Electric current is an ordered movement of charged particles. In solids, this is the movement of electrons (negatively charged particles) in liquid and gaseous bodies, this is the movement of ions (positively charged particles). Moreover, the current can be constant and variable, and they have a completely different movement of electric charges. In order to understand and master the topic of current flow in conductors, perhaps you first need to understand the basics of electrophysics in more detail. That's where I'll start.

So, how does electric current flow in general? We know that matter is made up of atoms. it elementary particles substances. The structure of the atom is similar to ours solar system where the nucleus of an atom is located in the center. It consists of tightly pressed together protons (positive electrical particles) and neutrons (electrically neutral particles). Around this core great speed electrons (smaller particles with a negative charge) rotate in their orbits. Different substances have different numbers of electrons and orbits in which they rotate. Atoms of solids have a so-called crystal lattice. This is the structure of matter, in which atoms are arranged in a certain order relative to each other.

And where can there be electricity? It turns out that in some substances (current conductors) the electrons that are the most distant from their nucleus can break away from the atom and go to the neighboring atom. This movement of electrons is called free. It's just that electrons move inside matter from one atom to another. But if an external electromagnetic field is connected to this substance (electrical conductor), thereby creating an electrical circuit, then all free electrons will begin to move in one direction. This is precisely the movement of electric current inside the conductor.

Now let's look at what constitutes direct and alternating current. So, direct current always moves in only one direction. As mentioned at the very beginning, electrons move in solids, and ions move in liquid and gaseous bodies. Electrons are negatively charged particles. Consequently, in solids, electric current flows from the minus to the plus of the power source (electrons move along the electrical circuit). In liquids and gases, the current moves in two directions at once, or rather, simultaneously, electrons flow to the plus, and ions (separate atoms that are not interconnected by a crystal lattice, they are each on their own) flow to the minus of the power source.

Scientists, on the other hand, officially considered that the movement occurs from plus to minus (on the contrary, than it actually happens). So, from a scientific point of view, it is correct to say that the electric current moves from plus to minus, but from a real point of view (electrophysical nature) it is more correct to believe that the current flows from minus to plus (in solids). Perhaps this was done for some convenience.

Now, with regard to alternating electric current. Here everything is a little more complicated. If, in the case of direct current, the movement of charged particles has only one direction (physically, electrons with a minus sign flow towards the plus), then with alternating current, the direction of movement periodically changes to the opposite. You have probably heard that in an ordinary city power supply, an alternating voltage of 220 volts and a standard frequency of 50 hertz. So these 50 hertz indicate that the electric current in one second has time to go through a full cycle 50 times, which has a sinusoidal shape. In fact, in one second, the direction of the current changes as much as 100 times (it changes twice in one cycle).

P.S. The direction of current in electrical circuits has importance. In many cases, if the circuit is designed for one direction of current, and you accidentally change it to the opposite one or connect an alternating current instead of direct current, then most likely the device will simply fail. Many semiconductors that work in circuits can break through and burn out when the current is reversed. So when connecting the power supply, the direction of the current must be strictly observed by you.

An electric current is formed in a substance only if there are free charged particles. The charge can be in the environment initially or be formed under the condition of the assistance of external factors (temperature, electro magnetic field, ionizers). The movement of charged particles is chaotic in the absence of an electromagnetic field, and when connected to two points of a substance, the potential differences turn into directed ones - from one substance to another.

The concept, essence and manifestations of electric current

Definition 1

An electric current is an ordered and directed movement of charged particles.

These particles can be:

• in gases - ions and electrons,
• in metals, electrons
• in electrolytes - anions and cations,
• in vacuum - electrons (under certain conditions),
• in semiconductors - holes and electrons (electron-hole conductivity).

Remark 1

This definition is often used. Electric current is a displacement current that occurs as a result of a change in the electric field over time.

Electric current can be expressed in the following manifestations:

1. heating conductors. Heat is not released in superconductors.
2. Changes in the chemical composition of some conductors. This manifestation can be mainly observed in electrolytes.
3. Formation of an electric field. Appears in all conductors without exception.

Figure 1. Electric current - ordered movement of charged particles. Author24 - online exchange of student papers

Classification of electric current

Definition 2

Conduction electric current is a phenomenon in which charged particles move inside the macroscopic elements of a particular medium.

Convection current is a phenomenon in which macroscopic charged bodies move (for example, charged drops of precipitation).

There are direct, alternating and pulsating electric currents and their various combinations. However, in such combinations, the term "electric" is often omitted.

