At today's meeting we will talk about electricity, which has become an integral part of modern civilization. Electric power has invaded all areas of our lives. And presence in every home household appliances using electric current is such a natural and integral part of everyday life that we take it for granted.

So, our readers are offered basic information about electric current.

What is electric current

Electric current means directed movement of charged particles. Substances containing a sufficient number of free charges are called conductors. A collection of all devices connected to each other using wires is called an electrical circuit.

IN Everyday life we use electricity passing through metal conductors. The charge carriers in them are free electrons.

Usually they rush chaotically between atoms, but the electric field forces them to move in a certain direction.

How does this happen

The flow of electrons in a circuit can be compared to the flow of water falling from high level to low. The role of level in electrical circuits is played by potential.

For current to flow in the circuit, a constant potential difference must be maintained at its ends, i.e. voltage.

It is usually denoted by the letter U and measured in volts (B).

Due to the applied voltage, an electric field is established in the circuit, which gives the electrons directional movement. The higher the voltage, the stronger the electric field, and therefore the intensity of the flow of directionally moving electrons.

The speed of propagation of electric current is equal to the speed of establishment of an electric field in the circuit, i.e. 300,000 km/s, but the speed of electrons barely reaches only a few mm per second.

It is generally accepted that current flows from a point with a higher potential, i.e., from (+) to a point with a lower potential, i.e., to (−). The voltage in the circuit is maintained by a current source, such as a battery. The sign (+) at its end means a lack of electrons, the sign (−) means their excess, since electrons are carriers of a negative charge. As soon as the circuit with the current source becomes closed, electrons rush from the place where there is an excess of them to the positive pole of the current source. Their path runs through wires, consumers, measuring instruments and other circuit elements.

Please note that the direction of the current is opposite to the direction of movement of the electrons.

Simply, the direction of the current, by agreement of scientists, was determined before the nature of the current in metals was established.

Some quantities characterizing electric current

Current strength. The electric charge passing through the cross-section of a conductor in 1 second is called current strength. The letter I is used to designate it and is measured in amperes (A).

Resistance. The next quantity you need to know about is resistance. It arises due to collisions of directionally moving electrons with ions crystal lattice. As a result of such collisions, electrons transfer part of their energy to the ions. kinetic energy. As a result, the conductor heats up and the current strength decreases. Resistance is symbolized by the letter R and is measured in ohms (ohms).

The resistance of a metal conductor is greater, the longer the conductor and smaller area its cross section. With the same length and diameter of the wire, conductors made of silver, copper, gold and aluminum have the least resistance. For obvious reasons, wires made of aluminum and copper are used in practice.

Power. When performing calculations for electrical circuits, it is sometimes necessary to determine the power consumption (P).

To do this, the current flowing through the circuit must be multiplied by the voltage.

The unit of power is the watt (W).

Direct and alternating current

The current provided by various batteries and accumulators is constant. This means that the current strength in such a circuit can only be changed in magnitude by changing different ways its resistance, and its direction remains unchanged.

But Most electrical appliances consume alternating current, that is, a current whose magnitude and direction continuously changes according to a certain law.

It is generated in power plants and then travels through high-voltage transmission lines into our homes and businesses.

In most countries, the frequency of current reversal is 50 Hz, i.e. it occurs 50 times per second. In this case, each time the current strength gradually increases, reaches a maximum, then decreases to 0. Then this process is repeated, but with the opposite direction of the current.

In the USA, all devices operate at a frequency of 60 Hz. An interesting situation has developed in Japan. There, one third of the country uses alternating current with a frequency of 60 Hz, and the rest - 50 Hz.

Caution - electricity

Electric shock can occur when using electrical appliances and from lightning strikes, since The human body is a good conductor of current. Electrical injuries are often caused by stepping on a wire lying on the ground or pushing away loose electrical wires with your hands.

Voltage above 36 V is considered dangerous to humans. If a current of only 0.05 A passes through a person’s body, it can cause involuntary muscle contraction, which will not allow the person to independently tear himself away from the source of the lesion. A current of 0.1 A is lethal.

Alternating current is even more dangerous because it has a stronger effect on humans. This friend and helper of ours in some cases turns into a merciless enemy, causing breathing problems and heart function, even to the point of complete cardiac arrest. It leaves terrible marks on the body in the form of severe burns.

How to help the victim? First of all, turn off the source of damage. And then take care of providing first aid.

Our acquaintance with electricity is coming to an end. Let’s add just a few words about sea creatures that have “electric weapons.” These are some types of fish, conger eel and stingray. The most dangerous of them is the conger eel.

You should not swim to it at a distance of less than 3 meters. His blow is not fatal, but consciousness can be lost.

