Glass at normal temperatures is a non-conductor of electricity. However, if it is heated to a temperature of several hundred degrees, it begins to noticeably conduct electricity. Experience can serve as a clear demonstration of this. A glass rod is connected to the city current circuit through a rheostat R, on the ends of which bare copper wire is wound L and B. At room temperature The resistance of a glass rod is many millions of ohms. Therefore, when the switch is closed, an insignificant current (microamperes or even less) will flow through the circuit, and there will be no glow in the light bulb L. But if the AB stick is heated on a gas burner to a temperature of 300-400 ° C, then its resistance will drop to several tens of ohms, and the light bulb filament will become hot. If you then remove the burner and at the same time short-circuit the light bulb with key K, the total resistance of the circuit will decrease and the current will increase. The glass rod will be heated by electric current and will glow until it glows brightly, as a result of which its resistance will further decrease and the current will increase. Eventually the stick will melt.

What explains the conductivity of glass? Glass is a highly supercooled liquid with enormous viscosity. It is also an electrolyte in which there are positive sodium ions Na\ When heated, when the glass softens and its viscosity decreases greatly, the ions in the glass acquire noticeable mobility. They are the current carriers in the glass. The following demonstration experiment can serve as proof of this.

In a heated crucible gas burner(Fig. 232), molten Chilean saltpeter NaN03 is placed, into which a cylinder of an incandescent vacuum bulb is immersed about one-third (gas-filled is not suitable). The light bulb filament glows DC. The carbon electrode a is attached to the positive end of the thread and is immersed in the molten saltpeter. It serves as an anode, relative to which all points of the light bulb filament have a lower potential. The saltpeter melt is partially dissociated into Na+ and NO ions. Under the influence of the potential difference between the anode and the filament of the bulb, Na+ ions move in the saltpeter in the direction from the anode to the bulb cylinder. Then they penetrate into the bulb through its walls. At these walls they are neutralized by electrons emitted by the hot filament and converted into neutral Na atoms. Evaporating, the latter are deposited on the cooler parts of the inner surface of the glass bulb of the light bulb, where a clearly visible mirror layer of sodium is formed.

1. 250V. 2. 55V. 3. 10V. 4. 45V.

Question 2.

What is the name given to the discharge that occurs in a gas tube when low pressures?

1. Arc. 2. Smoldering. 3. Spark. 4. Crown. 5. Plasma.

Question 3.

What is the process of emitting electrons from a heated metal cathode called?

1. Electrolysis. 2. Electrolytic dissociation.

3. Thermionic emission. 4. Impact ionization.

Question 4.

What is the induced emf in a 2 m long conductor moving in a magnetic field with

B = 10 T at a speed of 5 m/s along the magnetic induction lines.

1.0V. 2. 10 V. 3. 50 V. 4. 100 V.

Question 6.

Determine the inductance of the coil if, when an electric current of 5 A passes through it, a magnetic flux of 100 Wb appears near the coil.

1. 4 Gn. 2. 5 Gn. 3. 20 Gn. 4. 100 Gn.

Question 7.

What is the energy of the magnetic field of a coil with L = 200 mH when the current in it is equal to 5A?

1. 0.025 J. 2. 0.25 J. 3. 2.5 J. 4. 25 J.

Question 9.

When the frame rotates in a magnetic field, an emf appears at its ends, varying with time according to the law: e = 10 sin 8 t. What is the maximum value of the emf if all quantities in the equation are given in the SI system?

1. 4 V. 2. 5 V. 3. 8 V. 4. 10 V.

Question 10.

The effective value of the voltage in a section of the alternating current circuit is 100 V. What is approximately the amplitude value of the voltage in this section?

1. 100 V. 2. Approximately 142 V. 3. 200 V. 4. Approximately 284 V.

Question 11.

The oscillating circuit is connected to: an alternating current source. Under what condition does resonance occur in this oscillatory circuit?