There are several types of electric current:

1. A direct current is a current whose magnitude and direction change little over time.
2. An alternating current is a current whose direction and magnitude change progressively with time. Alternating current refers to current that is not direct. Among all varieties alternating current the main one is the one whose value can only change according to a sinusoidal law. The potential of each end of the conductor in this case changes with respect to the other end alternately from negative to positive, and vice versa. In doing so, it passes through all intermediate potentials. The result is a current that continuously changes direction. Moving in one direction, the current increases, reaching its maximum, which is called the amplitude value. After that, it goes down, for some period it is equal to zero, after which the cycle resumes.
3. A quasi-stationary current is an alternating current that changes relatively slowly; for its instantaneous values, the laws of direct currents are satisfied with sufficient accuracy. Similar laws are Kirchhoff's rules and Ohm's law. The quasi-stationary then in all sections of an unbranched network has the same strength. When calculating circuits of a given current, lumped parameters are taken into account. Quasi-stationary industrial currents are those in which the condition of quasi-stationarity along the line is not satisfied (except for currents in long-distance transmission lines).
4. A high-frequency alternating current is an electric current in which the condition of quasi-stationarity is no longer satisfied. It passes over the surface of the conductor and flows around it from all sides. This effect is called the skin effect.
5. A pulsating current is an electric current in which the direction remains constant, but only the magnitude changes.
6. Eddy currents or Foucault currents are closed electric currents that are located in a massive conductor and arise when the magnetic flux changes. Based on this, eddy currents are inductive. The sooner magnetic flux changes, the stronger the eddy currents become. They do not flow along the wires along certain paths, but close in the conductor and form vortex-like contours.

Due to the existence of eddy currents, the skin effect occurs when the magnetic flux and alternating electric current propagate along the surface layer of the conductor. Due to heating by eddy currents, there is a loss of energy, especially in the cores of AC coils. To reduce energy loss for eddy currents, the division of AC magnetic wires into separate plates is used, which are isolated from each other and are located perpendicular to the direction of eddy currents. Because of this, the possible contours of their paths are limited, and the magnitude of these currents is rapidly decreasing.

Electric current characteristics

Historically, it so happened that the direction of movement of positive charges in the conductor coincides with the direction of the current. If the natural carriers of electric current are negatively charged electrons, then the direction of the current will be opposite to the direction of positively charged particles.

The speed of charged particles directly depends on the charge and mass of the particles, the material of the conductor, the ambient temperature and the applied potential difference. The speed of purposeful movement is a value that is much less than the speed of light. Electrons in one second move in the conductor due to ordered movement less than one tenth of a millimeter. But, despite this, the speed of current propagation is equal to the speed of light and the speed of propagation of the front of electromagnetic waves.

The place where the speed of movement of electrons changes after a change in voltage moves with the speed of propagation of the electromagnetic wave.

Main types of conductors

In conductors, unlike dielectrics, there are free carriers of uncompensated charges. They are under the influence of the force of electrical potentials come into motion and form an electric current.

The current-voltage characteristic or, in other words, the dependence of current on voltage is main characteristic conductor. For electrolytes and metallic conductors, it takes the simplest form: the current strength is directly proportional to the voltage. This is Ohm's law.

In metals, current carriers are conduction electrons, which are considered as an electron gas. They clearly show the quantum properties of a degenerate gas.

Plasma is an ionized gas. In this case, with the help of ions and free electrons, an electric charge is transferred. Free electrons are formed under the influence of ultraviolet and X-ray radiation or heating.

Electrolytes are solid or liquid systems and substances in which there is a noticeable concentration of ions, which causes the passage of an electric current. In the process of electrolytic dissociation, ions are formed. The resistance of electrolytes decreases when heated due to an increase in the number of molecules that have decomposed into ions. As a result of the passage of an electric current through the electrolyte, the ions approach the electrodes and are neutralized, settling on them.

The physical laws of Faraday's electrolysis determine the mass of the substance that is released on the electrodes. There is also an electric current of electrons in a vacuum, used in cathode-ray devices.

Electricity — directed (ordered) motion of charged particles. Such particles can be: in metals - electrons, in electrolytes - ions (cations and anions), in gases - ions and electrons, in vacuum under certain conditions - electrons, in semiconductors - electrons and holes (electron-hole conductivity). Sometimes electric current is also called the displacement current resulting from a change in the electric field over time.

Electric current has the following manifestations:

• heating of conductors (there is no heat release in superconductors);
• change in the chemical composition of conductors (observed mainly in electrolytes);
• the creation of a magnetic field (manifested in all conductors without exception).

Classification:

If charged particles move inside macroscopic bodies relative to a particular medium, then such a current is called an electric conduction current. If macroscopic charged bodies are moving (for example, charged raindrops), then this current is called the convection current.

Distinguish variable(English alternating current, AC), constant(English direct current, DC) and throbbing electric currents, as well as their various combinations. In such terms, the word "electric" is often omitted.

D.C - current, the direction and magnitude of which change slightly with time.

