Conductivity
Superconductivity theory
When crystal lattices of solids are formed from atoms of various substances, valence electrons located in the outer orbits of the atoms interact with each other in different ways and, as a result, behave differently (see band
solid state superconductivity theory and theory
molecular orbitals). Thus, the freedom of valence electrons to move within a substance is determined by its molecular-crystalline structure. In general, according to their electrically conductive properties, all substances can (with some degree of convention) be divided into three categories, each of which has pronounced characteristics of the behavior of valence electrons under the influence of an external electric field.
Conductors
In some substances, valence electrons move freely between atoms. First of all, this category includes metals in which the electrons of the outer shells are literally “common property” of the atoms of the crystal lattice (see.
chemical bonds and electronic theory of conductivity).
If you apply an electrical voltage to such a substance (for example, connect the poles of a battery to its two ends), the electrons will begin to move unhindered in an orderly manner towards the south pole of the potential difference, thereby creating an electric current. Conducting substances of this kind are usually called conductors. The most common conductors in technology are, of course, metals, primarily copper and aluminum, which have minimal electrical resistance and are quite widespread in earthly nature. It is from them that high-voltage electrical cables and household electrical wiring are mainly made. There are other types of materials that have good electrical conductivity, such as salt, alkaline and acidic solutions, as well as plasma and some types of long organic molecules.
In this regard, it is important to remember that electrical conductivity can be caused by the presence in a substance not only of free electrons, but also of free positively and negatively charged ions of chemical compounds. In particular, even in ordinary tap water there are so many different salts dissolved, which decompose when dissolved into negatively charged cations and positively charged anions, that water (even fresh water) is a very good conductor, and this should not be forgotten when working with electrical equipment in conditions of high humidity - otherwise you can get a very noticeable electric shock.
Insulators
In many other substances (particularly glass, porcelain, plastics), electrons are tightly bound to atoms or molecules and
are not capable of free movement under the influence of externally applied electrical voltage. Such materials are called insulators.
Most often in modern technology, various plastics are used as electrical insulators. In fact, any plastic consists of polymer molecules - that is, very long chains of organic (hydrogen-carbon) compounds - which also form complex and very strong interweavings. The easiest way to imagine the structure of the polymer is in the form of a plate of long and thin noodles entangled and stuck together. In such materials, electrons are tightly bound to their ultra-long molecules and are not able to leave them under the influence of external voltage. Amorphous substances such as glass, porcelain or rubber, which do not have a rigid crystalline structure, also have good insulating properties. They are also often used as electrical insulators.
Both conductors and insulators play an important role in our technological civilization, which uses electricity as the main means of transmitting energy over distances. Electricity is carried through conductors from power plants to our homes and to various industrial enterprises, and insulators ensure our safety by protecting us from the harmful consequences of direct contact of the human body with high electrical voltage.
Semiconductors
Finally, there is a small category of chemical elements that occupy an intermediate position between metals and insulators (the most famous of them are silicon and germanium). In the crystal lattices of these substances, all valence electrons, at first glance, are connected by chemical bonds and, it would seem, there should be no free electrons left to ensure electrical conductivity. However, in reality the situation looks somewhat different, since some electrons are knocked out of their outer orbits as a result of thermal motion due to insufficient energy of their binding with atoms. As a result, at temperatures above absolute zero they still have a certain electrical conductivity under the influence of external voltage. Their conductivity coefficient is quite low (silicon conducts electric current millions of times worse than copper), but they still conduct some current, albeit insignificant. Such substances are called semiconductors.
As it turned out as a result of research, electrical conductivity in semiconductors, however, is due not only to the movement of free electrons (the so-called n-conductivity due to the directional movement of negatively charged particles). There is also a second mechanism of electrical conductivity - and a very unusual one. When an electron is released from the crystal lattice of a semiconductor due to thermal motion, a so-called hole is formed in its place - a positively charged cell of the crystal structure, which can at any moment be occupied by a negatively charged electron that has jumped into it from the outer orbit of a neighboring atom, where, in turn, , a new positively charged hole is formed. Such a process can continue for as long as desired, and from the outside (on a macroscopic scale) everything will look like the electric current under external voltage is caused not by the movement of electrons (which just jump from the outer orbit of one atom to the outer orbit of a neighboring atom), but by a directed migration of a positively charged hole (electron deficiency) towards the negative pole of the applied potential difference. As a result, a second type of conductivity is observed in semiconductors (the so-called hole or p-conductivity), which is, of course, also caused by the movement of negatively charged electrons, but from the point of view of the macroscopic properties of the substance, it appears to be a directed current of positively charged holes towards the negative pole.
