Arc electric discharge. Abstract: Arc discharge in gases

An important occasion elastic vibrations are the so-called torsional vibrations, in which the body turns back and forth about an axis passing through its center of gravity.

If, for example, you hang a disk on a wire (Fig. 18), turn it so that the wire twists, and then release it, the disk will begin to unwind, twist into reverse side etc., i.e. it will perform torsional vibrations. In this case, the transition of the kinetic energy of the moving disk into potential energy(strain energy) of the twisting wire and vice versa. Torsional vibrations often occur in engine shafts, in particular in the propeller shafts of motor vehicles, and under certain conditions, which will be discussed below, can be very harmful (§ 15).

Rice. 18. Torsional vibrations of a disk suspended on a wire

Hand and pocket watches cannot use a pendulum; they use a so-called balancer (Fig. 19) - a wheel to the axis of which a spiral spring (“hair”) is attached. The balancer periodically turns back and forth, and during these torsional vibrations the spring bends (unwinds and twists) in both directions from its equilibrium state. Thus, the balancer is a torsion pendulum.

Rice. 19. Clock balancer

For the period of torsional vibrations, the same laws remain valid as for the period of any elastic vibrations: the longer the period, the lower the rigidity of the system and the greater its mass (with a constant shape).

During torsional vibrations, not only the mass of the body is significant, but also its distribution relative to the axis of rotation. If, for example, we hang a dumbbell on a wire, consisting of a knitting needle on which two identical weights are symmetrically mounted (Fig. 20), then when moving the weights apart, the frequency of torsional vibrations will decrease. Although the mass of the dumbbell remains the same. Leaving the loads in the same places, but making them more massive, we will see that the frequency also becomes less.

Rice. 20. Dumbbell torsional vibrations

Torsional vibrations at large angles twisting (small angular amplitudes) are also harmonic. Their period is determined by the relation

where is the rigidity of the system. Numerically, the stiffness is equal to the torque, which gives a rotation per radian. If elastic forces are caused by twisting of a thread or wire, then this is the so-called torsional rigidity of these bodies. The value characterizes the distribution of mass relative to the axis of rotation (the so-called moment of inertia, which plays the same role in rotational motion as mass does in translational motion). For example, for a dumbbell where is the mass of each load, and is the distance from the weights to the axis of rotation.

Torsional vibrations are the rotational movement of a body suspended on an elastic thread (steel wire) around a vertical axis, expressed in its alternating deviation in one direction or another from the equilibrium position. The force quantity characterizing the dynamics of such movement is a consequence of the deformation of the thread. Deformation is a change in the shape and size of a body under the influence of forces applied to it. If, after the cessation of the force, the body returns to its original size and shape, then the deformation is called elastic. There are several types of deformation of bodies: tension, compression,

torsion, shear, bending. In this case, an opposing force arises inside the deformed body, equal in magnitude to the deforming force and called elastic force. Magnitude of elastic force F control . , which occurs during small deformations of any kind, is directly proportional to the magnitude of the deformation x, that is, a change in body size, which is generally expressed by the ratio:

F ex. = – kh (10)

Where k– proportionality coefficient, called elasticity coefficient. Relationship (10) is called Hooke's law. The minus sign indicates opposite directions of the elastic force and the magnitude of the deformation.

When a thread with a suspended body is twisted at a certain angle φ (Fig. 1), the reaction arising in the thread is a moment of force M, the modulus value of which is proportional to the angle φ, and the direction, as with any deformation, is opposite to it, i.e. twist angle:

M = – D φ (11)

Here: D – torsion modulus (elasticity coefficient), characterizing the elastic properties of the thread.

For the rotational motion of a body, Newton’s second law is written as:

M = Jε (12)

Where J– moment body inertia, ε = d 2 φ/ dt 2 – angular acceleration, M– total moment of forces, in in this case which is the moment of elastic force that occurs when an elastic thread (wire) is twisted at an angle φ. Substituting the expression for the moment of elastic force (11) into this equation, we obtain a second-order linear differential equation describing the motion of a body during torsional vibrations:

J (d 2 φ/ dt 2 ) + D φ = 0 (13)

The solution to this equation is:

φ = φ 0 sin(ω t + φ o ) (14)

Where φ – angle of rotation of the body at the moment of time t , φ 0 – maximum angle of rotation of the body during oscillations (amplitude of oscillations), φ o – initial phase oscillations, and the coefficient at t , constituting ω = (D / J ) 1/2 , – cyclic (or circular) oscillation frequency. Because, on the other hand,

the cyclic frequency of oscillations, by definition, is equal to 2π/ T (T– period of oscillation), then we can write the equality:

ω = (D / J ) 1/2 = 2π/T (15)

from which follows an equation connecting the period of torsional vibrations with the moment of inertia of a body suspended on an elastic thread and the torsional modulus of the thread:

T = 2π(J / D ) 1/2 (16)

Description of installation and measurement method:

To determine the moments of inertia of solid bodies, the installation shown in Fig. is used. 2.

Based on 1 electronic unit is located 2 with a millisecond watch and a full oscillation counter, and a reinforced column 3 , on which there are three brackets 4 , 5 , 6 . Brackets 4 And 6 have clamps for hanging the frame on elastic steel threads 7 . The frame consists of two beams 8 , connected by rods 9 . On rods using collet clamps 10 the movable beam is fixed in the desired position 11 ,

which allows (by tightening the screw 12 ) strengthen the studied bodies in a frame 13 , significantly different in external dimensions. On a steel plate attached to a bracket 5 , located electromagnet connected to the electronic unit 14 and photoelectric sensor 15 , as well as an angular scale (not shown in the figure), which serves to set the amplitude value of the twist angle.

Work on the installation is carried out in the following order:

Fix the electromagnet in a position that corresponds to a certain angle of twist of the frame (set by the teacher).

If required by the task, strengthen the body under study in a frame. To do this:

a) loosen the clamps 10 ;

b) moving the movable beam 11 , clamp the body between the conical protrusion of the lower beam and the cone of the screw 12 , tighten the clamps 10 ;

c) finally strengthen the body by tightening the screw 12 .

Turn on the supply voltage by pressing the "Network" key. At the same time, the light bulbs of the photoelectric sensor and the stopwatch indicator should light up.

Press the "Reset" key to reset the stopwatch indicator, generate a measurement permission signal and turn on the electromagnet winding.

Rotate the frame of the device so that in the deflected state it is fixed by an electromagnet.

Press the "Start" key. Pressing this key starts the stopwatch and turns off the electromagnet. At the same time, the frame is released and begins to perform torsional vibrations, which are counted in the electronic unit using signals coming from a photoelectric sensor 15 .

After committing the frame a certain number stop the oscillation system by pressing the "Stop" button. The indicators of the electronic unit indicate the number N perfect vibrations and total time t for which they were committed.

Task 1. Determination of torsional modulus.

When using a torsional vibration setup to determine moments of inertia solids it is necessary to know the value of the torsional modulus D suspension threads 7 framework. Since the elastic properties

material during torsion depend on many factors, then the value of the modulus D determined experimentally. This work uses a dynamic method for measuring the torsional modulus, based on the dependence of the period T torsional vibrations of a frame suspended on a wire, from elastic properties wire material.

