Presentation on the topic "conductors and dielectrics." Presentation on the topic "conductors and dielectrics" A dielectric weakens an external electric field

Conductors in an electric field Free charges - charged particles of the same sign, capable of moving under the influence of an electric field Bound charges - opposite charges included in the composition of atoms (or molecules) that cannot move under the influence of an electric field independently of each other substances conductors dielectrics semiconductors

Any medium weakens the electric field strength

The electrical characteristics of a medium are determined by the mobility of charged particles in it

Conductor: metals, solutions of salts, acids, moist air, plasma, human body

This is a body that contains a sufficient amount of free electrical charges inside that can move under the influence of an electric field.

If you introduce an uncharged conductor into an electric field, the charge carriers begin to move. They are distributed so that the electric field they create is opposite to the external field, that is, the field inside the conductor will be weakened. The charges will be redistributed until the conditions for equilibrium of charges on the conductor are met, that is:

a neutral conductor introduced into an electric field breaks the tension lines. They end at negative induced charges and begin at positive

The phenomenon of spatial separation of charges is called electrostatic induction. The self-field of the induced charges compensates for the external field inside the conductor with a high degree of accuracy.

If the conductor has an internal cavity, then the field will be absent inside the cavity. This circumstance is used when organizing the protection of equipment from electric fields.

The electrification of a conductor in an external electrostatic field by the separation of positive and negative charges already present in it in equal quantities is called the phenomenon of electrostatic induction, and the redistributed charges themselves are called induced. This phenomenon can be used to electrify uncharged conductors.

An uncharged conductor can be electrified by contact with another charged conductor.

The distribution of charges on the surface of conductors depends on their shape. The maximum charge density is observed at the points, and inside the recesses it is reduced to a minimum.

The property of electric charges to concentrate in the surface layer of a conductor has found application for obtaining significant potential differences by electrostatic method. In Fig. a diagram of an electrostatic generator used to accelerate elementary particles is shown.

A spherical conductor 1 of large diameter is located on an insulating column 2. A closed dielectric tape 3 moves inside the column, driving drums 4. From a high-voltage generator, an eclectic charge is transmitted through a system of pointed conductors 5 to the tape, on the back side of the tape there is grounding plate 6. Charges from the tape are removed by a system of points 7 and flow onto the conducting sphere. The maximum charge that can accumulate on a sphere is determined by leakage from the surface of the spherical conductor. In practice, with generators of a similar design, with a sphere diameter of 10–15 m, it is possible to obtain a potential difference of the order of 3–5 million volts. To increase the charge of the sphere, the entire structure is sometimes placed in a box filled with compressed gas, which reduces the intensity of ionization.

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On the surface of the sphere, cones cut out small spherical areas that can be considered flat. A r1r1 r2r2 S1S1 S2S2, or The cones are similar to each other, since the angles at the vertex are equal. From the similarity it follows that the areas of the bases are related as the squares of the distances from point A to the sites and, respectively. Thus,

Equipotential surfaces An approximate course of equipotential surfaces for a certain moment of cardiac excitation is shown in the figure. In an electric field, the surface of a conducting body of any shape is an equipotential surface. The dotted lines indicate equipotential surfaces, the numbers next to them indicate the potential value in millivolts.

Dielectric constant of substances Substance ε ε Gases and water vapor Nitrogen Hydrogen Air Vacuum Water vapor (at t=100 ºС) Helium Oxygen Carbon dioxide Liquids Liquid nitrogen (at t= –198.4 ºС) Gasoline Water Liquid hydrogen (at t= –252, 9 ºС) Liquid helium (at t= –269 ºC) Glycerin 1.0058 1.006 1.4 1.9–2.0 81 1.2 1.05 43 Liquid oxygen (at t= –192.4 ºС) Transformer oil Alcohol Ether Solids Diamond Waxed paper Dry wood Ice (at t= –10 ºС) Paraffin Rubber Mica Glass Titanium barium Porcelain Amber 1.5 2.2 26 4.3 5.7 2.2 2.2–3.7 70 1.9–2.2 3.0–6.0 5.7–7.2 6.0–10.4–6.8 2.8

Literature O. F. Kabardin “Physics. Reference materials". O. F. Kabardin “Physics. Reference materials". A. A. Pinsky “Physics. A textbook for 10th grade schools and classes with in-depth study of physics." A. A. Pinsky “Physics. A textbook for 10th grade schools and classes with in-depth study of physics." G. Ya. Myakishev “Physics. Electrodynamics classes". G. Ya. Myakishev “Physics. Electrodynamics classes". Magazine "Kvant". Magazine "Kvant".

