What is the central conductor of a tree? Dielectric properties of wood. piezoelectric properties of wood

A dielectric is a material or substance that practically does not transmit electricity. This conductivity is due to the small number of electrons and ions. These particles are formed in a non-conducting material only when high temperature properties are achieved. What a dielectric is will be discussed in this article.

Description

Each electronic or radio conductor, semiconductor or charged dielectric passes 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. Electric current in a dielectric is the movement of charged particles in a certain direction (can be positive or negative).

Types of currents

The electrical conductivity of dielectrics is based on:

• Absorption currents are a current that flows in a dielectric at a constant current until it reaches a state of equilibrium, changing direction when turned on and voltage is applied to it and when turned off. At alternating current tension in the dielectric will be present in it the entire time it is in action electric field.
• Electronic conductivity is the movement of electrons under the influence of a field.
• Ionic conductivity is the movement of ions. Found in solutions of electrolytes - salts, acids, alkalis, as well as in many dielectrics.
• Molion electrical conductivity is the movement of charged particles called molions. Found in colloidal systems, emulsions and suspensions. The phenomenon of the movement of molions in an electric field is called electrophoresis.

Classified by state of aggregation and chemical nature. The former are divided into solid, liquid, gaseous and solidifying. Based on their chemical nature, they are divided into organic, inorganic and organoelement materials.

According to the state of aggregation:

• Electrical conductivity of gases. Gaseous substances have a fairly 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 radiation, collisions of molecules and charged particles, thermal factors.
• Electrical conductivity of a liquid dielectric. Dependence factors: molecular structure, temperature, impurities, 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 in polar substances is also created using a liquid with dissociated ions. When comparing polar and non-polar liquids, the former have a clear advantage in conductivity. If you clean a liquid of impurities, this will help reduce its conductive properties. With an increase in conductivity and its temperature, a decrease in its viscosity occurs, leading to an increase in ion mobility.
• Solid dielectrics. Their electrical conductivity is determined by the movement of charged dielectric particles and impurities. In strong fields of electric current, 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 classified as conductors. If more than 108 Ohm*m - to dielectrics. There may be cases when the resistivity will be several times greater than the resistance of the conductor. In the range of 10-5-108 Ohm*m there is a semiconductor. Metal material is an excellent conductor of electric current.

Of the entire periodic table, only 25 elements are classified as non-metals, and 12 of them may have semiconductor properties. But, of course, in addition to the substances in 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 definite line between the values ​​of various substances and 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, because it is one of the most popular classes of electrical components. It has become quite clear that they can be used due to their properties in active and passive form.

In their passive form, the properties of dielectrics are used for use in electrical insulating materials.

IN active form They are used in ferroelectrics, as well as in materials for laser emitters.

Basic dielectrics

Commonly encountered 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 field of electric current. Liquid non-conducting substances are used in technology for pouring or impregnating materials. There are 3 classes of liquid dielectrics:

Petroleum oils are slightly viscous and mostly non-polar. They are often used in high-voltage equipment: 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 purer structure than capacitor oil. This type of dielectric is widely used in production, despite the high cost compared to analogue 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. And the liquid dielectric, which is based on organic silicon, is safe and environmentally friendly. This type does not cause metal rust and has low hygroscopic properties. There is a liquefied dielectric containing an organofluorine compound, which is especially popular due to its non-flammability, thermal properties and oxidative stability.

And the last type is vegetable oils. They are weakly polar dielectrics, these include flax, castor, tung, and hemp. Castor oil is highly hot and is used in paper capacitors. The remaining oils are evaporable. Evaporation in them is not due to natural evaporation, but chemical reaction called polymerization. Actively used in enamels and paints.

Conclusion

The article discussed in detail what a dielectric is. Were mentioned different kinds and their properties. Of course, in order to understand the subtlety of their characteristics, you will have to study the physics section about them in more depth.

Is wood a conductor or a dielectric? 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.
A living thing, although a bad one, is a conductor, and an ionic one at that (juices are an electrolyte)

Answer from really[guru]
depending on how old the tree is

Answer from Alexei[expert]
Dry-dielectric.