1. If the frequency of the AC source is less than the natural frequency

2. If the frequency of the AC source is equal to the natural frequency

oscillatory circuit.

3. If the frequency of the AC source is greater than the natural frequency

oscillations of the oscillatory circuit.

Question 12.

What physical phenomenon is the operating principle of a transformer based on?

1. On the creation of a magnetic field by moving electric charges.

2. On the creation of an electric field by moving electric charges.

3. On the phenomenon of electromagnetic induction.

Question 13.

Where will the vortex electric field intensity lines be directed as the magnetic field increases?

Question 14.

The transmitting and receiving Hertz vibrators are located mutually perpendicular. Will vibrations occur in the receiving vibrator?

1.Yes, very strong. 2. Yes, but weak. 3. Will not arise.

Question 15.

What device in A. S. Popov’s receiver serves as a sensitive indicator of electromagnetic waves?

1. Antenna. 2. Kogerer. 3. Electromagnet.

4. Grounding. 5. Coil. 6. Battery.

Question 16.

Why do they try to make the air gap between the armature and the generator inductor as small as possible?

1. To reduce the size of the generator.

2. To increase magnetic field leakage.

3. To reduce magnetic field leakage.

Question 17.

Which of the following radiations has the lowest frequency?

1. Ultra-violet rays. 2. Infrared rays.

3. Visible light. 4. Radio waves.

Question 19.

The detector radio receiver receives signals from a radio station operating on the wave

30 m. What is the frequency of oscillations in the oscillating circuit of a radio receiver?

1.10^ -7 Hz. 2.10^7 Hz. 3. 9*10^9 Hz.

Question 20.

Which radio waves provide the most reliable radio communication if the transmitting radio station has sufficient power?

1. Long waves. 2. Medium waves. 3.Short waves. 4. Ultrashort waves.

Glass was placed on the edge of the funnel and lowered into the water as shown in the figure. The glass is held in place by water pressure. When water of mass m is poured into a funnel, then

The glass plate came off and fell to the bottom. Will the glass remain in place if shot or sand of the same mass is poured into the funnel?
1. Will remain
2. It will fall off,
3. The result depends on the water level in the vessel,
4. The result depends on the weight of the glass.

We said that glass does not conduct electricity. However, this statement cannot be understood unconditionally. Careful observation shows that electric charges can pass through glass, as well as through any other dielectric. However, under the same conditions, an incomparably smaller electric charge passes through bodies called dielectrics over the same period than through conductors of the same size and shape. When we say that a substance is a dielectric, it only means that for its given applications we can neglect the charges passing through it.

So, for example, through the amber plug of an electroscope, despite the fact that amber is the best known dielectric, a certain amount of electricity still passes. However, the charge passing through the plug during the experiment is always negligible compared to the total charge of the electroscope, and therefore amber is a suitable dielectric for the electroscope. This is not at all what would be observed in an electroscope with porcelain insulation. In this case, the charges flowing through the porcelain plug during the experiment would be comparable to the charge of the electroscope, and we would see that the leaves of the electroscope noticeably fall off. Porcelain is an insufficient dielectric for these purposes. However, the same porcelain turns out to be an excellent material for technical insulators, since the charge passing through such an insulator over a certain period of time is negligible compared to the huge charges flowing through wires during the same time. We see that the division into conductors and dielectrics is arbitrary. And it may even turn out that the same substance in some cases should be considered as a dielectric, and in other cases as a conductor.

Until relatively recently, electrical engineering used almost exclusively either metals, through which charge spreads extremely easily, or dielectrics with very high insulating properties - such as porcelain, glass, ebonite, amber, etc. Wires are made from metals, and dielectrics - supports that prevent charge leakage from the wires. The vast majority of natural substances do not belong, however, to either group; these substances are so-called semiconductors, that is, in their properties they occupy an intermediate position between very good conductors and very good dielectrics. They are therefore little suitable for the manufacture of wires and for insulating supports. However, in recent decades, a number of very special properties of semiconductors have been discovered and studied, which has opened up the possibility of extremely important and promising applications in various fields of science and technology. These properties of semiconductors will be discussed in more detail in Chapter. IX.