The phenomenon of hole conduction is most easily illustrated using the example of a traffic jam. As the car stuck in it moves forward, a free space is formed in its place, which is immediately occupied by the next car, whose place is immediately occupied by a third car, etc. This process can be imagined in two ways: you can describe the rare movement of individual cars from among those standing in a long traffic jam; It is easier, however, to characterize the situation from the point of view of the episodic movement in the opposite direction of the few voids between the cars stuck in the traffic jam. It is guided by this analogy that physicists talk about hole conductivity, conventionally taking it for granted that electric current is conducted not due to the movement of numerous, but rarely moving negatively charged electrons, but due to the movement in the opposite direction of positively charged voids in the outer orbits of semiconductor atoms, which they agreed to call holes. Thus, the dualism of electron-hole conductivity is purely conditional, since from a physical point of view, the current in semiconductors is in any case determined exclusively by the directional movement of electrons.
Semiconductors have found wide practical application in modern radio electronics and computer technology precisely due to the fact that their conductive properties are easily and accurately controlled by changing external conditions.
electronic conductivity theory
The electrical conductivity of solids is due to the collective directed movement of free electrons
I.V.TRIGUBCHAK
Chemistry tutor
LESSON 6
10th grade(first year of study)
Continuation. For the beginning, see No. 22/2005; 1, 2, 3, 5/2006
Chemical bond. Structure of matter
Plan
1. Chemical bond:
covalent (non-polar, polar; single, double, triple);
ionic; metal; hydrogen; forces of intermolecular interaction.
2. Crystal lattices (molecular, ionic, atomic, metal).
Different substances have different structures. Of all substances known to date, only inert gases exist in the form of free (isolated) atoms, which is due to the high stability of their electronic structures. All other substances (and more than 10 million of them are currently known) consist of bonded atoms.
Chemical bonding is the forces of interaction between atoms or groups of atoms, leading to the formation of molecules, ions, free radicals, as well as ionic, atomic and metal crystal lattices. By its nature, a chemical bond is an electrostatic force. The main role in the formation of chemical bonds between atoms is played by them valence electrons, i.e. electrons of the outer level, least tightly bound to the nucleus. During the transition from the atomic state to the molecular state, energy is released associated with the filling of free orbitals of the outer electronic level with electrons to a certain stable state.
There are different types of chemical bonds.
A covalent bond is a chemical bond that occurs through the sharing of electron pairs. The theory of covalent bonds was proposed in 1916 by the American scientist Gilbert Lewis. Most molecules, molecular ions, free radicals and atomic crystal lattices are formed through covalent bonds. A covalent bond is characterized by length (the distance between atoms), direction (a certain spatial orientation of electron clouds during the formation of a chemical bond), saturation (the ability of atoms to form a certain number of covalent bonds), energy (the amount of energy that must be expended to break a chemical bond).
A covalent bond can be non-polar And polar. Non-polar covalent bond occurs between atoms with the same electronegativity (EO) (H 2, O 2, N 2, etc.). In this case, the center of the total electron density is at the same distance from the nuclei of both atoms. Based on the number of common electron pairs (i.e., multiplicity), single, double and triple covalent bonds are distinguished. If only one shared electron pair is formed between two atoms, then such a covalent bond is called a single bond. If two or three common electron pairs appear between two atoms, multiple bonds are formed - double and triple. A double bond consists of one -bond and one -bond. A triple bond consists of one -bond and two -bonds.
Covalent bonds, during the formation of which the area of overlapping electron clouds is located on the line connecting the nuclei of atoms, are called - connections. Covalent bonds, during the formation of which the area of overlapping electron clouds is located on both sides of the line connecting the nuclei of atoms, are called - connections.
Can participate in the formation of connections s- And s- electrons (H 2), s- And p-electrons (HCl), R- And
R-electrons (Cl 2). In addition, -bonds can be formed due to the overlap of “pure” and hybrid orbitals. Only R- And d-electrons.
The lines below show the chemical bonds in the molecules of hydrogen, oxygen and nitrogen:
where pairs of dots (:) are paired electrons; “crosses” (x) – unpaired electrons.