Answer from Yoadovnik[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 electric current, i.e. it 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 wood 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.
It would be more correct to ask the resistance of the tree.
Lightning is more likely to strike trees with deep roots. Why?
Trees with roots penetrating deep aquifers of the soil are better connected to the ground and therefore, under the influence of electrified clouds, significant charges of electricity flowing from the ground, having a sign opposite to the sign of the charge of the clouds, accumulate on them.
Thanks to its roots deeply buried in the soil, the oak is well grounded, so it is more often struck by lightning.
The electric current passes mainly between the bark and wood of the pine tree, that is, in those places where the most tree sap is concentrated, which conducts electricity well.
The trunk of a resinous tree, such as pine, has much greater resistance than the bark and subcortical layer. Therefore, in pine, the electric current of lightning passes mainly through the outer layers without penetrating inside. If lightning strikes a deciduous tree, then current flows inside it. The wood of these trees contains a lot of sap, which boils under the influence of electric current. The resulting vapors tear the tree apart.
The wooden support provides a significant insulation distance from the point of view of surge voltages (lightning resistance), can extinguish the power arc of the ceiling and provides high resistance to the ground fault circuit. These properties are used to reduce the number of lightning outages of overhead lines and ensure safety.
The impulse strength of the wooden support body 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 an overvoltage on an overhead line with an amplitude of hundreds of kilovolts. The presence of wooden supports eliminates the possibility of overlapping insulation and disconnecting the line in such cases.
The high resistance of wooden supports ensures increased safety of 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, and dry pine is on average 100 times more.
The high resistance of wood and the high contact resistance when a person touches a support with damaged insulation limits the current through a person to values ​​that are not life-threatening (40–100 mA).

In electricity, there are three main groups of materials: conductors, semiconductors and dielectrics. Their main difference is the ability to conduct current. In this article we will look at how these types of materials differ and how they behave in an electric field.

What is a conductor

A substance in which free charge carriers are present is called a conductor. The movement of free carriers is called thermal. The main characteristic of a conductor is its resistance (R) or conductivity (G) - the reciprocal of resistance.

Speaking in simple words– a conductor conducts current.

Such substances include metals, but if we talk about non-metals, then, for example, carbon is an excellent conductor and has found application in sliding contacts, for example, electric motor brushes. Wet soil, solutions of salts and acids in water, and the human body also conduct current, but their electrical conductivity is often less than that of copper or aluminum, for example.

Metals are excellent conductors due to a large number free charge carriers in their structure. Under the influence of an electric field, charges begin to move and also redistribute, and the phenomenon of electrostatic induction is observed.

What is a dielectric

Dielectrics are substances that do not conduct current, or conduct, but very poorly. There are no free charge carriers in them, because the bond of atomic particles is strong enough to form free charge carriers, therefore, under the influence of an electric field, no current arises in the dielectric.

Gas, glass, ceramics, porcelain, some resins, textolite, carbolite, distilled water, dry wood, rubber are dielectrics and do not conduct electric current. In everyday life, dielectrics are found everywhere, for example, they are used to make housings for electrical appliances, electrical switches, housings for plugs, sockets, etc. In power lines, insulators are made of dielectrics.

However, if certain factors are present, e.g. increased level humidity, electric field strength above the permissible value, etc., lead to the fact that the material begins to lose its dielectric functions and becomes a conductor. Sometimes you may hear phrases like “insulator breakdown” - this is the phenomenon described above.

In short, the main properties of a dielectric in the field of electricity are electrical insulation. It is the ability to prevent the flow of current that protects a person from electrical injuries and other troubles. The main characteristic of a dielectric is its electrical strength - a value equal to its breakdown voltage.

What is a semiconductor

A semiconductor conducts electric current, but not like metals, but subject to certain conditions - imparting energy to the substance in the required quantities. This is due to the fact that there are too few free charge carriers (holes and electrons) or none at all, but if you apply a certain amount of energy, they will appear. Energy can be various forms– electrical, thermal. Also, free holes and electrons in a semiconductor can appear under the influence of radiation, for example in the UV spectrum.

Where are semiconductors used? Transistors, thyristors, diodes, microcircuits, LEDs, etc. are made from them. Such materials include silicon, germanium, mixtures of different materials, such as gallium arsenide, selenium, and arsenic.

To understand why a semiconductor conducts electricity but not like metals, we need to consider these materials from the point of view of band theory.

Zone theory

Band theory describes the presence or absence of free charge carriers relative to certain energy layers. An energy level or layer is the amount of energy of electrons (atomic nuclei, molecules - simple particles), they are measured in Electron Volts (EV).

