Wood (wood) is an insulator: its electrical conductivity is very low at room temperature, especially if the wood is dry. When heated, the wood chars. Charcoal (graphite with a partially disordered lattice) is a conductor of electric current: far from the best, but a conductor. Our experiment is based on the described principle. We take a 220 V light bulb, turn on two electrodes (nails, steel wire, etc.) in series with it, which are located in parallel at a distance of about 1-2 cm. Plug it all into a socket. The lamp, of course, does not burn, since the circuit is open: the electrodes are separated by a centimeter of air. Place a few matches on top of the electrodes. The matches will connect the electrodes, but the wood is an insulator, so the lamp will not burn. We direct the flame of a gas burner at the lamp. The wood will catch fire and char, the coal will connect the two electrodes, and since coal is a conductor, the circuit will close and the lamp will light up. The gas burner will light the lamp.
It sounds simple, but in practice it's a little more complicated. Several nuances.
1. The tree must be completely charred.
The process of wood charring differs, for example, from the decomposition of calcium carbonate (into calcium oxide and carbon dioxide) by the fact that wood thermolysis goes through many stages. We are not satisfied with the products of the intermediate stages: the carbonization of the wood must be complete. A sign of this: the tree stops burning - the flame disappears, the tree only smolders (i.e. volatile combustible thermolysis products are no longer formed).
2. Matches may bend in the flame during heating, resulting in loss of contact with the electrodes. Sometimes further heating helps: the matches bend until they touch the electrode again. (It is possible that the heating process itself is also important to improve contact). It is necessary not to overdo it and not burn the coal completely.
In the process of charring, matches often fall, therefore, before the experiment, they must be placed on the electrodes so that neither end outweighs the other (loops on the electrodes are useful - see below).
3. In some cases, a charred match can be corrected and pressed against the electrodes with an ordinary match - to restore contact. It is advisable to make electrodes with "loops" at the ends, and insert matches into the loops: this improves contact.
4. During the experiment, the electrodes are covered with scale and soot. Between experiments, it is desirable to clean them off to improve contact (apparently, this is not necessary).
5. During the experiment, the bare electrodes are energized at 220 V. The experimenter has to manipulate these electrodes many times: place matches on them, correct charred matches, demonstrate with a multimeter that the electrodes are energized, etc. Not every experience goes well, so routine procedures need to be done again and again. As a result, it is easy to forget that the electrodes are live and accidentally touch them.
In the course of the experiments, I touched the energized electrodes twice. Once - with sweaty hands, standing with bare feet on linoleum. The palm twitched, I dropped the pliers and uttered a couple of "cultural" words. The second time I didn't feel anything at all. - Got off easy.
But if a person simultaneously touches bare wires and grounded objects (water pipe, central heating battery, etc.), the result can be fatal. It is especially bad if the hands are wet, because. electrical resistance The human body is concentrated mainly in the skin.
So, there is a 220 V lamp in the circuit, two electrodes are connected in series with it. The role of electrodes in various experiments was played by nails, large paper clips and steel wire. The electrodes are arranged in parallel and at the same level (so that matches or pieces of wood can be placed on top of them). To prove that the circuit is energized, I connect the electrodes with a screwdriver. The lamp lights up brightly. I remove the screwdriver - the lamp goes out.
I put several matches on the electrodes so that they connect them. The lamp does not burn because wood is an insulator. I direct the flame of the burner at the matches, charring them evenly along the entire length. When red coals remain from the matches, the circuit closes, the lamp lights up. At the point of contact of the match with the electrodes, a bluish electric arc often flashes, the match itself in some places remains red-hot. This is accompanied by a characteristic crackle. After a few seconds or tens of seconds, the match burns out, the contact is lost, the lamp goes out. But often the contact is restored in new places, the arc flashes again, sparks and crackles appear. The lamp lights up again: sometimes brightly and almost evenly, sometimes dimly and with flashes (depending on how good the contact is). If necessary, charred matches are corrected and pressed against the electrodes with an unburned match. If this does not work, they direct the flame of the burner to the charred matches.
