Presentation on the topic of current in metals. Electric current in metals, presentation for a physics lesson (grade 11) on the topic. Last presentation slide: Electric current in metals: resources used

Class: 11

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Lesson Objectives:

Expand the concept of the physical nature of electric current in metals, experimental confirmation of the electronic theory;

Continue the formation of natural scientific ideas on the topic being studied

Create conditions for the formation of cognitive interest and activity of students

Formation of skills;

Formation of communicative communication.

Equipment: SMART Board Notebook interactive complex, local computer network, Internet.

Lesson teaching method: combined.

Lesson epigraph:

Strive to comprehend science more and more deeply,
Thirst for the knowledge of the eternal.
Only the first knowledge will shine upon you,
You will find out: there is no limit to knowledge.

Ferdowsi
(Persian and Tajik poet, 940-1030)

Lesson plan.

I. Organizational moment

II. Group work

III. Discussion of results, installation of presentation

IV. Reflection

V. Homework

During the classes

Hello guys! Sit down. Today our work will take place in groups.

Group assignments:

I. Physical nature of charges in metals.

II. Experience of K.Rikke.

III. Experience of Stewart, Tolman. Mandelstam's experience, Papaleksi.

IV. Drude's theory.

V. Current-voltage characteristics of metals. Ohm's law.

VI. Dependence of conductor resistance on temperature.

VII. Superconductivity.

1. Electrical conductivity is the ability of substances to conduct electric current under the influence of an external electric field.

According to the physical nature of charges - carriers of electric current, electrical conductivity is divided into:

A) electronic

B) ionic,

B) mixed.

2. Each substance under given conditions is characterized by a certain dependence of the current strength on the potential difference.

Based on specific resistance, substances are usually divided into:

A) conductors (p< 10 -2 Ом*м)

B) dielectrics (p > 10 -8 Ohm*m)

B) semiconductors (10 -2 Ohm*m> p>10 -8 Ohm*m)

However, this division is conditional, since under the influence of a number of factors (heating, irradiation, impurities), the resistivity of substances and their current-voltage characteristics change, and sometimes very significantly.

3. The carriers of free charges in metals are electrons. Proven by classical experiments K. Riecke (1901) – German physicist; L.I. Mandelstam and N.D. Papaleksi (1913) - our compatriots; T. Stewart and R. Tolman (1916) - American physicists.

Experience of K. Rikke

Rikke stacked three pre-weighed cylinders (two copper and one aluminum) with polished ends so that the aluminum one was between the copper ones. The cylinders were then connected to a direct current circuit: a large current passed through them for a year. During that time, an electric charge equal to approximately 3.5 million C passed through the electric cylinders. The secondary interaction of the cylinders, carried out with up to 0.03 mg, showed that the mass of the cylinders did not change as a result of the experiment. When examining the contacting ends under a microscope, it was found that there were only minor traces of metal penetration, which did not exceed the results of the usual diffusion of atoms in solids. The experimental results indicated that ions do not participate in charge transfer in metals.

L.I. Mandelstam

N. . Papalexi

The experience of L. I. Mandelstam and N. D. Papaleksi

Russian scientists L. I. Mandelstam (1879-1949; founder of the school of radiophysicists) and N. D. Papaleksi (1880-1947; the largest Soviet physicist, academician, chairman of the All-Union Scientific Council on Radiophysics and Radio Engineering at the USSR Academy of Sciences) in 1913 staged the original experience. They took a coil of wire and began to twist it in different directions.

They will spin, for example, clockwise, then abruptly stop and then back.

They reasoned something like this: if electrons really have mass, then when the coil suddenly stops, the electrons should continue to move by inertia for some time. The movement of electrons along a wire is an electric current. It happened as we planned. We connected a telephone to the ends of the wire and heard a sound. Since sound is heard in the phone, therefore, current flows through it.

T. Stewart

Experience of T. Stewart and R. Tolman

Let's take a coil that can rotate around its axis. The ends of the coil are connected to a galvanometer using sliding contacts. If the coil, which is in rapid rotation, is sharply braked, then the free electrons in the wire will continue to move by inertia, as a result of which the galvanometer should register a current pulse.

Drude theory

Electrons in a metal are considered as an electron gas, to which the kinetic theory of gases can be applied. It is believed that electrons, like gas atoms in the kinetic theory, are identical solid spheres that move in straight lines until they collide with each other. It is assumed that the duration of an individual collision is negligible, and that no forces other than those arising at the moment of collision act between the molecules. Since an electron is a negatively charged particle, in order to comply with the condition of electrical neutrality, a solid must also contain particles of a different type - positively charged. Drude suggested that the compensating positive charge belonged to much heavier particles (ions), which he considered immobile. In Drude's time, it was not clear why there were free electrons and positively charged ions in a metal, and what these ions were. Only the quantum theory of solids could provide answers to these questions. For many substances, however, we can simply assume that the electron gas consists of outer valence electrons weakly bound to the nucleus, which are “freed” in the metal and are able to move freely throughout the metal, while the atomic nuclei with the electrons of the inner shells (atomic cores) remain unchanged and play the role of immobile positive ions of the Drude theory.

Electric current in metals

All metals are conductors of electric current and consist of a spatial crystal lattice, the nodes of which coincide with the centers of positive ions, and free electrons move chaotically around the ions.

Basic principles of the electronic theory of conductivity of metals.

1. A metal can be described by the following model: a crystal lattice of ions is immersed in an ideal electron gas consisting of free electrons. In most metals, each atom is ionized, so the concentration of free electrons is approximately equal to the concentration of atoms 10 23 - 10 29 m -3 and is almost independent of temperature.
2. Free electrons in metals are in continuous chaotic motion.
3. Electric current in a metal is formed only due to the ordered movement of free electrons.
4. Colliding with ions oscillating at the nodes of the crystal lattice, electrons give them excess energy. This is why conductors heat up when current passes.

Electric current in metals.

Superconductivity

The phenomenon of resistivity decreasing to zero at a temperature other than absolute zero is called superconductivity. Materials that exhibit the ability to transition to a superconducting state at certain temperatures other than absolute zero are called superconductors.

The passage of current in a superconductor occurs without loss of energy, therefore, once excited in a superconducting ring, the electric current can exist indefinitely without change.

Superconducting materials are already used in electromagnets. Research is underway aimed at creating superconducting power lines.