Alternating current - current, the magnitude and direction of which change with time. In a broad sense, alternating current is any current that is not direct. Among the alternating currents, the main one is the current, the value of which varies according to a sinusoidal law. In this case, the potential of each end of the conductor changes with respect to the potential of the other end of the conductor alternately from positive to negative and vice versa, while passing through all intermediate potentials (including the zero potential). As a result, a current arises that continuously changes direction: when moving in one direction, it increases, reaching a maximum, called the amplitude value, then decreases, at some point becomes zero, then increases again, but in the other direction and also reaches the maximum value , falls off to then pass through zero again, after which the cycle of all changes resumes.

Quasi-stationary current - “a relatively slowly changing alternating current, for the instantaneous values ​​​​of which the laws of direct currents are satisfied with sufficient accuracy” (TSB). These laws are Ohm's law, Kirchhoff's rules and others. Quasi-stationary current, as well as direct current, has the same current strength in all sections of an unbranched circuit. When calculating quasi-stationary current circuits due to the emerging e. d.s. capacitance and inductance inductions are taken into account as lumped parameters. Quasi-stationary are ordinary industrial currents, except for currents in long-distance transmission lines, in which the condition of quasi-stationarity along the line is not satisfied.

High frequency alternating current - current, in which the condition of quasi-stationarity is no longer satisfied, the current passes over the surface of the conductor, flowing around it from all sides. This effect is called the skin effect.

Ripple current - a current in which only the magnitude changes, but the direction remains constant.

Eddy currents (Foucault currents) - “closed electric currents in a massive conductor that arise when the magnetic flux penetrating it changes”, therefore eddy currents are induction currents. The faster the magnetic flux changes, the stronger the eddy currents. Eddy currents do not flow along certain paths in the wires, but, closing in the conductor, form vortex-like contours.

The existence of eddy currents leads to the skin effect, that is, to the fact that the alternating electric current and magnetic flux propagate mainly in the surface layer of the conductor. Eddy current heating of conductors leads to energy losses, especially in the cores of AC coils. To reduce energy losses due to eddy currents, the alternating current magnetic circuits are divided into separate plates, isolated from each other and located perpendicular to the direction of eddy currents, which limits the possible contours of their paths and greatly reduces the magnitude of these currents. At very high frequencies, instead of ferromagnets, magnetodielectrics are used for magnetic circuits, in which, due to the very high resistance, eddy currents practically do not occur.

Characteristics:

Historically, it is accepted that the direction of the current coincides with the direction of movement of positive charges in the conductor. In this case, if the only current carriers are negatively charged particles (for example, electrons in a metal), then the direction of the current is opposite to the direction of movement of charged particles.

The speed of the directed motion of particles in conductors depends on the material of the conductor, the mass and charge of the particles, the ambient temperature, the applied potential difference, and is much less than the speed of light. In 1 second, the electrons in the conductor move by ordered movement by less than 0.1 mm. Despite this, the propagation speed of the actual electric current is equal to the speed of light (the propagation speed of the electromagnetic wave front). That is, the place where the electrons change their speed of movement after a change in voltage moves with the speed of propagation of electromagnetic oscillations.

Main types of conductors:

Unlike dielectrics, conductors contain free carriers of uncompensated charges, which, under the action of a force, usually a difference in electrical potentials, set in motion and create an electric current. The current-voltage characteristic (dependence of current strength on voltage) is the most important characteristic of a conductor. For metallic conductors and electrolytes, it has the simplest form: the current strength is directly proportional to the voltage (Ohm's law).

Metals - here the current carriers are conduction electrons, which are usually considered as an electron gas, clearly showing the quantum properties of a degenerate gas.

Plasma - ionized gas. Electric charge is carried by ions (positive and negative) and free electrons, which are formed under the influence of radiation (ultraviolet, X-ray and others) and (or) heating.

electrolytes - "liquid or solid substances and systems in which ions are present in any noticeable concentration, causing the passage of an electric current." Ions are formed in the process of electrolytic dissociation. When heated, the resistance of electrolytes decreases due to an increase in the number of molecules decomposed into ions. As a result of the passage of current through the electrolyte, the ions approach the electrodes and are neutralized, settling on them. Faraday's laws of electrolysis determine the mass of the substance released on the electrodes.

There is also an electric current of electrons in a vacuum, which is used in cathode ray devices.

Static electricity. If yellow amber is rubbed with wool or fur, then amber acquires the property of attracting hair, leaves, and straws for a long time. The ability of amber to attract other substances to itself is caused by its charge. By the charge of bodies is meant an electric charge. Under certain conditions, the charge is stored on the charged bodies, so it is called static electricity.

The amount of electricity charged bodies and the distance between them affect their interaction. The rules that bodies obey when interacting are called Coulomb's law. It is formulated as follows: the force acting between two charged bodies is directly proportional to the amount of electricity on each of the bodies and inversely proportional to the square of the distance between the charges.

