Electric current Project of an 8th grade student of Municipal Educational Institution “Secondary School No. 4”, Kimry Ilya Ustinova 201 4-2015
Electric current is the ordered (directed) movement of charged particles.
The current strength is equal to the ratio of the electric charge q passing through the cross section of the conductor to the time of its passage t. I= I - current strength (A) q- electric charge (C) t- time (s) g t
Unit of measurement of current strength The unit of current strength is the current strength at which sections of parallel conductors 1 m long interact with a force of 2∙10 -7 N (0.0000002 N). This unit is called AMPERE (A). -7
Ampere Andre Marie Born on January 22, 1775 in Polemiers near Lyon into an aristocratic family. He received a home education. He was engaged in research into the connection between electricity and magnetism (Ampère called this range of phenomena electrodynamics). Subsequently he developed the theory of magnetism. Ampère died in Marseille on June 10, 1836.
Ammeter Ammeter is a device for measuring current. The ammeter is connected in series with the device in which the current is measured.
Current measurement Electrical circuit Electrical circuit diagram
Voltage is a physical quantity that shows how much work an electric field does when moving a unit positive charge from one point to another. A q U=
The unit of measurement is the electrical voltage at the ends of the conductor at which the work done to move an electric charge of 1 C along this conductor is equal to 1 J. This unit is called VOLT (V)
Alessandro Volta is an Italian physicist, chemist and physiologist, one of the founders of the doctrine of electricity. Alessandro Volta was born in 1745, the fourth child in the family. In 1801 he received the title of count and senator from Napoleon. Volta died in Como on March 5, 1827.
Voltmeter Voltmeter is a measuring device electrical voltage. The voltmeter is connected to the circuit parallel to the section of the circuit between the ends of which the voltage is measured.
Voltage measurement Electrical circuit diagram Electrical circuit
Electrical resistance Resistance is directly proportional to the length of the conductor, inversely proportional to its cross-sectional area and depends on the substance of the conductor. R = ρ ℓ S R- resistance ρ - resistivity ℓ - length of conductor S - cross-sectional area
The cause of resistance is the interaction of moving electrons with ions of the crystal lattice.
The unit of resistance is taken to be 1 ohm. the resistance of such a conductor in which, at a voltage at the ends of 1 volt, the current strength is equal to 1 ampere.
Ohm Georg OM (Ohm) Georg Simon (March 16, 1787, Erlangen - July 6, 1854, Munich), German physicist, author of one of the fundamental laws, Ohm began researching electricity. In 1852, Ohm received the post of full professor. Ohm died on July 6, 1854. In 1881, at the electrical engineering congress in Paris, scientists unanimously approved the name of the resistance unit - 1 Ohm.
Ohm's Law The current strength in a section of a circuit is directly proportional to the voltage at the ends of this section and inversely proportional to its resistance. I = u R
Determining conductor resistance R=U:I Measuring current and voltage Electrical circuit diagram
APPLICATION OF ELECTRIC CURRENT
WHAT IS ELECTRIC CURRENT IN METALS?
Electric current in metals – This 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.
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.
E.RIKKE'S EXPERIENCE
German physicist Karl Ricke conducted an experiment in which electricity I passed it through three pressed, polished cylinders for a year - copper, aluminum and copper again. After completion, it was found that there were only minor traces of mutual penetration of metals, which did not exceed the results of ordinary diffusion of atoms in solids. Measurements taken with high degree accuracy, 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. It is natural to assume that the current in metals is carried out by free electrons.
Karl Victor Eduard Rikke
EXPERIENCE L.I. MANDELSHTAM AND N.D. PAPALEXI
Russian scientists L.I. Mandelstam and N.D. Papaleksi carried out an original experiment in 1913. The coil with the wire began to be twisted in different directions. They will spin it clockwise, then abruptly stop it 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. And so it happened. We connected a telephone to the ends of the wire and heard a sound, which meant that current was flowing through it.
Mandelstam Leonid Isaakovich
Nikolay Dmitrievich Papalexi (1880-1947)
THE EXPERIENCE OF T. STEWART AND R. TOLMAN
The experience of Mandelstam and Papaleksi was repeated in 1916 by American scientists Tolman and Stewart.
- A coil with a large number of turns of thin wire was brought 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.