The image below shows three types of materials with their energy levels:

Note that for a conductor, the energy levels from the valence band to the conduction band are combined into a continuous diagram. The conduction band and valence band overlap each other, this is called the overlap zone. Depending on the presence of an electric field (voltage), temperature and other factors, the number of electrons may change. Thanks to the above, electrons can move in conductors, even if they are given some minimal amount of energy.

A semiconductor has a certain band gap between the valence band and the conduction band. The band gap describes how much energy must be supplied to a semiconductor for current to flow.

In a dielectric, the diagram is similar to the one that describes semiconductors, but the only difference is the band gap - it is many times larger here. The differences are due internal structure and substances.

We looked at the main three types of materials and gave their examples and features. Their main difference is their ability to conduct current. Therefore, each of them has found its own field of application: conductors are used to transmit electricity, dielectrics are used to insulate live parts, semiconductors are used for electronics. We hope the information provided has helped you understand what conductors, semiconductors and dielectrics are in an electric field, as well as how they differ from each other.

The ability to conduct electric current characterizes the electrical resistance of wood. In general, the total resistance of a wood sample placed between two electrodes is determined as the result of two resistances: volumetric and surface. Volume resistance numerically characterizes the obstacle to the passage of current through the thickness of the sample, and surface resistance determines the obstacle to the passage of current along the surface of the sample. Indicators of electrical resistance are volumetric and surface resistivity. The first of these indicators has the dimension ohm per centimeter (ohm x cm) and is numerically equal to the resistance when current passes through two opposite faces of a cube measuring 1X1X1 cm made of a given material (wood). The second indicator is measured in ohms and is numerically equal to the resistance of a square of any size on the surface of a wood sample when current is supplied to the electrodes delimiting two opposite sides of this square. Electrical conductivity depends on the type of wood and the direction of current movement. As an illustration of the order of magnitude of volumetric and surface resistance in Table. Some data is given.

comparative data on the specific volumetric and surface resistance of wood

To characterize electrical conductivity highest value has a specific volume resistivity. Resistance is highly dependent on the moisture content of the wood. As the moisture content of wood increases, the resistance decreases. A particularly sharp decrease in resistance is observed with an increase in the content of bound moisture from an absolutely dry state to the hygroscopic limit. In this case, the volumetric resistivity decreases by millions of times. A further increase in humidity causes a drop in resistance only tenfold. This is illustrated by the data in Table.

specific volumetric resistivity of wood in a completely dry state

the effect of humidity on the electrical resistance of wood

The surface resistance of wood also decreases significantly with increasing humidity. An increase in temperature leads to a decrease in the volumetric resistance of wood. Thus, the resistance of false sugi wood decreases by 2.5 times when the temperature rises from 22-23° to 44-45° C (about half), and the resistance of beech wood decreases by 3 times when the temperature rises from 20-21° to 50° C. At negative temperatures, the volumetric resistance of wood increases. The specific volumetric resistivity along the fibers of birch samples with a humidity of 76% at a temperature of 0°C was 1.2 x 10 7 ohm cm, and when cooled to a temperature of -24° C it turned out to be equal to 1.02 x 10 8 ohm cm. Impregnation of wood with mineral antiseptics (for example, zinc chloride) reduces the resistivity, while impregnation with creosote has little effect on the electrical conductivity. The electrical conductivity of wood is practical significance when it is used for communication poles, masts of high-voltage transmission lines, handles of power tools, etc. In addition, the design of electric moisture meters is based on the dependence of electrical conductivity on wood moisture content.

electrical strength of wood

Electrical strength is important when assessing wood as an electrically insulating material and is characterized by the breakdown voltage in volts per 1 cm of material thickness. The electrical strength of wood is low and depends on the species, humidity, temperature and direction. As humidity and temperature increase, it decreases; It is significantly lower along the fibers than across it. Data on the electrical strength of wood along and across the fibers are given in table.

electrical strength of wood along and across the grain

With a moisture content of pine wood of 10%, the following electrical strength in kilovolts per 1 cm of thickness was obtained: along the fibers 16.8; in the radial direction 59.1; in the tangential direction 77.3 (determination was made on samples with a thickness of 3 mm). As you can see, the electrical strength of wood along the grain is approximately 3.5 times less than across the grain; in the radial direction the strength is less than in the tangential direction, since the core rays reduce the breakdown voltage. An increase in humidity from 8 to 15% (halved) reduces the electrical strength across the fibers by about 3 times (on average for beech, birch and alder).