According to equation (16), the oscillation period T depends on the torsional modulus D, and from the moment of inertia J a system that oscillates. Therefore, measurements are carried out using reference bodies whose moments of inertia J t are known or easy to calculate. The oscillating system includes reference body and frame, moment of inertia J r which cannot be neglected. Since the reference body and the frame perform rotational motion around the same axis, then, according to equation (9), their moments of inertia are summed up:

J = J r + J T (17)

If we express the moment of inertia from equation (16), then:

J = J r + J T = T 2 D /(4  2 ) (18)

Since the moment of inertia of the frame J p is unknown, measurements are carried out in two stages, using two different reference bodies with moments of inertia J t1 and J t2. In this case we have a system of two equations:

J r + J t1 = T 1 2 D /(4  2 ) (19a)

J r + J t2 = T 2 2 D /(4  2 ) (19b)

Subtracting one from the other, we obtain an expression for the torsion modulus that does not contain unknown (or not experimentally determined) parameters:

D = 4  2 (J t1 – J t2 )/(T 1 2 – T 2 2 ) (20)

Measurements are carried out as follows:

Take the first reference body - one of the two disks. Measure its thickness with a caliper l and diameter. Get the disk radius value r. Weigh the disk on technical scales, or take the known

the value of its mass m(may be listed on disk or installation). Using formula (4), calculate the moment of inertia of the disk J t1 and enter this value in table 1.

Carry out operations in accordance with points 1-7 of the procedure for operating the installation (see section “Description of the installation and measurement method”). Received values N And t for the first body, enter in table 1.

Determine twice more N And t(operations according to points 4-7 of the operating procedure on the installation) and enter them in the same columns.

Repeat previous operations (1-4) for another disk, entering the values J t2, N, t, T 2 and T 2cf to table 1 (“Second body”).

Using the obtained values J t1, J t2, T 1 and T 2, using formula (20), calculate and enter into table 1 the value of the torsion modulus D.

Using the method of indirect measurements, find the absolute and relative measurement errors.

Table 1.

Task 2. Determination of the moment of inertia of an empty frame.

Free the frame from additional bodies and determine the values ​​three times N And t for an empty frame (similar to how it was done in the previous task for a frame with a body) and enter them in Table 2.

Determine three values ​​for the period of oscillation of an empty frame T 0 , calculate their average value T 0ср and enter this data into table 2.

There are three ways to determine the moment of inertia of an empty frame:

a) using the obtained value of the oscillation period T 0av and value D from table 1, calculate Jр0 according to a formula similar to formula (18):

J p0 = (T 0wed ) 2 D /(4  2 )

b) using values J t1, T 1 and D from table 1, calculate Jр1 according to the formula obtained from formula (19a):

J p1 = T 1 2 D /(4  2 ) – J t1

c) using values J t2, T 2 and D from table 1, calculate Jр2 according to the formula obtained from formula (19b):

J p2 = T 2 2 D /(4  2 ) – J t2

Compare the obtained values J p0, J p1 and J p2; calculate from them the average value of the moment of inertia of the empty frame Jр-ср and enter it into table 2.

Calculate absolute and relative error.

Table 2.

Task 3. Determination of the moment of inertia of a body.

Fix the body under study - a rectangular parallelepiped - in a frame, aligning one of the main axes of inertia of this body with the axis of rotation. Determine three times N And t(the same way as was done in previous tasks) and enter these values, as well as the corresponding values T = t/N, for axis No. I in table 3.

Repeat the operations performed for the other two main axes of inertia of the body under study. Received values N, t And T for axis No. II and for axis No. III enter in table 3.

For all three axes of the three values ​​of the period of oscillation of the frame with the body T calculate their average values T Wed and enter this data into Table 3.

Using the previously obtained average value of the moment of inertia of the frame Ј р‑ср (table 2), from the ratio Ј t = Ј – Ј р‑ср determine the values ​​of the moments of inertia of the body Ј t relative to its three main axes of inertia and enter these values ​​in Table 3.

Table 3.

Test questions to prepare for work:

What quantity is called the moment of inertia of a material point and a rigid body?

According to what law do torsional vibrations occur?

What is torsional modulus and what does it depend on?

What quantities are measured in this work, which ones are calculated and by what formulas?

Security questions to protect your work:

What is the moment of inertia and what is it physical meaning this size?

What are the units of moment of inertia?

What is elastic deformation?

Derive a formula to determine the torsion modulus.

What is the nature of torsional vibrations and according to what law do they occur?

What is the angular momentum of a body? How is it directed?

What is Steiner's theorem?

Write the formula for the period of torsional vibrations.

EDUCATIONAL EDITION

Makarov Igor Evgenievich, Doctor of Chemical Sciences professor

Yurik Tamara Konstantinovna, Ph.D. associate professor

1. Conditions for the occurrence and burning of an arc

Opening electrical circuit if there is current in it, it is accompanied by an electrical discharge between the contacts. If in the disconnected circuit the current and voltage between the contacts are greater than critical for the given conditions, then a arc, the duration of combustion of which depends on the parameters of the circuit and the conditions of deionization of the arc gap. The formation of an arc when copper contacts are opened is possible already at a current of 0.4-0.5 A and a voltage of 15 V.

Rice. 1. Location in a stationary arc DC voltage U(a) and tensionE(b).

In the arc there are distinguished the near-cathode space, the arc shaft and the near-anode space (Fig. 1). All stress is distributed between these areas U To, U sd, U A. The cathode voltage drop in a DC arc is 10-20 V, and the length of this section is 10-4-10-5 cm, so there is a high voltage near the cathode electric field(105-106 V/cm). At such high voltages, impact ionization occurs. Its essence lies in the fact that electrons, torn from the cathode by the forces of the electric field (field emission) or due to heating of the cathode (thermionic emission), are accelerated in the electric field and, when striking a neutral atom, give off their kinetic energy to it. If this energy is enough to remove one electron from the shell of a neutral atom, then ionization will occur. Formed free electrons and ions make up the plasma of the arc barrel.

Rice. 2. .

Plasma conductivity approaches the conductivity of metals [ at= 2500 1/(Ohm×cm)]/ A large current passes in the arc barrel and a high temperature is created. The current density can reach 10,000 A/cm2 or more, and the temperature - from 6000 K at atmospheric pressure up to 18000 K or more at high blood pressure.

High temperatures in the arc barrel lead to intense thermal ionization, which maintains high plasma conductivity.

Thermal ionization is the process of formation of ions due to the collision of molecules and atoms with high kinetic energy at high speeds their movements.

The greater the current in the arc, the lower its resistance, and therefore less voltage is required to burn the arc, i.e., it is more difficult to extinguish an arc with a high current.

With AC power supply voltage u cd changes sinusoidally, the current in the circuit also changes i(Fig. 2), and the current lags behind the voltage by approximately 90°. Arc voltage u d, burning between the contacts of the switch, intermittently. At low currents, the voltage increases to a value u h (ignition voltage), then as the current in the arc increases and thermal ionization increases, the voltage drops. At the end of the half-cycle, when the current approaches zero, the arc goes out at the quenching voltage u d. In the next half-cycle, the phenomenon repeats if measures are not taken to deionize the gap.

If the arc is extinguished by one means or another, then the voltage between the switch contacts must be restored to the supply voltage - u vz (Fig. 2, point A). However, since the circuit contains inductive, active and capacitive resistances, a transient process occurs, voltage fluctuations appear (Fig. 2), the amplitude of which U in,max can significantly exceed the normal voltage. For switching equipment, it is important how quickly the voltage in the AB section is restored. To summarize, the arc discharge is initiated by impact ionization and electron emission from the cathode, and after ignition, the arc is maintained by thermal ionization in the arc barrel.

In switching devices it is necessary not only to open the contacts, but also to extinguish the arc that arises between them.

In chains AC The current in the arc passes through zero every half-cycle (Fig. 2), at these moments the arc goes out spontaneously, but in the next half-cycle it can arise again. As the oscillograms show, the current in the arc becomes close to zero somewhat earlier than the natural transition through zero (Fig. 3, A). This is explained by the fact that when the current decreases, the energy supplied to the arc decreases, therefore, the arc temperature decreases and thermal ionization stops. Duration of dead time t n is small (from tens to several hundred microseconds), but plays important role in arc extinction. If you open the contacts during a dead pause and separate them at a sufficient speed to such a distance that the electrical breakdown, the circuit will be disconnected very quickly.