Butler Stuart Thomas
Richard Chase Tolman
CLASSICAL ELECTRONIC THEORY
The assumption that electrons are responsible for the electric current in metals existed even before the experiment of Stewart and Tolman. In 1900, the German scientist P. Drude, based on the hypothesis about the existence of free electrons in metals, created his electronic theory of metal conductivity, named after classical electron theory . According to this theory, electrons in metals behave like an electron gas, much like an ideal gas. It fills the space between the ions that form the metal crystal lattice
The figure shows the trajectory of one of the free electrons in the crystal lattice of a metal
BASIC PROVISIONS OF THE THEORY:
- The presence of a large number of electrons in metals contributes to their good conductivity.
- Under the influence of an external electric field, ordered movement is superimposed on the random movement of electrons, i.e. current arises.
- The strength of the electric current passing through a metal conductor is equal to:
- Since the internal structure of different substances is different, the resistance will also be different.
- With an increase in the chaotic movement of particles of a substance, the body heats up, i.e. heat release. The Joule-Lenz law is observed here:
l = e * n * S * Ū d
SUPERCONDUCTIVITY OF METALS AND ALLOYS
- Some metals and alloys have superconductivity, the property of having strictly zero electrical resistance when they reach a temperature below certain value(critical temperature).
The phenomenon of superconductivity was discovered by the Dutch physicist H. Kamerling - Ohness in 1911 for mercury (T cr = 4.2 o K).
AREA OF ELECTRIC CURRENT APPLICATION:
- obtaining strong magnetic fields
- transmission of electricity from source to consumer
- powerful electromagnets with superconducting windings in generators, electric motors and accelerators, in heating devices
Currently, there is a big problem in the energy sector associated with large losses during the transmission of electricity through wires.
Possible solution to the problem:
Construction of additional power lines - replacement of wires with larger cross-sections - increase in voltage - phase splitting
Presentation on physics on the topic: “Electric current” Completed by: Viktor_Sad Kapustin Lyceum No. 18; 10 IV grade Teacher I.A. Boyarina 1. Basic information about electric current 2. Current strength 3. Resistance 4. Voltage 5. Ohm's law for a section of a circuit 6. Ohm's law for a complete circuit 7. Connecting an ammeter and voltmeter 8. Tests
Electric current is the ordered movement of free electric charges under the influence of an electric field. Experience will help us understand this... To the beginning...
Current strength. Current strength is a physical quantity that shows the charge passing through a conductor per unit time. Mathematically, this definition is written in the form of a formula: I - current strength (A) q - charge (C) t - time (s) To measure the current strength, a special device is used - an ammeter. It is included in the open circuit in the place where the current strength needs to be measured. Unit of current measurement... Back to top...
Resistance. 1. The main electrical characteristic of a conductor is resistance. 2. Resistance depends on the material of the conductor and its geometric dimensions: R =? *(?/S), where? - specific resistance of the conductor (a value depending on the type of substance and its condition). The unit of resistivity is 1 Ohm * m. That's it in a nutshell. Now in more detail... To the beginning...
Voltage. Voltage is the potential difference between 2 points of an electrical circuit; in a section of a circuit that does not contain electromotive force, is equal to the product of the current strength and the resistance of the section. U = I * R To the beginning... That's it in a nutshell. Now more details...
Ohm's law for a section of a circuit: The current strength in a section of a circuit is directly proportional to the voltage at the ends of the conductor and inversely proportional to its resistance. I=U/R To the beginning... And to prove it?!
Ohm's law for a complete circuit: The current in a complete circuit is equal to the ratio of the circuit's emf to its total resistance. I = ? / (R + r), where? – EMF, and (R + r) – total resistance of the circuit (the sum of the resistances of the external and internal sections of the circuit). Back to top... More details...
Connecting an ammeter and voltmeter: The ammeter is connected in series with the conductor in which the current is measured. The voltmeter is connected in parallel to the conductor on which the voltage is measured. R R To the beginning...
An experiment explaining the determination of electric current: Two electrometers with large balls are placed at some distance from each other. One of them is electrified with a charged stick, which can be seen by the deflection of the arrow. Then they take the conductor by the insulating handle, in the middle of which a neon light bulb is soldered. Connect an electrified ball with a non-electrified one. The light flashes for a moment. Based on the deviations of the arrows on the electrometers, they come to the conclusion: the left ball loses part of its charge, and the right one acquires the same charge. Explain... Back to top...