Electrical strength (in kilovolts per 1 cm of thickness) of other materials is as follows: mica 1500, glass 300, bakelite 200, paraffin 150, transformer oil 100, porcelain 100. In order to increase the electrical strength of wood and reduce electrical conductivity when used in the electrical industry as an insulator it is impregnated with drying oil, transformer oil, paraffin, artificial resins; the effectiveness of such impregnation is evident from the following data on birch wood: impregnation with drying oil increases the breakdown voltage along the fibers by 30%, with transformer oil - by 80%, with paraffin - almost twice as much as the breakdown voltage for air-dry, unimpregnated wood.

dielectric properties of wood

The value showing how many times the capacity of the capacitor increases if the air gap between the plates is replaced with a gasket of the same thickness made of a given material is called the dielectric constant of this material. The dielectric constant (dielectric constant) for some materials is given in table.

dielectric constant of some materials

Data for wood show a marked difference between the dielectric constant along and across the grain; in the same time the dielectric constant across the fibers in the radial and tangential directions differs little. Dielectric constant in the field high frequency depends on the frequency of the current and the moisture content of the wood. With increasing current frequency, the dielectric constant of beech wood along the fibers at a humidity of 0 to 12% decreases, which is especially noticeable for a humidity of 12%. With increasing moisture content of beech wood, the dielectric constant along the fibers increases, which is especially noticeable at lower current frequencies.

In a high frequency field, wood heats up; The reason for heating is Joule heat losses inside the dielectric, occurring under the influence of an alternating electromagnetic field. This heating consumes part of the supplied energy, the value of which is characterized by the loss tangent.

The loss tangent depends on the direction of the field in relation to the fibers: along the fibers it is approximately twice as large as across the fibers. Across the fibers in the radial and tangential directions, the loss tangent varies little. The dielectric loss tangent, like the dielectric constant, depends on the frequency of the current and the moisture content of the wood. Thus, for absolutely dry beech wood, the loss tangent along the fibers first increases with increasing frequency, reaches a maximum at a frequency of 10 7 Hz, after which it begins to decrease again. At the same time, at a humidity of 12%, the loss tangent drops sharply with increasing frequency, reaches a minimum at a frequency of 10 5 Hz, and then increases just as sharply.

maximum value of the loss tangent for dry wood

With an increase in the moisture content of beech wood, the loss tangent along the fibers increases sharply at low (3 x 10 2 Hz) and high (10 9 Hz) frequencies and remains almost unchanged at a frequency of 10 6 -10 7 Hz.

Through a comparative study of the dielectric properties of pine wood and cellulose, lignin and resin obtained from it, it was found that these properties are determined mainly by cellulose. Heating of wood in a field of high-frequency currents is used in the processes of drying, impregnation and gluing.

piezoelectric properties of wood

On the surface of some dielectrics under the influence of mechanical stresses appear electric charges. This phenomenon, associated with the polarization of the dielectric, is called the direct piezoelectric effect. Piezoelectric properties were first discovered in crystals of quartz, tourmaline, Rochelle salt, etc. These materials also have the opposite piezoelectric effect, which consists in the fact that their sizes change under the influence of an electric field. Plates made from these crystals are widely used as emitters and receivers in ultrasonic technology.

These phenomena are found not only in single crystals, but also in a number of other anisotropic solid materials called piezoelectric textures. Piezoelectric properties have also been discovered in wood. It was found that the main carrier of piezoelectric properties in wood is its oriented component - cellulose. The intensity of wood polarization is proportional to the magnitude of mechanical stresses from applied external forces; the proportionality coefficient is called the piezoelectric modulus. Quantitative study of the piezoelectric effect, thus, comes down to determining the values ​​of piezoelectric moduli. Due to the anisotropy of the mechanical and piezoelectric properties of wood, these indicators depend on the direction of mechanical forces and the polarization vector.