The application of the phenomenon of superconductivity in widespread practice may become a reality in the coming years thanks to the discovery in 1986 of the superconductivity of ceramics - compounds of lanthanum, barium, copper and oxygen. The superconductivity of such ceramics persists up to temperatures of about 100 K.

Well done boys! They did an excellent job. It was a good presentation. Thank you for the lesson!

Literature.

1. Gorbushin Sh.A. Basic notes for studying physics for a secondary school course. – Izhevsk “Udmurtia”, 1992.
2. Lanina I.Ya. Formation of cognitive interests of students in physics lessons: A book for teachers. – M.: Education, 1985.
3. Physics lesson in a modern school. Creative search for teachers: A book for teachers / Comp. E.M. Braverman / Edited by V.G. Razumovsky.- M.: Education, 1993
4. Digelev F.M. From the history of physics and the life of its creators: A book for students. - M.: Education, 1986.
5. Kartsev V.L. Adventures of great equations. - 3rd edition - M.: Znanie, 1986. (Life of wonderful ideas).

Description of the presentation by individual slides:

1 slide

Slide description:

ELECTRIC CURRENT IN METALS The presentation was developed by the teacher of CS and PT Karakasheva I.V. St. Petersburg 2016

2 slide

Slide description:

Lesson objectives: Educational: to introduce students to the conductivity of metals and its technical uses; reveal the concept of the physical nature of electric current in metals; continue the formation of natural scientific ideas on the topic being studied; create conditions for the formation of cognitive interest; expand the scientific and technical horizons of students Developmental: create conditions for the development of communication skills; create conditions for the development of students’ analytical abilities, the ability to analyze, compare, compare, generalize, and draw conclusions; create conditions for the development of memory, attention, imagination Educational: promote the development of the ability to defend one’s point of view; promote the development of a culture of relationships when working in a team

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Slide description:

What is called metal? The most famous of the early definitions of metal was given in the middle of the 18th century by M.V. Lomonosov: “Metal is a light body that can be forged. There are only six such bodies: gold, silver, copper, tin, iron and lead.” Two and a half centuries later, much has become known about metals. More than 75% of all elements of D.I. Mendeleev’s table are metals, and finding an absolutely accurate definition for metals is an almost hopeless task.

4 slide

Slide description:

In 1900, the German scientist P. Drude, based on the hypothesis of the existence of free electrons in metals, created the electronic theory of metal conductivity. This theory was developed in the works of the Dutch physicist H. Lorentz (1904) and is called classical electronic theory. She gave a simple and visual explanation of most of the electrical and thermal properties of metals. Paul Drude Karl Ludwig - German physicist Hendrik Anton Lorenz - Dutch physicist Classical electron theory

5 slide

Slide description:

The movement of electrons obeys the laws of classical mechanics. Electrons do not interact with each other. Electrons interact only with ions of the crystal lattice; this interaction is reduced to collision. In the intervals between collisions, electrons move freely. Conduction electrons form an “electron gas”, similar to an ideal gas. “Electronic gas” obeys the laws of ideal gas. During any collision, the electron transfers all the accumulated energy. Basic principles of the theory

6 slide

Slide description:

The metal has a crystal lattice, at the nodes of which there are positive ions that oscillate around the equilibrium position, and free electrons that can move throughout the entire volume of the conductor (electron gas, subject to the laws of an ideal gas) Structure of the metal

7 slide

Slide description:

The average speed of thermal motion of electrons at room temperature is approximately 105 m/s. Structure of a metal In a metal, in the absence of an electric field, electrons move chaotically and collide, most often with ions of the crystal lattice.

8 slide

Slide description:

Electric current in metals Under the influence of an electric field, free electrons begin to move in an orderly manner between the ions of the crystal lattice. Electric current flows through a conductor due to the presence of free electrons in it that have escaped from atomic orbits

Slide 9

Slide description:

Electric current in metals Electric current in metals is the ordered movement of electrons under the influence of an electric field. When current flows through a metal conductor, no substance transfer occurs; therefore, metal ions do not take part in the transfer of electric charge. This was confirmed in the experiments of the German physicist E. Ricke in 1901.

10 slide

Slide description:

Experiments by E.Rikke In these experiments, an electric current of 0.1 A was passed for a year through three well-polished cylinders pressed against each other. The total charge passed through the cylinders during this time exceeded 3.5 MK. After completion, it was found that there were only minor traces of mutual penetration of metals, not exceeding the results of ordinary diffusion of atoms in solids. Measurements showed that the mass of each cylinder remained unchanged. Since the masses of copper and aluminum atoms differ significantly from each other, the mass of the cylinders would have to change noticeably if the charge carriers were ions. Therefore, free charge carriers in metals are not ions. The huge charge that passed through the cylinders was apparently carried by particles that are the same in both copper and aluminum.

11 slide

Slide description:

Experimental proof of the existence of free electrons in metals Experimental proof that the current in metals is created by free electrons was given in experiments by L.I. Mandelstam and N.D. Papaleksi (1913, the results were not published), as well as the experiments of T. Stewart and R. Tolman (1916). L.I. Mandelstam 1879-1949 N. D. Papaleksi 1880-1947 T. Stewart

12 slide

Slide description:

The coil connected to the telephone spun around its axis in different directions and slowed down sharply. If electrons really have mass, then when the coil suddenly stops, the electrons should continue to move by inertia for some time. The movement of electrons through a wire is an electric current, and the phone should make a sound. Since sound is heard in the phone, therefore, current flows through it. But no measurements or quantitative calculations were made in these experiments. Experience of L.I. Mandelstam and N.D. Papaleksi (1912)

Slide 13

Slide description:

Experience of T. Stewart and R. Tolman A coil with a large number of turns of thin wire was driven into rapid rotation around its axis. The ends of the coil were connected using flexible wires to a sensitive ballistic galvanometer. The untwisted coil was sharply slowed down, and a short-term current arose in the circuit due to the inertia of the charge carriers. The total charge flowing through the circuit was measured by the deflection of the galvanometer needle.

Slide 14

Slide description:

The experiment of T. Stewart and R. Tolman The direction of the current indicated that it was caused by the movement of negatively charged particles. By measuring the charge passing through the galvanometer during the entire existence of the current in the circuit, T. Stewart and R. Tolman experimentally determined the specific charge of the particles. He turned out to be equal

15 slide

Slide description:

Volt - ampere characteristic of metals Electric current in metals Charge carriers - electrons Conductivity - electronic The conductor through which the current flows heats up. A conductor through which current flows has a magnetic effect on surrounding bodies.