Slide 2

Conductors and dielectrics in an electric field Charged particles that can move freely in an electric field are called free charges, and substances containing them are called conductors. Conductors are metals, liquid solutions and molten electrolytes. Free charges in a metal are the electrons of the outer shells of atoms that have lost contact with them. These electrons, called free electrons, can move freely through the metal body in any direction. Under electrostatic conditions, i.e., when electric charges are stationary, the electric field strength inside the conductor is always zero. Indeed, if we assume that there is still a field inside the conductor, then the free charges located in it will be acted upon by electric forces proportional to the field strength, and these charges will begin to move, which means the field will cease to be electrostatic. Thus, there is no electrostatic field inside the conductor.

Slide 3

Substances that have no free charges are called dielectrics or insulators. Examples of dielectrics include various gases, some liquids (water, gasoline, alcohol, etc.), as well as many solids (glass, porcelain, plexiglass, rubber, etc.). There are two types of dielectrics - polar and non-polar. In a polar dielectric molecule, positive charges are located predominantly in one part (the “+” pole), and negative charges are located in the other (the “-” pole). In a non-polar dielectric, positive and negative charges are equally distributed throughout the molecule. Electric dipole moment is a vector physical quantity that characterizes the electrical properties of a system of charged particles (charge distribution) in the sense of the field it creates and the action of external fields on it. The simplest system of charges that has a certain (independent of the choice of origin) non-zero dipole moment is a dipole (two point particles with opposite charges of the same size)

Slide 4

The absolute value of the electric dipole moment of a dipole is equal to the product of the magnitude of the positive charge and the distance between the charges and is directed from the negative charge to the positive, or: where q is the magnitude of the charges, l is a vector with the beginning in the negative charge and the end in the positive. For a system of N particles, the electric dipole moment is: The system units for measuring the electric dipole moment do not have a special name. In SI it is simply Kl·m. The electric dipole moment of molecules is usually measured in debyes: 1 D = 3.33564·10−30 C m.

Slide 5

Dielectric polarization. When a dielectric is introduced into an external electric field, a certain redistribution of the charges that make up the atoms or molecules occurs in it. As a result of such redistribution, excess uncompensated bound charges appear on the surface of the dielectric sample. All charged particles that form macroscopic bound charges are still part of their atoms. Bound charges create an electric field, which inside the dielectric is directed opposite to the vector of the external field strength. This process is called dielectric polarization. As a result, the total electric field inside the dielectric turns out to be less than the external field in absolute value. A physical quantity equal to the ratio of the modulus of the external electric field strength in a vacuum E0 to the modulus of the total field strength in a homogeneous dielectric E is called the dielectric constant of the substance:

Slide 6

There are several mechanisms for the polarization of dielectrics. The main ones are orientation and deformation polarization. Orientational or dipole polarization occurs in the case of polar dielectrics consisting of molecules in which the centers of distribution of positive and negative charges do not coincide. Such molecules are microscopic electric dipoles - a neutral combination of two charges, equal in magnitude and opposite in sign, located at some distance from each other. For example, a water molecule, as well as molecules of a number of other dielectrics (H2S, NO2, etc.) have a dipole moment. In the absence of an external electric field, the axes of molecular dipoles are randomly oriented due to thermal motion, so that on the surface of the dielectric and in any volume element the electric charge is on average zero. When a dielectric is introduced into an external field, a partial orientation of molecular dipoles occurs. As a result, uncompensated macroscopic bound charges appear on the surface of the dielectric, creating a field directed towards the external field

Slide 7

The polarization of polar dielectrics strongly depends on temperature, since the thermal movement of molecules plays the role of a disorienting factor. The figure shows that in an external field, oppositely directed forces act on the opposite poles of a polar dielectric molecule, which try to rotate the molecule along the field strength vector.