During the dead pause, the ionization intensity drops significantly, since thermal ionization does not occur. In switching devices, in addition, artificial measures are taken to cool the arc space and reduce the number of charged particles. These deionization processes lead to a gradual increase in the electrical strength of the gap u pr (Fig. 3, b).

A sharp increase in the electrical strength of the gap after the current passes through zero occurs mainly due to an increase in the strength of the near-cathode space (in AC circuits 150-250V). At the same time, the recovery voltage increases u V. If at any time u pr > u the gap will not be pierced, the arc will not light up again after the current passes through zero. If at some point u pr = u c, then the arc re-ignites in the gap.

Rice. 3. :

A– extinction of the arc when the current naturally passes through zero; b– increase in the electrical strength of the arc gap when the current passes through zero

Thus, the task of extinguishing the arc comes down to creating such conditions that the electrical strength of the gap between the contacts u there was more tension between them u V.

The process of increasing voltage between the contacts of the device being switched off can be different character depending on the parameters of the switched circuit. If a circuit with a predominance of active resistance is turned off, then the voltage is restored according to an aperiodic law; if inductive reactance predominates in the circuit, then oscillations occur, the frequencies of which depend on the ratio of capacitance and inductance of the circuit. Oscillatory process leads to significant speeds of voltage recovery, and the greater the speed du V/ dt, the more likely it is that the gap will break down and the arc will re-ignite. To facilitate the conditions for extinguishing the arc, active resistances are introduced into the disconnected current circuit, then the nature of the voltage recovery will be aperiodic (Fig. 3, b).

3. Methods for extinguishing arcs in switching devices up to 1000IN

In switching devices up to 1 kV, the following arc extinguishing methods are widely used:

Lengthening the arc with rapid divergence of contacts.

The longer the arc, the greater the voltage required for its existence. If the power source voltage is lower, the arc goes out.

Dividing a long arc into a number of short ones (Fig. 4, A).
As shown in Fig. 1, the arc voltage is the sum of the cathode voltage U k and anode U and voltage drops and arc shaft voltage U sd:

U d= U k+ U a+ U sd= U e+ U sd.

If a long arc that occurs when the contacts open is pulled into an arc-extinguishing grid made of metal plates, then it will split into N short arcs. Each short arc will have its own cathode and anode voltage drops U e. The arc goes out if:

U n U uh,

Where U- mains voltage; U e - the sum of the cathode and anode voltage drops (20-25 V in a DC arc).

The AC arc can also be divided into N short arcs. At the moment the current passes through zero, the near-cathode space instantly acquires an electrical strength of 150-250 V.

The arc goes out if

Arc extinction in narrow slots.

If an arc burns in a narrow gap formed by an arc-resistant material, then due to contact with cold surfaces, intensive cooling and diffusion of charged particles occurs in environment. This leads to rapid deionization and arc extinction.

Rice. 4.

A– dividing a long arc into short ones; b– drawing the arc into a narrow slot in the arc-extinguishing chamber; V– rotation of the arc in a magnetic field; G– arc extinction in oil: 1 – fixed contact; 2 – arc trunk; 3 – hydrogen shell; 4 – gas zone; 5 – oil vapor zone; 6 – moving contact

Movement of an arc in a magnetic field.

An electric arc can be considered as a conductor carrying current. If the arc is in a magnetic field, then it is acted upon by a force determined by the left-hand rule. If you create a magnetic field directed perpendicular to the axis of the arc, then it will receive forward motion and will be pulled inside the gap of the arc chute (Fig. 4, b).

In a radial magnetic field, the arc will receive rotational motion (Fig. 4, V). A magnetic field can be created permanent magnets, special coils or the circuit of live parts itself. Fast rotation and the movement of the arc contributes to its cooling and deionization.

The last two methods of extinguishing the arc (in narrow slots and in a magnetic field) are also used in disconnecting devices with voltages above 1 kV.

4. The main methods of extinguishing the arc in devices above 1kV.

In switching devices over 1 kV, methods 2 and 3 described in paragraphs are used. 1.3. and the following arc extinguishing methods are also widely used:

1. Arc extinction in oil .

If the contacts of the disconnecting device are placed in oil, then the arc that occurs during opening leads to intense gas formation and evaporation of the oil (Fig. 4, G). A gas bubble is formed around the arc, consisting mainly of hydrogen (70-80%); rapid decomposition of the oil leads to an increase in pressure in the bubble, which contributes to its better cooling and deionization. Hydrogen has high arc-quenching properties. Contacting directly with the arc shaft, it contributes to its deionization. Inside the gas bubble there is a continuous movement of gas and oil vapor. Arc extinguishing in oil is widely used in circuit breakers.

2. Gas-air blowing .

Arc cooling is improved if a directed movement of gases is created - blasting. Blowing along or across the arc (Fig. 5) promotes the penetration of gas particles into its barrel, intense diffusion and cooling of the arc. Gas is created during the decomposition of oil by an arc (oil switches) or solid gas-generating materials (autogas blast). It is more effective to blow with cold, non-ionized air coming from special compressed air cylinders (air switches).

3. Multiple current circuit break .

Switching off large currents at high voltages is difficult. This is explained by the fact that when large values With the added energy and recovery voltage, deionization of the arc gap becomes more complicated. Therefore, in switches high voltage multiple arc breaks are used in each phase (Fig. 6). Such switches have several extinguishing devices designed for part of the rated value. yarn. The number of breaks per phase depends on the type of switch and its voltage. In 500-750 kV circuit breakers there can be 12 breaks or more. To facilitate arc extinction, the recovery voltage must be evenly distributed between the breaks. In Fig. Figure 6 schematically shows an oil switch with two breaks per phase.

When a single-phase short circuit is disconnected, the recovering voltage will be distributed between the breaks as follows:

U 1/U 2 = (C 1+C 2)/C 1

Where U 1 ,U 2 - stresses applied to the first and second breaks; WITH 1 – capacitance between the contacts of these gaps; C 2 – capacity of the contact system relative to the ground.

Rice. 6. Voltage distribution over breaks in the switch: a – voltage distribution over breaks in the oil switch; b – capacitive voltage dividers; c – active voltage dividers.

Because WITH 2 is much more C 1, then the voltage U 1 > U 2 and, therefore, extinguishing devices will operate under different conditions. To equalize the voltage, capacitances or active resistances are connected parallel to the main contacts of the circuit breaker (MC) (Fig. 16, b, V). The values ​​of capacitances and active shunt resistances are selected so that the voltage at the breaks is distributed evenly. In switches with shunt resistances, after extinguishing the arc between the main circuits, the accompanying current, limited in value by the resistances, is broken by the auxiliary contacts (AC).

Shunt resistances reduce the rate of rise of the recovery voltage, which makes it easier to extinguish the arc.

4. Arc extinction in vacuum .

Highly rarefied gas (10-6-10-8 N/cm2) has an electrical strength tens of times greater than gas at atmospheric pressure. If the contacts open in a vacuum, then immediately after the first passage of the current in the arc through zero, the strength of the gap is restored and the arc does not light up again.

5. Arc extinction in gases high pressure .

Air at a pressure of 2 MPa or more has high electrical strength. This makes it possible to create fairly compact devices for extinguishing an arc in a compressed air atmosphere. The use of high-strength gases, such as sulfur hexafluoride SF6 (SF6 gas), is even more effective. SF6 gas not only has greater electrical strength than air and hydrogen, but also better arc-extinguishing properties even at atmospheric pressure.