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Slide description:

Dependence of conductor resistance on temperature Resistance is a physical quantity that characterizes the ability of a conductor to resist the establishment of electric current in it. Specific resistance is the resistance of a cylindrical conductor of unit length and unit cross-sectional area. When heated, the dimensions of the conductor change little, but mainly the resistivity changes.

Slide 17

Slide description:

Dependence of conductor resistance on temperature Specific resistance of a conductor depends on temperature: where ro is the resistivity at 0 degrees, t is the temperature, α is the temperature coefficient of resistance

18 slide

Slide description:

Dependence of conductor resistance on temperature For metal conductors, as the temperature increases, the resistivity increases, the resistance of the conductor increases, and the electric current in the circuit decreases. The resistance of a conductor with a change in temperature can be calculated using the formula: R = Ro (1 + α t), where Ro is the resistance of the conductor at 0 degrees Celsius t is the temperature of the conductor α is the temperature coefficient of resistance

Slide 19

Slide description:

Application of current in metals Transfer of electricity from source to consumers In electric motors and generators In heating devices

20 slide

Slide description:

Contradictions of classical electronic theory Classical electronic theory explains the existence of electrical resistance of metals, Ohm's and Joule-Lenz's laws. However, in a number of issues, the classical electronic theory leads to conclusions that are in conflict with experiment. This theory cannot explain why the molar heat capacity of metals, as well as the molar heat capacity of dielectric crystals, is equal to 3R, where R is the universal gas constant (Dulong and Petit's law). The presence of free electrons does not affect the heat capacity of metals. Classical electronic theory also cannot explain the temperature dependence of the resistivity of metals. The theory gives the relation, while from experiment the dependence ρ ~ T is obtained. However, the most striking example of the discrepancy between theory and experiment is superconductivity.

21 slides

Slide description:

Superconductivity According to classical electronic theory, the resistivity of metals should monotonically decrease with cooling, remaining finite at all temperatures. This dependence is actually observed experimentally at relatively high temperatures. At temperatures of the order of several kelvins, the resistivity of many metals ceases to depend on temperature and reaches a certain limiting value. In 1911, the Dutch scientist Geike Kamerling-0nnes discovered that when the temperature of mercury drops to 4.1 K, its resistivity abruptly decreases to zero. (1853-1926) Geike Kamerling -0nnes, Dutch scientist

22 slide

Slide description:

Superconductivity At a certain temperature Tcr, different for different substances, the resistivity abruptly decreases to zero. This phenomenon is called superconductivity. Materials that exhibit the ability to transition to a superconducting state at certain temperatures other than absolute zero are called superconductors. Dependence of resistivity ρ on absolute temperature T at low temperatures: a – normal metal; b – superconductor

Slide 23

Slide description:

Superconductivity G. Kamerlingh Onnes was awarded the Nobel Prize in Physics in 1913 “for his studies of the properties of matter at low temperatures.” Later it was found that more than 25 chemical elements - metals - become superconductors at very low temperatures. The lowest temperature is for tungsten - 0.012 K, the highest for niobium - 9 K. Superconductivity is observed not only in pure metals, but also in many chemical compounds and alloys and some semiconductors. Moreover, the elements themselves that make up the superconducting compound may not be superconductors. For example, NiBi, Au2Bi, PdTe, PtS and others. At the same time, such “good” conductors as copper and silver do not become superconductors at low temperatures.

24 slide

Slide description:

Superconductivity The first theoretical explanation of superconductivity was given in 1935 by brothers Fritz and Heinz London. A more general theory was constructed in 1950 by L. D. Landau and V. L. Ginzburg. However, these theories did not reveal the detailed mechanisms of superconductivity. Superconductivity was first explained at the microscopic level in 1957 in the work of American physicists John Bardeen, Leon Cooper and John Schrieffer. The central element of their theory, called the BCS theory, is the so-called Cooper pairs of electrons. It was later discovered that superconductors are divided into two large families: type I superconductors (which, in particular, include mercury) and type II (which are usually alloys of different metals). The work of A. A. Abrikosov in the 1950s played a significant role in the discovery of type II superconductivity.

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Slide description:

Superconductivity In 1962, the English physicist Brian Josephson discovered the effect that received his name. In 1986, Karl Müller and Georg Bednorz discovered a new type of superconductors, called high-temperature superconductors. In early 1987, it was shown that compounds of lanthanum, strontium, copper and oxygen (La-Sr-Cu-O) experience a jump in resistance to almost zero at a temperature of 36 K. In early March 1987, a superconductor was obtained for the first time at temperatures above boiling of liquid nitrogen (77.4 K): it was discovered that the compound of yttrium, barium, copper and oxygen (Y-Ba-Cu-O) has this property.

26 slide

Slide description:

Superconductivity In 1988, a ceramic compound (a mixture of thallium, calcium, barium and copper oxides) with a critical temperature of 125 K was created. In 2003, a ceramic compound Hg-Ba-Ca-Cu-O(F) was discovered, the critical temperature for which is 138 K. Moreover, at a pressure of 400 kbar, the same compound is a superconductor at temperatures up to 166 K. In 2015, a new record was set for the temperature at which superconductivity is achieved. For H2S (hydrogen sulfide) at a pressure of 100 GPa, a superconducting transition was recorded at a temperature of 203 K (-70°C).

Slide 27

Slide description:

Properties of superconductors Since there is no resistance in superconductivity, no heat is generated when an electric current passes through a conductor. This property of superconductors is widely used. For each superconductor, there is a critical current value that can be achieved in the conductor without violating its superconductivity. This happens because when current passes, a magnetic field is created around the conductor. And the magnetic field destroys the superconducting state. Therefore, superconductors cannot be used to produce an arbitrarily strong magnetic field. When energy passes through a superconductor, there is no loss of energy. One of the areas of research by modern physicists is the creation of superconducting materials at room temperatures.

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Slide description:

Superconductivity Currently, over 500 pure elements and alloys are known that exhibit the property of superconductivity. Based on their behavior in sufficiently strong magnetic fields, they are divided into type 1 and type 2 superconductors. Type I superconductors completely displace the magnetic field. Type 1 superconductors include all superconducting elements except Nb and V, and some alloys.