If desired, 3-4 matches or 1-2 can be used in the experiment.
A dielectric is a material or substance that practically does not transmit electricity. Such conductivity is obtained due to a small number of electrons and ions. These particles are formed in a non-conductive material only when high temperature properties are achieved. About what a dielectric is and will be discussed in this article.
Description
Each electronic or radio conductor, semiconductor or charged dielectric passes an electric current through itself, but the peculiarity of the dielectric is that even at high voltages above 550 V, a small current will flow in it. An electric current in a dielectric is the movement of charged particles in a certain direction (it can be positive or negative).
Types of currents
The electrical conductivity of dielectrics is based on:
- Absorption currents - a current that flows in a dielectric at a constant current until it reaches an equilibrium state, changing direction when it is turned on and when voltage is applied to it and when it is turned off. With alternating current, the tension in the dielectric will be present in it all the time while it is in operation. electric field.
- Electronic electrical conductivity - the movement of electrons under the influence of a field.
- Ionic electrical conductivity - is the movement of ions. It is found in electrolyte solutions - salts, acids, alkalis, as well as in many dielectrics.
- Molionic electrical conductivity is the movement of charged particles called molions. It is found in colloidal systems, emulsions and suspensions. The phenomenon of the movement of molions in an electric field is called electrophoresis.
Classify by state of aggregation and chemical nature. The first are divided into solid, liquid, gaseous and solidifying. By chemical nature, they are divided into organic, inorganic and organoelement materials.
By state of aggregation:
- Electrical conductivity of gases. Gaseous substances have a rather low current conductivity. It can occur in the presence of free charged particles, which appears due to the influence of external and internal, electronic and ionic factors: X-ray and radioactive species, collision of molecules and charged particles, thermal factors.
- Electrical conductivity of a liquid dielectric. Dependence factors: molecular structure, temperature, impurities, the presence of large charges of electrons and ions. The electrical conductivity of liquid dielectrics largely depends on the presence of moisture and impurities. The conductivity of electricity of polar substances is created even with the help of a liquid with dissociated ions. When comparing polar and non-polar liquids, the former have a clear advantage in conductivity. If the liquid is cleaned of impurities, this will contribute to a decrease in its conductive properties. With an increase in conductivity and its temperature, a decrease in its viscosity occurs, leading to an increase in the mobility of ions.
- solid dielectrics. Their electrical conductivity is determined as the movement of charged dielectric particles and impurities. In strong electric current fields, electrical conductivity is revealed.
Physical properties of dielectrics
When the specific resistance of the material is less than 10-5 Ohm * m, they can be attributed to conductors. If more than 108 Ohm * m - to dielectrics. There are cases when the resistivity will be many times greater than the resistance of the conductor. In the interval 10-5-108 Ohm*m there is a semiconductor. Metallic material is an excellent conductor of electric current.
Of the entire periodic table, only 25 elements belong to non-metals, and 12 of them, possibly, will have semiconductor properties. But, of course, in addition to the substances of the table, there are many more alloys, compositions or chemical compounds with the property of a conductor, semiconductor or dielectric. Based on this, it is difficult to draw a certain line between the values of various substances with their resistances. For example, at a reduced temperature factor, a semiconductor will behave like a dielectric.
Application
The use of non-conductive materials is very extensive, as it is one of the most commonly used classes of electrical components. It became quite clear that they can be used due to their properties in an active and passive form.
In a passive form, the properties of dielectrics are used for use in electrical insulating material.
In active form, they are used in ferroelectrics, as well as in materials for emitters of laser technology.
Basic dielectrics
Common types include:
- Glass.
- Rubber.
- Oil.
- Asphalt.
- Porcelain.
- Quartz.
- Air.
- Diamond.
- Pure water.
- Plastic.
What is a liquid dielectric?