Let's think about what happens in this experiment: Since the charge of one ball decreased and the charge of the other increased, this means that electric charges passed through the conductor that connected the balls, which was accompanied by the glow of the light bulb. In this case, we say that an electric current flows through the conductor. What makes charges move along a conductor? There can only be one answer - an electric field. Any current source has two poles, one pole is positively charged, the other is negatively charged. When a current source operates, an electric field is created between its poles. When a conductor is connected to these poles, an electric field created by the current source also appears in it. Under the influence of this electric field, free charges inside the conductor begin to move along the conductor from one pole to the other. An ordered movement of electric charges occurs. This is electric current. If the conductor is disconnected from the current source, the electric current stops. To the beginning...
The unit of current is 1 ampere (1 A = 1 C/s). The unit of current is 1 ampere (1 A = 1 C/s). To establish this unit, the magnetic action of current is used. It turns out that conductors carrying parallel, identically directed currents are attracted to each other. This attraction is stronger, the longer the length of these conductors and the smaller the distance between them. 1 ampere is taken to be the strength of a current that causes between two thin infinitely long parallel conductors located in a vacuum at a distance of 1 m from each other, an attraction with a force of 0.0000002 N for each meter of their length. And on the right you see an ammeter: Back to the beginning...
Let's assemble a circuit from a light bulb and a current source. When the circuit is closed, the light will, of course, light up. Now let's connect a piece of steel wire to the chain. The light will become dimmer. Let us now replace the steel wire with nickel wire. The filament intensity of the light bulb will decrease further. In other words, we observed a weakening of the thermal effect of the current or a decrease in the current power. The conclusion follows from experience: an additional conductor connected in series to the circuit reduces the current in it. In other words, the conductor provides resistance to current. Different conductors (pieces of wire) offer different resistance to current. So, the resistance of a conductor depends on the type of substance from which the conductor is made. Back to top... Are there other reasons that affect conductor resistance?
Consider the experiment depicted in the figure. The letters A and B indicate the ends of the thin nickel wire, and the letter K indicates the moving contact. By moving it along the wire, we change the length of the section that is included in the chain (section AK). By moving pin K to the left, we will see that the light bulb will burn brighter. Moving the contact to the right will cause the light to glow dimmer. From this experiment it follows that a change in the length of the conductor included in the circuit leads to a change in its resistance. To the top... What devices are there for changing the length of a conductor?
There are special devices - rheostats. The principle of their operation is the same as in the experiment with wire we considered. The only difference is that to reduce the size of the rheostat, the wire is wound on a porcelain cylinder fixed in the body, and the moving contact (they say: “slider” or “slider”) is mounted on a metal rod, which also serves as a conductor. So, a rheostat is an electrical device whose resistance can be changed. Rheostats are used to regulate the current in a circuit. And the third reason affecting the resistance of a conductor is its cross-sectional area. As it increases, the resistance of the conductor decreases. The resistance of conductors also changes as their temperature changes. To the beginning...
The same current passes through both lamps: 0.4 A. But the large lamp burns brighter, that is, it works with more power than the small one. It turns out that the power can be different with the same current strength? In our case, the voltage created by the rectifier is less than the voltage created by the city power grid. Therefore, when the current strength is equal, the current power in the circuit with a lower voltage is less. According to international agreement, the unit of electrical voltage is 1 volt. This is the voltage that, at a current of 1 A, creates a current of 1 W. To the beginning... Vol - this is understandable. We all know 220 V, which should not be touched. But how to measure these 220?
To measure voltage, a special device is used - a voltmeter. It is always connected parallel to the ends of the section of the circuit on which the voltage is to be measured. Appearance school demonstration voltmeter is shown in the figure on the right. To the beginning...
Let us establish the dependence of current on voltage experimentally: The figure shows an electrical circuit consisting of a current source - a battery, an ammeter, a spiral of nickel wire, a key and a voltmeter connected in parallel to the spiral. Close the circuit and note the instrument readings. Then a second battery of the same type is connected to the first battery and the circuit is closed again. The voltage on the coil will double, and the ammeter will show twice the current. With three batteries, the voltage on the coil triples, and the current increases by the same amount. Thus, experience shows that no matter how many times the voltage applied to the same conductor increases, the current strength in it increases by the same amount. In other words, the current in a conductor is directly proportional to the voltage at the ends of the conductor. Well, then... We can go back to the beginning...
To answer the question of how the current strength in a circuit depends on resistance, let us turn to experience. The figure shows an electrical circuit in which the current source is a battery. Conductors with different resistances are included in this circuit in turn. The voltage at the ends of the conductor is maintained constant during the experiment. This is monitored using the voltmeter readings. The current in the circuit is measured with an ammeter. The table below shows the results of experiments with three different conductors: Continue experiment... Back to top...