If this message was useful to you, I would be glad to see you

Introduction

electromagnetic induction circuit current

The first knowledge of electricity, many centuries ago, related to electrical “charges” produced through friction. Already in ancient times, people knew that amber, rubbed with wool, acquired 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 had exactly the same properties. Bodies that, like amber, after rubbing, can attract light objects, he called electrified. This word is derived from the Greek electron - “amber”. Currently, we say that bodies in this state have electrical charges, and the bodies themselves are called “charged.”

Electric charges always arise when different substances come into close contact. If the bodies are solid, then their close contact is prevented by microscopic protrusions and irregularities that are present on their surface. By squeezing such bodies and rubbing them against each other, we bring together their surfaces, which without pressure would only touch at a few points. In some bodies, electrical charges can move freely between different parts, but in others this is impossible. 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 found 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 positive and negative. This kind of current will not last long, because the electrified body will run out of charge. For the continued 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 electric current is when one end of the wire is connected to an electrified body, and the other to the ground.

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

> Basic 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 transferred by the electrons. This net charge is called the amount of electricity passing through a conductor.

In particular, the chemical effect of electric current depends on the amount of electricity, i.e. The more charge passed through the electrolyte solution, the more substance will be deposited 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.

Current strength is a quantity that is equal to the ratio of the electric charge passing 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 is expressed in C/s. This unit is called ampere (A). In order to measure the current 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 connected in series to the circuit.

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

Voltage is the ratio of the work done by the current in a certain section of an electrical circuit to the charge flowing through the same section of the circuit. Current work is measured in joules (J), charge - in coulombs (C). In this regard, the unit of measurement for voltage will become 1 J / C. This unit was called the volt (V).

In order for voltage to arise in an electrical circuit, a current source is needed. When the circuit is open, voltage is present only at the terminals of the current source. If this current source is included in the circuit, voltage will also arise in individual sections of the circuit. In this regard, a current will appear in the circuit. That is, we can briefly say the following: if there is no voltage in the circuit, there is no current. In order to measure voltage, an electrical measuring instrument called a voltmeter is used. In its appearance, it resembles the previously mentioned ammeter, with the only difference being that the letter V is written on the voltmeter scale (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 various conductors and an ammeter to the electrical circuit, you can notice that when using different conductors, the ammeter gives different readings, i.e. 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. It was named Ohm in honor of the German physicist. As a rule, larger units are used in physics: kilo-ohm, mega-ohm, etc. The resistance of a conductor is usually denoted by the letter R, the length of the conductor is L, and the cross-sectional area is S. In this case, the resistance can be written as a formula:

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 m 2. Specific resistance is expressed in Ohms 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 resistivity will become Ohm x mm2/m.

According to the table. 1 it becomes clear that copper has the lowest electrical resistivity, and metal alloy has the highest. In addition, dielectrics (insulators) have high resistivity.

Electrical capacity. We already know that two conductors isolated from each other can accumulate electrical charges. This phenomenon is characterized by a physical quantity 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 capacity. The unit of electrical capacitance is the farad (F). In practice, fractions of this unit are used: microfarad (μF) and picofarad (pF).

If you take two conductors isolated from each other and place them at a short distance from one another, you will 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, the capacitance of the latter can be significantly increased. On all capacitors, in addition to their capacity, the voltage for which these devices are designed must be indicated.

Work and power of electric current. From the above it is clear that electric current does some 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, i.e. allows electrolysis, etc. We can say that the work of current in a certain section of the circuit is equal to the product of current, voltage and time during which the work was performed. Work is measured in joules, voltage in volts, current in amperes, time in seconds. In this regard, 1 J = 1B x 1A x 1 s. From this it turns out that in order to measure the work of electric current, three instruments should be used at once: an ammeter, a voltmeter and a clock. But this is cumbersome and ineffective. Therefore, usually, the work of electric current is measured with electric meters. This device contains all of the above devices.

The power of the electric current is equal to the ratio of the work of the current to the time during which it was performed. Power is designated 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 engineers express the work of current in kilowatt-hours (kWh).

Characteristics

Historically, it was accepted that the direction of the current coincides with the direction of movement of positive charges in the conductor. Moreover, 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 the charged particles. .

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

Basic quantities of electric current

Amount of electricity and current. 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 transferred by the electrons. This net charge is called the amount of electricity passing through a conductor.

In particular, the chemical effect of electric current depends on the amount of electricity, i.e., the greater the charge passed through the electrolyte solution, the more substance will be deposited 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.

Current strength is a quantity that is equal to the ratio of the electric charge passing 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 is expressed in C/s. This unit is called ampere (A). In order to measure the current 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 connected in series to the circuit.