Electrically charged bodies, being at a distance from each other, experience the action of a certain force. The space in which these forces act is called the electric force field. Inside an electric field, forces act in a certain direction. The lines along which the electric forces of the field act are called power lines. For their direction at any point of the field, the direction in which the positive charge will move in this field is taken. Therefore, the electric field of an isolated negative charge is directed towards the charge (Fig. 1), and the lines of forces acting between the positive and negative charges are directed towards the negative charge. The lines of force of like charges repel each other (Fig. 2).

Rice. one
Rice. 2

Electric current and the direction of electron movement. When studying the laws of electric current, it was first assumed that the electric current is directed from positively to negatively charged bodies. With the help of later research, it was found that electrons pass from negatively charged to positively charged or neutral bodies.

However, the first provision took root, which formed the basis of all electrical measurements and electrical engineering practice. But despite this, in modern conditions there is a rule that defines electric current as a flow of electrons directed from minus to plus.

Electric potential. The forces acting on the bodies tend to bring them to a position in which the potential energy of the bodies will be the smallest (for example, spilled water flows down to the lowest places, steam moves in a pipe from a point with a lower to a point with a higher potential energy). For message potential energy water can be raised to a certain height. These provisions also apply to electricity.

An electric potential can be created by subtracting or adding electrons to a neutral body. In the first case, the body acquires a positive charge, i.e., the potential of the body increases (work has been done to remove the electron), in the second - a negative charge and its potential will be negative. Electricity flows from a higher to a lower potential.

It is possible to discharge a body from an electric charge by connecting it to the ground, i.e. grounding the body. The electric charges of the body, due to their mutual repulsion, tend to be evenly distributed on the charged body and the earth. However, due to the fact that the earth is incomparably larger than a charged body, all the charges from it will go into the earth and the body will become neutral, that is, electrically safe.

DC electrical circuit. Electric current, the value of which does not change with time, is called constant. An electric current source with linear wires connected to it and a current consumer form a closed electrical circuit through which an electric current flows. The simplest electrical circuit has a source and consumer of electric current and two linear wires connecting them (Fig. 3). Batteries, generators - electrical machines driven by mechanical motors, galvanic cells and a number of other devices are used as sources of direct electric current. Electric current consumers can be electric heaters, a welding arc, light bulbs, etc.

Rice. 3

Capacitors. At the same pressure, a larger volume can contain more gas. Some analogy can be carried through with an electric charge. The larger the conductor, the greater its capacity for electric charges, i.e., the greater its electrical capacitance.

Single conductors have low capacitance. Therefore, capacitors are used to form a reserve of electric charges. A capacitor is a device that, with a relatively small size, is capable of accumulating large electrical charges. In its simplest form, a capacitor consists of two metal plates separated by a dielectric (air, mica, waxed paper, etc.). Depending on the type of dielectric, the capacitor is called air, paper, mica, etc. One plate of the capacitor is charged with positive charges, and the other with negative charges. The strong mutual attraction holds the charges, allowing a large amount of charge to accumulate in the capacitor.

The capacitance of a capacitor depends on the area of ​​its plates. A capacitor with larger plates can hold more charges.

The basic unit of measure for electrical capacitance is the farad (f). In practice, smaller units are used: microfarad ( 1 microfarad = 0.000001 f ), picofarad ( 1 pf = 0.000 001 microfarads ).

In engineering, capacitors are used in various electrical and radio circuits.

Electromotive force of the current source. Voltage. If two vessels with different levels of water are connected by a tube, then the water will pass into a vessel with a lower level. By pouring water into one of the vessels, it is possible to ensure that the water flows continuously through the tube. A similar picture is observed in the electrical circuit. During the passage of electric current in the circuit at the poles of the current source, it is necessary to maintain a potential difference.

The force that maintains the potential difference, ensuring the passage of current through the electrical circuit, is called the electromotive force and is conventionally denoted e. d.s. The potential difference required to conduct current through an electrical circuit is called the voltage between the ends of the electrical target.

Voltage is generated by a current source. In an open circuit, voltage exists at the poles or terminals of the current source. When a current source is included in the circuit, voltage appears in certain sections of the circuit, which determines the current in the circuit. No voltage, no current in the circuit.

Electrical resistance. When an electric current occurs in a circuit, free electrons move along the conductor under the influence of electric field forces. The movement of electrons is hindered by the atoms and molecules of the conductors encountered on the way, i.e., the electrical circuit resists the passage of electric current. The electrical resistance of a conductor is the property of a body or medium to convert electrical energy into thermal energy when an electric current passes through it.