1. Arc formation.

arc discharge .

.

4. Temperature and radiation of individual parts of the arc discharge.

tric arc.

and ultra-high pressure.

III. Application of arc discharge.

1. Modern methods of electrical processing.

2. Electric arc welding.

3.Plasma technology.

4.Plasma welding.
IV. Conclusion.

An arc discharge in the form of a so-called electric (or voltaic) arc was first discovered in 1802 by the Russian scientist, professor of physics at the Military Medical-Surgical Academy in St. Petersburg, and later academician of the St. Petersburg Academy of Sciences Vasily Vladimirovich Petrov. In one of the books he published, Petrov describes his first observations of the electric arc in the following words:

“If two or three charcoals are placed on a glass tile or on a bench with glass legs... and if metal insulated guides... communicated with both poles of a huge battery are brought closer to each other at a distance of one to three lines, then between them appears a very bright white light or flame, from which these coals ignite faster or slower and from which the dark peace can be quite clearly illuminated...”

The path to the electric arc began in ancient times. Even the Greek Thales of Miletus, who lived in the sixth century BC, knew the property of amber to attract light objects such as feathers, straw, hair when rubbed, and even create sparkles. Until the seventeenth century, this was the only way to electrify bodies, which had no practical application. Scientists were looking for an explanation for this phenomenon.

The English physicist William Gilbert (1544-1603) found that other bodies (for example, rock crystal, glass), like amber, have the property of attracting light objects after rubbing. He called these properties electrical, introducing this term into use for the first time (in Greek, amber is electron).

The burgomaster of Magdeburg, Otto von Guericke (1602-1686), designed one of the first electric machines. It was an electrostatic machine, which was a sulfur ball mounted on an axis. One of the poles was... the inventor himself. When the handle was rotated, bluish sparks flew out from the palms of the satisfied burgomaster with a slight crackling sound. Later, Guericke's machine was improved by other inventors. The sulfur ball was replaced by a glass one, and instead of the researcher’s palms, leather pads were used as one of the poles.

Of great importance was the invention in the eighteenth century of the Leyden jar-capacitor, which made it possible to store electricity. It was a glass vessel filled with water, wrapped in foil. A metal rod passed through a stopper was immersed in water.

The American scientist Benjamin Franklin (1706-1790) proved that water does not play any role in the collection of electrical charges; dielectric glass has this property.

Electrostatic machines have become quite widespread, but only as fun gizmos. There were, however, attempts to treat patients with electricity, but it is difficult to say what the physiotherapeutic effect of such treatment was.

The French physicist Charles Coulomb (1736-1806), the founder of electrostatics, established in 1785 that the force of interaction between electric charges is proportional to their magnitudes and inversely proportional to the square of the distance between them.

In the forties of the eighteenth century, Benjamin Franklin put forward the theory that there is only one kind of electricity - a special electrical matter consisting of tiny particles capable of penetrating into matter. If a body has an excess of electrical matter, it is charged positively; if there is a deficiency, the body is negatively charged. Franklin introduced the plus and minus signs into practice, as well as the terms: capacitor, conductor, charge.

Original theories about the nature of electricity were made by M. V. Lomonosov (1711-1765), Leonhard Euler (1707-1783), Franz Apinus (1724-1802) and other scientists. By the end of the eighteenth century, the properties and behavior of stationary charges had been sufficiently studied and to some extent explained. However, nothing was known about electric current-moving charges, since there was no device that could make a large number of charges move. The currents received from the electrostatic machine were too small to be measured.

1 . If you increase the current in a glow discharge, reducing the external resistance, then at a high current, the voltage at the tube terminals begins to fall, the discharge quickly develops and turns into an arc. In most cases, the transition occurs abruptly and almost often leads to a short circuit. By selecting the resistance of the external circuit, it is possible to stabilize the transition form of the discharge and observe, at certain pressures, the continuous transition of the glow discharge into an arc. In parallel with the voltage drop between the electrodes of the tube, there is an increase in the cathode temperature and a gradual decrease in the cathode drop.

The use of the usual method of igniting an arc by moving the electrodes apart is due to the fact that the arc burns at relatively low voltages of tens of volts, while to ignite a glow discharge a voltage of the order of tens of kilovolts is needed at atmospheric pressure. The ignition process when moving the electrodes apart is explained by local heating of the electrodes due to the formation of poor contact between them at the moment the circuit breaks.

The question of the development of an arc when a circuit breaks is technically important not only from the point of view of obtaining “useful” arcs, but also from the point of view of combating “harmful” arcs, for example, with the formation of an arc when a switch is opened. Let L be the self-inductance of the circuit, W be its resistance, ع be the e.m.f. current source, U(I) is a function of the current-voltage characteristic of the arc. Then we must have: ع= L dI/dt+WI+U(I) (1) or

LdI/dt=(ع-WI)-U(I)=∆ (2).

The difference (ع - WI) is nothing more than the ordinate of the direct resistance AB (Fig. 1), and U(I) is the ordinate of the arc characteristic for a given I. So that dI/dt is negative, i.e. So that the current I certainly decreased over time and no stable arc formed between the electrodes of the switch, it is necessary that

Fig.1. The relative position of the resistance line and the current-voltage characteristic curve of a steady arc for the cases: a) when the arc cannot occur when the circuit breaks; b) when an arc occurs during a break in the current range corresponding to points P and Q.

∆ ع-WI took place.

To do this, the characteristic with all its points must lie above the resistance line (Fig. 1, a). This simple conclusion does not take into account the capacitances in the circuit and applies only to direct current.

The point of intersection of the resistance line with the current-voltage characteristic curve of a steady arc corresponds to the lowest limit of direct current strength at which an arc can occur when the circuit breaks (Fig. 1, b). In the case of a switch opening an alternating current arc that goes out with each voltage transition through zero, it is essential that the conditions present in the discharge gap during opening do not allow the arc to re-ignite with a subsequent increase in the voltage of the current source. This requires that as the voltage increases, the discharge gap is sufficiently deionized. In switches of strong alternating currents, enhanced deionization is artificially achieved by introducing special electrodes that suck out charged gas particles due to bipolar diffusion, as well as by using mechanical blowing or by exposing the discharge to a magnetic field. At high voltages, oil switches are used.

2 . The cathode spot, stationary on the carbon cathode, on the surface of liquid mercury is in continuous rapid motion. The position of the cathode spot on the surface of liquid mercury can be fixed using a metal pin immersed in the mercury and protruding slightly from it.

In the case of a small distance between the anode and the cathode, the thermal radiation of the anode greatly affects the properties of the cathode spot. At a sufficiently large distance of the anode from the carbon cathode, the dimensions of the cathode spot tend to some constant limiting value, and the area occupied by the cathode spot on the carbon electrode in air is proportional to the current strength and corresponds to an atmospheric pressure of 470 A/cm². For a mercury arc 4000 a/cm² was found in vacuum.

As the pressure decreases, the area occupied by the cathode spot on the carbon cathode at a constant current increases.

The sharpness of the visible boundary of the cathode spot is explained by the fact that a relatively slow decrease in temperature with distance from the center of the spot corresponds to a rapid drop in both light radiation and thermionic emission, and this is equivalent to sharp “optical” and “electrical” boundaries of the spot.

When an arc burns in air, the carbon cathode becomes sharp, while on the carbon anode, if the discharge does not cover the entire front area of ​​the anode, a round depression is formed - positive arc crater.

The formation of a cathode spot is explained as follows. The distribution of space charges in a thin layer near the cathode is such that the discharge requires the smaller the cross-section of the discharge channel to maintain it, the smaller the potential difference. Therefore, the discharge at the cathode must contract.