In the first experiment, the resistance of the conductor is 1 Ohm and the current in the circuit is 2 A. The resistance of the second conductor is 2 Ohms, i.e. twice as much, and the current is half as strong. And finally, in the third case, the circuit resistance increased four times and the current decreased by the same amount. Let us recall that the voltage at the ends of the conductors in all three experiments was the same, equal to 2 V. Summarizing the results of the experiments, we come to the conclusion: the current strength in the conductor is inversely proportional to the resistance of the conductor. Let's express our two experiences in graphs: Back to top...
The internal section of the circuit, like the external one, provides some resistance to the current passing through it. It is called the internal resistance of the source. For example, the internal resistance of a generator is due to the resistance of the windings, and the internal resistance of galvanic cells is due to the resistance of the electrolyte and electrodes. Let's consider the simplest electrical circuit consisting of a current source and resistance in external circuit. The internal section of the circuit, located inside the current source, as well as the external one, has electrical resistance. We will denote the resistance of the external section of the circuit by R, and the resistance of the internal section by r. To the beginning... Let's continue...
And how Ohm derived his law for a complete circuit: EMF in closed circuit is equal to the sum of the voltage drops in the external and internal sections. Let us write, according to Ohm’s law, expressions for the voltages in the external and internal sections of the circuit. By adding up the resulting expressions and expressing the current strength from the resulting equality, we obtain a formula reflecting Ohm’s law for the complete circuit. To the beginning...
Tests: 1. The figure shows the scale of an ammeter connected to an electrical circuit. What is the current in the circuit? A. 12 ± 1 A B. 18 ± 2 A C. 14 ± 2 A 2. A proton flies into the space between two charged bars. What trajectory will it follow? A. 1 B. 2 C. 3 D. 4 3. The girl measured the current strength in the device at different meanings voltage at its terminals. The measurement results are presented in the figure. What was most likely the current value in the device at 0 V? A. 0 mA B. 5 mA D. 10 mA Back to top...
The answer is not correct... Bad tests... I want to go to the beginning... This is, of course, sad, but maybe we can try again?!
Bravo!!! It's right!!! Too easy for me... So back to the beginning... I love this kind of game! Let's repeat!!!
Electric current Electric current is the ordered (directed) movement of electric charges. Conduction current (current in conductors) is the movement of microcharges in a macrobody. Convection current is the movement of macroscopic charged bodies in space. Current in a vacuum is the movement of microcharges in a vacuum.
Electric current In a conductor, under the influence of an applied electric field, free electric charges move: positive - along the field, negative - against the field. Charge carriers perform a complex movement: 1) chaotic with an average speed v ~ (10 3 ÷ 10 4 m/s), 2) directed with an average speed v ~ E (fractions of mm/s).
Thus, the average speed of directional motion of electrons is much less than the average speed of their chaotic motion. The low average speed of directed motion is explained by their frequent collisions with ions of the crystal lattice. At the same time, any change in the electric field is transmitted along the wires at a speed equal to the speed of propagation of the electromagnetic wave - (3·10 8 m/s). Therefore, the movement of electrons under the influence of an external field occurs along the entire length of the wire almost simultaneously with the application of the signal.
When charges move, their equilibrium distribution is disrupted. Consequently, the surface of the conductor is no longer equipotential and the electric field strength vector E is not directed perpendicular to the surface, since for the movement of charges it is necessary that E τ 0 on the surface. For this reason, there is an electric field inside the conductor, which is equal to zero only in the case of an equilibrium distribution charges on the surface of a conductor.
Conditions for the appearance and existence of conduction current: 1. The presence of free charge carriers in the medium, i.e. charged particles that can move. In a metal these are conduction electrons; in electrolytes – positive and negative ions; in gases - positive, negative ions and electrons.
Conditions for the appearance and existence of conduction current: 2. The presence in the medium of an electric field, the energy of which would be spent on the movement of electric charges. In order for the current to last, the energy of the electric field must be replenished all the time, i.e. you need a source of electrical energy - a device in which any energy is converted into the energy of an electric field.