Slide 8

The deformation (or elastic) mechanism manifests itself during the polarization of nonpolar dielectrics, the molecules of which do not possess a dipole moment in the absence of an external field. During electronic polarization under the influence of an electric field, the electronic shells of non-polar dielectrics are deformed - positive charges are displaced in the direction of the vector and negative charges in the opposite direction. As a result, each molecule turns into an electric dipole, the axis of which is directed along the external field. Uncompensated bound charges appear on the surface of the dielectric, creating their own field directed towards the external field. This is how the polarization of a non-polar dielectric occurs. An example of a non-polar molecule is the methane molecule CH4. In this molecule, the quadruple ionized carbon ion C4– is located in the center of a regular pyramid, at the vertices of which there are hydrogen ions H+. When an external field is applied, the carbon ion is displaced from the center of the pyramid, and the molecule develops a dipole moment proportional to the external field.

Slide 9

In the case of solid crystalline dielectrics, a type of deformation polarization is observed - the so-called ionic polarization, in which ions of different signs that make up the crystal lattice, when an external field is applied, are displaced in opposite directions, as a result of which bound (uncompensated) charges appear on the crystal faces. An example of such a mechanism is the polarization of a NaCl crystal, in which the Na+ and Cl– ions form two sublattices nested inside each other. In the absence of an external field, each unit cell of a NaCl crystal is electrically neutral and does not have a dipole moment. In an external electric field, both sublattices are displaced in opposite directions, i.e., the crystal is polarized.

Slide 10

The figure shows that an external field acts on a molecule of a non-polar dielectric, moving opposite charges inside it in different directions, as a result of which this molecule becomes similar to a molecule of a polar dielectric, oriented along the field lines. The deformation of non-polar molecules under the influence of an external electric field does not depend on their thermal motion, therefore the polarization of a non-polar dielectric does not depend on temperature.

Slide 11

Fundamentals of the band theory of solids Band theory is one of the main sections of the quantum theory of solids, describing the movement of electrons in crystals, and is the basis of the modern theory of metals, semiconductors and dielectrics. The energy spectrum of electrons in a solid differs significantly from the energy spectrum of free electrons (which is continuous) or the spectrum of electrons belonging to individual isolated atoms (discrete with a specific set of available levels) - it consists of individual allowed energy bands separated by bands of forbidden energies. According to Bohr's quantum mechanical postulates, in an isolated atom the energy of an electron can take strictly discrete values ​​(the electron has a certain energy and is located in one of the orbitals).

Slide 12

In the case of a system of several atoms united by a chemical bond, the electronic energy levels are split in an amount proportional to the number of atoms. The measure of splitting is determined by the interaction of the electron shells of atoms. With a further increase in the system to the macroscopic level, the number of levels becomes very large, and the difference in the energies of electrons located in neighboring orbitals is correspondingly very small - the energy levels are split into two almost continuous discrete sets - energy zones.

Slide 13

The highest of the allowed energy bands in semiconductors and dielectrics, in which at a temperature of 0 K all energy states are occupied by electrons, is called the valence band, the next one is the conduction band. Based on the principle of the relative arrangement of these zones, all solids are divided into three large groups: conductors - materials in which the conduction band and valence band overlap (there is no energy gap), forming one zone called the conduction band (thus, the electron can move freely between them, having received any permissibly low energy); dielectrics - materials in which the zones do not overlap and the distance between them is more than 3 eV (in order to transfer an electron from the valence band to the conduction band, significant energy is required, so dielectrics practically do not conduct current); semiconductors - materials in which the bands do not overlap and the distance between them (band gap) lies in the range of 0.1–3 eV (in order to transfer an electron from the valence band to the conduction band, less energy is required than for a dielectric, therefore pure semiconductors are weakly conductive).

Slide 14

The band gap (the energy gap between the valence and conduction bands) is a key quantity in band theory and determines the optical and electrical properties of a material. The transition of an electron from the valence band to the conduction band is called the process of generation of charge carriers (negative - electron, and positive - hole), and the reverse transition is called the process of recombination.