Polarization of this type occurs in the electric current field. Liquid non-conductive substances are used in engineering for pouring or impregnating materials. There are 3 classes of liquid dielectrics:
Petroleum oils are low viscosity and mostly non-polar. They are often used in high-voltage instruments: high-voltage water. is a non-polar dielectric. Cable oil has found application in the impregnation of insulating paper wires with a voltage of up to 40 kV, as well as metal-based coatings with a current of more than 120 kV. Transformer oil has a cleaner structure than capacitor oil. This type of dielectric is widely used in production, despite the high cost compared to analog substances and materials.
What is a synthetic dielectric? Currently, it is banned almost everywhere due to its high toxicity, as it is produced on the basis of chlorinated carbon. A liquid dielectric based on organic silicon is safe and environmentally friendly. This type does not cause metal rust and has the properties of low hygroscopicity. There is a fluidized dielectric containing an organofluorine compound that is particularly popular for its non-combustibility, thermal properties, and oxidative stability.
And the last type is vegetable oils. They are weakly polar dielectrics, these include flaxseed, castor, tung, hemp. Castor oil is highly heated and is used in paper capacitors. The rest of the oils are evaporated. Evaporation in them is not caused by natural evaporation, but by a chemical reaction called polymerization. It is actively used in enamels and paints.
Conclusion
The article discussed in detail what a dielectric is. Various species and their properties have been mentioned. Of course, in order to understand the subtlety of their characteristics, you will have to study the section of physics about them in more depth.
Electrical conductivity. The ability of wood to conduct electricity is inversely related to its electrical resistance.
The impedance of a wood sample placed between two electrodes is defined as the resultant of two resistances: volume and surface. Highest value to characterize the electrical conductivity of the material has the first type of resistance, an indicator of which is volume resistivity which has the dimension of Ohm cm and is numerically equal to the resistance when current passes through two opposite faces of a cube with dimensions of 1x1x1 cm of the published material (wood).
Wood belongs to dielectrics (10 8 -10 17 ohm cm). For it, methods for measuring the resistance of solid dielectrics at constant voltages are applicable. Taking into account the specifics of wood, these methods were used by TsNIIMOD in the development of GOST 18408-73.
Different breeds have different electrical conductivity, but in all breeds it is several times greater along the fibers than across the fibers.
As the moisture content of the wood increases, the resistance decreases. A particularly sharp decrease in resistance (by tens of millions of times) is observed with an increase in the content of bound water, i.e., during the transition from an absolutely dry state of wood to the saturation limit of cell walls Wbp. . A further increase in humidity causes a drop in resistance only by tens or hundreds of times. This explains the decrease in the accuracy of moisture determination by electric moisture meters in the region above W b.p. .
An increase in the temperature of wood leads to a decrease in its volumetric resistance. On average, it is considered that an increase in the temperature of wood for every 12 ° C causes a decrease in resistance by about half.
The electrical conductivity of wood is taken into account when wood is used for communication poles, masts of high-voltage transmission lines, power tool handles, etc.
Electrical strength. This is the name of the ability of wood to resist breakdown, i.e., a decrease in resistance at high voltages. To determine the electrical strength of wood at an alternating voltage with a frequency of 50 Hz, TsNIIMOD developed GOST 18407-73. An indicator of electrical strength is E pr - the ratio of breakdown voltage to the thickness of the material, kV / mm.
The electrical strength of absolutely dry wood along the fibers is 1.3-1.5 kV / mm, which is 4-7 times less than across the fibers. With increasing humidity, the electrical strength decreases markedly. According to BelTI, the strength decreases by 2 times when the humidity changes from 10 to 14%. The electrical strength of wood compared to other solid insulating materials is low (for glass E pr \u003d 30, for polyethylene - 40 kV / mm). To increase the electrical strength, wood is impregnated with paraffin, drying oil, artificial resins and other substances.
Dielectric properties. Wood in an alternating electric field exhibits its dielectric properties, which are characterized by two indicators. The first of them - the relative permittivity ε - is numerically equal to the ratio of the capacitance of a capacitor with a wood gasket to the capacitance of a capacitor with an air gap between the electrodes. The second indicator - the tangent of the dielectric loss angle tan δ - determines the proportion of the input power that is absorbed by the wood and converted into heat.
