Conditions for the existence of electric current: characteristics and actions. What is electric current

Charge in motion. It can take the form of a sudden discharge of static electricity, such as lightning. Or it could be a controlled process in generators, batteries, solar or fuel cells. Today we will look at the concept itself " electricity"and the conditions for the existence of electric current.

Electric Energy

Most of the electricity we use comes in the form of alternating current from the electrical grid. It is created by generators that work according to Faraday's law of induction, due to which a changing magnetic field can induce an electric current in a conductor.

Generators have rotating coils of wire that pass through magnetic fields as they rotate. As the coils rotate, they open and close relative magnetic field and create an electric current that changes direction at every turn. The current passes through a full cycle back and forth 60 times per second.

Generators can be powered by steam turbines heated by coal, natural gas, oil or a nuclear reactor. From the generator, the current passes through a series of transformers, where its voltage increases. The diameter of the wires determines the amount and intensity of current they can carry without overheating and losing energy, and the voltage is limited only by how well the lines are insulated from ground.

It is interesting to note that the current is carried by only one wire and not two. Its two sides are designated as positive and negative. However, since the polarity of alternating current changes 60 times per second, they have other names - hot (main power lines) and ground (running underground to complete the circuit).

Why is electric current needed?

There are many uses for electric current: it can light up your home, wash and dry your clothes, lift your garage door, make water boil in a kettle and enable other household items that make our lives much easier. However, the ability of current to transmit information is becoming increasingly important.

When your computer connects to the Internet, only a small amount of electrical current is used, but that's something without modern man can't imagine his life.

The concept of electric current

Like a river flow, a flow of water molecules, an electric current is a flow of charged particles. What is it that causes it, and why doesn't it always go in the same direction? When you hear the word "flowing", what do you think of? Perhaps it will be a river. This is a good association because it is for this reason that electric current gets its name. It is very similar to the flow of water, but instead of water molecules moving along a channel, charged particles move along a conductor.

Among the conditions necessary for the existence of electric current, there is a point that requires the presence of electrons. Atoms in a conductive material have many of these free charged particles floating around and between the atoms. Their movement is random, so there is no flow in any given direction. What is needed for electric current to exist?

Conditions for the existence of electric current include the presence of voltage. When it is applied to a conductor, all free electrons will move in the same direction, creating a current.

Curious about electric current

What's interesting is that when electrical energy is transferred through a conductor at the speed of light, the electrons themselves move much slower. In fact, if you walked slowly next to a conductive wire, your speed would be 100 times faster than the electrons. This is due to the fact that they do not need to travel huge distances to transfer energy to each other.

Direct and alternating current

Today there are two widely used different types current - direct and alternating. In the first, electrons move in one direction, from the “negative” side to the “positive” side. Alternating current pushes electrons back and forth, changing the direction of flow several times per second.

Generators used in power plants to produce electricity are designed to produce alternating current. You've probably never noticed that the lights in your home actually flicker because the current direction changes, but it happens too quickly for your eyes to detect.

What are the conditions for the existence of direct electric current? Why do we need both types and which one is better? This good questions. The fact that we still use both types of current suggests that they both serve specific purposes. Back in the 19th century, it was clear that efficient transmission of power over long distances between a power plant and a home was only possible at very high voltages. But the problem was that sending really high voltage was extremely dangerous for people.

The solution to this problem was to reduce the tension outside the home before sending it inside. To this day, direct electric current is used for long distance transmission, mainly due to its ability to be easily converted into other voltages.

How does electric current work?

The conditions for the existence of electric current include the presence of charged particles, a conductor, and voltage. Many scientists have studied electricity and discovered that there are two types of electricity: static and current.

It is the second that plays a huge role in Everyday life any person, since it represents an electric current that passes through a circuit. We use it daily to power our homes and much more.

What is electric current?