– the current strength is numerically equal to the charge passing through the cross section of the conductor per unit time. In SI: . The movement of charge carriers of one sign is equivalent to the movement of charge carriers of the opposite sign in the opposite direction. If the current is created by two types of carriers:
Outside forces. Electromotive force. Voltage If the current carriers in a circuit are affected only by the force of the electrostatic field, then the carriers move, which leads to equalization of potentials at all points of the circuit and to the disappearance of the electric field. Therefore, for the existence of direct current, it is necessary to have a device in the circuit that creates and maintains a potential difference φ due to the work of forces of non-electrical origin. Such devices are called current sources (generators - mechanical energy is converted; batteries - the energy of a chemical reaction between electrodes and electrolyte).
Outside forces. Electromotive force. Third-party forces of non-electric origin acting on charges from current sources. Due to the field of external forces, electric charges move inside the current source against the forces of the electrostatic field. Consequently, a potential difference is maintained at the ends of the external circuit and a constant current flows in the circuit.
Outside forces. Electromotive force. External forces do work to move electric charges. Electromotive force (emf - E) is a physical quantity determined by the work done by external forces when moving a single positive charge
Ohm's law for a homogeneous section of a circuit A section of a circuit that does not contain a source of emf is called homogeneous. Ohm's law in integral form: the current is directly proportional to the voltage drop across a homogeneous section of the circuit and inversely proportional to the resistance of this section.
Ohm's law is not a universal relationship between current and voltage. a) Current in gases and semiconductors obeys Ohm’s law only at small U. b) Current in vacuum does not obey Ohm’s law. Boguslavsky-Langmuir law (law 3/2): I ~ U 3/2. c) in an arc discharge - as the current increases, the voltage drops. Disobedience to Ohm's law is due to the dependence of resistance on current.
Ohm's Law In SI, resistance R is measured in ohms. The value of R depends on the shape and size of the conductor, as well as on the properties of the material from which it is made. For a cylindrical conductor: where ρ is the electrical resistivity [Ohm m], for metals its value is about 10 –8 Ohm m.
The resistance of a conductor depends on its temperature: α is the temperature coefficient of resistance, for pure metals (at not very low temperaturesα 1 / 273 K -1, ρ 0, R 0 – respectively, the resistivity and resistance of the conductor at t = 0 o C. This dependence ρ(t) is explained by the fact that with increasing temperature the intensity of the chaotic movement of positive ions of the crystal lattice increases, directed the movement of electrons is slowed down.
Ohm's law for a non-uniform section of a circuit Non-uniform is a section of a circuit containing a source of emf. A closed circuit contains a source of emf, which in direction 1–2 promotes the movement of positive charges. E is the field strength of Coulomb forces, E st is the field strength of external forces.
Ohm's law for an inhomogeneous section of a circuit The work done by Coulomb and external forces to move a single positive charge q 0+ is the voltage drop (voltage). Since points 1, 2 were chosen arbitrarily, the resulting relations are valid for any two points of the electrical circuit:
Work and power of electric current Joule-Lenz Law When free electrons collide with ions of a crystal lattice, they transfer to the ions excess kinetic energy, which they acquire during accelerated movement in an electric field. As a result of these collisions, the amplitude of vibrations of the ions near the nodes of the crystal lattice increases (the thermal movement of the ions becomes more intense). Consequently, the conductor heats up: temperature is a measure of the intensity of the chaotic movement of atoms and molecules. The released heat Q is equal to the work done by the current A.
Kirchhoff's laws Used to calculate branched DC circuits. An unbranched electrical circuit is a circuit in which all elements of the circuit are connected in series. An electrical circuit element is any device included in an electrical circuit. An electrical circuit node is a point in a branched circuit where more than two conductors converge. A branch of a branched electrical circuit is a section of a circuit between two nodes.
Kirchhoff's second law (generalized Ohm's law): in any closed circuit, arbitrarily chosen in a branched electrical circuit, the algebraic sum of the products of the current strengths I i and the resistance of the corresponding sections R i of this circuit is equal to the algebraic sum of the emf. in the circuit.
Kirchhoff's second law The current is considered positive if its direction coincides with the conventionally selected direction of traversal of the circuit. E.m.f. is considered positive if the direction of the bypass is from – to + the current source, i.e. e.m.f. creates a current coinciding with the direction of bypass.
The procedure for calculating a branched circuit: 1. Arbitrarily select and indicate on the drawing the direction of the current in all sections of the circuit. 2. Count the number of nodes in the chain (m). Write Kirchhoff's first law for each of the (m-1) nodes. 3. Select arbitrarily closed contours in the circuit, arbitrarily select directions for traversing the contours. 4. Write Kirchhoff’s second law for contours. If the chain consists of p-branches and m-nodes, then the number of independent equations of Kirchhoff’s 2nd law is (p-m+1).