The dielectric constant absolutely dry wood with increasing density increases. So, for balsa wood (ρ 0 \u003d 130 kg / m 3) the dielectric constant across the fibers in the frequency range 10-10 11 Hz is on average 1.3, and for hornbeam (ρ 0 \u003d 800 kg / m 3) - 2, 6. The permeability along the fibers is 1.4 times higher on average. With an increase in wood moisture, e increases, since for water the value of this indicator in the frequency range 10-10 11 Hz is 81-7.5. According to G. I. Torgovnikov, at a humidity of 10% and a temperature of 20 ° C for wood with a density of ρ 0 \u003d 500 kg / m 3 at a frequency of 10 4 Hz it is 4.2, at a frequency of 10 10 Hz - 2.0, and at humidity 60% - respectively equal to 65 and 6.6. An increase in temperature from -40 to 100 ° C for absolutely dry wood leads to a slight increase (about 1.3 times). Increasing the temperature of wet wood leads to a more significant increase.
Loss tangent also depends on the density of the wood. Across the fibers, tg δ at a density of ρ 0 = 500 kg / m 3 and room temperature in the frequency range of 10-10 5 Hz is 0.005-0.007, and at a density of ρ 0 = 800 kg / m 3 this figure is 0.007-0.025. Along the fibers tg δ is higher than across the fibers, on average 1.7 times. With increasing humidity, tg δ increases. The frequency dependences of this indicator are complex. So, for wood with a density of ρ 0 = 500 kg / m 3 at a temperature of 20 ° C and a humidity of 80%, the value of tg δ at a frequency of 10 3 Hz reaches 74, at a frequency of 10 8 Hz it decreases to 0.2, and in the region of microwave frequencies (10 10 Hz) increases to 0.34. An increase in the temperature of absolutely dry wood causes a decrease in tg δ, but this indicator increases in the microwave region. For wet wood (W=25%), heating leads to a significant increase in tg δ, but in the microwave region it changes insignificantly.
With dielectric heating, the temperature rises simultaneously throughout the entire volume of wood. This method of heating is practical use in the processes of drying, gluing and impregnation of wood. Heating in the microwave field can be used for drying wood, for surface thawing of logs before debarking and sawing.
Piezoelectric Properties. On the surface of anisotropic plates of crystals (quartz, tourmaline, Rochelle salt), when stretched or compressed, electric charges appear: positive on one side and negative on the other. Electric charges arise under the action of mechanical forces, pressure, therefore this phenomenon is called direct piezoelectric effect(the word "piezo" means pressure). These materials also have an inverse piezoelectric effect - their dimensions change under the influence of an electric field. Plates made of these crystals are widely used as emitters and receivers in ultrasonic technology.
Studies by V. A. Bazhenov showed that wood containing an oriented component, cellulose, also has such properties. The greatest piezoelectric effect is observed when a compressive and tensile load is applied at an angle of 45° to the fibers. Loads directed strictly along or across the fibers do not cause this effect. The piezoelectric effect is especially noticeable in dry wood, with increasing humidity it decreases and already at a moisture content of 6-8% it almost completely disappears. As the temperature rises to 100°C, the effect increases. The higher the modulus of elasticity of wood, the less its piezoelectric effect.
This phenomenon allows a deeper study of the fine structure of wood, to characterize the degree of anisotropy of natural wood and new wood materials. It is used in the development of non-destructive methods for quality control of wood.
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When electricity appeared in our life, few people knew about its properties and parameters, and various materials were used as conductors, it was noticeable that with the same voltage value of the current source, the consumer had a different voltage value. It was clear that this was influenced by the type of material used as a conductor. When scientists took up the issue of studying this problem, they came to the conclusion that electrons are charge carriers in the material. And the ability to conduct electric current is isolated by the presence of free electrons in the material. It was found that in some materials 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. The vast majority of electrical devices and apparatus is 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 lamp coils.