Various substances have different amount electrons and different arrangement of atoms. Therefore, the resistance of a conductor depends on the material from which it is made. Good conductors are silver , copper, . They have great resistance iron, coal. Along with this, the resistance depends on the length and cross-sectional area of ​​the conductor. The longer the conductor with the same cross section, the greater its resistance, and vice versa: the larger the conductor section with the same length, the lower its resistance.

Heating increases the resistance of most metals and alloys. For pure metals, this increase is about 4% for every 10° temperature increase. Only some special metal alloys ( manganin , constantan etc.) almost do not change their resistance with increasing temperature.

Rheostats. Devices with which, by changing the resistance, you can adjust the current strength in the circuit, called rheostats. There are several types of rheostats, for example: sliding contact rheostat, lever rheostat, tube rheostat, etc.

Rice. four

The sliding contact rheostat is arranged as follows (Fig. 4). A wire made of metal with high resistivity is wound on a cylinder made of insulator, terminals are attached to the ends of the wire to connect the rheostat to the circuit. A slider is attached to the top of the cylinder on a metal rod, tightly touching the turns of the wire. The rheostat is connected to the circuit using one of the terminals on the rheostat wire and the terminal on the metal rod of the slider. By moving the slider in one direction or another, increase or decrease the length of the included wire and thereby change the resistance of the circuit.

Lever-type rheostat, consists of a series of wire spirals mounted on an insulator frame. On one side of the frame, the ends of the spirals are connected to a series of metal contacts. The metal handle, rotating around the axis, can be tightly pressed against one or another contact. Depending on the position of the handle, a different number of spirals can be included in the chain.

Measurement of current, voltage and resistance. Experiments show that the more electricity flows through the conductor at the same time, the stronger the effect of the current. Therefore, the electric current is determined by the amount of electricity flowing through the cross section of the conductor per unit time. The amount of electricity flowing through the cross section of a conductor in 1 sec, is called the strength of the electric current. The unit of current is taken 1 a , i.e. the strength of such a current at which in 1 sec passes through the cross section of the conductor 1 pendant electricity. Ampere is denoted by the letter a . The unit of current, the ampere, is named after the French scientist Ampère.

The English physicist Faraday, studying the phenomenon of the passage of current through liquid conductors, found that the weight amount of substances released at the same time on the electrodes is directly proportional to the amount of electricity passed through the solution. Based on this, a unit of quantity of electricity was established.

A unit of electricity is taken as the amount of electricity that, when passing through a solution of silver salt, is released at the electrode 1.118 mg silver. This unit is called a kulan.

Based on the definition of electric current, its strength can be determined by the formula

I - current strength in the circuit;

Q - the amount of electricity flowing\u003e in value, in pendants;

T - the time of passage of electricity in the circuit in sec.

In technology, there is also such a thing as current density.

current density called the ratio of the magnitude of the current to the cross-sectional area of ​​\u200b\u200bthe conductor. Typically, the cross-sectional area of ​​conductors is given in square millimeters, so the current density is measured in a/mm 2 .

Consider an electric circuit consisting of a current source, conductors and a light bulb connected in series. The strength of the current in all sections of this circuit is the same, which means that the amount of electricity flowing through the wires and the hair of the light bulb at the same time is the same. However, the amount of energy released in individual sections of the circuit is different. This is easy to verify if you touch the wires that supply current to the lamp with your hand - they are cold, while the hair of the lamp is hot. The release of different amounts of energy in different parts of the circuit is caused by the fact that there are different voltages in these parts of the circuit.

The voltage in a given section of the circuit shows how much energy will be released in this section when a unit of electricity passes through it.

The unit of voltage is taken to be such a voltage at which a 1 joule energy ( 1 kg m=9.8 joules ) if 1 coulomb of electricity flows through this section. The unit of voltage is called volt ohm and abbreviated as in . Voltage unit "volt" named after the Italian scientist Volta.

If in any part of the circuit the voltage is 1 in, this means that with the passage of each pendant of electricity through this section, 1 joule energy.

When measuring high voltages, a unit is used called kilovolt and abbreviated sq. . A kilovolt is a thousand times larger than a volt: 1 kv=1000 in . Used to measure small voltages. millivolt (mv ) is a unit one thousand times smaller than a volt: 1 mV = 0.001 V .

A source of electric current, included in an electrical target, expends energy to overcome the resistance of the circuit. The unit of resistance is called ohm in honor of the German scientist Ohm, who discovered the laws of electric current; ohm - electrical resistance between two points of a linear conductor, in which the potential difference in 1 in produces current in 1 a . Electrical resistance is indicated by two letters ohm .

When measuring high resistances, much larger units are used than ohm : kiloohm (com ) and megom (mgom ). 1 com \u003d 1000 ohm ,1 mg = 1,000,000 ohms .