Slide 29

ELECTRIC CURRENT IN METALS

Slide 2

Fundamentals of the electronic theory of conductivity At the beginning of the 20th century, the classical electronic theory of conductivity of metals was created (P. Drude, 1900, H. Lorenz, 1904), which provided a simple and visual explanation of most of the electrical and thermal properties of metals. Paul Drude Karl Ludwig - German physicist Hendrik Anton Lorenz - Dutch physicist

Slide 3

The movement of electrons obeys the laws of classical mechanics. Electrons do not interact with each other. Electrons interact only with ions of the crystal lattice; this interaction is reduced to collision. In the intervals between collisions, electrons move freely. Conduction electrons form an “electron gas”, similar to an ideal gas. “Electronic gas” obeys the laws of ideal gas. During any collision, the electron transfers all the accumulated energy. Classical electronic theory of Drude - Lorentz.

Slide 4

Electric current in metals Ions of the metal crystal lattice do not take part in the creation of current. Their movement during the passage of current would mean the transfer of matter along the conductor, which is not observed. For example, in the experiments of E. Riecke (1901), the mass and chemical composition of the conductor did not change when current passed for a year.

Slide 5

Conclusion: There is no transfer of matter => 1) Metal ions do not take part in the transfer of electric charge. 2) Charge carriers are particles that are part of all metals. Riecke's experiment in 1901.

Slide 6: Electrons interact not with each other, but with ions of the crystal lattice. With each collision, the electron transfers its kinetic energy

Slide 7

Experimental proof that the current in metals is created by free electrons was given in experiments by L.I. Mandelstam and N.D. Papaleksi (1913, the results were not published), as well as T. Stewart and R. Tolman (1916). They discovered that when a rapidly rotating coil suddenly stops, an electric current arises in the coil conductor, created by negatively charged particles - electrons.

Slide 8

The experiment of Mandelstam and Papaleksi Conclusion: Electric charge carriers move by inertia 1913

Slide 9

Experience of Tolman and Stewart Conclusions: Charge carriers in a metal are negatively charged particles. Ratio = > Electric current in metals is due to the movement of electrons 1916

10

Slide 10: Ions undergo thermal vibrations near the equilibrium position - the nodes of the crystal lattice. Free electrons move chaotically and during their movement collide with ions of the crystal lattice

11

Slide 11

A metal conductor consists of: positively charged ions oscillating around the equilibrium position, and 2) free electrons that can move throughout the entire volume of the conductor. In a metal, in the absence of an electric field, conduction electrons move chaotically and collide, most often with ions of the crystal lattice. The collection of these electrons can be approximately considered as a kind of electron gas, subject to the laws of an ideal gas. The average speed of thermal motion of electrons at room temperature is approximately 105 m/s.

12

Slide 12

Dependence of conductor resistance R on temperature: When heated, the dimensions of the conductor change little, but the resistivity changes mainly. The resistivity of a conductor depends on temperature: where rho is the resistivity at 0 degrees, t is the temperature, is the temperature coefficient of resistance (i.e., the relative change in the resistivity of the conductor when it is heated by one degree)

13

Slide 13

For all metallic conductors α > 0 and varies slightly with temperature. For most metals, in the temperature range from 0° to 100°C, the coefficient α varies from 3.3⋅10–3 to 6.2⋅10–3 K–1 (Table 1). For chemically pure metals, there are special alloys whose resistance practically does not change when heated, for example, manganin and constantan. Their temperature coefficients of resistance are very small and equal to 1⋅10–5 K–1 and 5⋅10–5 K–1, respectively.

14

Slide 14

Thus, for metal conductors, with increasing temperature, the resistivity increases, the resistance of the conductor increases and the electric current in the circuit decreases. The resistance of a conductor with a change in temperature can be calculated using the formula: R = Ro (1 + t) where Ro is the resistance of the conductor at 0 degrees Celsius t - the temperature of the conductor - the temperature coefficient of resistance

15

Slide 15: Conductor Resistance

Resistance is a physical quantity that characterizes the degree of resistance of a conductor to the directional movement of charges. Specific resistance is the resistance of a cylindrical conductor of unit length and unit cross-sectional area. Superconductivity is a physical phenomenon consisting in an abrupt drop in resistance to zero at a certain critical temperature (Tcr) - resistivity, - conductor length, S - cross-sectional area = (1 + ∆ T) - resistivity at t = 20 0 C; - temperature coefficient of resistance = 1/ 273 0 K -1 ∆ T – temperature change T, K 0 metal superconductor T cr 293

16

Slide 16

Superconductivity is a property of many conductors, consisting in the fact that their electrical resistance abruptly drops to zero when cooled below a certain critical temperature Tk, characteristic of a given material. S. is found in more than 25 metal elements, in a large number of alloys and intermetallic compounds, as well as in some semiconductors.

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Slide 17

In 1911, the Dutch physicist Kamerlingh Onnes discovered that when mercury is cooled in liquid helium, its resistance first changes gradually, and then sharply drops to zero at a temperature of 4.2 K.

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Slide 18

G. Kamerlingh Onnes was awarded the Nobel Prize in Physics in 1913 “for his studies of the properties of matter at low temperatures.” Later it was found that more than 25 chemical elements - metals - become superconductors at very low temperatures. Each of them has its own critical temperature for transition to a state with zero resistance. Its lowest value is for tungsten - 0.012 K, the highest for niobium - 9 K. Superconductivity is observed not only in pure metals, but also in many chemical compounds and alloys. Moreover, the elements themselves that make up the superconducting compound may not be superconductors. For example, NiBi, Au2Bi, PdTe, PtSb and others. Until 1986, superconductors were known to have this property at very low temperatures - below –259 °C. In 1986-1987, materials were discovered with a transition temperature to the superconducting state of about –173 °C. This phenomenon is called high-temperature superconductivity, and to observe it, liquid nitrogen can be used instead of liquid helium.

