What is current. Ohm's law

Electric current is charged particles that can move in an orderly manner in any conductor. This movement occurs under the influence of an electric field. The occurrence of electric charges occurs almost constantly. This is especially pronounced when different substances are in contact with each other.

If complete free movement of charges relative to each other is possible, then these substances are conductors. When such movement is not possible, this category of substances is considered insulators. Conductors include all metals with varying degrees of conductivity, as well as hydrochloric and acid solutions. Insulators can be natural substances in the form of ebonite, amber, various gases and quartz. They can be of artificial origin, for example, PVC, polyethylene and others.

Electric current values

As a physical quantity, current can be measured by its basic parameters. According to the results of measurements, the possibility of using electricity in a particular area is determined.

There are two types of electric current - direct and alternating. The first one always remains unchanged in time and direction, and in the second case, there are changes in these parameters over a certain period of time.

Electric current is now used in every building, knowing current characteristics in the electrical network at home, you should always remember that it is life-threatening.

Electric current is the effect of the directed movement of electric charges (in gases - ions and electrons, in metals - electrons), under the influence of an electric field.

The movement of positive charges along the field is equivalent to the movement of negative charges against the field.

Usually, the direction of the electric charge is taken as the direction of the positive charge.

• current power;
• voltage;
• current strength;
• current resistance.

Current power.

Power of electric current is the ratio of the work done by the current to the time during which this work was done.

The power that an electric current develops in a section of the circuit is directly proportional to the magnitude of the current and voltage in this section. Power (electric-three-che-sky and me-ha-no-che-sky) from-me-rya-et-xia in Watts (W).

Current power does not depend on the time of the pro-the-ka-niya of the electric-tri-che-th current in the circuit, but define-de-la-is-sya as a pro-of-ve-de -ne voltage to current strength.

voltage.

Electric voltage is a value that shows how much work an electric field has done when moving a charge from one point to another. In this case, the voltage in different parts of the circuit will be different.

For example: the voltage on the section of the empty wire will be very small, and the voltage on the section with any load will be much greater, and the magnitude of the voltage will depend on the amount of work done by the current. Measure the voltage in volts (1 V). To determine the voltage, there is a formula: U \u003d A / q, where

• U - voltage,
• A is the work done by the current to move the charge q to a certain section of the circuit.

Current strength.

current strength called the number of charged particles that flow through the cross section of the conductor.

A-priory current strength directly proportional to voltage and inversely proportional to resistance.

The strength of the electric current measured with an instrument called an ammeter. The amount of electric current (the amount of charge carried) is measured in amperes. To increase the range of designations for the unit of change, there are multiplicity prefixes such as micro-microampere (μA), miles - milliamp (mA). Other prefixes are not used in everyday life. For example: they say and write "ten thousand amperes", but they never say or write 10 kiloamperes. Such values ​​are not used in everyday life. The same can be said about nanoamps. Usually they say and write 1 × 10-9 Amps.

current resistance.

electrical resistance called a physical quantity that characterizes the properties of the conductor that prevent the passage of electric current and is equal to the ratio of the voltage at the ends of the conductor to the strength of the current flowing through it.

Resistance for AC circuits and for alternating electromagnetic fields is described in terms of impedance and wave resistance. current resistance(often denoted by the letter R or r) is considered the resistance of the current, within certain limits, a constant value for a given conductor. Under electrical resistance understand the ratio of the voltage at the ends of the conductor to the strength of the current flowing through the conductor.

Conditions for the occurrence of electric current in a conductive medium:

1) the presence of free charged particles;

2) if there is an electric field (there is a potential difference between two points of the conductor).

Types of influence of electric current on a conductive material.

1) chemical - a change in the chemical composition of conductors (occurs mainly in electrolytes);

2) thermal - the material is heated through which the current flows (this effect is absent in superconductors);

3) magnetic - the appearance of a magnetic field (occurs in all conductors).

The main characteristics of the current.

1. The current strength is denoted by the letter I - it is equal to the amount of electricity Q passing through the conductor in time t.