Slide 15

Semiconductors are substances whose band gap is on the order of several electron volts (eV). For example, diamond can be classified as a wide-gap semiconductor, and indium arsenide can be classified as a narrow-gap semiconductor. Semiconductors include many chemical elements (germanium, silicon, selenium, tellurium, arsenic and others), a huge number of alloys and chemical compounds (gallium arsenide, etc.). The most common semiconductor in nature is silicon, making up almost 30% of the earth's crust. A semiconductor is a material that, in terms of its specific conductivity, occupies an intermediate position between conductors and dielectrics and differs from conductors in the strong dependence of the specific conductivity on the concentration of impurities, temperature and exposure to various types of radiation. The main property of a semiconductor is an increase in electrical conductivity with increasing temperature.

Slide 16

Semiconductors are characterized by both the properties of conductors and dielectrics. In semiconductor crystals, electrons need about 1-2 10−19 J (approximately 1 eV) of energy to be released from an atom versus 7-10 10−19 J (approximately 5 eV) for dielectrics, which characterizes the main difference between semiconductors and dielectrics . This energy appears in them as the temperature increases (for example, at room temperature, the energy level of thermal motion of atoms is 0.4·10−19 J), and individual electrons receive energy to be separated from the nucleus. They leave their nuclei, forming free electrons and holes. With increasing temperature, the number of free electrons and holes increases, therefore, in a semiconductor that does not contain impurities, the electrical resistivity decreases. Conventionally, elements with an electron binding energy of less than 2-3 eV are considered semiconductors. The electron-hole conductivity mechanism manifests itself in native (that is, without impurities) semiconductors. It is called the intrinsic electrical conductivity of semiconductors.

Slide 17

The probability of electron transition from the valence band to the conduction band is proportional to (-Eg/kT), where Eg is the band gap. At a large value of Eg (2-3 eV), this probability turns out to be very small. Thus, the division of substances into metals and non-metals has a very definite basis. In contrast, the division of nonmetals into semiconductors and dielectrics does not have such a basis and is purely conditional.

Slide 18

Intrinsic and impurity conductivity Semiconductors in which free electrons and “holes” appear during the ionization of the atoms from which the entire crystal is built are called semiconductors with intrinsic conductivity. In semiconductors with intrinsic conductivity, the concentration of free electrons is equal to the concentration of “holes”. Impurity conductivity Crystals with impurity conductivity are often used to create semiconductor devices. Such crystals are made by introducing impurities with atoms of a pentavalent or trivalent chemical element

Slide 19

Electronic semiconductors (n-type) The term "n-type" comes from the word "negative", which refers to the negative charge of the majority carriers. An impurity of a pentavalent semiconductor (for example, arsenic) is added to a tetravalent semiconductor (for example, silicon). During the interaction, each impurity atom enters into a covalent bond with silicon atoms. However, there is no place for the fifth electron of the arsenic atom in saturated valence bonds, and it breaks off and becomes free. In this case, charge transfer is carried out by an electron, not a hole, that is, this type of semiconductor conducts electric current like metals. Impurities that are added to semiconductors, causing them to become n-type semiconductors, are called donor impurities.

Slide 20

Hole semiconductors (p-type) The term “p-type” comes from the word “positive”, which denotes the positive charge of the majority carriers. This type of semiconductor, in addition to the impurity base, is characterized by the hole nature of conductivity. A small amount of atoms of a trivalent element (such as indium) is added to a tetravalent semiconductor (such as silicon). Each impurity atom establishes a covalent bond with three neighboring silicon atoms. To establish a bond with the fourth silicon atom, the indium atom does not have a valence electron, so it grabs a valence electron from the covalent bond between neighboring silicon atoms and becomes a negatively charged ion, resulting in the formation of a hole. The impurities that are added in this case are called acceptor impurities.

Slide 21

Slide 22

The physical properties of semiconductors are the most studied in comparison with metals and dielectrics. To a large extent, this is facilitated by a huge number of effects that cannot be observed in either one or another substance, primarily related to the structure of the band structure of semiconductors and the presence of a fairly narrow band gap. Semiconductor compounds are divided into several types: simple semiconductor materials - the chemical elements themselves: boron B, carbon C, germanium Ge, silicon Si, selenium Se, sulfur S, antimony Sb, tellurium Te and iodine I. Germanium, silicon and selenium. The rest are most often used as dopants or as components of complex semiconductor materials. The group of complex semiconductor materials includes chemical compounds that have semiconductor properties and include two, three or more chemical elements. Of course, the main incentive for studying semiconductors is the production of semiconductor devices and integrated circuits.