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

Voltage is the ratio of the work done by the current in a certain section of an electrical circuit to the charge flowing through the same section of the circuit. Current work is measured in joules (J), charge - in coulombs (C). In this regard, the unit of measurement for voltage will become 1 J/C. This unit was called the volt (V).

In order for voltage to arise in an electrical circuit, a current source is needed. When the circuit is open, voltage is present only at the terminals of the current source. If this current source is included in the circuit, voltage will also arise in individual sections of the circuit. In this regard, a current will appear in the circuit. That is, we can briefly say the following: if there is no voltage in the circuit, there is no current. In order to measure voltage, an electrical measuring instrument called a voltmeter is used. In its appearance, it resembles the previously mentioned ammeter, with the only difference being that the letter V is written on the voltmeter scale (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 various conductors and an ammeter to the electrical circuit, you can notice that when using different conductors, the ammeter gives different readings, i.e. 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. It was named Ohm in honor of the German physicist. As a rule, larger units are used in physics: kilo-ohm, mega-ohm, etc. The resistance of a conductor is usually denoted by the letter R, the length of the conductor is L, and the cross-sectional area is S. In this case, the resistance can be written as a formula:

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. Specific resistance is expressed in Ohms 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. Figure 1 shows the resistivities of some materials.

Amount of electricity and current. 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 transferred by the electrons. This net charge is called the amount of electricity passing through a conductor.

Current strength is a quantity that is equal to the ratio of the electric charge passing 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 is expressed in C/s. This unit is called ampere (A). In order to measure the current in a circuit, an electrical measuring device called an ammeter is used.

Electrical voltage. Voltage is the ratio of the work done by the current in a certain section of an electrical circuit to the charge flowing through the same section of the circuit. Current work is measured in joules (J), charge - in coulombs (C). In this regard, the unit of measurement for voltage will become 1 J/C. This unit was called the volt (V).

Electrical resistance. When using different conductors, the current strength available in the electrical circuit is different. Different conductors have different electrical resistance, which is a physical quantity. The resistance of a conductor is usually denoted by the letter R, the length of the conductor is L, and the cross-sectional area is S. In this case, the resistance can be written as a formula:

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. Specific resistance is expressed in Ohms 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.

Electrical capacity. 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 capacity. The unit of electrical capacitance is the farad (F). In practice, fractions of this unit are used: microfarad (μF) and picofarad (pF).

Work and power of electric current. Work is measured in joules, voltage in volts, current in amperes, time in seconds. In this regard, 1 J = 1B x 1A x 1s. From this it turns out that in order to measure the work of electric current, three instruments should be used at once: an ammeter, a voltmeter and a clock.

The power of the electric current is equal to the ratio of the work of the current to the time during which it was performed. Power is designated 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 engineers express the work of current in kilowatt-hours (kWh).

A magnetic field- a force field acting on moving electric charges and on bodies with a magnetic moment, regardless of the state of their motion, the magnetic component of the electric magnetic field

A magnetic field can be created by the current of charged particles and/or the magnetic moments of electrons in atoms (and the magnetic moments of other particles, although to a noticeably lesser extent) (permanent magnets).

In addition, it appears in the presence of a time-varying electric field.

The main strength characteristic of the magnetic field is magnetic induction vector (magnetic field induction vector). From a mathematical point of view - vector field that defines and specifies the physical concept of a magnetic field. Often, for brevity, the magnetic induction vector is simply called a magnetic field (although this is probably not the most strict use of the term).

Another fundamental characteristic of the magnetic field (alternative to magnetic induction and closely interrelated with it, almost equal to it in physical value) is vector potential .

Magnetic field can be called a special type of matter, through which interaction occurs between moving charged particles or bodies with a magnetic moment.

Magnetic fields are necessary (in the context special theory relativity) a consequence of the existence of electric fields.

Together, magnetic and electric field form an electromagnetic field, the manifestations of which are, in particular, light and all other electromagnetic waves.

An electric current (I) passing through a conductor creates a magnetic field (B) around the conductor.

• From the point of view of quantum field theory, magnetic interaction is how special case electromagnetic interaction is carried by a fundamental massless boson - a photon (a particle that can be represented as a quantum excitation of an electromagnetic field), often (for example, in all cases of static fields) - virtual.

Magnetic field energy

The increment in magnetic field energy density is equal to:

H- magnetic field strength,

B- magnetic induction

In the linear tensor approximation, magnetic permeability is a tensor (we denote it) and multiplication of a vector by it is tensor (matrix) multiplication:

Or in components .

The energy density in this approximation is equal to:

Components of the magnetic permeability tensor,

A tensor, represented by a matrix, inverse matrix magnetic permeability tensor,