I=Q/t

The current strength is determined by an ammeter.

The voltage is determined by a voltmeter.

3. Resistance R of the conductive material.

The resistance depends on:

a) on the cross section of the conductor S, on its length l and material (denoted by the specific resistance of the conductor ρ);

R=pl/S

b) on temperature t°С (or Т): R = R0 (1 + αt),

• where R0 is the resistance of the conductor at 0°С,
• α - temperature coefficient of resistance;

c) to obtain various effects, conductors can be connected both in parallel and in series.

Table of current characteristics.

Compound	Sequential	Parallel
Conserved value	I 1 \u003d I 2 \u003d ... \u003d I n I \u003d const	U 1 \u003d U 2 \u003d ... U n U \u003d const
Total value	voltage e=Ast/q The value equal to the expended work done by external forces to move a positive charge along the entire circuit, including the current source, to the charge, is called the electromotive force of the current source (EMF): e=Ast/q Current characteristics must be known when repairing electrical equipment.

Electricity

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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 instrument; 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?

The resistance of a conductor is a physical quantity that characterizes the properties of a 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?

The 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 of the electrostatic field (positive from minus to plus), and throughout the rest of the circuit they are set in motion by the 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

The first discoveries related to the work of electricity began in the 7th century BC. The ancient Greek philosopher Thales of Miletus revealed that when amber is rubbed against wool, it is subsequently able to attract lightweight objects. From Greek "electricity" is translated as "amber". In 1820, André-Marie Ampère established the law of direct current. In the future, the magnitude of the current, or what the electric current is measured in, began to be denoted in amperes.

Term meaning

The concept of electric current can be found in any physics textbook. electric current- this is an ordered movement of electrically charged particles in a direction. To understand to a simple layman what an electric current is, you should use the dictionary of an electrician. In it, the term stands for the movement of electrons through a conductor or ions through an electrolyte.

Depending on the movement of electrons or ions inside the conductor, the following are distinguished: types of currents:

• constant;
• variable;
• intermittent or pulsating.

Basic measurements

The strength of the electric current- the main indicator used by electricians in their work. The strength of the electric current depends on the magnitude of the charge that flows through the electrical circuit for a set period of time. The more electrons flowed from one beginning of the source to the end, the greater will be the charge transferred by the electrons.

A quantity that is measured as the ratio of the electric charge flowing through the cross section of particles in a conductor to the time it passes. The charge is measured in coulombs, the time is measured in seconds, and one unit of the strength of the current of electricity is determined by the ratio of charge to time (coulomb to second) or in amperes. The determination of the electric current (its strength) occurs by connecting two terminals in series to the electrical circuit.

When the electric current is working, the movement of charged particles is carried out with the help of an electric field and depends on the strength of the movement of electrons. The value on which the work of the electric current depends is called voltage and is determined by the ratio of the work of the current in a particular part of the circuit and the charge passing through the same part. The volt unit is measured with a voltmeter when the two terminals of the instrument are connected in parallel to the circuit.

The value of electrical resistance is directly dependent on the type of conductor used, its length and cross section. It is measured in ohms.

Power is determined by the ratio of the work of the movement of currents to the time when this work occurred. Measure power in watts.

Such a physical quantity as capacitance is determined by the ratio of the charge of one conductor to the potential difference between the same conductor and the neighboring one. The lower the voltage when the conductors receive an electric charge, the greater their capacitance. It is measured in farads.

The value of the work of electricity at a certain interval of the chain is found using the product of the current strength, voltage and the time period at which the work was carried out. The latter is measured in joules. The determination of the work of the electric current occurs with the help of a meter that connects the readings of all quantities, namely voltage, force and time.

Electrical safety engineering

Knowing the rules of electrical safety will help prevent an emergency and protect human health and life. Since electricity tends to heat the conductor, there is always the possibility of a situation dangerous to health and life. For home security must adhere following simple but important rules:

1. Network insulation must always be in good working order to avoid overloads or the possibility of short circuits.
2. Moisture should not get on electrical appliances, wires, shields, etc. Also, a humid environment provokes short circuits.
3. Be sure to make grounding for all electrical devices.
4. It is necessary to avoid overloading the electrical wiring, as there is a risk of ignition of the wires.

