Electric current in various environments. Electric current in metals

Application of superconductors: Powerful electromagnets that operate without energy consumption. (Particle accelerators.) If it were possible to create superconducting materials at temperatures close to room temperature, lossless transmission of electricity would become possible.

Liquids: conductors (solutions of acids, alkalis and salts); conductors (solutions of acids, alkalis and salts); dielectrics (distilled water, kerosene...) dielectrics (distilled water, kerosene...) semiconductors (molten sulfides, molten selenium). semiconductors (sulfide melts, molten selenium).

The degree of dissociation (the proportion of molecules that have broken up into ions) Depends on: the concentration of the solution; solution concentration; dielectric constant of the solution; dielectric constant of the solution; temperature (increases with increasing temperature). temperature (increases with increasing temperature).

Electric current in liquids Directed movement of positive ions to the cathode and negative ions to the anode Directed movement of positive ions to the cathode and negative ions to the anode In liquid metals - movement of positive ions to the cathode and electrons to the anode. In liquid metals - the movement of positive ions to the cathode and electrons to the anode.

The mass of the substance released on the electrode when a charge of 1 C is transferred through the solution. The mass of the substance released on the electrode when a charge of 1 C is transferred through the solution. The ratio of the mass of an ion of a substance to its charge. The ratio of the mass of an ion of a substance to its charge.

Faraday's constant Faraday's constant The charge that must be passed through a solution of a 1-valent substance in order for 1 mole of a substance to be released at the electrode. A charge that must be passed through a solution of a 1-valent substance in order for 1 mole of the substance to be released at the electrode.

Application of electrolysis Electroplating (coating). Electroplating (coating). Galvanoplasty (making copies of relief objects). Galvanoplasty (making copies of relief objects). Refining (cleaning) of metals. Refining (cleaning) of metals. Obtaining pure metals from melts of natural compounds. Obtaining pure metals from melts of natural compounds.

“Physics “Uniform linear motion”” - Equation of motion. Solution. Types of movement. Uniform straight motion. Build graphs. What is called mechanical movement. What kind of movement is called rectilinear? A value equal to the distance traveled per unit time. Speed ​​graph. Path schedule. The simplest type of movement. Equation of body motion. Coordinate graph.

““Internal energy” grade 10” - Internal energy of a monatomic ideal gas. Change in internal energy. The unit of energy is the Joule. Pressure. Isothermal process. Let's repeat the isoprocess graphs. Isoprocess graphs. Average kinetic energy of one atom. A thermodynamic system consists of a large number of microparticles. Two definitions of internal energy. Molecular-kinetic interpretation of the concept of internal energy.

“Energy Saving Program” - Gaps in window frames. Analysis of consumption of fuel and energy resources. Increasing energy efficiency. Color TV. Questionnaire. The problem of wise use of energy. Traffic lights. Energy consumption. Fridge. Energy efficiency program. Rational use of energy. Smart consumption. With respect to energy saving. Tap. Huge heat loss. Islands. Energy problems of humanity.

“Law of conservation of body momentum” - Projectile. Study the “body impulse”. Complete the drawing. Collection of problems. Graphic interpretation. Steel bullet. Newton. Direction of the impulse. Physical warm-up. System of interacting bodies. Motivation to learn new material. Core. Plan for studying a physical quantity. Nature. Problem solving. Law of conservation of momentum. Let us consider a system of two interacting bodies. Human. Experimental confirmation of the law.

“Field strength” - Which arrow in the figure indicates the direction of the electric field strength vector. Field strength of a point charge. Indicate the point at which the field strength can be zero. Creators of electrodynamics. The electrostatic field is created by a system of two balls. Thanks to the principle of superposition, to find the field strength of a system of charged particles at any point, it is enough to know the expression for the field strength of a point charge.

"Faraday" - Experimental research. First independent research. Induction current. Current. Transformer. Electric motor. Royal Institution. Black circle. Getting to know the biography. Closing moments. Start of work at the Royal Institution. Faraday is rightfully considered one of the founders of electrical engineering. Change in magnetic field. Faraday clearly demonstrated the temperature difference between individual parts of the flame.

Slide 1

Presentation on the topic: “Electric current in various media”

Performed by Alisa Kravtsova, ML No. 1, Magnitogorsk, 2009.

Slide 2

Electric current can flow in five different media:

Metals Vacuum Semiconductors Liquids Gases

Slide 3

Electric current in metals:

Electric current in metals is the ordered movement of electrons under the influence of an electric field. Experiments show that when current flows through a metal conductor, no substance is transferred, therefore, metal ions do not take part in the transfer of electric charge.

