of heat. This heat acts at different depths in the tissues, and the effect lasts for some time after the completion of the procedure.

Application of HF currents in medical practice

The influence of high-frequency currents on the central nervous system is sedative and on the autonomic - sympatholytic, in general, high-frequency currents have a relaxing effect on the nervous system. The same can be said about their effect on the smooth muscles of the bronchi, where the antispasmodic effect is combined with the anti-inflammatory effect.

HF currents are indicated for pain syndromes with neuralgia, neuritis, radiculitis, etc. The analgesic effect is due to an increase in the pain threshold of skin receptors and inhibition of the transmission of pain signals through the nerves.

Procedures using high-frequency currents are effective for slow tissue overgrowth in wounds, bedsores and trophic diabetes. This mechanism of action is associated with the induction of endogenous vasodilator heat. In spastic conditions such as Buerger's disease or Raynaud's syndrome, HF currents can also relieve some of the symptoms.

In another case, the effect of high frequency currents on blood vessels is tonic and is used in the treatment of varicose veins and hemorrhoids. Sometimes the bactericidal effect of high-frequency currents is used to treat infected wounds. The bactericidal and antimicrobial action of HF currents has indirect mechanisms that increase local blood flow, stimulate and accelerate the phase of the inflammatory process.

Contraindications to the use of all types of currents in medicine are large metal objects in tissues, implanted pacemakers, pregnancy, a tendency to bleeding, and some others.

Ultra-high frequency currents

Ultra-high frequency currents are another group of high-frequency currents. They also work on the principle of endogenous heat generation and targeted activation of metabolism in certain tissues. Their action is used in response to a variety of pathological processes. The time of one procedure is on average 10-15 minutes, and the courses vary in length depending on the result achieved.

Irradiation of the kidney with ultra-high frequency currents in acute and chronic glomerulonephritis gives a vasodilating and anti-inflammatory effect, acting on the vessels, and increases diuresis. On the other hand, radiation to the adrenal glands naturally stimulates the production of corticosteroids and is used in the treatment of certain autoimmune diseases.

The third group of high-frequency currents used in medicine is centimeter high-frequency currents. Microwave waves affect the blood, lymph and parenchymal organs. Centimeter waves have a depletion effect 3-4 centimeters deep into the surface of the body.

The principle of operation of all types of high-frequency currents is associated with the formation of endogenous heat. The latter has a different effect on various organs. The difference between the currents in frequency determines the depth of penetration of heat into the body and the preference for treatment of a certain type of tissue, with more or less water content. Treatment with HF currents must strictly correspond to the type of pathology, location and type of tissue.

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Low-frequency currents in physiotherapy

Low frequency current is specified from one to 1000 hertz. Within this range, depending on the frequency, the effects of LF currents differ. Most medical equipment uses low frequency currents with a frequency of 100-150 Hz.

In general, the therapeutic effect of impulse currents of low frequency can be divided into irritating and overwhelming. What will be the effect of such therapy depends mainly on the frequency of the current. Low frequency currents affect electrically excitable structures such as nerves and muscles.

The application of low-frequency currents is carried out by means of electrodes that are placed on injured muscles, a diseased part of the body, or elsewhere. In most cases, the electrodes are applied to the skin. However, it is possible to introduce them into the vagina, rectum, or implantation in certain muscle groups and the medullary canal, and even in the brain.

Normal excitation of nerve and muscle cells is achieved by changing the charge on either side of the positive and negative electrodes. The application of an electric current with certain characteristics near excitable structures has a stimulating effect on them. The local mode of action of the current is due to a change in the charge of the cell membrane.

Application of low-frequency currents in medicine

Low-frequency currents are used to stimulate muscles with preserved innervation, for example, when, during immobilization after bone fractures, hypotrophy and hypotension (low tone) of the muscles in the immobilized area develop. This is because the muscles do not move and are not stimulated by nerves.

In these cases, the applied low frequency current causes a portion of the muscle fiber to contract, which improves blood flow and, to a certain extent, helps prevent severe malnutrition. However, in order to achieve this effect, electrical stimulation must be used frequently enough.

In other cases, muscle stimulation can be impaired by innervation (paralysis, paresis). It is necessary to reuse low-frequency currents, but with their different physical characteristics. The goal is to stimulate the muscles and restore the integrity of the nerve.

Electrical stimulation can be applied not only to the skeleton, but also for various diseases of smooth muscles, such as postoperative intestinal atony, postpartum uterine atony, etc. Another application of this method is the stimulation of the venous wall during varicose veins and hemorrhoids. Contraindications for stimulation with low-frequency currents are pregnancy, pacemakers and some other conditions.

The second main application of low-frequency currents is to reduce pain in neuralgia, myalgia, tendonitis, headaches and other conditions. The most common method is transcutaneous electrical nerve stimulation. With this type of stimulation, there is an effect on specific very sensitive nerve fibers that block the transmission of pain information at the level of the spinal cord. The duration of one session of such therapy ranges from 10 minutes to 1-2 hours. The most suitable frequency for achieving an analgesic effect is around 100 Hz.

Denial of responsibility: The information presented in this article about the use of low and high frequency currents in physiotherapy is intended only to inform the reader. It cannot be a substitute for the advice of a healthcare professional.

DEPARTMENT OF EDUCATION AND SCIENCE OF THE KEMEROVSK REGION

State educational institution of secondary vocational education

Kemerovo Vocational Technical School

High frequency currents.

Prepared by: physics teachers

Shcherbunova Evgeniya Olegovna and

Kolabina Galina Alekseevna

Kemerovo

What are high frequency currents?

Currents with a frequency higher than 10,000 Hz are called high frequency currents (HFC). They are received using electronic devices.

If you place a conductor inside a coil through which a high frequency current flows, then eddy currents will arise in the conductor. Eddy currents heat up 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. The high heating rate is very convenient when rolling and forging metal. By choosing the shape of the coils, you can solder and weld parts at the best temperature conditions.

Induction Melting Furnace

The current i flowing through the conductor creates a magnetic field B. At very high frequencies, the influence of the vortex electric field E, generated by the change in the field B, becomes noticeable.

The influence of the E field 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 workpiece. After rapid cooling, a non-fragile product with a hard surface is obtained.

Hardening machine

For more details, see here: Induction Heating and Quenching Units

The action of high frequency currents on dielectrics

Dielectrics are acted upon by a high-frequency electric field, placing them between the capacitor plates. Part of the energy of the electric field is spent in this case for heating the dielectric. Heating with HFC 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. Direct and low-frequency currents over several milliamperes are deadly for humans. The high frequency current (≈ 1 MHz), even at 1 A, causes only tissue heating and is used for treatment.

"Electroknife" is a high-frequency device widely used in medicine. It cuts tissue and seals the blood vessels.

Other applications of high frequency currents

Grain treated with HDTV before sowing significantly increases the yield.

Induction heating of gas plasma allows high temperatures to be obtained.

A 2400 MHz field in a microwave electric oven cooks the soup right on the plate for 2-3 minutes.

The action of the mine detector is based on the change in the parameters of the oscillatory circuit when the coil is brought to the 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 general educational institutions of secondary vocational education [Text] / V.F. Dmitrieva. –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. ru/ window, free. - Title 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. - Title from the screen. - (Date of treatment: 11.11.2014).

Portal of natural sciences ”[Electronic resource]. - Access mode: http://e-science.ru/physics, free. - Title from the screen. - (Date of treatment: 11.11.2014).