When electrical charges circulate in a circuit from one place to another, an electric current is created. The conditions for the existence of electric current include, in addition to charged particles, the presence of a conductor. Most often this is a wire. Its circuit is a closed circuit in which current passes from the power source. When the circuit is open, he cannot complete the journey. For example, when the light in your room is off, the circuit is open, but when the circuit is closed, the light is on.

Current power

The conditions for the existence of electric current in a conductor are greatly influenced by voltage characteristics such as power. This is a measure of how much energy is used over a certain period of time.

There are many different units that can be used to express this characteristic. However, electrical power is almost measured in watts. One watt is equal to one joule per second.

Electric charge in motion

What are the conditions for the existence of electric current? It can take the form of a sudden discharge of static electricity, such as lightning or a spark from friction with woolen fabric. More often, however, when we talk about electric current, we're talking about a more controlled form of electricity that makes lights burn and appliances work. Most of the electrical charge is carried by negative electrons and positive protons within an atom. However, the latter are mainly immobilized inside atomic nuclei, so the work of transferring charge from one place to another is done by electrons.

Electrons in a conducting material such as a metal are largely free to move from one atom to another along their conduction bands, which are the highest electron orbits. Sufficient electromotive force or voltage creates a charge imbalance that can cause electrons to flow through a conductor in the form of an electric current.

If we draw an analogy with water, then take, for example, a pipe. When we open the valve at one end to allow water to flow into the pipe, we do not have to wait for that water to make its way all the way to the end. We get water at the other end almost instantly because the incoming water pushes the water that is already in the pipe. This is what happens when there is an electric current in a wire.

Electric current: conditions for the existence of electric current

Electric current is usually thought of as a flow of electrons. When the two ends of a battery are connected to each other using a metal wire, this charged mass passes through the wire from one end (electrode or pole) of the battery to the opposite. So, let's name the conditions for the existence of electric current:

1. Charged particles.
2. Conductor.
3. Voltage source.

However, not all so simple. What conditions are necessary for the existence of electric current? This question can be answered in more detail by considering the following characteristics:

• Potential difference (voltage). This is one of the mandatory conditions. There must be a potential difference between the 2 points, meaning that the repulsive force that is created by the charged particles at one place must be greater than their force at another point. Voltage sources are generally not found in nature, and the electrons are distributed in environment fairly evenly. Nevertheless, scientists managed to invent certain types of devices where these charged particles can accumulate, thereby creating the very necessary voltage (for example, in batteries).
• Electrical resistance (conductor). This is the second important condition, which is necessary for the existence of electric current. This is the path along which charged particles travel. Only those materials that allow electrons to move freely act as conductors. Those who do not have this ability are called insulators. For example, a metal wire will be an excellent conductor, while its rubber sheath will be an excellent insulator.

Having carefully studied the conditions for the emergence and existence of electric current, people were able to tame this powerful and dangerous element and direct it for the benefit of humanity.

Metals in the solid state, as is known, have a crystalline structure. Particles in crystals are arranged in a certain order, forming a spatial (crystalline) lattice.

Positive ions are located at the nodes of the metal crystal lattice, and free electrons move in the space between them. Free electrons are not associated with the nuclei of their atoms (Fig. 53).

Rice. 53. Metal crystal lattice

The negative charge of all free electrons is equal in absolute value to the positive charge of all lattice ions. Therefore in normal conditions metal is electrically neutral. Free electrons move randomly in it. But if an electric field is created in a metal, then free electrons will begin to move directionally under the influence of electrical forces. An electric current will occur. In this case, the random movement of electrons is preserved, just as the random movement of a flock of midges is preserved when, under the influence of the wind, it moves in one direction.

So, electric current in metals is the ordered movement of free electrons.

Mandelstam Leonid Isaakovich (1879-1944)
Russian physicist, academician. He made a significant contribution to the development of radiophysics and radio engineering.

Papaleksi Nikolai Dmitrievich (1880-1947)
Russian physicist, academician. He was engaged in research in the field of radio engineering, radio physics, and radio astronomy.