Directly adjacent to the cathode spot is a part of the discharge called the negative cathode brush or negative flame. The length of the cathode brush in the arc at low pressure is determined by the distance over which the fast primary electrons fly, having received their velocities in the region of the cathode potential drop.

Between the negative brush and the positive column there is an area similar to the Faraday dark space of a glow discharge. In Petrov's arc in the air, in addition to the negative brush, there is a positive flame and a number of halos. Spectral analysis indicates the presence of a number of chemical compounds (cyanine and nitrogen oxides) in these flames and halos.

With a horizontal arrangement of the electrodes and high gas pressure, the positive column of the arc discharge bends upward under the influence of convection currents of the gas heated by the discharge. This is where the name arc discharge comes from.

3 . In the Petrov arc, high temperature and high pressure do not make it possible to use the probe method to measure the potential distribution.

The potential drop between the arc electrodes consists of the cathode drop and Uk, the anodic drop Ua and the drop in the positive column. The sum of the cathode and anode potential drops can be determined by bringing the anode and cathode closer together until the positive column disappears and measuring the voltage between the electrodes. In the case of an arc at low pressure, it is possible to determine the potential values ​​​​at two points of the arc column using the method of probe characteristics, calculate the longitudinal potential gradient from here and then calculate both the anodic and cathodic potential drop.

It has been established that in an arc discharge at atmospheric pressure the sum of the cathode and anode drops is approximately the same value as the ionization potential of the gas or vapor in which the discharge occurs.

In the technique of using the Petrov arc with carbon electrodes, the empirical Ayrton formula is usually used:

U=a+bl+(c+dl)/I (3)

Here U is the voltage between the electrodes, I is the current strength in the arc, l is the length of the arc, a, b, c and d are four constants. The characteristic formula (3) is established for an arc between carbon electrodes in air. By l we mean the distance between the cathode and the plane drawn through the edges of the positive crater.

Let us rewrite formula (4) in the form

U=a+c/I+l(b+d/I). (4)

In (4), terms containing the factor l correspond to the potential drop in the positive column; the first two terms represent the sum of the cathode and anode drop Uк+Uа. The constants in (3) depend on the air pressure and the cooling conditions of the electrodes, and, consequently, on the size and shape of the coals.

In the case of an arc discharge in an evacuated vessel filled with metal vapor (for example, mercury), the vapor pressure depends on the temperature of the coldest parts of the vessel and therefore the course of the characteristic strongly depends on the cooling conditions of the entire tube.

The dynamic characteristics of an arc discharge are very different from the static ones. The type of dynamic characteristic depends on the speed of change of the arc mode. In practice, the most interesting characteristic of the arc is when powered by alternating current. Simultaneous oscillography of current and voltage gives the picture shown in Fig. 2. The characteristic of the arc drawn from these curves for the entire period has

view shown in Fig. 3. The dotted line shows the voltage progression in the absence of a discharge.

Fig.4. Dynamic characteristics

arc discharge tick on

low frequency alternating current.

Is. 3. Oscillogram of current and voltage of arc discharge on alternating current

low frequency. Points A, B, C, etc.

correspond to the points indicated by those

the same letters in Fig. 4.

The cathode, which has not yet had time to cool after the discharge that took place in the previous half-cycle of the current, from the very beginning of the half-cycle, when the external emf. passes through zero and emits electrons. From point O to point A, the characteristic corresponds to a non-self-sustaining discharge, the source of which is the electrons emitted by the cathode. At point A the arc is ignited. After point A, the discharge current increases rapidly. If there is resistance in the external circuit, the voltage between the arc electrodes drops, although the emf. current source (dotted line in Fig. 3), running through a sinusoid, increases even more. As the voltage and current supplied by the external source decrease, the discharge current begins to decrease.

With a decrease in the current in the arc, the voltage between its electrodes may increase again depending on the external resistance, but part of the BC characteristic in Fig. 4 may be horizontal or have the opposite slope. At point C the arc goes out.

After point C, the non-self-sustaining discharge current decreases to zero along with a decrease in the voltage between the electrodes.

P
after the voltage passes through

zero, the role of the cathode begins to be played by the previous anode and the picture is repeated with opposite signs of current and voltage.

Fig.5. Change in dynamic characteristics at increased frequency of alternating current superimposed on direct current.

If an alternating voltage of an amplitude less than the voltage of the direct current feeding the arc is applied to the electrodes of an arc fed by direct current, then the characteristic takes the form of a closed loop covering the static characteristic Sun on both sides. As the frequency of alternating current increases, the axis of this loop rotates, the loop itself is flattened and, finally, tends to take the form of a straight line segment OA, passing through the origin of coordinates (Fig. 5). At a very low frequency, the loop of the dynamic characteristic turns into a segment of the static characteristic of the VS, since all internal parameters of the discharge, in particular the concentration of ions and electrons, manage to take on values ​​at each point of the characteristic that correspond to a stationary discharge for given U and I. Vice versa , with a very rapid change, the discharge parameters do not have time to change at all, therefore I turns out to be proportional and, which corresponds to the straight line OA passing through the origin of coordinates. Thus, with an increase in the frequency of the alternating current, the characteristic loop (Fig. 5) becomes in all its increasing points.

Due to the possibility of complete ionization of gas in an arc

discharge there is a question of arc breakage at low gas pressure

and very strong currents. A significant decrease in gas density due to electrophoresis and suction of ions to the walls plays a significant role in the phenomenon of arc breakage, especially in places where the discharge gap is greatly narrowed. In practice, this leads to the need to avoid excessive contractions when constructing mercury rectifiers for very high currents.

Electricians who dealt with electric arcs for the first time

tried to apply Ohm's law in this case as well. To obtain calculation results according to Ohm's law that agree with reality, they had to introduce the concept of the inverse electromotive force of the arc. By analogy with phenomena in galvanic cells, the expected appearance of this emf. called arc polarization. The question of reverse emf. The arcs are devoted to the works of Russian scientists D. A. Lachinov and V. F. Mitkevich. Further development of ideas about electrical discharges in gases showed that such a formulation of the question is purely formal and can be successfully replaced by the idea of ​​​​the falling characteristic of the arc. The validity of this point of view is confirmed by the failure of all attempts to directly detect experimentally the reverse emf. electric arc.

4 . In the case of an arc in air between carbon electrodes

The radiation from hot electrodes, mainly from the positive crater, predominates.

The radiation of the anode, like the radiation of a solid body, has

continuous spectrum. Its intensity is determined by the temperature of the anode. The latter is a characteristic value for an arc in atmospheric air with an anode made of any given material, since the temperature of the anode does not depend on the current strength and is determined solely by the melting or distillation temperature of the anode material. The melting or sublimation temperature depends on the pressure under which the melting or sublimating body is located. Therefore, the anode temperature, and therefore the intensity of the positive crater radiation, depends on the pressure at which the arc burns. In this regard, classical experiments with a carbon arc under pressure are known, which led to very high temperatures.

On the change in the temperature of a positive crater with pressure

This drawing contains points for pressures from 1 atm

and above, serves as confirmation of the assumption that the temperature of the positive crater is determined by the temperature of melting or sublimation of the anode substance, since in this case there should be a linear relationship between ln r and 1/T. The deviation from the linear dependence at lower pressures is explained by the fact that at pressures below 1 atm, the amount of heat released at the anode is insufficient for

Rice. 6. Change in temperature of the carbon anode of an electric dkg in air with a change in pressure. The scale along the ordinate axis is logarithmic.

The temperature of the cathode spot of the Petrov arc is always several

hundreds of degrees below the temperature of the positive crater.