Slide 4

The experiments of Tolman and Stewart provide evidence that metals have electronic conductivity

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 G. The untwisted coil was sharply slowed down, and a short-term current arose in the circuit due to the inertia of the electrons.

Slide 5

Conclusion: 1.charge carriers in metals are electrons;

2. the process of formation of charge carriers - socialization of valence electrons; 3.current strength is directly proportional to voltage and inversely proportional to conductor resistance - Ohm’s law is satisfied; 4. technical application of electric current in metals: windings of motors, transformers, generators, wiring inside buildings, power transmission networks, power cables.

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Electric current in a vacuum

Vacuum is a highly rarefied gas in which the mean free path of a particle is greater than the size of the vessel, that is, the molecule flies from one wall of the vessel to the other without colliding with other molecules. As a result, there are no free charge carriers in the vacuum, and no electric current occurs. To create charge carriers in a vacuum, the phenomenon of thermionic emission is used.

Slide 7

THERMAL ELECTRON EMISSION is the phenomenon of “evaporation” of electrons from the surface of a heated metal.

A metal spiral coated with metal oxide is brought into a vacuum, it is heated with an electric current (incandescent circuit) and electrons evaporate from the surface of the spiral, the movement of which can be controlled using an electric field.

Slide 8

The slide shows the inclusion of a two-electrode lamp

This lamp is called a vacuum diode

Slide 9

This electron tube is called a vacuum TRIOD.

It has a third electrode - a grid, the sign of the potential on which controls the flow of electrons.

Slide 10

Conclusions: 1. charge carriers – electrons;

2. the process of formation of charge carriers – thermionic emission; 3.Ohm's law is not fulfilled; 4.technical application – vacuum tubes (diode, triode), cathode ray tube.

Slide 11

Electric current in semiconductors

When heated or illuminated, some electrons become able to move freely within the crystal, so that when an electric field is applied, directional movement of electrons occurs. Semiconductors are a cross between conductors and insulators.

Semiconductors are solid substances whose conductivity depends on external conditions (mainly heating and lighting).

Slide 12

As the temperature decreases, the resistance of metals decreases. In semiconductors, on the contrary, the resistance increases with decreasing temperature and near absolute zero they practically become insulators.

Dependence of resistivity ρ of a pure semiconductor on absolute temperature T.

Slide 13

Intrinsic conductivity of semiconductors

Germanium atoms have four weakly bound electrons in their outer shell. They are called valence electrons. In a crystal lattice, each atom is surrounded by its four nearest neighbors. The bond between atoms in a germanium crystal is covalent, that is, it is carried out by pairs of valence electrons. Each valence electron belongs to two atoms. The valence electrons in a germanium crystal are much more strongly bound to the atoms than in metals; Therefore, the concentration of conduction electrons at room temperature in semiconductors is many orders of magnitude lower than in metals. Near absolute zero temperature in a germanium crystal, all electrons are occupied in the formation of bonds. Such a crystal does not conduct electric current.

Slide 14

Formation of an electron-hole pair

With increasing temperature or increasing illumination, some of the valence electrons may receive energy sufficient to break covalent bonds. Then free electrons (conduction electrons) will appear in the crystal. At the same time, vacancies are formed in places where bonds are broken, which are not occupied by electrons. These vacancies are called “holes.”

Slide 15

Impurity conductivity of semiconductors

The conductivity of semiconductors in the presence of impurities is called impurity conductivity. There are two types of impurity conductivity - electronic and hole conductivity.

Slide 16

Electronic and hole conductivity.

If the impurity has a valence greater than the pure semiconductor, then free electrons appear. Conductivity – electronic, donor impurity, n-type semiconductor.

If the impurity has a valence lower than that of the pure semiconductor, then bond breaks—holes—appear. Conductivity is hole, acceptor impurity, p-type semiconductor.

Slide 17

Conclusions: 1. charge carriers – electrons and holes;

2. the process of formation of charge carriers - heating, illumination or the introduction of impurities; 3.Ohm's law is not fulfilled; 4.technical application – electronics.

Slide 18

Electric current in liquids

Electrolytes are commonly called conducting media in which the flow of electric current is accompanied by the transfer of matter. The carriers of free charges in electrolytes are positively and negatively charged ions. Electrolytes are aqueous solutions of inorganic acids, salts and alkalis.

Slide 19

The resistance of electrolytes decreases with increasing temperature, since the number of ions increases with increasing temperature.

Graph of electrolyte resistance versus temperature.