The greatest piezoelectric effect is observed under compressive and tensile loads at an angle of 45° to the fibers. Mechanical stresses directed strictly along or across the fibers do not cause a piezoelectric effect in wood. In table The values ​​of piezoelectric moduli for some rocks are given. The maximum piezoelectric effect is observed in dry wood; with increasing humidity it decreases and then completely disappears. Thus, already at a humidity of 6-8% the magnitude of the piezoelectric effect is very small. With an increase in temperature to 100° C, the value of the piezoelectric modulus increases. With low elastic deformation (high elastic modulus) of wood, the piezoelectric modulus decreases. The piezoelectric modulus also depends on a number of other factors; however greatest influence its value is influenced by the orientation of the cellulose component of the wood.

piezoelectric wood modules

The discovery allows for a deeper study of the fine structure of wood. Indicators of the piezoelectric effect can serve as quantitative characteristics of cellulose orientation and are therefore very important for studying the anisotropy of natural wood and new wood materials with properties specified in certain directions.

All materials existing in nature differ in their electrical properties. Thus, from the entire variety of physical substances, dielectric materials and conductors of electric current are separated into separate groups.

What are conductors?

A conductor is a material whose peculiarity is the presence of freely moving charged particles that are distributed throughout the substance.

Substances that conduct electric current are molten metals and the metals themselves, undistilled water, salt solution, moist soil, and the human body.

Metal is the best conductor of electric current. Also among non-metals there are good conductors, for example carbon.

All conductors of electric current that exist in nature are characterized by two properties:

• resistance indicator;
• electrical conductivity indicator.

Electrical conductivity is the characteristic (ability) of a physical substance to conduct current. Therefore, the properties of a reliable conductor are low resistance to the flow of moving electrons and, consequently, high electrical conductivity. That is, the best conductor is characterized by a high conductivity index.

For example, cable products: copper cable has greater electrical conductivity compared to aluminum.

What are dielectrics?

Dielectrics are physical substances, in which there are no electrical charges at low temperatures. The composition of such substances includes only atoms of a neutral charge and molecules. The charges of a neutral atom have a close connection with each other, and therefore are deprived of the possibility of free movement throughout the substance.

The best dielectric is gas. Other non-conducting materials include glass, porcelain, ceramic products, as well as rubber, cardboard, dry wood, resins and plastics.

Dielectric objects are insulators whose properties mainly depend on the state of the surrounding atmosphere. For example, at high humidity, some dielectric materials partially lose their properties.

Conductors and dielectrics are widely used in electrical engineering to solve various problems.

For example, all cable and wire products are made of metals, usually copper or aluminum. The sheath of wires and cables is polymer, as are the plugs of all electrical appliances. Polymers are excellent dielectrics that do not allow charged particles to pass through.

Silver, gold and platinum products are very good conductors. But their negative characteristic, which limits their use, is their very high cost.

Therefore, such substances are used in areas where quality is much more important than the price paid for it (defense industry and space).

Copper and aluminum products are also good conductors and are not so expensive. Consequently, the use of copper and aluminum wires is widespread.

Tungsten and molybdenum conductors have less good properties, so they are mainly used in incandescent light bulbs and high-temperature heating elements. Poor electrical conductivity can significantly impair the operation of an electrical circuit.

Dielectrics also differ in their characteristics and properties. For example, some dielectric materials also contain free electrical charges, albeit in small quantities. Free charges arise due to thermal vibrations of electrons, i.e. An increase in temperature still, in some cases, provokes the separation of electrons from the nucleus, which reduces the insulating properties of the material. Some insulators are characterized by a large number of “stripped” electrons, which indicates poor insulating properties.

The best dielectric is complete vacuum, which is very difficult to achieve on planet Earth.

Fully purified water also has high dielectric properties, but this does not even exist in reality. It is worth remembering that the presence of any impurities in the liquid gives it the properties of a conductor.

The main criterion for the quality of any dielectric material is the degree of compliance with the functions assigned to it in a specific electrical circuit. For example, if the properties of the dielectric are such that the current leakage is very insignificant and does not cause any damage to the operation of the circuit, then the dielectric is reliable.

What is a semiconductor?

Semiconductors occupy an intermediate place between dielectrics and conductors. The main difference between conductors is the dependence of the degree of electrical conductivity on temperature and the amount of impurities in the composition. Moreover, the material has the characteristics of both a dielectric and a conductor.

With increasing temperature, the electrical conductivity of semiconductors increases, and the degree of resistance decreases. As the temperature drops, the resistance tends to infinity. That is, when the temperature reaches zero, semiconductors begin to behave like insulators.

Semiconductors are silicon and germanium.