The properties of conductors in relation to their electrical resistance are evaluated by resistivity. Resistivity is the resistance of a conductor with a length 1m with a cross section in 1 mm 2 . Resistivity is also measured in ohms.

If one large galvanic cell is included in an electric circuit consisting of a light bulb and an ammeter, you will notice that a very weak current flows through the circuit and the light bulb filament does not glow. As soon as we replace the galvanic cell with a fresh battery from a flashlight, the current in the circuit increases and the filament of the light bulb glows brightly. By measuring the voltage at the ends of the circuit when the cell and battery are turned on, we will see that when the battery is turned on, the voltage is much higher.

It follows that the strength of the current in the conductor increases with increasing voltage at the ends of the conductor. By including two light bulbs in series instead of one, we double the resistance of the circuit. Now we see that the current in the circuit has decreased. Studying the dependence of current strength on resistance and voltage, the German scientist Ohm found that the current strength in a conductor is directly proportional to the voltage at the ends of the conductor and inversely proportional to the resistance of the conductor. This relationship between current strength, voltage and resistance is called Ohm's law, which is one of the basic laws of electric current.

Ohm's law is expressed by the following formula:

Where I - current in a ;

V - voltage in in ;

R - resistance in ohm .

Ohm's law doesn't just apply to dc. chain, but also on any part of it. The current in any section of the electrical circuit is equal to the voltage at the ends of this section, divided by its resistance.

Serial connection in an electrical circuit. In most cases, the electrical circuit consists of several current consumers (Fig. 5). The connection of current consumers, in which the end of one conductor is connected to the beginning of another, the end of the other to the beginning of the third, etc., is called serial.

Rice. 5

Since the resistance is directly proportional to the length of the conductor, the resistance of the circuit is equal to the sum of the resistances of the individual conductors, since the inclusion of several conductors increases the length of the current path. The current in individual sections of the circuit will be the same. Therefore, the voltage drop in each section will be proportional to the resistance of this section.

Parallel connection in an electrical circuit they call such a connection when the beginnings of all conductors are connected at one point, and their ends at another point (Fig. 6). With a parallel connection, there are several paths for the passage of electric current (Fig. 6). The current between consumers connected in parallel is distributed in inverse proportion to the resistances of consumers. If individual consumers have the same resistance, they will have the same current. The lower the resistance of an individual consumer, the more current will pass through it.

Fig.6

The sum of the currents of the individual sections in a parallel circuit is equal to the total current at the branching point of the circuit.

If in a series-connected circuit, the connection of new consumers of electric current increases the resistance of the circuit, when connected in parallel, it decreases: the connected new resistance increases the total conductor cross section, consisting of the sum of the cross sections of the conductors of all consumers. And as you know, the larger the cross section of the conductor at a constant length, the lower the resistance.

Neglecting the resistance of the connecting wires, we can assume that the voltage of the current source is applied to each consumer of the parallel circuit. Therefore, the advantage of a parallel connection is the independence of the operation of each current consumer. You can turn off any consumer without interrupting the flow of current through the rest. By changing the resistance of one of the consumers, we change the current in its circuit. For other consumers, the current will not change.

Rice. 7

Mixed connection in an electrical circuit. Very often in electrical circuits there is a mixed connection. A mixed connection is a connection in which there is both a serial and a parallel connection of electric current consumers (Fig. 7). To determine the resistance of several conductors connected in a mixed circuit, first find the resistance of parallel or series-connected conductors, and then replace them with one conductor with a resistance equal to that found. In this way, the circuit is simplified, leading it to a single conductor, the resistance of which is equal to the total resistance of the complex circuit.

Work and power of electric current. An electric current can do work. The ability of a body to do work is called its energy. By means of electric motors, the current sets in motion electric trains, machine tools. Due to the energy of the electric current, mechanical work is performed. If the conductor through which the current passes is heated, the energy of the current is converted into heat. With various manifestations of the current, a transformation is observed electrical energy into other forms of energy.

In a closed electrical circuit, a current flows, which represents the movement of electric charges. To transfer charges in an electrical circuit, a source of electrical energy expends a certain amount of energy or performs work equal to the product of the circuit voltage and the amount of electricity transferred through the circuit.

If a section of the electrical circuit has flowed Q coulombs of electricity, and the voltage across it is V , then the work done on this section of the chain BUT will be equal to:

A \u003d QV j.

At current Ia during T seconds passes through the cross section of the conductor IT = Q coulombs of electricity. Therefore, the work of the current in Ia at voltage V during T seconds will be equal to:

A=IVT.

The work of the current is usually estimated by its power. The power of the current is numerically equal to the work that the current produces in 1 sec. Therefore, the current power will be equal to:

joules in 1 sec.