Slide 1
Lecture plan 1. The concept of conduction current. Current vector and current strength. 2. Differential form of Ohm's law. 3. Serial and parallel connection of conductors. 4. The reason for the appearance of an electric field in a conductor, the physical meaning of the concept of external forces. 5. Derivation of Ohm's law for the entire circuit. 6. Kirchhoff's first and second rules. 7. Contact potential difference. Thermoelectric phenomena. 8. Electric current in various environments. 9. Current in liquids. Electrolysis. Faraday's laws.
Slide 2
Electric current is the orderly movement of electric charges. Current carriers can be electrons, ions, and charged particles. If an electric field is created in a conductor, then free electric charges in it will begin to move - a current appears, called conduction current. If a charged body moves in space, then the current is called convection. 1. The concept of conduction current. Current vector and current strength
Slide 3
The direction of current is usually taken to be the direction of movement of positive charges. For the occurrence and existence of current it is necessary: 1. the presence of free charged particles; 2.presence of an electric field in the conductor. The main characteristic of current is the current strength, which is equal to the amount of charge passing through the cross-section of the conductor in 1 second. Where q is the amount of charge; t – charge transit time; Current strength is a scalar quantity.
Slide 4
Electric current over the surface of a conductor can be distributed unevenly, so in some cases the concept of current density is used. The average current density is equal to the ratio of the current strength to the cross-sectional area of the conductor. Where j is the change in current; S – change in area.
Slide 5
Current Density
Slide 6
In 1826, the German physicist Ohm experimentally established that the current strength J in a conductor is directly proportional to the voltage U between its ends. Where k is the proportionality coefficient, called electrical conductivity or conductivity; [k] = [Sm] (Siemens). The quantity is called the electrical resistance of the conductor. Ohm's law for a section of an electrical circuit that does not contain a current source 2. Differential form of Ohm's law
Slide 7
We express from this formula R Electrical resistance depends on the shape, size and substance of the conductor. The resistance of a conductor is directly proportional to its length l and inversely proportional to its cross-sectional area S Where characterizes the material from which the conductor is made and is called the resistivity of the conductor.
Slide 8
Let us express : The resistance of the conductor depends on the temperature. As the temperature increases, the resistance increases. WhereR0 is the resistance of the conductor at 0С; t – temperature; – temperature coefficient of resistance (for metal 0.04 deg-1). The formula is also valid for resistivity. Where0 is the resistivity of the conductor at 0С.
Slide 9
At low temperatures (
Slide 10
Let's rearrange the terms of the expression Where I/S=j – current density; 1/= – specific conductivity of the conductor substance; U/l=E – electric field strength in the conductor. Ohm's law in differential form.
Slide 11
Ohm's law for a homogeneous section of a chain. Differential form of Ohm's law.
Slide 12
3. Series and parallel connection of conductors
Series connection of conductors I=const (according to the law of conservation of charge); U=U1+U2 Rtot=R1+R2+R3 Rtot=Ri R=N*R1 (For N identical conductors) R1 R2 R3
Slide 13
Parallel connection conductors U=const I=I1+I2+I3 U1=U2=U R1 R2 R3 For N identical conductors
Slide 14
4. The reason for the appearance of electric current in the conductor. The physical meaning of the concept of external forces To maintain a constant current in a circuit, it is necessary to separate positive and negative charges in the current source; for this, forces of non-electrical origin, called external forces, must act on the free charges. Due to the field created by external forces, electric charges move inside the current source against the forces of the electrostatic field.
Slide 15
Due to this, a potential difference is maintained at the ends of the external circuit and in the circuit goes constant electricity. Extraneous forces cause the separation of unlike charges and maintain a potential difference at the ends of the conductor. An additional electric field of external forces in a conductor is created by current sources (galvanic cells, batteries, electric generators).
Slide 16
EMF of a current source The physical quantity equal to the work of external forces to move a single positive charge between the poles of the source is called the electromotive force of the current source (EMF).
Slide 17
Ohm's Law for a non-uniform section of a circuit
Slide 18
5. Derivation of Ohm's law for a closed electrical circuit
Let a closed electrical circuit consist of a current source with , with internal resistance r and an external part with resistance R. R is external resistance; r – internal resistance. where is the voltage across the external resistance; A – work on moving charge q inside the current source, i.e. work on internal resistance.