Safety precautions when working with electricity involves the use of rubberized gloves, mittens, rugs, discharge devices, grounding devices for work areas, circuit breakers or fuses with thermal and current protection.

Experienced electricians, when there is a possibility of electric shock, work with one hand, and the other is in their pocket. Thus, the hand-to-hand circuit is interrupted in case of involuntary contact with the shield or other grounded equipment. In case of ignition of equipment connected to the network, extinguish the fire exclusively with powder or carbon dioxide extinguishers.

Application of electric current

Electric current has many properties that allow it to be used in almost all spheres of human activity. Ways to use electric current:

Electricity is the most environmentally friendly form of energy today. In the conditions of the modern economy, the development of the electric power industry is of planetary importance. In the future, if there is a shortage of raw materials, electricity will take a leading position as an inexhaustible source of energy.

First of all, it is worth finding out what constitutes an electric current. Electric current is the ordered movement of charged particles in a conductor. In order for it to arise, an electric field must first be created, under the influence of which the above-mentioned charged particles will begin to move.

The first information about electricity, which appeared many centuries ago, related to electrical "charges" obtained through friction. Already in ancient times, people knew that amber, worn on wool, acquires the ability to attract light objects. But only at the end of the 16th century, the English physician Gilbert studied this phenomenon in detail and found out that many other substances have exactly the same properties. Bodies capable, like amber, after being rubbed to attract light objects, he called electrified. This word is derived from the Greek electron - "amber". At present, we say that there are electric charges on bodies in this state, and the bodies themselves are called "charged."

Electric charges always arise when different substances are in close contact. If the bodies are solid, then their close contact is prevented by microscopic protrusions and irregularities that exist on their surface. By squeezing such bodies and rubbing them together, we bring their surfaces together, which without pressure would touch only at a few points. In some bodies, electric charges can move freely between different parts, while in others this is not possible. In the first case, the bodies are called "conductors", and in the second - "dielectrics, or insulators." Conductors are all metals, aqueous solutions of salts and acids, etc. Examples of insulators are amber, quartz, ebonite and all gases that are under normal conditions.

Nevertheless, it should be noted that the division of bodies into conductors and dielectrics is very arbitrary. All substances conduct electricity to a greater or lesser extent. Electric charges are either positive or negative. This kind of current will not last long, because the electrified body will run out of charge. For the continuous existence of an electric current in a conductor, it is necessary to maintain an electric field. For these purposes, electric current sources are used. The simplest case of the occurrence of an electric current is when one end of the wire is connected to an electrified body, and the other to the ground.

Electric circuits supplying current to lighting bulbs and electric motors did not appear until after the invention of batteries, which dates back to about 1800. After that, the development of the doctrine of electricity went so fast that in less than a century it became not just a part of physics, but formed the basis of a new electrical civilization.

The main quantities of electric current

The amount of electricity and current strength. The effects of electric current can be strong or weak. The strength of the electric current depends on the amount of charge that flows through the circuit in a certain unit of time. The more electrons moved from one pole of the source to the other, the greater the total charge carried by the electrons. This total charge is called the amount of electricity passing through the conductor.

The amount of electricity depends, in particular, on the chemical effect of the electric current, i.e., the greater the charge passed through the electrolyte solution, the more the substance will settle on the cathode and anode. In this regard, the amount of electricity can be calculated by weighing the mass of the substance deposited on the electrode and knowing the mass and charge of one ion of this substance.