Slide 23

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1. In the absence of an external field, particles are distributed inside the substance in such a way that the electric field they create is equal to zero. 2. In the presence of an external field, a redistribution of charged particles occurs, and a substance’s own electric field arises, which consists of the external E0 field and the internal E/ created by the charged particles of the substance? What substances are called conductors? 3. Conductors -

• substances with the presence of free charges that participate in thermal movement and can move throughout the entire volume of the conductor

• 4. In the absence of an external field in the conductor, the “-” free charge is compensated by the “+” charge of the ionic lattice. In an electric field, occurs redistribution free charges, as a result of which uncompensated “+” and “-” charges appear on its surface
• This process is called electrostatic induction, and the charges that appear on the surface of the conductor are induction charges.

5. The total electrostatic field inside the conductor is equal to zero 6. All internal areas of a conductor introduced into an electric field remain electrically neutral 7. This is the basis electrostatic protection– devices sensitive to the electric field are placed in metal boxes to eliminate the influence of the field. ? What substances are called dielectrics? 8. There are no free electric charges in dielectrics (insulators). They consist of neutral atoms or molecules. Charged particles in a neutral atom are bound to each other and cannot move under the influence of an electric field throughout the entire volume of the dielectric.

• 8. There are no free electric charges in dielectrics (insulators). They consist of neutral atoms or molecules. Charged particles in a neutral atom are bound to each other and cannot move under the influence of an electric field throughout the entire volume of the dielectric.

9. When a dielectric is introduced into an external electric field, a redistribution of charges occurs in it. As a result, excess uncompensated related charges. 10. Bound charges create an electric field that inside the dielectric is directed opposite to the vector of the external field strength. This process is called dielectric polarization. 11. A physical quantity equal to the ratio of the modulus of the external electric field strength in a vacuum to the modulus of the total field strength in a homogeneous dielectric is called dielectric constant substances. ε =E0/E 12. Polar dielectrics - consisting of molecules in which the centers of distribution of “+” and “-” charges do not match. 13. Molecules are microscopic electric dipoles - a neutral combination of two charges, equal in magnitude and opposite in sign, located at some distance from each other. 14. Examples of polar dielectrics:

• Water, alcohol,
• nitric oxide (4)

15. When a dielectric is introduced into an external field, a partial orientation of the dipoles occurs. As a result, uncompensated bound charges appear on the surface of the dielectric, creating a field directed towards the external field. 16. Non-polar dielectrics– substances in the molecules of which the centers of distribution of “+” and “-” charges match up. 17. Uncompensated bound charges appear on the surface of the dielectric, creating their own field E/ directed towards the external field E0 Polarization of a non-polar dielectric 18. Examples of non-polar dielectrics:

• inert gases, oxygen, hydrogen, benzene, polyethylene.

1. What is the electric field inside the conductor?

• A) Potential energy of charges
• B) Kinetic energy of charges
• B) zero

A) These are substances in which charged particles cannot move under the influence of an electric field.

• A) These are substances in which charged particles cannot move under the influence of an electric field.
• B) These are substances in which charged particles can move under the influence of an electric field.

A) 1 4. What is called polarization?

• A) This is a displacement of positive and negative bound charges of the dielectric in opposite directions
• B) This is a displacement of positive and negative bound charges of the dielectric in one direction
• B) This is the arrangement of positive and negative charges of the dielectric in the middle

5. Where is the static charge of the conductor concentrated?

• A) inside the conductor
• B) On its surface

7. WHAT IS DIELECTRIC CONTINUITY? 8. Non-polar dielectrics are dielectrics in which the centers of distribution of positive and negative charges...

• 8. Non-polar dielectrics are dielectrics in which the centers of distribution of positive and negative charges...

A) The fact that the electric field inside the conductor is maximum.