The insulating properties of a substance also depend on its state and can vary greatly. In Fig. 6 shows an experiment showing that glass completely loses its insulating properties when high temperature. Let's cut one of the wires going to the light bulb, and, having peeled off the insulation, screw the resulting ends to a glass rod. When the current is turned on, the light bulb does not light up, since at room temperature glass is a fairly good dielectric. If, however, the glass rod is heated strongly with a torch, the light bulb begins to glow; therefore, current passes through the heated glass rod. In this case, one more phenomenon can be observed. An electric current passing through a glass rod heats it up, and the stronger the current, the more significantly it heats it up. Therefore, if you take a light bulb powerful enough, i.e., such that a strong electricity, then this current will greatly heat up the wand. The burner can be removed, and the glass will remain hot and highly conductive; The heating of the glass increases all the time, and eventually the glass will melt.

When electricity appeared in our lives, few people knew about its properties and parameters, and they used various materials, it was noticeable that for the same voltage value of the current source, the consumer had a different voltage value. It was clear that this is influenced by the type of material used as a conductor. When scientists began to study this problem, they came to the conclusion that the charge carriers in the material are electrons. And the ability to conduct electric current is determined by the presence of free electrons in the material. It was found that some materials have these electrons a large number of, while others don’t have them at all. Thus, there are materials that and some do not have this ability.
Based on the above, all materials were divided into three groups:

• conductors;
• semiconductors;
• dielectrics;

Each of the groups has found wide application in electrical engineering.

Conductors

Guides are materials that conduct electric current well; they are used for the manufacture of wires, cable products, contact groups, windings, tires, conductive cores and tracks. Overwhelming majority electrical devices and devices are made on the basis of conductive materials. Moreover, I will say that the entire electric power industry could not exist without these substances. The group of conductors includes all metals, some liquids and gases.

It is also worth mentioning that among the conductors there are super conductors, the resistance of which is almost zero; such materials are very rare and expensive. And conductors with high resistance - tungsten, molybdenum, nichrome, etc. Such materials are used to make resistors, heating elements and spirals of lighting lamps.

But the lion's share in the electrical field belongs to ordinary conductors: copper, silver, aluminum, steel, and various alloys of these metals. These materials have found the widest and most extensive use in electrical engineering, especially copper and aluminum, since they are relatively cheap and their use as conductors of electric current is most appropriate. Even copper is limited in its use; it is used as winding wires, multi-core cables, and more critical devices, copper busbars are even less common. But aluminum is considered the king among electrical conductors, although it has a higher resistivity than copper, but this is compensated by its very low cost and resistance to corrosion. It is widely used in power supply, cable products, overhead lines, busbars, general wires, etc.

Semiconductors

Semiconductors, something between conductors and semiconductors. Their main feature is their dependence to conduct electric current from external conditions. The key condition is the presence of various impurities in the material, which provide the ability to conduct electric current. Also with a certain arrangement of two semiconductor materials. Based on these materials at the moment, many semiconductor devices have been produced: LEDs, transistors,semistors, thyristors, stabistors, various microcircuits. There is an entire science dedicated to semiconductors and devices based on them: electronic engineering. All computers, mobile devices. What can I say, almost all of our equipment contains semiconductor elements.

Semiconductor materials include: silicon, germanium, graphite, gr aphen, indium, etc.

Dielectrics

Well, the last group of materials is dielectrics , substances that are not capable of conducting electric current. Such materials include: wood, paper, air, oil, ceramics, glass, plastics, polyethylene, polyvinyl chloride, rubber, etc. Dielectrics are widely used due to their properties. They are used as an insulating material. They protect the contact of two live parts and prevent direct human contact with these parts. The role of dielectrics in electrical engineering is no less important than the role of conductors, since they provide stable, safe work all electrical and electronic devices. All dielectrics have a limit up to which they are unable to conduct electric current, this is called the breakdown voltage. This is an indicator at which the dielectric begins to pass electric current, while heat is released and the dielectric itself is destroyed. This breakdown voltage value is different for each dielectric material and is given in reference materials. The higher it is, the better, the more reliable the dielectric is considered.