But the lion's share in the electrical field belongs to ordinary conductors: copper, silver, aluminum, steel, various alloys of these metals. These materials have found the widest and greatest application 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 rarer. But aluminum is considered the king among the conductors of electric current, even if it has a higher resistivity than copper, but this is offset by its very low cost and corrosion resistance. 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 just 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 a whole science devoted 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 aphene, indium, etc.
Dielectrics
Well, the last group of materials is dielectrics Substances that are not capable of conducting electricity. 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 current-carrying parts, do not allow a person to directly touch these parts. The role of a dielectric in electrical engineering is no less important than the role of conductors, as they ensure the stable, safe operation of all electrical and electronic devices. All dielectrics have a limit to which they are not able to conduct electric current, it is called breakdown voltage. This is an indicator at which the dielectric begins to pass an electric current, while heat is released and the dielectric itself is destroyed. This value of the breakdown voltage for each dielectric material is different and is given in reference materials. The higher it is, the better, the dielectric is considered more reliable.
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 is from a few 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 electric power industry and electrical engineering. They are also closely related to each other.
Is wood a conductor or an insulator? and got the best answer
Answer from Lena Malikova[active]
dielectric. but only dry.
Answer from 2 answers[guru]
Hello! Here is a selection of topics with answers to your question: is wood a conductor or a dielectric??
Answer from Andrey Ryzhov[guru]
dielectric
Answer from www[newbie]
dielectric
Answer from white rabbit[guru]
Dry - dielectric.
Living - albeit bad, but a conductor, moreover - ionic (juices - electrolyte)
Answer from yyyyyyyyyyyyyyyyyyyyyyyyyyyying[guru]
how old is the tree
Answer from Alexei[expert]
Dry dielectric.
Answer from Eadovnik[guru]
The electrical conductivity of wood mainly depends on its moisture content, species, grain direction and temperature. Wood in a dry state does not conduct electricity, i.e., is a dielectric, which allows it to be used as an insulating material.
For example, paper impregnated with something is used in capacitors and transformers.
I myself often insert a fuse using a notebook sheet.
But a tree is never dry.
I still remember how I was shocked when I took a dry screwdriver with a wooden handle and reached into the switch.
And it is more correct to ask the resistance of the tree.
Lightning is more likely to strike trees with roots that penetrate deep into the soil. Why?
Trees with roots penetrating into deep aquifers of the soil are better connected to the earth and therefore, under the influence of electrified clouds, significant charges of electricity flowing from the earth accumulate on them, having a sign opposite to that of the cloud charge.
Due to its roots deep into the soil, the oak is well grounded, so it is more likely to be struck by lightning.
Electric current passes mainly between the bark and wood of the pine, that is, in those places where the most tree sap is concentrated, which conducts electricity well.
The trunk of a resinous tree, such as a pine tree, has a much greater resistance than the bark and subcortex. Therefore, in pine, the electric current of lightning passes mainly through the outer layers, without penetrating inside. If lightning strikes a deciduous tree, then the current flows inside it. The wood of these trees contains a lot of juice, which boils under the influence of an electric current. The resulting pairs break the tree.
A wooden pole provides a significant insulating distance in terms of surge voltages (lightning resistance), can extinguish a power arc of the ceiling and provides a high resistance to the earth fault circuit. These properties are used to reduce the number of lightning outages of overhead lines and ensure safety.
The impulse strength of the body of a wooden support is more than 200 kV/m. This property is extremely useful in areas with high thunderstorm activity. A lightning strike, even at a considerable distance from the line, can induce overvoltages on overhead lines with an amplitude of hundreds of kilovolts. The presence of wooden poles excludes overlapping insulation and disconnecting the line in such cases.
The high resistance of wooden poles ensures increased safety of the lines for people in the event of damage to the main insulation. The resistance of the support body is highly dependent on moisture. For example, the minimum resistance of wet pine is about 20 kOhm/m, while dry pine is on average 100 times greater.
The high wood resistance and high contact resistance when a person touches a support with damaged insulation limit the current through a person to non-life threatening values (40–100 mA).