The current strength is a quantity that is equal to the ratio of the electric charge that has passed through the cross section of the conductor to the time of its flow. The unit of charge is the coulomb (C), time is measured in seconds (s). In this case, the unit of current strength is expressed in C/s. This unit is called the ampere (A). In order to measure the current strength in a circuit, an electrical measuring device called an ammeter is used. For inclusion in the circuit, the ammeter is equipped with two terminals. It is included in the circuit in series.

electrical voltage. We already know that electric current is an ordered movement of charged particles - electrons. This movement is created with the help of an electric field, which does a certain amount of work. This phenomenon is called the work of an electric current. In order to move more charge through an electric circuit in 1 second, the electric field must do more work. Based on this, it turns out that the work of an electric current should depend on the strength of the current. But there is another value on which the work of the current depends. This value is called voltage.

Voltage is the ratio of the work of the current in a certain section of the electrical circuit to the charge flowing through the same section of the circuit. The current work is measured in joules (J), the charge is measured in pendants (C). In this regard, the unit of voltage measurement will be 1 J/C. This unit is called the volt (V).

In order for a voltage to appear in an electrical circuit, a current source is needed. In an open circuit, voltage is present only at the current source terminals. If this current source is included in the circuit, voltage will also appear in certain sections of the circuit. In this regard, there will also be a current in the circuit. That is, briefly we can say the following: if there is no voltage in the circuit, there is no current. In order to measure voltage, an electrical measuring device called a voltmeter is used. In its appearance, it resembles the previously mentioned ammeter, with the only difference being that the letter V is on the scale of the voltmeter (instead of A on the ammeter). The voltmeter has two terminals, with the help of which it is connected in parallel to the electrical circuit.

Electrical resistance. After connecting all kinds of conductors and an ammeter to an electrical circuit, you can notice that when using different conductors, the ammeter gives different readings, that is, in this case, the current strength available in the electrical circuit is different. This phenomenon can be explained by the fact that different conductors have different electrical resistance, which is a physical quantity. In honor of the German physicist, she was named Ohm. As a rule, larger units are used in physics: kiloohm, megaohm, etc. The conductor resistance is usually denoted by the letter R, the conductor length is L, the cross-sectional area is S. In this case, the resistance can be written as a formula:

R = R * L/S

where the coefficient p is called resistivity. This coefficient expresses the resistance of a conductor 1 m long with a cross-sectional area equal to 1 m2. Resistivity is expressed in Ohm x m. Since wires, as a rule, have a rather small cross section, their areas are usually expressed in square millimeters. In this case, the unit of resistivity will be Ohm x mm2/m. In the table below. 1 shows the resistivity of some materials.

Table 1. Electrical resistivity of some materials
Material	p, Ohm x m2/m	Material	p, Ohm x m2/m
Copper	0,017	Platinum iridium alloy	0,25
Gold	0,024	Graphite	13
Brass	0,071	Coal	40
Tin	0,12	Porcelain	1019
Lead	0,21	Ebonite	1020
Metal or Alloy
Silver	0,016	Manganin (alloy)	0,43
Aluminum	0,028	Constantan (alloy)	0,50
Tungsten	0,055	Mercury	0,96
Iron	0,1	Nichrome (alloy)	1,1
Nickel (alloy)	0,40	Fechral (alloy)	1,3
		Chromel (alloy)	1,5

According to Table. 1, it becomes clear that copper has the smallest electrical resistivity, and an alloy of metals has the largest. In addition, dielectrics (insulators) have high resistivity.

Electrical capacitance. We already know that two conductors isolated from each other can accumulate electric charges. This phenomenon is characterized by a physical quantity, which is called electrical capacitance. The electrical capacitance of two conductors is nothing more than the ratio of the charge of one of them to the potential difference between this conductor and the neighboring one. The lower the voltage when the conductors receive a charge, the greater their capacitance. The farad (F) is taken as the unit of electrical capacitance. In practice, fractions of this unit are used: microfarad (µF) and picofarad (pF).

If you take two conductors isolated from each other, place them at a small distance from one another, you get a capacitor. The capacitance of a capacitor depends on the thickness of its plates and the thickness of the dielectric and its permeability. By reducing the thickness of the dielectric between the plates of the capacitor, it is possible to greatly increase the capacitance of the latter. On all capacitors, in addition to their capacitance, the voltage for which these devices are designed must be indicated.