19

Slide 19: Superconductivity

Academician V.L. Ginzburg, Nobel laureate for his work on superconductivity

20

Slide 20: Superconductivity of metals and alloys

For many metals and alloys at temperatures close to T = 0 K, a sharp decrease in resistivity is observed - this phenomenon is called superconductivity of metals. It was discovered by the Dutch physicist H. Kamerling - Ohness in 1911 for mercury (T cr = 4.2 o K). T P 0

21

Slide 21: General information

About half of the metals and several hundred alloys have the property of superconductivity. Superconducting properties depend on the type of crystal structure. Changing it can transform a substance from a normal to a superconducting state. The critical temperatures of isotopes of elements that pass into the superconducting state are related to the masses of the isotopes by the relation: T e (M e) 1/2 = const (isotope effect) A strong magnetic field destroys the effect of superconductivity. Therefore, when placed in a magnetic field, the property of superconductivity may disappear.

22

Slide 22: Reaction to impurities

The introduction of an impurity into a superconductor reduces the abruptness of the transition to the superconducting state. In normal metals, the current disappears after about 10 -12 s. In a superconductor, the current can circulate for years (theoretically 105 years!).

23

Slide 23: The physical nature of superconductivity

The phenomenon of superconductivity can be understood and justified only with the help of quantum concepts. They were presented in 1957 by American scientists J. Bardin, L. Cooper, J. Schrieffer and Soviet academician N.N. Bogolyubov. In 1986, high-temperature superconductivity of compounds of lanthanum, barium and other elements was discovered (T = 100 0 K is the boiling point of liquid nitrogen).

24

Slide 24

However, zero resistance is not the only distinguishing feature of superconductivity. It is also known from Drude’s theory that the conductivity of metals increases with decreasing temperature, that is, the electrical resistance tends to zero.

Pushing off from a stationary superconductor, the magnet floats up on its own and continues to hover until external conditions remove the superconductor from the superconducting phase. As a result of this effect, a magnet approaching a superconductor will "see" a magnet of reverse polarity of exactly the same size, which causes levitation.

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Slide 27: Applications of superconductivity

1. Powerful electromagnets with superconducting windings are constructed, which create a magnetic field without consuming electricity over a long period of time, because no heat is released. 2. Superconducting magnets are used in particle accelerators, magnetohydrodynamic and generators that convert the energy of a stream of hot ionized gas moving in a magnetic field into electrical energy. 3. High-temperature superconductivity in the near future will lead to a technical revolution in radio electronics and radio engineering. 4. If it is possible to create superconductors at room temperature, then generators and electric motors will become extremely compact and it will be possible to transmit electricity over long distances without losses.

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Last presentation slide: ELECTRIC CURRENT IN METALS: Resources used:

http://www.physbook.ru/index.php/ T._Electronic_conductivity_of_metals http://class-fizika.narod.ru/10_9.htm

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Slide captions:

Electric current in metals Grade 11 Teacher Kechkina N.I. MBOU "Secondary school No. 12" Dzerzhinsk

Ohm's law from the point of view of electronic theory Electric current in metals is caused by the movement of free electrons. Experiment by E. Rikke Result: penetration of copper into aluminum was not detected. Experiments by L.I. Mandelstam and N.D. Papaleksi 1912 R. Tolman and T. Stewart 1916 C-cylinder; Ш – brushes (contacts); OO ’ - isolated semi-axes Result: when stopped, the galvanometer needle deviated, recording the current. Based on the direction of the current, it was determined that negative particles move by inertia. The largest charge is electrons.

The mean free path λ is the average distance between two successive collisions of electrons with defects. Electrical resistance is a violation of the periodicity of the crystal lattice. Reasons: thermal movement of atoms; presence of impurities. Electron scattering. Measure of dispersion Classical electronic theory of Lorentz (electrical conductivity of metals): There are free electrons in a conductor that move continuously and chaotically; Each atom loses 1 electron to become an ion; λ is equal to the distance between ions in the crystal lattice of the conductor. e – electron charge, Cl n – number of electrons passing through the cross section of the conductor in units. time m – electron mass, kg u – root mean square speed of random motion of electrons, m/s γ

The Joule-Lenz law from the point of view of electronic theory γ The Joule-Lenz law in differential form. The classical electronic theory of Lorentz explains Ohm's and Joule-Lenz's laws, which are confirmed experimentally. A number of conclusions are not confirmed experimentally. BUT Specific resistance (the reciprocal of conductivity) is proportional to the square root of the absolute temperature. The classical electronic theory of Lorentz has limits of applicability. Experiments ρ~ T

On the topic: methodological developments, presentations and notes

Electric current in metals

The most convincing evidence of the electronic nature of current in metals was obtained in experiments with the inertia of electrons. The idea of ​​such experiments and the first qualitative results belong to Russian physicists...

Topic “Electric current in metals” Lesson goal: Continue studying the nature of electric current in metals, experimentally study the effect of electric current. Lesson objectives: Educational - ...

Lecturer: Ph.D. Sc., Associate Professor
Veretelnik Vladimir Ivanovich

Electric current in metals

1.
2.
3.
4.
5.
Tolman-Stewart experiment.
Classical conduction theory
metals - Drude-Lorentz theory.
Ohm's law and Joule-Lenz's law from
classical theory of electrical conductivity.
Superconductivity.
Electron-hole transition.
Transistors.

Electric current in metals

Electric current in metals is
ordered movement of electrons under
action of an electric field.
The most convincing evidence
the electronic nature of current in metals was
obtained in experiments with electron inertia
(The experience of Tolman and Stewart).
Coil with a large number of turns of thin
the wire was driven into rapid rotation
around its axis.
Coil ends with flexible wires
were attached to the sensitive
ballistic galvanometer.

Electric current in metals

Untwisted reel sharply
slowed down, and a problem appeared in the chain
short-term current due to
inertia of charge carriers.
The total charge flowing through the circuit is
measured by needle drop
galvanometer.

Electric current in metals

When braking the rotating coil for each
charge carrier e acts as a braking force, which
plays the role of an external force, that is, a force
non-electrical origin.
External force per unit charge, according to
definition is the field strength Est
third party forces:
Consequently, in the circuit when the coil is braking
electromotive force arises:

Electric current in metals

where l is the length of the coil wire. During braking
coil, a charge q will flow through the circuit equal to:
Here I is the instantaneous value of the current in the coil, R is
total resistance of the circuit, υ0 – initial linear
wire speed.
Hence the specific charge e/m of free current carriers
in metals is equal to:
According to modern data, the electron charge modulus
(elementary charge) is equal to

Electric current in metals

Specific charge
Good electrical conductivity of metals
due to high concentration
free electrons, equal in order
quantities for the number of atoms per unit volume.
Assumption about what kind of electric current
in metals electrons are responsible, arose
much earlier than the experiments of Tolman and Stewart.
Back in 1900, the German scientist P. Drude
basis of the hypothesis about the existence of free
electrons in metals created electron
theory of conductivity of metals.