High arc cord temperatures cannot be detected

using a thermocouple or bolometer. Currently

Spectral spectra are used to determine the temperature in the arc

At high current strengths, the gas temperature in Petrov's arc

may be higher than the anode temperature and reaches 6000° K. Such high gas temperatures are characteristic of all cases of arc discharge at atmospheric pressure. In the case of very high pressures (tens and hundreds of atmospheres), the temperature in the central parts of the detached positive arc column reaches 10,000° K. In an arc discharge at low pressures, the gas temperature in the positive column is of the same order as in the positive column of a glow discharge.

The temperature of the positive arc crater is higher than the temperature of the cathode, because at the anode all the current is carried by electrons bombarding and heating the anode. Electrons

donate to the anode not only everything purchased in the anode area

drop in kinetic energy, but also the work function (“hidden-

heat of evaporation" of electrons). On the contrary, to the cathode

falls and is bombarded and heated by a small number of positive ions compared to the number of electrons hitting the anode at the same current strength. The rest of the current on the cathode is carried out by electrons, upon the exit of which in the case

thermionic arc, heat is expended on the work function

vaya energy of the cathode.

5 . Due to the fact that the arc has a falling characteristic, it can be used as a generator of continuous oscillations. The diagram of such an arc generator is shown in Fig. 7. Conditions for generating oscillations in this

With
heme can be deduced from consideration

friction conditions of stability of the

national rank for given

parameters of the external circuit.

Let the electromotive force

DC source, pi-

Rice. 7. Schematic diagram of an arc generator.

voltage between electrodes

tubes U, the force of stationary current

flow through the discharge tube in this mode is equal to I, the cathode-anode capacitance of the tube plus the capacitance of all supply wires C, self-inductance in the circuit L, the resistance through which current is supplied from the source, R. Under steady-state DC mode, we will have:

ع= UO+IR(5)

Let us assume that this stationary regime is violated. Bit

the current at any given time is equal to I+i, Where i-small value, and the potential difference between the electrodes is equal to U.

Let us introduce the notation

(dU/d i)i=0 is equal to the tangent of the tangent to the current-voltage characteristic at the operating point corresponding to the mode we initially selected (current I). Let's see how it will change further i. If i will increase, then this discharge mode is unstable; if, on the contrary, i decreases infinitely, then the discharge mode is stable.

Let us turn to the current-voltage characteristic of the considered

discharge gap U= f(I+i) - Current flows through the tube

I+i and capacity WITH is charging (or discharging). Difference

potentials on the capacitance WITH is balanced in this case

not only by the voltage across the discharge gap, but also by the emf.

self-inductance of the circuit. Let I+i2 -total current through the resistance-

tion R. Let us denote the current charging the capacitance C by i1 ; instantly

the real value of the potential difference across the capacitance C- through U1. The potential difference between the arc electrodes will be U0 +iU’.

ع =U1 +(i+I2 )R, (6)

U1 -U0 =U'i+Ldi/dt, (7)

i2 =i1 +i. (8)

Additional charge Q on capacity C compared to stationary mode:

Q=∫i 1 dt=(U 1 -U 0)C. (9)

Subtracting (5) from (6), we find:

U 1 -U 0 =-i 2 R (10)

Expressions (7), (8) and (10) give:

U"i+Ldi/dt=-R(i+i 1 ) . (11)

Expressions (7) and (9) give:

1/C∫ i 1 dt=U'i+Ldi/dt. (12)

Differentiating (12) with respect to t and inserting the result into (11), we find:

U’i+Ldi/dt=-iR-RCU’di/dt-RLCdІ i/dtІ . (13)

dІ i/dtІ +(1/CR+U’/L)di/dt + 1/LC(U’/R+1)i=0 (14)

Formula (14) is a differential equation,

to which the additional current is subject i.

As is known, the complete integral of equation (14) has the form:

i=А1е^r1t+А2е^r2t, (15)

where r1 and r2 are the roots of the characteristic equation, determined by the formula

r=-1/2(1/CR+U’/L)+ √1/4(1/CR+U’/L)І-1/LC(U’/R+1). (16)

If the radical value in (16) is greater than zero, then r1 and r2

both are real, i changes aperiodically according to an exponential law, and solution (15) corresponds to an aperiodic change in current. In order for current oscillations to occur in the circuit we are considering, it is necessary that r 1 and r 2 be complex quantities, i.e.

1/LC(U’/R+1)>1/4(1/CR+U’/L)І(17)

In this case, (15) can be represented as

i=A 1 e -δt+jωt +A 2 e -δt-jωt , (18)

δ=1/2(1/CR+U’/L); i=√-1.

At δ δ > 0 they quickly decay, and the discharge at a constant current will be stable.

Thus, in order for undamped oscillations to eventually be established in the circuit under consideration, it is necessary that

(1/CR+U’/L) (19)

Since P, L and C are essentially positive quantities, then

inequality (19) can be satisfied only under the condition:

From here we conclude that oscillations in the circuit under consideration

can arise only with a falling current-voltage characteristic

risk of discharge.

Investigation of the conditions under which r1 and r2 are valid

and both are less than zero, leading to discharge stability conditions

DC:

(1/CR+U’/L)>0And (21)

U'/R+1>0 . (22)

Conditions (21) and (22) are general conditions

Stability of discharge powered by constant voltage. From

(21) it follows that with increasing current-voltage characteristic

According to statistics, the discharge is always stable.

Combining this requirement with condition (22), we find that

with a falling characteristic, the discharge can be stable

only when

When directly applying the formulas of this paragraph

to the question of generating oscillations using an arc we have to

take U" from the “average characteristic”, built on the basis of the ascending and descending branches of the dynamic characteristic.

With a periodic change in the current strength in the Petrov arc due to

the temperature and density of the gas and the speed of aerodynamic flows change. When selecting the appropriate mode, these

changes lead to acoustic vibrations

in the surrounding air. The result is a so-called singing arc, reproducing pure musical tones.

6 . With increasing gas pressure and increasing current density, the temperature along the axis of the positive column, detached from the walls of the discharge tube, rises more and more. Ionization processes begin to take on a character more and more consistent with purely thermal ionization. The average kinetic energy of plasma electrons approaches the average kinetic energy of neutral gas particles. The plasma becomes close in its properties to isothermal

chesky plasma. All this allows us to solve the problem of finding

various discharge parameters, including the number of longitudinal field gradient depending on the discharge current density, based on thermodynamic relationships.

The starting points of the theory of the positive arc column

The second discharge at high and ultra-high pressure is the Sag equation for thermal ionization in the form

αІp=AT 5/2 e -eUi/kT (24)

and Boltzmann's theorem in the form of the relation

n a =nge (-eU a /kT) (25)

Here α is the degree of ionization, p is the gas pressure, A is the constant,

T-gas temperature, U i -ionization potential, k-constant

Boltzmann, “n a is the concentration of excited atoms, n-concentration

tration of normal atoms, U a -excitation potential, g-relative

determination of the statistical weights g a /g n of the excited and normal states of the atom. The temperature of the electron gas is assumed to be equal to the temperature of the neutral gas. To simplify the problem, only one “average” level of excitation is taken into account. The discharge tube is assumed to be located vertically. In any other position, convection gas flows distort the axial symmetry of the gas regime.

Let us denote the inner radius of the discharge tube by R1, and the distance of any point from the axis of the tube by r. Let's carry out

at a distance of one centimeter from one another there are two sections perpendicular to the axis of the tube, and we select an elementary volume between them using two concentric cylinders with radii r and r+dr (Fig. 8). Let us denote the amount of energy released by the discharge per unit time per unit tube length by N1, and the amount of energy per unit volume under consideration by dN1. Amount of energy emitted per unit
time is a gas enclosed

per unit length of the entire tube and

in the elementary

volume, denoted by S1 and dS1.