Slide 19
Then since, we rewrite the expression for : , Since according to Ohm’s law for a closed electrical circuit ( = IR) IR and Ir are the voltage drop on the external and internal sections of the circuit,
Slide 20
That is Ohm's law for a closed electrical circuit. In a closed electrical circuit, the electromotive force of the current source is equal to the sum of the voltage drops in all sections of the circuit.
Slide 21
6. Kirchhoff's first and second rules The first Kirchhoff rule is the condition for constant current in the circuit. The algebraic sum of the current strength in the branching node is equal to zero where n is the number of conductors; Ii – currents in conductors. Currents approaching the node are considered positive, and currents leaving the node are considered negative. For node A, the first Kirchhoff rule will be written:
Slide 22
Kirchhoff's first rule A node in an electrical circuit is the point at which at least three conductors converge. The sum of the currents converging at a node is equal to zero - Kirchhoff’s first rule. Kirchhoff's first rule is a consequence of the law of conservation of charge - electric charge cannot accumulate in a node.
Slide 23
Kirchhoff's second rule Kirchhoff's second rule is a consequence of the law of conservation of energy. In any closed circuit of a branched electrical circuit, the algebraic sum Ii of the resistance Ri of the corresponding sections of this circuit is equal to the sum of the emf i applied in it
Slide 24
Kirchhoff's second rule
Slide 25
To create an equation, you need to select the direction of traversal (clockwise or counterclockwise). All currents coinciding in direction with the circuit bypass are considered positive. The EMF of current sources is considered positive if they create a current directed towards bypassing the circuit. So, for example, Kirchhoff’s rule for parts I, II, III. I I1r1 + I1R1 + I2r2 + I2R2 = – 1 –2 II–I2r2 – I2R2 + I3r3 + I3R3= 2 + 3 IIII1r1 + I1R1 + I3r3 + I3R3 = – 1 + 3 Based on these equations, the circuits are calculated.
Slide 26
7. Contact potential difference. Thermoelectric phenomena Electrons, which have the greatest kinetic energy, can fly out of the metal into the surrounding space. As a result of the emission of electrons, an “electron cloud” is formed. There is a dynamic equilibrium between the electron gas in the metal and the “electron cloud”. The work function of an electron is the work that must be done to remove an electron from a metal into airless space. The surface of the metal is an electrical double layer, similar to a very thin capacitor.
Slide 27
The potential difference between the capacitor plates depends on the work function of the electron. Where is the electron charge; – contact potential difference between the metal and the environment; A – work function (electron-volt – E-V). The work function depends on the chemical nature of the metal and the condition of its surface (pollution, moisture).
Slide 28
Volta's laws: 1. When two conductors made of different metals are connected, a contact potential difference arises between them, which depends only on the chemical composition and temperature. 2. The potential difference between the ends of a circuit consisting of metal conductors connected in series, located at the same temperature, does not depend on the chemical composition of the intermediate conductors. It is equal to the contact potential difference that arises when the outermost conductors are directly connected.
Slide 29
Let's consider a closed circuit consisting of two metal conductors 1 and 2. The emf applied to this circuit is equal to the algebraic sum of all potential jumps. If the temperatures of the layers are equal, then =0. If the temperatures of the layers are different, for example, then Where is a constant characterizing the properties of the contact of two metals. In this case, a thermoelectromotive force appears in a closed circuit, directly proportional to the temperature difference between both layers.
Slide 30
Thermoelectric phenomena in metals are widely used to measure temperature. For this, thermoelements or thermocouples are used, which are two wires made of various metals and alloys. The ends of these wires are soldered. One junction is placed in a medium whose temperature T1 needs to be measured, and the second junction is placed in a medium with a constant known temperature. Thermocouples have a number of advantages over conventional thermometers: they allow you to measure temperatures in a wide range from tens to thousands of degrees of the absolute scale.
Slide 31
Gases under normal conditions are dielectrics R => ∞, consisting of electrically neutral atoms and molecules. When gases are ionized, electric current carriers (positive charges) appear. Electric current in gases is called gas discharge. To carry out a gas discharge, there must be an electric or magnetic field to the tube with ionized gas.