The parameter characterizing the ability to conduct electric current is resistivity R , unit [ Ohm ] and conductivity, reciprocal of resistance. The higher this parameter, the worse the material conducts electric current. For conductors it ranges from several tenths to hundreds of Ohms. In dielectrics, the resistance reaches tens of millions of ohms.

All three types of materials are widely used in the power industry and electrical engineering. And they are also closely interconnected with each other.

Glass at normal conditions, i.e. in the solid state, is an insulator, and this feature is widely used. For example, metal contacts - inputs - in devices are soldered directly into glass. However, when molten, glass conducts electricity. As the temperature increases and the glass softens, its electrical resistance decreases, and it varies for different glasses. Glass with a small content of alkali metal ions, as well as glass containing low-mobility ions, have the highest electrical resistance.

Electrical resistivity in SP units is expressed in Ohm-m. The tables most often show the specific volume resistance of glass at temperatures of 100, 250 and 350 °C. In addition, the temperature at which the volumetric resistivity of the glass becomes equal to 100 MΩ-cm is given; Conventionally, this temperature is designated T To -- 100.

The ability of glasses to change electrical resistance when heated is used for soldering using high-frequency currents. This method is especially convenient for soldering and mounting large glass products. After heating the parts to be soldered with a gas torch until they soften, a high-frequency current is supplied and the parts are “welded.”

This property of glass must always be taken into account when manufacturing electrodes, installing electrical inputs, etc. If metal inputs are soldered into the glass, then they are the electrodes of the capacitor, where the glass is a dielectric. Dissipated on the capacitor plates due to dielectric losses Electric Energy turns into heat.

Often the voltage supplied to the contacts reaches tens of kilovolts, and therefore there is always a danger of overheating the glass between the contacts. In this case, the glass can become a conductor, a short circuit or partial electrolysis of the glass will occur. The silicates that make up glass undergo electrolysis when a potential difference is applied, as a result of which the homogeneity of the glass composition is disrupted and its properties deteriorate. In addition, when large currents are passed through the bushings along the soldered metal electrodes, bubbles appear, cracks form, and the vacuum tightness of the junction is disrupted. A sign by which the onset of electrolysis can be detected is a change in the color of the junction, and in lead glasses - the release of metallic lead on the surface of the electrodes.

Glass electrolysis intensifies With an increase in the potential difference across the inputs and with an increase in temperature.

In this case, the glass may soften as a result of overheating and, if the device operates at reduced pressure, the contact insertion site may be deformed under the influence of atmospheric pressure, even depressurization of the device is possible.

Taking into account all of the above, when installing the device, you should carefully select the required types of glass. The greater the dielectric losses, the greater the potential for overheating. Dielectric losses are directly proportional to the frequency of alternating current and the product of the dielectric loss tangent and the dielectric constant of the material. The last product is called the loss coefficient. For soldering electrodes, glass with the lowest loss coefficient should be selected, and for using glass as a dielectric, glass with the highest resistivity should be selected. Thus, lead, borosilicate, Pyrex, aluminosilicate and quartz glasses have the highest electrical resistance.

It is also very important to know the surface resistance of glass. This property is determined by the state of the glass surface - contamination and adsorbed water film. Glasses containing a large number of alkali metal ions easily absorb water vapor and carbon dioxide contained in the air. In this case, a “carbonate film” is formed on the surface of the glass, which is a conductor of electricity, as a result of which the surface resistance of the glass decreases. The surface electrical resistance of glass can also decrease as a result of contamination of the glass surface with particles of substances and dust.

Such surface-contaminated glass becomes a conductor of electricity rather than an insulator.