If a covalent bond is formed between atoms with different EO, then the center of the total electron density is shifted towards the atom with a higher EO. In this case there is covalent polar bond. A diatomic molecule connected by a covalent polar bond is a dipole - an electrically neutral system in which the centers of positive and negative charges are located at a certain distance from each other.
The graphical view of the chemical bonds in hydrogen chloride and water molecules is as follows:
where the arrows indicate the shift in the total electron density.
Polar and nonpolar covalent bonds are formed by an exchange mechanism. In addition, there are donor-acceptor covalent bonds. The mechanism of their formation is different. In this case, one atom (donor) provides a lone pair of electrons, which becomes the shared electron pair between itself and another atom (acceptor). When forming such a bond, the acceptor provides a free electron orbital.
The donor-acceptor mechanism of covalent bond formation is illustrated using the example of the formation of ammonium ion:
Thus, in the ammonium ion, all four bonds are covalent. Three of them are formed by the exchange mechanism, one by the donor-acceptor mechanism. All four connections are equivalent, which is due to sp 3 -hybridization of the orbitals of the nitrogen atom. The valency of nitrogen in the ammonium ion is IV, because it forms four bonds. Consequently, if an element forms bonds through both exchange and donor-acceptor mechanisms, then its valence is greater than the number of unpaired electrons and is determined by the total number of orbitals in the outer electronic layer. For nitrogen in particular, the highest valency is four.
Ionic bond – chemical bond between ions due to the forces of electrostatic attraction. An ionic bond is formed between atoms having a large EO difference (> 1.7); in other words, it is the bond between typical metals and typical nonmetals. The theory of ionic bonding was proposed in 1916 by the German scientist Walter Kossel. By giving up their electrons, metal atoms turn into positively charged ions - cations; non-metal atoms, accepting electrons, turn into negatively charged ions - anions. An electrostatic attraction occurs between the resulting ions, which is called ionic bonding. Ionic bonding is characterized by non-directionality and non-saturation; For ionic compounds, the concept of “molecule” does not make sense. In the crystal lattice of ionic compounds, around each ion there is a certain number of ions with opposite charges. The compounds NaCl and FeS are characterized by a cubic crystal lattice.
The formation of an ionic bond is illustrated below using sodium chloride as an example:
An ionic bond is an extreme case of a polar covalent bond. There is no sharp boundary between them; the type of bond between atoms is determined by the difference in electronegativity of the elements.
When simple substances - metals - are formed, atoms quite easily give up electrons from the outer electronic level. Thus, in metal crystals, some of their atoms are in an ionized state. At the nodes of the crystal lattice there are positively charged metal ions and atoms, and between them there are electrons that can move freely throughout the crystal lattice. These electrons become common to all atoms and ions of the metal and are called "electron gas". The bond between all positively charged metal ions and free electrons in the metal crystal lattice is called metal bond.
The presence of a metallic bond determines the physical properties of metals and alloys: hardness, electrical conductivity, thermal conductivity, malleability, ductility, metallic luster. Free electrons can carry heat and electricity, so they are the reason for the main physical properties that distinguish metals from non-metals - high electrical and thermal conductivity.
Hydrogen bond occurs between molecules that contain hydrogen and atoms with high EO (oxygen, fluorine, nitrogen). Covalent bonds H–O, H–F, H–N are highly polar, due to which an excess positive charge accumulates on the hydrogen atom, and an excess negative charge on the opposite poles. Between oppositely charged poles, forces of electrostatic attraction - hydrogen bonds - arise. Hydrogen bonds can be either intermolecular or intramolecular. The energy of a hydrogen bond is approximately ten times less than the energy of a conventional covalent bond, but nevertheless, hydrogen bonds play an important role in many physicochemical and biological processes. In particular, DNA molecules are double helices in which two chains of nucleotides are linked by hydrogen bonds.