Inside the tube there is

Rice. 8. Volume element in an axially symmetric discharge.

heat through the gas in the direction

from the axis to the wall. Let us denote by dL1 the excess of the amount of heat leaving the volume element under consideration through its outer boundary per unit of time over the amount of heat penetrating into the same volume per unit of time through its internal boundary from the side of the tube axis. Let us assume that the convection flows of gas are strictly vertical and do not violate the thermal regime of the gas.

The condition for the thermal balance of the elemental element under consideration

volume will be written in general form like this:

dN1 =dL1 +dS1 . (26)

Due to the presence of axial symmetry, all quantities characterized by

representing the state of the gas and the discharge mode are the same for

points located at the same distance r from the axis.

Since the area of ​​the base of the elementary element under consideration

volume is equal to 2пrdr, then for the power released in this

volume, we can write:

dN 1 =2 nri r E z dr, (27)

where i r is the current density at a distance r from the axis, and E z is the longitudinal field gradient, the same over the entire cross section of the tube. Denoting the coefficient of thermal conductivity of a gas at temperature T by λ t, we write for dL 1, neglecting terms of higher order of smallness:

dL 1 =2п(r+dr)(λ t dT/dr) r+dr -2пr(λ t dT/dr) r =2пd(rλ t dT/dr)/dr (28)

Let us assume that the energy emitted by the gas leaves entirely

discharge gap without noticeable reabsorption in the gas. This

The assumption can be made because the resonant radiation absorbed by the gas at high pressure constitutes only a small fraction of the total radiation of the gas. Since the energy emitted per unit time is proportional to the concentration of excited atoms na, then for dS 1 we can write:

dS 1 =2пrCn a dr, (29)

where C is a constant factor independent of T. Substitution

values ​​(29) and (28) in (26) gives:

2 nri r E z dr=2nd(rλ T dT/dr)dr/dr + 2nrCn a dr (30)

Neglecting the small fraction of current attributable to the polar

resident ions, and denoting the mobility of electrons through K e, we can write:

i=n e eK e E z . (31)

If we denote the right side of the Sag equation (24) by f 1 (T), and replace p on the left side with nkT, where n is the concentration of neutral gas particles, then we find:

α 2 = f 1 (T)/ nkT. (32)

n is proportional to the mass of gas contained in a unit length

tube, g 1 and inversely proportional to the square of the tube radius R1 and the gas temperature at a given point:

n=C 1 g 1 /TR 1 2 (33)

Therefore, instead of (32) we can write:

α= R 1 √f1(T)/C1k/ √g 1 =R 1 f 2 (T)/√g 1 (34)

According to Langevin's equation, the speed of electron motion

in a gas in a field of intensity E z is equal to:

u=K e E z =aeλ e E z /mv (35)

where v is the arithmetic mean speed of thermal movement

electrons, is directly proportional to the square root of the temperature of the electron gas, while λ e is inversely proportional to n. Hence,

K e =C 2 /nT 1/2 (36)

According to the definition of α:

From (31), (34), (37) and (36) it follows:

i r =E z R i C 2 f 2 (T)/g 1 1/2 T 1/2 (38)

where T is the gas temperature at a distance r from the axis. From (38)

and (27) follows:

dN 1 =2пrE r 2 R 1 C 2 f 2 (T)dr/g 1 1/2 T 1/2 =2пrE z 2 R 1 f 3 (T)dr/g 1 1/2 ,(39)

According to the Boltzmann equation (25):

n a =nge (-eU a /kT) =C 1 gg 1 e (-eU a /kT) /TR 1 2 =g 1 f 4 (T)/ R 1 2, (40)

where f 4 (T)= C 1 ge (-eU a /kT) /T.

Inserting this value of n a into (29) and replacing Cf 4 (T) through f 5 (T), we find:

dS 1 =g 1 2пrf 5 (T)dr/R 1 2 . (41)

Substituting (39), (28) and (41) into (26) gives

E r 2 R 1 f 3 (T)/g 1 1/2 =d(rλ t dT/dr)/rdr+g 1 f 5 (T)dr/R 1 2 (42)

In equation (42), f 3 (T) and f 5 (T), as well as λ t, are functions of the variable T alone. Therefore, (42) is

differential equation connecting the variables T and r.

Boundary conditions that the solution must satisfy

of this equation are: a) at r=R the condition T=T st, where T st is the temperature of the discharge tube wall; b) at r=0, the condition dT/dr = 0, since on the axis of the tube the gas temperature has a maximum value.

All quantities characterizing the discharge are functions

from just one T. Therefore, the solution to equation (42) could

would provide a complete solution to all quantitative issues associated with this type of discharge. However, the significance of equation (42) lies mainly in the fact that by moving to dimensionless quantities it leads to similarity laws characteristic of a given type of discharge, which make it possible to transfer quantitative results established experimentally for the same values ​​of N 1, R 1 and g 1 for the discharge mode at other values ​​of these quantities. This technique is similar to that used to solve some problems of hydrodynamics, also only on the basis of the analysis of the differential equation and experimental measurements on models constructed in accordance with the similarity laws of hydrodynamics. In this case, two discharges are similar, in which at the corresponding points, characterized by the same value of the ratio r/R 1, the gas temperature is the same.

Practically the most significant are the following

two laws of similarity:

The first law of similarity of a high-pressure unlaced arc discharge. Two high-pressure arc discharges in cylindrical tubes of different diameters (2R 1 ≠ 2R 1 "), filled with gas so that one centimeter of the length of both tubes accounts for the same amount of gas (g 1 = g 1 '), are similar in the case if N 1 =N 1 ', i.e. if the power consumed per unit length of the tube is the same in both cases.

The second law of similarity for an unlaced high-pressure arc discharge. Two high-pressure arc discharges in mercury vapor in cylindrical tubes of different diameters (2R 1 ≠ 2R 1 "), filled with mercury vapor so that for one centimeter of the length of each tube there are different amounts of mercury vapor (g 1 ≠g 1 ') , are similar if the powers N 1 and N 1 ' consumed per unit length of each tube satisfy the relation

N 1 /N 1 '=8.5+5.7g 1 /8.5+5.7g 1' (43)

It is assumed that the mercury in the discharge has completely passed into the vapor state. The coefficients 8.5 and 5.75 were determined experimentally.

The type of discharge described in this chapter also includes

and the positive column (flame) of the Petrov arc, representing

is a cord of isothermal plasma. In this case, the boundary

conditions on the tube walls disappear and must be replaced

conditions in the boundary layer of the cord.

Currently, in addition to the arc discharge in mercury vapor

ultra-high pressure (up to 100 atm and more), arc discharge in inert gases Ne, Ar, Kr and Xe at pressures up to 20 atm and higher has also been studied and found technical application.

INTRODUCTION.

Properties of arc discharge.

1. Arc formation.

2. Cathode spot. Appearance and individual parts

arc discharge .

3. Potential distribution and current-voltage

arc discharge characteristic .

4. Temperature and radiation of individual parts of the blowing discharge.

5. Generation of continuous oscillations using electrical

tric arc.

6. Positive arc discharge at high

and ultra-high pressure.

Application of arc discharge.

1. Modern methods of electrical processing. Among modern technological processes, one of the most common is electric welding. Welding allows you to weld, solder, glue, and spray not only metals, but also plastics, ceramics and even glass. The range of application of this method is truly immense - from the production of powerful cranes, construction metal structures, equipment for nuclear and other power plants, the construction of large-tonnage ships, nuclear icebreakers to the production of the finest microcircuits and various household products. In a number of industries, the introduction of welding has led to a radical change in technology. Thus, a real revolution in shipbuilding was the development of the continuous construction of ships from large welded sections. Many shipyards in the country are now building large-capacity all-welded tankers. Electric welding made it possible to solve the problems of creating gas pipelines designed to operate in northern conditions at a pressure of 100-120 atmospheres. Employees of the Electric Welding Institute named after. E. O. Paton was offered the original

ginal method of manufacturing pipes based on welding technology intended for such gas pipelines. Of these

pipes with walls up to 40 millimeters thick and assemble highly reliable gas pipelines crossing continents.