Slide 32
Gas ionization is the disintegration of a neutral atom into a positive ion and electron under the influence of an ionizer ( external influences– strong heating, ultraviolet and x-rays, radioactive radiation, when bombarding atoms (molecules) of gases with fast electrons or ions). Ion electron atom neutral
Slide 33
A measure of the ionization process is the ionization intensity, measured by the number of pairs of oppositely charged particles appearing in a unit volume of gas in a unit time period. Impact ionization is the separation of one or more electrons from an atom (molecule), caused by the collision of electrons or ions accelerated by an electric field in a discharge with atoms or molecules of a gas.
Slide 34
Recombination is the joining of an electron with an ion to form a neutral atom. If the action of the ionizer stops, the gas again becomes dialectic. electron ion
Slide 35
1. A non-self-sustaining gas discharge is a discharge that exists only under the influence of external ionizers. Current-voltage characteristics of a gas discharge: as U increases, the number of charged particles reaching the electrode increases and the current increases to I = Ik, at which all charged particles reach the electrodes. In this case, U=Uk saturation current Where e is the elementary charge; N0 – maximum number pairs of monovalent ions formed in a volume of gas in 1 s.
Slide 36
2. Self-sustaining gas discharge – a discharge in a gas that persists after the external ionizer stops operating. Maintained and developed due to impact ionization. A non-self-sustaining gas discharge becomes independent at Uз – ignition voltage. The process of such a transition is called electrical breakdown of the gas. There are:
Slide 37
Corona discharge – occurs at high pressure and in a sharply inhomogeneous field with a large curvature of the surface, used in the disinfection of agricultural seeds. Glow discharge – occurs at low pressures, used in gas-light tubes and gas lasers. Spark discharge - at P = Ratm and at large electric fields - lightning (currents up to several thousand Amperes, length - several kilometers). Arc discharge - occurs between closely spaced electrodes, (T = 3000 °C - at atmospheric pressure. Used as a light source in powerful spotlights, in projection equipment.
Slide 38
Plasma is a special state of aggregation of a substance, characterized by a high degree of ionization of its particles. Plasma is divided into: – weakly ionized ( – fractions of a percent – upper layers of the atmosphere, ionosphere); – partially ionized (several%); – fully ionized (sun, hot stars, some interstellar clouds). Artificially created plasma is used in gas-discharge lamps, plasma sources of electrical energy, and magnetodynamic generators.
Slide 39
Emission phenomena: 1. Photoelectron emission - the ejection of electrons from the surface of metals in a vacuum under the influence of light. 2. Thermionic emission - the emission of electrons by solid or liquid bodies when they are heated. 3. Secondary electron emission - a counter flow of electrons from a surface bombarded by electrons in a vacuum. Devices based on the phenomenon of thermionic emission are called electron tubes.
Slide 40
In solids, an electron interacts not only with its own atom, but also with other atoms of the crystal lattice, and the energy levels of the atoms are split to form an energy band. The energy of these electrons may lie within shaded regions called allowed energy bands. Discrete levels are separated by areas of prohibited energy values - forbidden zones (their width is commensurate with the width of the forbidden zones). Differences in Electrical Properties various types solids is explained by: 1) the width of the forbidden energy bands; 2) different filling of allowed energy bands with electrons
Slide 41
Many liquids conduct electricity very poorly (distilled water, glycerin, kerosene, etc.). Aqueous solutions of salts, acids and alkalis conduct electricity well. Electrolysis is the passage of current through a liquid, causing the release of substances that make up the electrolyte on the electrodes. Electrolytes are substances with ionic conductivity. Ionic conductivity is the ordered movement of ions under the influence of an electric field. Ions are atoms or molecules that have lost or gained one or more electrons. Positive ions are cations, negative ions are anions.
Slide 42
An electric field is created in the liquid by electrodes (“+” – anode, “–” – cathode). Positive ions (cations) move towards the cathode, negative ions move towards the anode. The appearance of ions in electrolytes is explained by electrical dissociation - the disintegration of molecules of a soluble substance into positive and negative ions as a result of interaction with the solvent (Na+Cl-; H+Cl-; K+I-...). The degree of dissociation α is the number of molecules n0 dissociated into ions to the total number of molecules n0. During the thermal movement of ions, reverse process recombination of ions, called recombination.
Slide 43
M. Faraday's laws (1834). 1. The mass of the substance released on the electrode is directly proportional electric charge q passed through the electrolyte or Where k is the electrochemical equivalent of the substance; equal to the mass of the substance released when a unit amount of electricity passes through the electrolyte. Where I is the direct current passing through the electrolyte.
Slide 46
THANK YOU FOR YOUR ATTENTION
View all slides
Loading...