Table
| Feature of the crystal lattice | Lattice type | |||
|---|---|---|---|---|
| Molecular | Ionic | Nuclear | Metal | |
| Particles at lattice nodes | Molecules | Cations and anions | Atoms | Metal cations and atoms |
| The nature of the connection between particles | Intermolecular interaction forces (including hydrogen bonds) | Ionic bonds | Covalent bonds | Metal connection |
| Bond strength | Weak | Durable | Very durable | Various strengths |
| Distinctive physical properties of substances | Low-melting or sublimating, low hardness, many soluble in water | Refractory, hard, many soluble in water. Solutions and melts conduct electric current | Very refractory, very hard, practically insoluble in water | High electrical and thermal conductivity, metallic luster |
| Examples of substances | Iodine, water, dry ice | Sodium chloride, potassium hydroxide, barium nitrate | Diamond, silicon, boron, germanium | Copper, potassium, zinc, iron |
Intermolecular hydrogen bonds between water and hydrogen fluoride molecules can be depicted (by dots) as follows:
Substances with hydrogen bonds have molecular crystal lattices. The presence of a hydrogen bond leads to the formation of molecular associates and, as a consequence, to an increase in the melting and boiling points.
In addition to the listed main types of chemical bonds, there are also universal forces of interaction between any molecules that do not lead to the breaking or formation of new chemical bonds. These interactions are called van der Waals forces. They determine the attraction of molecules of a given substance (or various substances) to each other in liquid and solid states of aggregation.
Different types of chemical bonds determine the existence of different types of crystal lattices (table).
Substances consisting of molecules have molecular structure. These substances include all gases, liquids, as well as solids with a molecular crystal lattice, such as iodine. Solids with an atomic, ionic or metal lattice have non-molecular structure, they have no molecules.
Test on the topic “Chemical bonding. Structure of matter"
1. How many electrons are involved in the formation of chemical bonds in an ammonia molecule?
a) 2; b) 6; at 8; d) 10.
2. Solids with an ionic crystal lattice are characterized by low:
a) melting point; b) binding energy;
c) solubility in water; d) volatility.
3. Arrange the substances below in order of increasing polarity of covalent bonds. In your answer, indicate the sequence of letters.
a) S 8; b) SO 2; c) H 2 S; d) SF 6.
4. What particles form a sodium nitrate crystal?
a) Na, N, O atoms; b) ions Na +, N 5+, O 2–;
c) NaNO 3 molecules; d) Na +, NO 3 – ions.
5. Indicate the substances that have atomic crystal lattices in the solid state:
a) diamond; b) chlorine;
c) silicon(IV) oxide; d) calcium oxide.
6. Indicate the molecule with the highest binding energy:
a) hydrogen fluoride; b) hydrogen chloride;
c) hydrogen bromide; d) hydrogen iodide.
7. Select pairs of substances in which all bonds are covalent:
a) NaCl, HCl; b) CO 2, NO;
c) CH 3 Cl, CH 3 K; d) SO 2, NO 2.
8. In which row are the molecules arranged in order of increasing bond polarity?
a) HBr, HCl, HF; b) NH 3, PH 3, AsH 3;
c) H 2 Se, H 2 S, H 2 O; d) CO 2, CS 2, CSe 2.
9. A substance whose molecules contain multiple bonds is:
a) carbon dioxide; b) chlorine;
c) water; d) ethanol.
10. Which physical property is not affected by the formation of intermolecular hydrogen bonds?
a) electrical conductivity;
b) density;
c) boiling point;
d) melting point.
Key to the test
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| b | G | a B C D | G | a, c | A | b, d | a, c | A | A |
Problems on gases and gas mixtures
Level A
1. Gaseous sulfur oxide at a temperature of 60 °C and a pressure of 90 kPa has a density of 2.08 g/l. Determine the formula of the oxide.
Answer. SO2.
2. Find the volume fractions of hydrogen and helium in a mixture whose relative density in air is 0.1.
Answer. 55% and 45%.
3. We burned 50 liters of a mixture of hydrogen sulfide and oxygen with a relative hydrogen density of 16.2. The resulting substance was passed through 25 ml of a 25% sodium hydroxide solution (the density of the solution is 1280 kg/m3). Determine the mass of the resulting acidic salt.
Answer. 20.8 g.
4. A mixture of sodium nitrate and calcium carbonate was thermally decomposed. The resulting gases (volume 11.2 l) in the mixture had a relative hydrogen density of 16.5. Determine the mass of the initial mixture.
Answer. '82
5. At what molar ratio of argon and nitrogen can a gas mixture with a density equal to that of air be obtained?
The initial mixture contains Ar and N 2 .
According to the conditions of the problem (mixture) = (air).
M(air) = M(mixtures) = 29 g/mol.
Using the usual ratio:
we get the following expression:
Let (mixture) = 1 mol. Then (Ar) = X mol, (N 2) = (1 – X) mole.
Answer. (Ar) : (N 2) = 1: 11.