The unit of measure for power is watt (Tue ). One watt is the power of the current 1 a at a voltage of 1 in . Therefore, as the current and voltage increase, the power increases. To determine the power of an electric current, it is necessary to multiply the voltage in volts by the current in amperes.

Along with the watt, power is often measured kilowatt (1 kW = 1000 watts ), hectowatt (1 GW=100 W ), milliwatt (1 mW=0.001 W ) and microwatt (1 μW = 0.000 001 W ).

The work of an electric current can be determined if its power is multiplied by the time it takes the current to pass: power is the work in 1 sec . taken as the basic unit of work. watt second (Tue sec), i.e. work current power 1 watt during 1 sec . The larger units are watt hour (1 Wh=3600 Ws ), hectowatt hour (1 GWh =100 Wh ), kilowatt-hour (1 kWh= 1000 Wh ).

Lenz-Joule law. Russian Academician Lenz and English physicist Joule, independently of each other, found that in the process of passing an electric current through a conductor, the amount of heat released by the conductor is directly proportional to the square of the current strength, the resistance of the conductor and the time of passage of the current. This rule is called deputy Lenz - Joule and expressed by the formula

Q \u003d 0.24I 2 Rt ,

de Q - the amount of heat in feces ;

0,24 - coefficient of proportionality, causing the current to be expressed in a, voltage in in, and the resistance is ohm ;

I - current in a ;

R - conductor resistance in ohm ;

t - the time during which the current flowed through the conductor, in sec .

Electric arc. If the ends of two conductors connected to a source of electric current are brought together, a spark is formed between them. By spreading the ends, instead of a spark, we get an electric arc that creates a strong and dazzling light. If carbon rods are attached to the ends of the conductors, an electric arc will also occur between them. The occurrence of the arc is explained as follows.

With an increase in the temperature of the carbon rods, the speed of movement of the electrons in the coal increases. With strong heating, the speed of movement of free electrons increases so much that when the coals move apart, the electrons fly out of the rods into the interelectrode space. As a result of the action of the emitted electrons on neutral atoms and the intense radiation of light by the heated ends of the electrodes, the air between the electrodes ceases to be electrically neutral, i.e., a gas gap is created between the ends of the separated electrodes, which conducts electric current well, and an electric discharge occurs.

The ability of current to create an electric arc is successfully used in welding. Replacing one of the carbon electrodes with a welded product, we get an electric arc burning between this product and the second carbon electrode. However, at present, the method of welding with a metal electrode has received the greatest use. In this case, a metal electrode is used instead of a carbon electrode. The welding arc burns between the workpiece to be welded and the metal electrode. After melting the metal electrode, it is replaced by a new one.

Short circuit. The emergency operation of an electrical circuit, when, due to a decrease in its resistance, the current in it increases sharply against the normal one, is called a short circuit. A short circuit is obtained if a conductor or device, etc., is included in the electrical circuit. with very little resistance compared to the resistance of the circuit. Due to the small resistance, a current will flow through the circuit that is much higher than that for which the circuit is designed. Such a current will cause the release of a large amount of heat, which will lead to charring and burning of the wire insulation, melting of the wire material, damage to electrical measuring instruments, melting of the contacts of switches, knife switches, etc. Even the power source can be damaged. Therefore (due to the dangerous destructive consequences of a short circuit, certain conditions must be observed during the installation and operation of electrical installations.

In order to avoid a sudden and dangerous increase in current in an electrical circuit during a short circuit, the circuit is protected by fuses. The fuse is a fusible wire connected in series in the circuit. When the current increases beyond a certain value, the fuse wire heats up and melts, the electrical circuit automatically breaks and the current in it stops. Fusible links for different sections of the protected wires and for different energy consumers are taken different. Fuses can do the job provided they are properly selected.

Rice. eight

According to their design, the fuses are divided into cork (Fig. 8, a), plate (Fig. 8, b) and tubular (Fig. 8, c). the wires of the open circuit are connected. In plate fuses, the fuse-link is fixed on an insulating base with the help of lugs and screws. The wires of the open circuit are led to the screws. In tubular fuses, the fusible part is placed inside easily removable porcelain tubes.

In circuits with high current and voltage, fuses are rarely used. In these cases, arrange another automatic protection.

Electricity

What is called electric current?

The ordered (directed) movement of charged particles is called electric current. Moreover, an electric current, the strength of which does not change with time, is called constant. If the direction of current movement changes and changes. in magnitude and direction are repeated in the same sequence, then such a current is called alternating.

What causes and maintains the ordered movement of charged particles?

Causes and maintains the orderly movement of charged particles electric field. Does electric current have a certain direction?
It has. The direction of the electric current is taken as the movement of positively charged particles.

Is it possible to directly observe the movement of charged particles in a conductor?