Electric current in metals

This theory was developed in the works of the Dutch
physics by H. Lorentz and is called classical
electronic theory.
According to this theory, electrons in metals behave
like an electron gas, much like an ideal gas
gas.
Electron gas fills the space between the ions,
forming a metal crystal lattice
Due to interaction with ions, electrons can
leave the metal only by overcoming the so-called
potential barrier.
The height of this barrier is called the work function.
At ordinary (room) temperatures, electrons do not
enough energy to overcome potential
barrier.

Electric current in metals

According to the Drude–Lorentz theory,
electrons have the same average
energy of thermal motion, as well as
monatomic ideal molecules
gas
This allows us to estimate the average
speed of thermal movement
electrons according to the formulas of molecular kinetic theory.
At room temperature it
turns out to be approximately equal to 105 m/s.

Electric current in metals

When applying external
electric field in
metal conductor except
thermal motion of electrons
their orderly appears
movement (drift), that is
electricity.

Electric current in metals

Estimation of drift speed
shows that for metal
conductor with a cross section of 1 mm2, along which
a current of 10 A flows, this value lies in
within 0.6–6 mm/s.
So the average speed
ordered movement of electrons in
metal conductors for many
orders of magnitude less than their average speed
thermal movement.

Electric current in metals

Low drift speed does not contradict
the experimental fact that the current in the entire circuit
DC is installed practically
instantly.
Closing the circuit causes propagation
electric field with a speed c = 3·108 m/s.
After a time of the order of l / s (l is the length of the chain)
a stationary one is installed along the chain
electric field distribution and in it
orderly movement begins
electrons.

Electric current in metals

In classical electronic theory of metals
it is assumed that the movement of electrons
obeys Newton's laws of mechanics.
This theory neglects the interaction
electrons among themselves, and their interaction
with positive ions are reduced only to
collisions.
It is also assumed that for each
collision, the electron transfers to the lattice all
energy accumulated in the electric field and
so after the collision he starts
movement with zero drift speed.

Electric current in metals

Although all these assumptions are
very close, classical electronic
the theory qualitatively explains the laws of electrical
current in metal conductors.
Ohm's law. In the interval between collisions on
electron acts on a force equal in magnitude eE to
As a result, it gains acceleration
Therefore, by the end of the free run, the drift
electron speed is

Electric current in metals

where τ is the free travel time,
which, to simplify calculations
assumed to be the same for everyone
electrons.
Average drift speed
equal to half the maximum
values:

Electric current in metals

Consider a conductor of length l and cross-section S with
electron concentration n.
The current in a conductor can be written as:
where U = El is the voltage at the ends of the conductor.
The resulting formula expresses Ohm's law for
metal conductor.
Electrical resistance of the conductor
equals:

Electric current in metals

Resistivity ρ and specific
conductivity σ are expressed
ratios:
Joule-Lenz law. By the end
free path of electrons
acquired under the influence of the field
kinetic energy

Electric current in metals

According to the assumptions made,
all this energy is transferred to the lattice when
collision and turns into heat.
During the time Δt, each electron
experiences Δt/τ collisions.
In a conductor with cross section S and length l
there are nSl electrons.
It follows that what is allocated in
conductor during time Δt heat is equal to:

Electric current in metals

This ratio expresses
Joule-Lenz law.
Thus, classical electronic
theory explains existence
electrical resistance of metals,
Ohm's and Joule-Lenz's laws.
However, in a number of issues the classical
electron theory leads to conclusions
in conflict with experience.

Electric current in metals

This theory cannot, for example, explain why
molar heat capacity of metals, as well as molar
the heat capacity of dielectric crystals is 3R,
where R is the universal gas constant (law
Dulong and Petit.)
Classical electron theory cannot either
explain the temperature dependence of specific
metal resistance.
The theory gives
while from the experiment
the dependence ρ ~ T is obtained.
However, the most striking example of the divergence between theory and
experiments is superconductivity.

Electric current in metals

At some certain
temperature Tcr, different for different
substances, resistivity
decreases abruptly to zero.
The critical temperature of mercury is
4.1 K, aluminum 1.2 K, tin 3.7 K.
Superconductivity is not observed
only for elements, but also for many
chemical compounds and alloys.

Electric current in metals

For example, a compound of niobium with tin
(Ni3Sn) has a critical temperature
18 K.
Some substances that pass through
low temperatures into superconducting
condition, are not conductors
at normal temperatures.
At the same time so “good”
conductors like copper and silver are not
become superconductors when
low temperatures.

Electric current in metals

Substances in superconducting
condition have
exceptional properties.
Almost the most important of them
them is the ability
long time (many years)
maintain without attenuation
electric current excited in
superconducting circuit.

Electric current in metals

Classical electron theory is not
is able to explain the phenomenon
superconductivity. Explanation
the mechanism of this phenomenon was given
only 60 years after its discovery
based on quantum mechanical
representations.
Scientific interest in superconductivity
increased as new ones were discovered
materials with higher
critical temperatures.

Electric current in metals

A significant step in this direction occurred in
1986, when it was discovered that one complex
ceramic connection Tcr = 35 K.
Already in the next 1987, physicists were able to create
new ceramics with a critical temperature of 98 K,
exceeding the temperature of liquid nitrogen (77 K).
The phenomenon of transition of substances into superconducting
state at temperatures exceeding the temperature
boiling point of liquid nitrogen was called
high temperature superconductivity.
In 1988, a ceramic connection was created on
based on elements Tl–Ca–Ba–Cu–O with critical
temperature 125 K.
It should be noted that until now the mechanism
high temperature superconductivity ceramic
materials is not fully understood.

1.
2.
3.
4.
The qualitative difference between semiconductors and
metals
Electron-hole mechanism
conductivity of pure unadulterated
semiconductors.
Electronic and hole conductivity
impurity semiconductors. Donor and
acceptor impurities.
Electron-hole transition.
Semiconductor diode. Transistor.