Slide 20

Electrolysis phenomenon

This is the release on the electrodes of substances included in electrolytes; Positively charged ions (anions) under the influence of an electric field tend to the negative cathode, and negatively charged ions (cations) tend to the positive anode. At the anode, negative ions give up extra electrons (oxidation reaction). At the cathode, positive ions receive the missing electrons (reduction reaction).

Slide 21

Faraday's laws of electrolysis.

The laws of electrolysis determine the mass of a substance released during electrolysis at the cathode or anode during the entire period of passage of electric current through the electrolyte.

k is the electrochemical equivalent of the substance, numerically equal to the mass of the substance released on the electrode when a charge of 1 C passes through the electrolyte.

Slide 22

Conclusion: 1. charge carriers – positive and negative ions;

2. the process of formation of charge carriers - electrolytic dissociation; 3.electrolytes obey Ohm’s law; 4. Application of electrolysis: production of non-ferrous metals (removal of impurities - refining); electroplating - obtaining coatings on metal (nickel plating, chrome plating, gold plating, silver plating, etc.); galvanoplasty - obtaining peelable coatings (relief copies).

Slide 23

Electric current in gases

Let's charge the capacitor and connect its plates to the electrometer. The charge on the capacitor plates lasts indefinitely; there is no charge transfer from one capacitor plate to another. Therefore, the air between the capacitor plates does not conduct current. Under normal conditions, there is no conduction of electric current by any gases. Let us now heat the air in the gap between the plates of the condenser by introducing a lit burner into it. The electrometer will indicate the appearance of current, therefore, at high temperatures, part of the neutral gas molecules breaks up into positive and negative ions. This phenomenon is called gas ionization.

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Electrical properties of substances Conductors Semiconductors Dielectrics Conduct electric current well These include metals, electrolytes, plasma ... The most used conductors are Au, Ag, Cu, Al, Fe ... They practically do not conduct electric current These include plastics, rubber, glass, porcelain, dry wood, paper... In terms of conductivity, they occupy an intermediate position between conductors and dielectrics Si, Ge, Se, In, As. Different substances have different electrical properties, but according to electrical conductivity they can be divided into 3 main groups: Substances

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The nature of electric current in metals Electric current in metal conductors does not cause any changes in these conductors, except for their heating. The concentration of conduction electrons in a metal is very high: in order of magnitude it is equal to the number of atoms per unit volume of the metal. Electrons in metals are in continuous motion. Their random movement resembles the movement of ideal gas molecules. This gave reason to believe that electrons in metals form a kind of electron gas. But the speed of random movement of electrons in a metal is much greater than the speed of molecules in a gas (it is approximately 105 m/s). Electric current in metals

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Papaleksi-Mandelshtam experiment Description of the experiment: Purpose: to find out what the conductivity of metals is. Installation: coil on a rod with sliding contacts, connected to a galvanometer. The course of the experiment: the coil spun at high speed, then stopped abruptly, and the galvanometer needle was observed to be thrown back. Conclusion: the conductivity of metals is electronic. Electric current in metals

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Metals have a crystalline structure. At the nodes of the crystal lattice there are positive ions that perform thermal vibrations near the equilibrium position, and free electrons move chaotically in the space between them. The electric field imparts acceleration to them in the direction opposite to the direction of the field strength vector. Therefore, in an electric field, randomly moving electrons are displaced in one direction, i.e. move in an orderly manner. - - - - - - - - - - Electric current in metals

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Dependence of conductor resistance on temperature As the temperature increases, the resistivity of the conductor increases. The resistance coefficient is equal to the relative change in the resistance of the conductor when heated by 1K. Electric current in metals

Slide 9

Intrinsic conductivity of semiconductors Impurity conductivity of semiconductors p – n junction and its properties

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Semiconductors Semiconductors are substances whose resistivity decreases with increasing temperature. Intrinsic conductivity of semiconductors. Impurity conductivity of semiconductors p–n junction and its properties. Electric current in semiconductors.

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Intrinsic conductivity of semiconductors Let us consider the conductivity of semiconductors based on silicon Si Silicon is a 4-valence chemical element. Each atom has 4 electrons in the outer electron layer, which are used to form pair-electronic (covalent) bonds with 4 neighboring atoms. Under normal conditions (low temperatures), there are no free charged particles in semiconductors, so the semiconductor does not conduct electric current Si Si Si Si Si - - - - - - - - Electric current in semiconductors

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Let's consider changes in a semiconductor with increasing temperature. As the temperature increases, the energy of the electrons increases and some of them leave the bonds, becoming free electrons. In their place remain uncompensated electric charges (virtual charged particles), called holes. Si Si Si Si Si - - - - - - + free electron hole + + - - Electric current in semiconductors