6. The density of the gas mixture consisting of nitrogen and oxygen is 1.35 g/l. Find the volume fractions of gases in the mixture in %.
Answer. 44% and 56%.
7. The volume of the mixture containing hydrogen and chlorine is 50 ml. After the formation of hydrogen chloride, 10 ml of chlorine remains. Find the composition of the initial mixture in % by volume.
Answer. 40% and 60%.
Answer. 3%.
9. When adding which gas to a mixture of equal volumes of methane and carbon dioxide, its hydrogen density: a) will increase; b) will decrease? Give two examples in each case.
Answer.
M(mixtures of CH 4 and CO 2) = 30 g/mol; a) Cl 2 and O 2; b) N 2 and H 2.
10.
There is a mixture of ammonia and oxygen. When adding which gas to this mixture, its density is:
a) will increase; b) will decrease? Give two examples in each case.
Answer.
17 < M r(mixtures of NH 3 + O 2)< 32; а) Cl 2
и C 4 H 10 ; б) H 2 и Нe.
11. What is the mass of 1 liter of a mixture of carbon dioxide and carbon dioxide if the content of the first gas is 35% by volume?
Answer. 1.7 g.
12. 1 liter of a mixture of carbon dioxide and carbon dioxide at no. has a mass of 1.43 g. Determine the composition of the mixture in % by volume.
Answer. 74.8% and 25.2%.
Level B
1. Determine the relative density of air by nitrogen if all the oxygen contained in the air is converted into ozone (assume that the air contains only nitrogen and oxygen).
Answer. 1,03.
2. When a very common gas A is introduced into a glass vessel containing gas B, which has the same density as gas A, only wet sand remains in the vessel. Identify gases. Write equations for laboratory methods for obtaining them.
Answer. A – O 2, B – SiH 4.
2NaNO 3 2NaNO 2 + O 2,
Mg 2 Si + 4H 2 O = 2Mg(OH) 2 + SiH 4.
3. In a gas mixture consisting of sulfur dioxide and oxygen, with a relative density for hydrogen of 24, part of the sulfur dioxide reacted, and a gas mixture was formed with a relative density for hydrogen 25% greater than the relative density of the original mixture. Calculate the composition of the equilibrium mixture in % by volume.
Answer. 50% SO 3, 12.5% SO 2, 37.5% O 2.
4. The density of ozonized oxygen according to ozone is 0.75. How many liters of ozonated oxygen will be required to burn 20 liters of methane (n.o.)?
Answer. 35.5 l.
5. There are two vessels filled with mixtures of gases: a) hydrogen and chlorine; b) hydrogen and oxygen. Will the pressure in the vessels change when an electric spark is passed through these mixtures?
Answer. a) Will not change; b) will decrease.
(CaSO 3) = 1 mol,
Then y= (Ca(HCO 3) 2) = 5 mol.
The resulting gas mixture contains SO 2 and CO 2.
Answer. D air (mixtures) = 1.58.
7. The volume of the mixture of carbon monoxide and oxygen is 200 ml (n.s.). After all the carbon monoxide has been burned and brought to normal conditions. the volume of the mixture decreased to 150 ml. How many times will the volume of the gas mixture decrease after passing it through 50 g of a 2% potassium hydroxide solution?
Answer. 3 times.
When crystal lattices of solids are formed from atoms of various substances, valence electrons located in the outer orbits of atoms interact with each other in different ways and, as a result, behave differently ( cm. Band theory of conductivity of solids and Theory of molecular orbitals). Thus, the freedom of valence electrons to move within a substance is determined by its molecular-crystalline structure. In general, according to their electrically conductive properties, all substances can (with some degree of convention) be divided into three categories, each of which has pronounced characteristics of the behavior of valence electrons under the influence of an external electric field.
Conductors
In some substances, valence electrons move freely between atoms. First of all, this category includes metals in which the electrons of the outer shells are literally in the “common property” of the atoms of the crystal lattice ( cm. Chemical bonds and Electronic theory of conductivity). If you apply electrical voltage to such a substance (for example, connect the poles of a battery to its two ends), the electrons will begin an unhindered, orderly movement in the direction of the south pole potential difference, thereby creating an electric current. Conductive substances of this kind are usually called conductors. The most common conductors in technology are, of course, metals, primarily copper and aluminum, which have minimal electrical resistance and are quite widespread in earthly nature. It is from them that high-voltage electrical cables and household electrical wiring are mainly made. There are other types of materials that have good electrical conductivity, such as salt, alkaline and acidic solutions, as well as plasma and some types of long organic molecules.