Work and power of electric current. From the foregoing, it is clear that the electric current does a certain amount of work. When electric motors are connected, the electric current makes all kinds of equipment work, moves trains along the rails, illuminates the streets, heats the home, and also produces a chemical effect, that is, it allows electrolysis, etc. We can say that the work of the current in a certain section of the circuit is equal to the product current, voltage and time during which the work was done. Work is measured in joules, voltage in volts, current in amperes, and time in seconds. In this regard, 1 J = 1V x 1A x 1s. From this it turns out that in order to measure the work of an electric current, three devices should be used at once: an ammeter, a voltmeter and a clock. But this is cumbersome and inefficient. Therefore, usually, the work of electric current is measured by electric meters. The device of this device contains all of the above devices.

The power of an electric current is equal to the ratio of the work of the current to the time during which it was performed. Power is denoted by the letter "P" and is expressed in watts (W). In practice, kilowatts, megawatts, hectowatts, etc. are used. In order to measure the power of the circuit, you need to take a wattmeter. Electrical work is expressed in kilowatt-hours (kWh).

Basic laws of electric current

Ohm's law. Voltage and current are considered the most convenient characteristics of electrical circuits. One of the main features of the use of electricity is the rapid transportation of energy from one place to another and its transfer to the consumer in the desired form. The product of the potential difference and the current strength gives power, i.e., the amount of energy given off in the circuit per unit time. As mentioned above, to measure the power in an electrical circuit, it would take 3 devices. Is it possible to do with one and calculate the power from its readings and some characteristic of the circuit, such as its resistance? Many people liked this idea, they considered it fruitful.

So, what is the resistance of a wire or a circuit as a whole? Does a wire, like water pipes or pipes in a vacuum system, have a constant property that might be called resistance? For example, in pipes, the ratio of the pressure difference creating flow divided by the flow rate is usually a constant characteristic of the pipe. In the same way, the heat flow in a wire is subject to a simple relationship, which includes the temperature difference, the cross-sectional area of ​​the wire, and its length. The discovery of such a relationship for electrical circuits was the result of a successful search.

In the 1820s, the German schoolteacher Georg Ohm was the first to start looking for the above ratio. First of all, he aspired to fame and fame, which would allow him to teach at the university. That was the only reason he chose a field of study that offered particular advantages.

Om was the son of a locksmith, so he knew how to draw metal wire of different thicknesses, which he needed for experiments. Since in those days it was impossible to buy a suitable wire, Om made it with his own hands. During the experiments, he tried different lengths, different thicknesses, different metals and even different temperatures. All these factors he varied in turn. In Ohm's time, batteries were still weak, giving a current of variable magnitude. In this regard, the researcher used a thermocouple as a generator, the hot junction of which was placed in a flame. In addition, he used a crude magnetic ammeter, and measured potential differences (Ohm called them "voltages") by changing the temperature or the number of thermal junctions.

The doctrine of electrical circuits has just received its development. After the invention of batteries around 1800, it began to develop much faster. Various devices were designed and manufactured (quite often by hand), new laws were discovered, concepts and terms appeared, etc. All this led to a deeper understanding of electrical phenomena and factors.

Updating knowledge about electricity, on the one hand, caused the emergence of a new field of physics, on the other hand, was the basis for the rapid development of electrical engineering, i.e. batteries, generators, power supply systems for lighting and electric drive, electric furnaces, electric motors, etc. were invented , other.

Ohm's discoveries were of great importance both for the development of the theory of electricity and for the development of applied electrical engineering. They made it easy to predict the properties of electrical circuits for direct current, and later for alternating current. In 1826, Ohm published a book in which he outlined the theoretical conclusions and experimental results. But his hopes were not justified, the book was met with ridicule. This happened because the method of rough experimentation seemed little attractive in an era when many people were fond of philosophy.

Omu had no choice but to leave his position as a teacher. He did not achieve an appointment at the university for the same reason. For 6 years, the scientist lived in poverty, without confidence in the future, experiencing a feeling of bitter disappointment.