Slide 13

Thus, the electric current in semiconductors represents the ordered movement of free electrons and positive virtual particles - holes. Dependence of resistance on temperature R (Ohm) t (0C) metal R0 semiconductor As the temperature increases, the number of free charge carriers increases, the conductivity of semiconductors increases, and the resistance decreases. Electric current in semiconductors

Slide 14

Donor impurities The intrinsic conductivity of semiconductors is clearly insufficient for the technical application of semiconductors. Therefore, to increase conductivity, impurities are introduced into pure semiconductors (doped), which are donor and acceptor Si Si - - - As - - - Si - Si - - When doping 4-valent silicon Si with 5-valent arsenic As, one of the 5 electrons of arsenic becomes free. As is a positive ion. There is no hole! Such a semiconductor is called an n-type semiconductor; the main charge carriers are electrons, and the arsenic impurity that produces free electrons is called a donor impurity. Electric current in semiconductors

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Acceptor impurities Such a semiconductor is called a p-type semiconductor, the main charge carriers are holes, and the indium impurity that produces holes is called acceptor. If silicon is doped with trivalent indium, then indium lacks one electron to form bonds with silicon, i.e. a hole is formed. The base gives electrons and holes in equal numbers. The impurity is just holes. Si - Si - In - - - + Si Si - - Electric current in semiconductors

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Slide 17

Distilled water does not conduct electricity. Dip a crystal of table salt into distilled water and, lightly stirring the water, close the circuit. We will find that the light comes on. When salt is dissolved in water, free electric charge carriers appear. Electric current in liquids

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How do free carriers of electric charges arise? When a crystal is immersed in water, the water molecules are attracted to the positive sodium ions located on the surface of the crystal by their negative poles. To negative chlorine ions, water molecules turn positive poles. Electric current in liquids

Slide 19

Electrolytic dissociation is the breakdown of molecules into ions under the action of a solvent. The only mobile charge carriers in solutions are ions. A liquid conductor in which only ions are mobile charge carriers is called an electrolyte. Electric current in liquids

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How does current pass through the electrolyte? Let's lower the plates into the vessel and connect them to a current source. These plates are called electrodes. The cathode is a plate connected to the negative pole of the source. Anode is a plate connected to the positive pole of the source. Electric current in liquids

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Under the influence of electric field forces, positively charged ions move towards the cathode, and negative ions move towards the anode. At the anode, negative ions give up their extra electrons, and at the cathode, positive ions receive the missing electrons. Electric current in liquids

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Electrolysis At the cathode and anode, substances that are part of the electrolyte solution are released. The passage of electric current through an electrolyte solution, accompanied by chemical transformations of the substance and its release on the electrodes, is called electrolysis. Electric current in liquids

Slide 23

Law of electrolysis The mass m of the substance released on the electrode is directly proportional to the charge Q passing through the electrolyte: m = kQ = kIt. This is the law of electrolysis. The value of k is called the electrochemical equivalent. Faraday's experiments showed that the mass of the substance released during electrolysis depends not only on the magnitude of the charge, but also on the type of substance. Electric current in liquids

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Gases in their normal state are dielectrics because they consist of electrically neutral atoms and molecules and therefore do not conduct electricity. The insulating properties of gases are explained by the fact that atoms and molecules of gases in their natural state are neutral, uncharged particles. From here it is clear that in order to make a gas conductive, it is necessary in one way or another to introduce into it or create in it free charge carriers - charged particles. In this case, two cases are possible: either these charged particles are created by the action of some external factor or introduced into the gas from the outside - non-independent conductivity, or they are created in the gas by the action of the electric field itself existing between the electrodes - independent conductivity. Electric current in gases Electric current in gases

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Only ionized gases containing electrons, positive and negative ions can be conductors. Ionization is the process of separating electrons from atoms and molecules. Ionization occurs under the influence of high temperatures and various radiations (X-rays, radioactive, ultraviolet, cosmic rays), due to the collision of fast particles or atoms with atoms and gas molecules. The resulting electrons and ions make the gas a conductor of electricity. Ionization processes: electron impact thermal ionization photoionization Electric current in gases

Slide 27

Types of independent discharges Depending on the processes of formation of ions in the discharge at different gas pressures and voltages applied to the electrodes, several types of independent discharges are distinguished: glow spark corona arc Electric current in gases

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Glow discharge Glow discharge occurs at low pressures (in vacuum tubes). The discharge is characterized by a high electric field strength and a corresponding large potential drop near the cathode. It can be observed in a glass tube with flat metal electrodes soldered at the ends. Near the cathode there is a thin luminous layer called the cathode luminous film Electric current in gases