No. But the presence of an electric current can be judged by the actions and phenomena with which it is accompanied. For example, a conductor along which charged particles move is heated, and in the space surrounding the conductor, a magnetic field is formed and the magnetic needle near the conductor with electric current turns. In addition, the current passing through gases causes them to glow, and passing through solutions of salts, alkalis and acids, it decomposes them into constituent parts.

What determines the strength of an electric current?

The strength of the electric current is determined by the amount of electricity passing through the cross section of the conductor per unit time.
To determine the current strength in a circuit, it is necessary to divide the amount of electricity flowing by the time during which it has flowed.

What is the unit of current?

The unit of current strength is taken to be the strength of an unchanging current, which, passing through two parallel rectilinear conductors of infinite length of an even small cross section, located at a distance of 1 m from one another in a vacuum, would cause a force between these conductors equal to 2 Newtons per meter. This unit was named Ampere in honor of the French scientist Ampère.

What is the unit of quantity of electricity?

A Coulomb (Ku) is taken as a unit of electricity, which passes in one second at a current strength of 1 Ampere (A).

What instrument is used to measure electric current?

The strength of the electric current is measured by devices called ammeters. The ammeter scale is calibrated in amperes and fractions of an ampere according to the readings of accurate standard instruments. The current strength is counted according to the indications of the arrow, which moves along the scale from zero division. The ammeter is connected in series to the electrical circuit, using two terminals or clamps available on the device. What is electric voltage?
The voltage of an electric current is the potential difference between two points in an electric field. It is equal to the work done by the forces of the electric field when moving a positive charge equal to unity from one point of the field to another.

The basic unit of voltage measurement is Volt (V).

What instrument measures the voltage of an electric current?

The voltage of the electric current is measured by the device; rum, which is called a voltmeter. A voltmeter is connected in parallel in an electric circuit. Formulate Ohm's law on the circuit section.

What is conductor resistance?

Conductor resistance is physical quantity characterizing the properties of the conductor. The unit of resistance is the ohm. Moreover, a resistance of 1 ohm has a wire in which a current of 1 A is set at a voltage at its ends of 1 V.

Does the resistance in conductors depend on the magnitude of the electric current flowing through them?

The resistance of a homogeneous metal conductor of a certain length and cross section does not depend on the magnitude of the current flowing through it.

What determines the resistance in electrical conductors?

Resistance in conductors of electric current depends on the length of the conductor, its cross-sectional area and the type of conductor material (material resistivity).

Moreover, the resistance is directly proportional to the length of the conductor, inversely proportional to the cross-sectional area and depends, as mentioned above, on the material of the conductor.

Does resistance in conductors depend on temperature?

Yes, it depends. An increase in the temperature of a metal conductor causes an increase in the speed of thermal motion of particles. This leads to an increase in the number of collisions of free electrons and, consequently, to a decrease in the mean free path, as a result of which the specific conductivity decreases and the resistivity of the material increases.

The temperature coefficient of resistance of pure metals is approximately 0.004 °C, which means an increase in their resistance by 4% with an increase in temperature by 10 °C.

With an increase in temperature in the electrolyte coal, the mean free path also decreases, while the concentration of charge carriers increases, as a result of which their resistivity decreases with increasing temperature.

Formulate Ohm's law for a closed circuit.

The current strength in a closed circuit is equal to the ratio of the electromotive force of the circuit to its total resistance.

This formula shows that the current strength depends on three quantities: the electromotive force E, the external resistance R and the internal resistance r. The internal resistance does not have a noticeable effect on the current strength if it is small compared to the external resistance. In this case, the voltage at the terminals of the current source is approximately equal to the electromotive force (EMF).

What is electromotive force (EMF)?

The electromotive force is the ratio of the work of external forces to move the charge along the circuit to the charge. Like potential difference, electromotive force is measured in volts.

What forces are called external forces?

Any forces acting on electrically charged particles, with the exception of potential forces of electrostatic origin (ie, Coulomb), are called extraneous forces. It is due to the work of these forces that charged particles acquire energy and then give it away when moving in the conductors of an electrical circuit.

Third-party forces set in motion charged particles inside a current source, generator, battery, etc.

As a result, charges of the opposite sign appear at the terminals of the current source, and a certain potential difference between the terminals. Further, when the circuit is closed, the formation of surface charges begins to act, creating an electric field throughout the circuit, which appears as a result of the fact that when the circuit is closed, a surface charge arises almost immediately on the entire surface of the conductor. Inside the source, the charges move under the action of external forces against the forces electrostatic field(positive from minus to plus), and throughout the rest of the circuit they are set in motion by an electric field.

Rice. 1. Electrical circuit: 1- source, electricity (battery); 2 - ammeter; 3 - successor of energy (laying on incandescent); 4 - electrical wires; 5 - single-pole ruSidnik; 6 - fuses

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