• A) The fact that the electric field inside the conductor is maximum.
• B) on the fact that there is no electric field inside the conductor

10. What is a dipole?

• A) This is a positively charged system of charges
• B) This is a negatively charged system of charges
• B) This is a neutral system of charges

CONDUCTORS AND DIELECTRICS IN AN ELECTRIC FIELD

Basic course

• Conductors are substances that contain free electrical charges that can move under the influence of an arbitrarily weak electric field.

CONDUCTORS

IONIZED

GASES

METALS

ELECTROLYTES

Electrostatic protection– a phenomenon according to which it is possible to shield an electric field by “hiding” from it inside a closed shell made of electrically conductive material (for example, metal).

Electrostatic protection.

The phenomenon was discovered by Michael Faraday in 1836. He noticed that an external electric field could not get inside a grounded metal cage. Principle of operation Faraday cages lies in the fact that under the influence of an external electric field, free electrons located in the metal begin to move and create a charge on the surface of the cell that completely compensates for this external field.

Dielectrics (or insulators) are substances that conduct electricity relatively poorly (compared to conductors).

• In dielectrics, all electrons are bound, i.e., they belong to individual atoms, and the electric field does not tear them off, but only slightly shifts them, i.e., polarizes them. Therefore, an electric field can exist inside the dielectric; the dielectric has a certain influence on the electric field

Dielectrics are divided into polar And non-polar .

Polar dielectrics

consist of molecules in which the centers of distribution of positive and negative charges do not coincide. Such molecules can be represented as two identical in modulus opposite point molecules charges , located at some distance from each other, called dipole .

Non-polar dielectrics

consist of atoms and molecules in which the centers of distribution of positive and negative charges coincide.

Polarization of polar dielectrics.

• Placing a polar dielectric in an electrostatic field (for example, between two charged plates) leads to a reversal and displacement of previously chaotically oriented dipoles along the field.

The reversal occurs under the influence of a pair of forces applied from the field to two dipole charges.

The displacement of dipoles is called polarization. However, due to thermal motion, only partial polarization occurs. Inside the dielectric, the positive and negative charges of the dipoles compensate each other, and on the surface of the dielectric a bound charge appears: negative on the side of the positively charged plate, and vice versa.

Polarization of non-polar dielectrics

A non-polar dielectric in an electric field is also polarized. Under the influence of an electric field, positive and negative charges in a molecule are shifted in opposite directions, so that the centers of charge distribution are displaced, like those of polar molecules. The axis of the field-induced dipole is oriented along the field. Bound charges appear on the dielectric surfaces adjacent to the charged plates.

A polarized dielectric itself creates an electric field.

This field weakens the external electric field inside the dielectric

The degree of this attenuation depends on the properties of the dielectric.

A decrease in the strength of the electrostatic field in a substance compared to the field in a vacuum is characterized by the relative dielectric constant of the medium.

Conductors in an electric field

Dielectrics in an electric field

1. There are free electrons

1. There are no free charge carriers.

2.electrons collect on the surface of the conductor

2. In an electric field, molecules and atoms rotate so that on one side an excess positive charge appears in the dielectric, and on the other - a negative one

3. There is no electric field inside the conductor

3. The electric field inside the conductor weakens by ε times.

4. A conductor can be divided into 2 parts in an electric field, and each part will be charged with different signs.

4. A dielectric can be divided into 2 parts in an electric field, but each of them will be uncharged

Control questions

1 . What substances are called conductors?

2What electric charges are called free?

3.What particles are carriers of free charges in metals?

4.What happens in a metal placed in an electric field?

5. How the dawn communicated to him is distributed over the conductor d?

CONTROL QUESTIONS.

6. If a conductor in an electric field is divided into two parts, how will these parts be charged?

7.What principle is electrostatic protection based on?

8.What substances are called dielectrics?

9.What types of dielectrics are there? What is the difference?

10.Explain the behavior of a dipole in an external electric field.

11. How polarization of dielectrics occurs.

12. If a dielectric placed in an electric field is divided in half, what will be the charge of each part?

13. A negatively charged cloud passes over a lightning rod. Explain, based on electronic concepts, why a charge appears at the tip of the lightning rod. What is his sign?