But gradually his works gained fame first outside of Germany. Om was respected abroad, his research was used. In this regard, compatriots were forced to recognize him in their homeland. In 1849 he received a professorship at the University of Munich.

Ohm discovered a simple law that establishes a relationship between current strength and voltage for a piece of wire (for part of the circuit, for the entire circuit). In addition, he made rules that allow you to determine what will change if you take a wire of a different size. Ohm's law is formulated as follows: the current strength in a section of the circuit is directly proportional to the voltage in this section and inversely proportional to the resistance of the section.

Joule-Lenz law. Electric current in any part of the circuit performs a certain work. For example, let's take some section of the circuit, between the ends of which there is a voltage (U). By the definition of electric voltage, the work done when moving a unit of charge between two points is equal to U. If the current strength in a given section of the circuit is i, then the charge it will pass in time t, and therefore the work of the electric current in this section will be:

A = Uit

This expression is valid for direct current in any case, for any section of the circuit, which may contain conductors, electric motors, etc. Current power, i.e. work per unit time, is equal to:

P \u003d A / t \u003d Ui

This formula is used in the SI system to determine the unit of voltage.

Let us assume that the section of the circuit is a fixed conductor. In this case, all the work will turn into heat, which will be released in this conductor. If the conductor is homogeneous and obeys Ohm's law (this includes all metals and electrolytes), then:

U=ir

where r is the resistance of the conductor. In this case:

A = rt2i

This law was first empirically derived by E. Lenz and, independently of him, by Joule.

It should be noted that the heating of conductors finds numerous applications in engineering. The most common and important among them are incandescent lighting lamps.

Law of electromagnetic induction. In the first half of the 19th century, the English physicist M. Faraday discovered the phenomenon of magnetic induction. This fact, having become the property of many researchers, gave a powerful impetus to the development of electrical and radio engineering.

In the course of experiments, Faraday found out that when the number of magnetic induction lines penetrating a surface bounded by a closed loop changes, an electric current arises in it. This is the basis of perhaps the most important law of physics - the law of electromagnetic induction. The current that occurs in the circuit is called inductive. Due to the fact that electric current occurs in the circuit only in the case of external forces acting on free charges, then with a changing magnetic flux passing over the surface of a closed circuit, these same external forces appear in it. The action of external forces in physics is called the electromotive force or induction EMF.

Electromagnetic induction also appears in open conductors. In the case when the conductor crosses the magnetic field lines, a voltage appears at its ends. The reason for the appearance of such a voltage is the induction EMF. If the magnetic flux passing through the closed circuit does not change, the inductive current does not appear.

Using the concept of “EMF of induction”, one can talk about the law of electromagnetic induction, i.e., the EMF of induction in a closed loop is equal in absolute value to the rate of change of the magnetic flux through the surface bounded by the loop.

Lenz's rule. As we already know, an inductive current occurs in the conductor. Depending on the conditions of its appearance, it has a different direction. On this occasion, the Russian physicist Lenz formulated the following rule: the induction current that occurs in a closed circuit always has such a direction that the magnetic field it creates does not allow the magnetic flux to change. All this causes the appearance of an induction current.

Induction current, like any other, has energy. This means that in the event of an induction current, electrical energy appears. According to the law of conservation and transformation of energy, the above-mentioned energy can only arise due to the amount of energy of some other type of energy. Thus, Lenz's rule fully corresponds to the law of conservation and transformation of energy.

In addition to induction, the so-called self-induction can appear in the coil. Its essence is as follows. If a current appears in the coil or its strength changes, then a changing magnetic field appears. And if the magnetic flux passing through the coil changes, then an electromotive force arises in it, which is called the EMF of self-induction.

According to Lenz's rule, the EMF of self-induction when the circuit is closed interferes with the current strength and does not allow it to increase. When the EMF circuit is turned off, self-induction reduces the current strength. In the case when the current strength in the coil reaches a certain value, the magnetic field stops changing and the self-induction EMF becomes zero.