Electric current in semiconductors

Semiconductors include
many chemical elements (germanium,
silicon, selenium, tellurium, arsenic, etc.),
a huge number of alloys and
chemical compounds.
Almost all inorganic substances
the world around us -
semiconductors.
The most common in nature
silicon is a semiconductor
making up about 30% of the earth's crust.

Electric current in semiconductors

Qualitative difference
semiconductors from metals
manifests itself primarily in
specific
temperature resistance.

Electric current in semiconductors

This course of the dependence ρ(T) shows that
that semiconductors have a concentration
no free charge carriers
remains constant but increases with
rising temperature.
Let us consider this mechanism qualitatively
using the example of germanium (Ge).
In a silicon (Si) crystal, the mechanism
similar.

Electric current in semiconductors

Germanium atoms have four weak
bound electrons in the outer shell.
They are called valence electrons.
In a crystal lattice, each atom
surrounded by four closest neighbors.
Bonding between atoms in a germanium crystal
is covalent, i.e. it is carried out
pairs of valence electrons.
Each valence electron belongs to two
atoms.

Electric current in semiconductors

Valence electrons in a germanium crystal
much more strongly bound to atoms than in
metals
Therefore, the electron concentration
conductivity at room temperature in
semiconductors are many orders of magnitude smaller,
than metals.
Near absolute zero temperature in
In a germanium crystal, all electrons are occupied in
formation of connections.
Such an electric current crystal does not
conducts.

Electric current in semiconductors

Pair-electron bonds in a crystal
germanium and the formation of an electron-hole pair.

Electric current in semiconductors

As the temperature rises, some
some valence electrons can
get enough energy to
breaking covalent bonds.
Then free ones will appear in the crystal
electrons (conduction electrons).
At the same time, in places where connections are broken
vacancies are created that are not filled
electrons.
These vacancies are called
"holes".

Electric current in semiconductors

Vacant position may be filled
valence electron from the neighboring
pairs, then the hole moves to
a new place in the crystal.
If a semiconductor is placed in
electric field, then into an ordered
the movement involves not only
free electrons, but also holes,
who behave positively
charged particles.

Electric current in semiconductors

Therefore, the current I in the semiconductor
consists of electronic In and
hole IP currents:
I = In + Ip.
Electron-hole mechanism
conductivity appears only
in pure (i.e. without impurities)
semiconductors. It is called
own electric
conductivity of semiconductors.

Electric current in semiconductors

If there are impurities
electrical conductivity of semiconductors
changes a lot.
For example, adding phosphorus impurities to
silicon crystal in the amount of 0.001
atomic percent reduces specific
resistance by more than five
orders of magnitude.
Such a strong influence of impurities can
be explained based on the above
above ideas about the structure
semiconductors.

Electric current in semiconductors

A necessary condition for sharp
Reducing resistivity
semiconductor upon introduction of impurities
is the difference in valence of atoms
impurities from the valency of the main
atoms of the crystal.
Conductivity of semiconductors at
the presence of impurities is called
impurity conductivity.

Electric current in semiconductors

There are two types of impurity
conductivity – electronic and
hole conductivity.
Electronic conductivity
occurs when a crystal
germanium with tetravalent
atoms introduced pentavalent
atoms (for example, arsenic atoms,
As).

Electric current in semiconductors

Electric current in semiconductors

Electric current in semiconductors

The four valence electrons of the arsenic atom
included in the formation of covalent bonds with
four neighboring germanium atoms.
The fifth valence electron turned out to be redundant.
It is easily detached from the arsenic atom and
becomes free.
An atom that has lost an electron becomes
positive ion located at the site
crystal lattice.

Electric current in semiconductors

An impurity of atoms with valency,
exceeding the valency of the main atoms
semiconductor crystal is called
donor admixture.
As a result of its introduction into the crystal
there is a significant number of free
electrons.
This leads to a sharp decrease in specific
semiconductor resistance - in thousands and
even millions of times.
Conductor resistivity with
high content of impurities may
approach resistivity
metal conductor.

Electric current in semiconductors

Such conductivity
conditioned by free
electrons is called
electronic, but a semiconductor,
possessing electronic
conductivity is called
n-type semiconductor.

Electric current in semiconductors

Hole conduction occurs when
germanium crystal introduced trivalent
atoms (for example, indium atoms, In).

Electric current in semiconductors

In Fig. shows the indium atom that created with
using their valence electrons
covalent bonds with only three neighboring
germanium atoms.
To form a bond with the fourth atom
germanium the indium atom does not have an electron.
This missing electron could be
captured by an indium atom from a covalent bond
neighboring germanium atoms.
In this case, the indium atom turns into
negative ion located at the site
crystal lattice, and in a covalent
bonds between neighboring atoms, a vacancy is formed.

Electric current in semiconductors

An admixture of atoms capable of capturing
electrons, called acceptor
impurity.



As a result of the introduction of an acceptor impurity into
crystal, many covalent bonds are broken
connections and vacancies (holes) are formed.
Electrons can jump to these places from
neighboring covalent bonds, which leads to
chaotic wandering of holes throughout the crystal.

Electric current in semiconductors

Hole concentration in a semiconductor with
acceptor impurity significantly
exceeds the concentration of electrons, which
arose due to the mechanism of its own
electrical conductivity of a semiconductor: np >> nn.
This type of conductivity is called
hole conductivity.
Impurity semiconductor with hole
conductivity is called a semiconductor
p-type.
The main free charge carriers in
p-type semiconductors are holes.

Electric current in semiconductors

It should be emphasized that the hole
conductivity in reality
due to relay movement
by vacancies from one germanium atom to
other electrons that
make a covalent bond.
For n- and p-type semiconductors the law
Ohm is performed in certain
ranges of current and voltage at
condition of constant concentrations
free media.

In modern electronic technology
semiconductor devices play
exceptional role.
Over the past three decades they have almost
completely replaced electric vacuum
devices.
Any semiconductor device has
one or more electron-hole
transitions.
An electron-hole junction (or n–p junction) is the region of contact between two
semiconductors with different types
conductivity.

Electron-hole transition. Transistor

When two semiconductors n- and
p-types the diffusion process begins:
holes from the p-region move to the n-region, and electrons, on the contrary, from the n-region to the p-region.
As a result, in the n-region near the zone
contact concentration decreases
electrons and arises positively
charged layer.
In the p-region the concentration decreases
holes and occurs negatively
charged layer.