In this regard, it is important to remember that electrical conductivity can be caused by the presence in a substance not only of free electrons, but also of free positively and negatively charged ions of chemical compounds. In particular, even in ordinary tap water there are so many different salts dissolved that, when dissolved, decompose into negatively charged cations and positively charged anions that water (even fresh water) is a very good conductor, and this should not be forgotten when working with electrical equipment in conditions of high humidity - otherwise you can get a very noticeable electric shock.
Insulators
In many other substances (in particular, glass, porcelain, plastics), electrons are tightly bound to atoms or molecules and are not capable of free movement under the influence of externally applied electrical voltage. Such materials are called insulators.
Most often in modern technology, various plastics are used as electrical insulators. In fact, any plastic consists of polymer molecules- that is, very long chains of organic (hydrogen-carbon) compounds - which, moreover, form complex and very strong mutual interweavings. The easiest way to imagine the polymer structure is in the form of a plate of long, thin noodles tangled and stuck together. In such materials, electrons are tightly bound to their ultra-long molecules and are not able to leave them under the influence of external voltage. They also have good insulating properties. amorphous substances such as glass, porcelain or rubber that do not have a rigid crystalline structure. They are also often used as electrical insulators.
Both conductors and insulators play an important role in our technological civilization, which uses electricity as the main means of transmitting energy over a distance. Electricity is carried through conductors from power plants to our homes and to various industrial enterprises, and insulators ensure our safety by protecting us from the harmful consequences of direct contact of the human body with high electrical voltage.
Semiconductors
Finally, there is a small category of chemical elements that occupy an intermediate position between metals and insulators (the most famous of them are silicon and germanium). In the crystal lattices of these substances, all valence electrons, at first glance, are connected by chemical bonds, and it would seem that there should be no free electrons left to ensure electrical conductivity. However, in reality the situation looks somewhat different, since some electrons are knocked out of their outer orbits as a result of thermal motion due to insufficient energy of their binding with atoms. As a result, at temperatures above absolute zero they still have a certain electrical conductivity under the influence of external voltage. Their conductivity coefficient is quite low (silicon conducts electric current millions of times worse than copper), but they still conduct some current, albeit insignificant. Such substances are called semiconductors.
As it turned out as a result of research, electrical conductivity in semiconductors, however, is due not only to the movement of free electrons (the so-called n-conductivity due to the directed movement of negatively charged particles). There is also a second mechanism of electrical conductivity - and a very unusual one. When an electron is released from the crystal lattice of a semiconductor due to thermal movement, a so-called hole- a positively charged cell of a crystal structure, which can at any moment be occupied by a negatively charged electron that has jumped into it from the outer orbit of a neighboring atom, where, in turn, a new positively charged hole is formed. Such a process can continue for as long as desired - and from the outside (on a macroscopic scale) everything will look like the electric current under external voltage is caused not by the movement of electrons (which just jump from the outer orbit of one atom to the outer orbit of a neighboring atom), but by a directed migration of a positively charged hole (electron deficiency) towards the negative pole of the applied potential difference. As a result, a second type of conductivity is observed in semiconductors (the so-called hole or p-conductivity), caused, of course, also by the movement of negatively charged electrons, but, from the point of view of the macroscopic properties of matter, appears to be a directed current of positively charged holes towards the negative pole.
The phenomenon of hole conduction is most easily illustrated using the example of a traffic jam. As the car stuck in it moves forward, a free space is formed in its place, which is immediately occupied by the next car, whose place is immediately occupied by a third car, etc. This process can be imagined in two ways: one can describe the rare advance of individual cars from the number of people stuck in a long traffic jam; It is easier, however, to characterize the situation from the point of view of episodic progress in the opposite direction of a few voids between cars stuck in a traffic jam. It is guided by this analogy that physicists talk about hole conductivity, conventionally taking it for granted that electric current is conducted not due to the movement of numerous, but rarely moving negatively charged electrons, but due to the movement in the opposite direction of positively charged voids in the outer orbits of semiconductor atoms, which they agreed to call “holes.” Thus, the dualism of electron-hole conductivity is purely conditional, since from a physical point of view, the current in semiconductors, in any case, is determined exclusively by the directional movement of electrons.