Soviet scientists and specialists made a great contribution to the development of electric welding. Continuing and creatively developing the legacy of his great predecessors - V. V. Petrova, N. N. Benardos, N. G. Slavyanov, they created the science of the theoretical foundations of welding technology and developed a number of new technological processes. The whole world knows the names of academicians E. O. Paton, V. P. Vologdin, K. K. Khrenov, N. N.

Rykalina and others.

Currently, electric arc, electroslag and plasma arc welding are widely used.

2. Arc welding. The simplest method is manual arc welding. The holder is connected to one pole of the current source with a flexible wire, and the product to be welded is connected to the other. A carbon or metal electrode is inserted into the holder. When the electrode briefly touches the workpiece, an arc is ignited, which melts the base metal and the electrode rod, forming a weld pool that produces a weld when solidified.

Manual arc welding requires highly qualified workers and does not have the best working conditions, but with its help you can weld parts in any spatial position, which is especially important when installing metal structures. The productivity of manual welding is relatively low and depends largely on such a simple part,

as an electrode holder. And now, like a hundred years ago,

The search for its best design continues. A series of simple and reliable electrode holders were produced by Leningrad innovators M. E. Vasiliev and V. S. Shumsky.

When arc welding, protecting the weld metal from oxygen and nitrogen in the air is of great importance. Actively interacting with the molten metal, oxygen and nitrogen in the atmospheric air form oxides and nitrides, which reduce the strength and ductility of the welded joint.

There are two ways to protect the welding site: introducing various substances into the electrode material and electrode coating (internal protection) and introducing inert gases and carbon monoxide into the welding zone, covering the welding site with fluxes (external protection).

In 1932, at the Moscow Electromechanical Institute of Railway Transport Engineers, under the leadership of Academician K.K. Khrenov, underwater electric arc welding was carried out for the first time in the world. However, back in 1856, L.I. Shpakovsky first conducted an experiment in melting copper electrodes immersed in water with an arc. On the advice of D. A. Lachinov, who received an underwater arc, N. N. Benardos in 1887 carried out underwater cutting of metal. It took 45 years to

the first experiment was scientifically substantiated and turned into a method.

And on October 16, 1969, an electric arc burst into space for the first time. This is how this outstanding event was reported in the Izvestia newspaper; “The crew of the Soyuz-6 spacecraft, consisting of Lieutenant Colonel G. S. Shonin and flight engineer V. N. Kubasov, carried out experiments on carrying out welding work in space. The purpose of these experiments was to determine the characteristics of welding various metals in outer space... Several types of automatic welding were carried out in turn.” And yes-

lee: “The experiment carried out is unique and is of great importance for science and technology in the development of technology for welding and installation work in space” ...

3. Plasma technology. This technology is based

using a high temperature arc. She

includes plasma welding, cutting, surfacing and plasma mechanical processing.

How to improve arc performance? To do this, it is necessary to obtain an arc with a higher concentration of energy, i.e., the arc must be focused. This was achieved in 1957-1958, when at the Institute of Metallurgy. A. A. Baykov created equipment for plasma-arc cutting.

How to increase the arc temperature? Probably in the same way as they increase the pressure of a water or air stream by passing it through a narrow channel.

Passing through the narrow channel of the torch nozzle, the arc is compressed by a stream of gas (neutral, oxygen-containing) or a mixture of gases and is drawn into a thin stream. At the same time, its properties change sharply: the temperature of the arc discharge reaches

50,000 degrees, specific power reaches 500 or more kilowatts per square centimeter. The ionization of plasma in a gas column is so great that its electrical conductivity turns out to be almost the same as that of metals.

A compressed arc is called a plasma arc. With its help, plasma welding, cutting, guiding, spraying, etc. are carried out. To produce a plasma arc, special generators have been created - plasmatrons.

A plasma arc, like a regular one, can be of direct or indirect action. The direct action arc is closed on the product, the indirect action arc is closed on the second electrode, which is the nozzle. In the second case, it is not an arc that breaks out of the nozzle, but a plasma jet that arises due to heating by the arc and subsequent ionization of the plasma-forming gas. The plasma jet is mainly used for plasma spraying and processing of non-conductive materials.

The gas surrounding the arc also performs a heat-protective function.

The greatest load in a plasma torch is carried by the nozzle. The higher its heat resistance, the greater the current that can be obtained in an indirect plasmatron. The outer layer of plasma-forming gas has a relatively low temperature, so it protects the nozzle from destruction.

A significant increase in the temperature of the plasma-forming gas in direct plasma torches can lead to electrical breakdown and the occurrence of a double arc - between the cathode and the nozzle and between the nozzle and the product. In this case, the nozzle usually fails.

4. Plasma welding. There are two designs of plasmatrons. In some designs, gas is supplied along the arc, and good compression is achieved. In other designs, the gas covers the arc in a spiral, due to which it is possible to obtain a stable arc in the nozzle channel and ensure reliable protection of the nozzle by the wall layer of gas.

In direct plasma torches, the arc is not excited immediately, since the air gap between the cathode and the product is too large. First, the so-called pilot, or auxiliary, arc is excited between the cathode and the nozzle. It develops from a spark discharge, which occurs under the influence of a high-frequency voltage created by an oscillator. The gas flow blows out a pilot arc, it touches the metal being processed, and then the main arc is ignited. After this, the oscillator is turned off and the pilot arc goes out. If this does not happen, a double arc may occur.

The weld area during plasma welding, as with other types of welding, is protected from the action of ambient air. To do this, in addition to the plasma-forming gas, a protective gas is supplied into a special nozzle: argon or cheaper and more common carbon dioxide. Carbon dioxide is often used not only for protection, but also for plasma formation. Sometimes plasma welding is carried out under a layer of flux.

Plasma arc welding can be performed either automatically or manually. Currently, this method has become quite widespread. Many factories have introduced plasma welding of aluminum alloys and steels. Significant savings resulted from the use of single-pass plasma welding of aluminum instead of multi-pass argon-arc welding.

ki. Welding is carried out in an automatic installation using carbon dioxide as a plasma-forming and protective agent.

In modern life, the use of electrical energy has become widespread. Achievements of electrical engineering are used in all spheres of practical human activity: in industry, agriculture, transport, medicine, everyday life, etc. Advances in electrical engineering have a significant impact on the development of radio engineering, electronics, telemechanics, automation, computer technology -nicks, cybernetics. All this became possible as a result of the construction of powerful power plants, electrical networks, the creation of new electrical power systems, and the improvement of electrical devices. The modern electrical industry produces machines and devices for the production, transmission, conversion, distribution and consumption of electricity, a variety of electrical equipment and technological equipment, electrical measuring instruments and telecommunications, regulating, monitoring and control equipment for automatic control systems, medical and scientific equipment, electrical appliances and machines and much more. In recent years, various methods of electrical processing have received further development: electric welding, plasma cutting and surfacing of metals, plasma mechanical and electrical discharge processing. From the above

It is clear that the study of discharge in gas is of great importance for general scientific and technical progress. Therefore, there is no need to stop there, but it is necessary to continue research, looking for the unknown, thereby further stimulating the construction of new theories.

Khabarovsk State Pedagogical University

COURSE WORK

"ARC DISCHARGE IN GASES"

Completed by: student 131gr. FMF

Zyulyev M. N.