Electron-hole transition. Transistor

Thus, at the semiconductor boundary
an electrical double layer is formed,
whose electric field prevents
process of diffusion of electrons and holes
towards each other

Electron-hole transition. Transistor

The n–p junction has an amazing
property of one-sided
conductivity.
If a semiconductor with an n–p junction
connected to a current source so that
source positive pole
connected to the n-region, and
negative – with p-region, then
field strength in the blocking layer
increases.

Electron-hole transition. Transistor

Holes in the p-region and electrons in the n-region will shift away from the n–p junction, thereby increasing
concentrations of minority carriers in
barrier layer.
The current through the n–p junction is practically not
coming.
The voltage applied to the n–p junction in
This case is called the reverse.

Electron-hole transition. Transistor

Very minor inverse
the current is due only to its own
conductivity
semiconductor materials,
i.e. the presence of a small
concentrations of free
electrons in the p-region and holes in
n-regions.

Electron-hole transition. Transistor

If the n–p junction is connected to
source so that it is positive
the pole of the source was connected to the p-region, and the negative pole to the n-region, then the voltage
electric field in the blocking layer
will decrease, which makes it easier
transition of main carriers through
contact layer.

Electron-hole transition. Transistor

Holes from the p-region and electrons from
n-regions, moving towards each other
friend, will cross the n–p junction, creating a current in the direct
direction.
The current strength through the n–p junction in this
case will increase with
increasing source voltage.

Electron-hole transition. Transistor

The ability of an n–p junction to pass
current is practically only in one
direction is used in devices,
which are called
semiconductor diodes.
Semiconductor diodes
made from silicon crystals
or Germany.
During their manufacture, a crystal with any type of conductivity is melted into
admixture providing another type
conductivity.

Electron-hole transition. Transistor

Typical current-voltage
characteristics of silicon diode

Electron-hole transition. Transistor

Semiconductor devices are not
one, but with two n–p junctions
are called transistors.
Transistors are of two types:
p–n–p transistors and n–p–n transistors.

Electron-hole transition. Transistor

For example, a germanium transistor
p–n–p type is
small plate of germanium
with a donor impurity, i.e. from
n-type semiconductor.
This record creates two
areas with an acceptor impurity,
i.e. areas with hole
conductivity.

Electron-hole transition. Transistor

In an n–p–n-type transistor, the main
germanium plate has
p-type conductivity, and those created on
There are two regions with n-type conductivity.
The plate of the transistor is called the base
(B), one of the areas with
opposite type of conductivity
– collector (K), and the second –
emitter (E).

Electron-hole transition. Transistor

1.
2.
3.
4.
Electrolytes. Charge carriers in
electrolytes.
Electrolysis. Electrolytic
dissociation.
Faraday's law for electrolysis.
Faraday's combined law for
electrolysis.

Electric current in electrolytes

Electrolytes are commonly called
conducting media in which
flow of electric current
accompanied by transfer
substances.
Carriers of free charges in
electrolytes are
positive and negative
charged ions.

Electric current in electrolytes

The main representatives
electrolytes widely used in
technology are aqueous solutions
inorganic acids, salts and
grounds.
Passage of electric current through
electrolyte is accompanied by the release
substances on the electrodes.
This phenomenon is called
electrolysis.

Electric current in electrolytes

Electric current in electrolytes
represents the movement of ions of both
signs in opposite directions.
Positive ions move towards
negative electrode (cathode),
negative ions to positive
electrode (anode).
Ions of both signs appear in water
solutions of salts, acids and alkalis in
as a result of the splitting of part of the neutral
molecules.
This phenomenon is called electrolytic
dissociation.

Electric current in electrolytes

For example, copper chloride CuCl2
dissociates in aqueous solution into
copper and chlorine ions:
When connecting electrodes to
current source ions under the influence
electric field begin
orderly movement:
positive copper ions move towards
cathode, and negatively charged
chlorine ions - to the anode.

Electric current in electrolytes

Upon reaching the cathode, copper ions are neutralized
excess cathode electrons and
transform into neutral atoms
deposited on the cathode.
Chlorine ions, reaching the anode, give off
one electron.
After this, neutral chlorine atoms
combine in pairs to form molecules
chlorine Cl2.
Chlorine is released at the anode in the form of bubbles.

Electric current in electrolytes

The law of electrolysis was experimentally
established by the English physicist M. Faraday in
1833.
Faraday's law determines quantities
primary products released into
electrodes during electrolysis:
Mass m of substance released on
electrode, is directly proportional to the charge Q,
passed through the electrolyte:
m = kQ = kIt.
The value k is called electrochemical
equivalent.

Electric current in electrolytes

Mass of substance released on the electrode
equal to the mass of all ions arriving at
electrode:
Here m0 and q0 are the mass and charge of one ion,
– number of ions arriving at the electrode at
passing charge Q through the electrolyte.
Thus, the electrochemical equivalent
k is equal to the ratio of the mass m0 of a given ion
substance to its charge q0.

Electric current in electrolytes

Since the charge of an ion is equal to the product
valence of substance n on
elementary charge e (q0 = ne), then
expression for electrochemical
the equivalent of k can be written as:
F = eNA – Faraday's constant.
F = eNA = 96485 C/mol.

Electric current in electrolytes

Faraday's constant numerically
equal to the charge required
pass through electrolyte for
discharge on the electrode of one
mole of monovalent substance.
Faraday's law for electrolysis
takes the form:

Control questions

1.
2.
3.
4.
5.
6.
Charge carriers in metals.
Brief information about classical theory
conductivity of metals (Drude-Lorentz theory).
Ohm's law from classical theory (brief
conclusion).
Joule-Lenz law from classical theory
conductivity (brief conclusion).
What physical problems cannot be explained
classical theory of conductivity of metals.
Brief information about superconductivity.

Control questions

1.
2.
3.
4.
5.
6.
7.
8.
Electrons and holes. How are they formed in pure
semiconductors?
Conduction mechanism of pure semiconductors.
Donor and acceptor semiconductors.
Conduction mechanism of impurity semiconductors.
How to implement electron and hole
conductivity in semiconductors.
What is an electron-hole transition?
Explain why electron-hole transition
can rectify alternating current.
Transistor.

Control questions

What charge carriers are there in
electrolytes?
2. What are electrolytes? What's happened
electrolytic dissociation?
3. Faraday's law for electrolysis.
4. United law of electrolysis
Faraday.