Semiconductors have found wide practical application in modern radio electronics and computer technology precisely due to the fact that their conductive properties are easily and accurately controlled by changing external conditions.
All substances, according to their ability to conduct electric current, are conventionally divided into conductors and dielectrics. Semiconductors occupy an intermediate position between them. Conductors are understood as substances in which there are free charge carriers that can move under the influence of an electric field. Conductors are metals, solutions or molten salts, acids and alkalis. Metals, due to their unique properties of electrical conductivity, are widely used in electrical engineering. Copper and aluminum wires are mainly used to transmit electricity, and in exceptional cases, silver. Since 2001. Electrical wiring is supposed to be done only with copper wires. Aluminum wires are still used because of their low cost, as well as in cases where their use is completely justified and does not pose a danger. Aluminum wires are approved for powering stationary consumers with a known in advance guaranteed power, for example, pumps , air conditioners, fans, household sockets with a load of up to 1 kW, as well as for external electrical wiring (overhead lines, underground cables, etc.). Only copper-based wires are allowed in homes. Metals in the solid state have a crystalline structure. Particles in crystals are arranged in a certain order, forming a spatial (crystalline) lattice. Positive ions are located at the nodes of the crystal lattice, and free electrons move in the space between them, which are not associated with the nuclei of their atoms. Flow of free electrons are called electron gas. Under normal conditions, the metal is electrically neutral, because. the total negative charge of all free electrons is equal in absolute value to the positive charge of all lattice ions. The carriers of free charges in metals are electrons. Their concentration is quite high. These electrons participate in random thermal movement. Under the influence of an electric field, free electrons begin ordered movement along the conductor. the fact that electrons in metals serve as carriers of electric current was proven by simple experiment by the German physicist Karl Ricke back in 1899. He took three cylinders of the same radius: copper, aluminum and copper, placed them one after another, pressed them with their ends and included them in a tram line , and then passed an electric current through them for more than a year. After that, he examined the contact points of the metal cylinders and did not find aluminum atoms in copper, but no copper atoms in aluminum, i.e. there was no diffusion. From this he concluded that when an electric current passes through a conductor, the ions remain motionless, and only free electrons move, which are the same for all substances and are not associated with differences in their physicochemical properties. So, electric current in metal conductors is the ordered movement of free electrons under the influence of an electric field. The speed of this movement is small - a few millimeters per second, and sometimes even less. But as soon as an electric field arises in the conductor, it moves at enormous speed. close to the speed of light in a vacuum (300,000 fps), spreads along the entire length of the conductor. Simultaneously with the propagation of the electric field, all electrons begin to move in one direction along the entire length of the conductor. So, for example, when the circuit of an electric lamp is closed, they begin to move in an orderly manner and electrons present in the lamp coil. When they talk about the speed of propagation of electric current in a conductor, they mean the speed of propagation of the electric field along the conductor. An electric signal sent, for example, along wires from Moscow to Vladivostok (a distance of approximately 8000 km), arrives there in approximately 0.03 s. Dielectrics or insulators are substances in which there are no free charge carriers, and therefore they do not conduct electric current. Such substances are classified as ideal dielectrics. For example, glass, porcelain, earthenware and marble are good insulators in a cold state. Crystals of these materials have ionic structure, i.e. consist of positively and negatively charged ions. Their electrical charges are bound in a crystal lattice and are not free, which makes these materials dielectrics. In real conditions, dielectrics conduct electric current, not very weakly. To ensure their conductivity, a very high voltage must be applied. The conductivity of dielectrics is less than that of conductors. This is due to the fact that under normal conditions, the charges in dielectrics are bound into stable molecules and they do not state, as in conductors, it is easy to break off and become free. The electric current passing through dielectrics is proportional to the electric field strength. At a certain critical value of the electric field strength, electrical breakdown occurs. The value is called the dielectric strength of the dielectric and is measured in V/cm. Many dielectrics due to Their high electrical strength is used mainly as electrical insulating materials. Semiconductors do not conduct electric current at low voltages, but when the voltage increases, they become electrically conductive. Unlike conductors (metals), their conductivity increases with increasing temperature. This is especially noticeable, for example, in transistor radios, which do not work well in hot weather. Semiconductors are characterized by a strong dependence of electrical conductivity on external influences. Semiconductors are widely used in various electrical devices, since their electrical conductivity can be controlled.