Evidence that the current in metals is caused by electrons was provided by the experiments of our country’s physicists Leonid Isaakovich Mandelstam and Nikolai Dmitrievich Papaleksi, as well as the American physicists Balfour Stewart and Robert Tolman.

The speed of movement of the electrons themselves in the conductor under the influence electric field small - a few millimeters per second, and sometimes even less. But as soon as an electric field arises in the conductor, it spreads along the entire length of the conductor at a tremendous speed, close to the speed of light in vacuum (300,000 km/s).

Simultaneously with the propagation of the electric field, all electrons begin to move in one direction along the entire length of the conductor. So, for example, when the circuit of an electric lamp is closed, the electrons present in the spiral of the lamp also begin to move in an orderly manner.

Comparing the electric current with the flow of water in a water pipe, and the distribution of the electric field with the distribution of water pressure will help to understand this. When water rises into a water tower, the pressure (pressure) of the water spreads very quickly throughout the entire water supply system. When we open the tap, the water is already under pressure and immediately begins to flow. But the water that was in it flows from the tap, and the water from the tower will reach the tap much later, since the movement of water occurs at a lower speed than the spread of pressure.

When we talk about the speed of propagation of electric current in a conductor, we mean the speed of propagation of the electric field along the conductor.

An electrical signal sent, for example, along wires from Moscow to Vladivostok (s = 8000 km), arrives there in approximately 0.03 s.

Questions

1. How can we explain that under normal conditions a metal is electrically neutral?
2. What happens to the electrons of a metal when an electric field appears in it?
3. What is electric current in metal?
4. What speed do they mean when they talk about the speed of propagation of electric current in a conductor?

Exercise

Using the Internet, find how fast electrons move in metals. Compare it to the speed of light.

Electricity

First of all, it is worth finding out what electric current is. Electric current is the ordered movement of charged particles in a conductor. 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 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, electric charges can move freely between various parts, 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

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. to his 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. Resistivity is expressed in Ohms x m. Since wires, as a rule, have a fairly small cross-section, their areas are usually expressed in square millimeters. In this case, the unit resistivity will become Ohm x mm2/m. In the table below. Figure 1 shows the resistivities of some materials.

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).

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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 connecting electric motors, 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 done by the current on a certain section of the circuit is equal to the product 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 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. 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).

Basic laws of electric current

Ohm's law. Voltage and current are considered the most useful 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 required form. The product of the potential difference and the current 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, 3 devices would be needed. Is it possible to get by with just one and calculate the power from its readings and some characteristic of the circuit, such as its resistance? Many people liked this idea and found it fruitful.

So what is the resistance of a wire or circuit as a whole? Does a wire, like water pipes or vacuum system pipes, have a permanent property that could be called resistance? For example, in pipes, the ratio of the pressure difference producing flow divided by the flow rate is usually a constant characteristic of the pipe. Similarly, heat flow in a wire is governed by a simple relationship involving 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 begin searching for the above relationship. First of all, he strived for fame and fame, which would allow him to teach at the university. That is why he chose an area of ​​research that promised special advantages.

Om was the son of a mechanic, so he knew how to draw metal wire of different thicknesses, which he needed for experiments. Since it was impossible to buy suitable wire in those days, Om made it himself. During his experiments, he tried different lengths, different thicknesses, different metals and even different temperatures. He varied all these factors one by one. In Ohm's time, batteries were still weak and produced inconsistent current. 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 study of electrical circuits has just begun to develop. After batteries were invented 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, became the reason for the emergence of a new field of physics, on the other hand, it 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 great value both for the development of the study of electricity and for the development of applied electrical engineering. They made it possible to easily predict the properties of electrical circuits for direct current, and subsequently for alternating current. In 1826, Ohm published a book in which he outlined theoretical conclusions and experimental results. But his hopes were not justified; the book was greeted with ridicule. This happened because the method of crude experimentation seemed unattractive in an era when many were interested in philosophy.

He had no choice but to leave his teaching position. He did not achieve an appointment to 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 Germany. Om was respected abroad and benefited from his research. In this regard, his compatriots were forced to recognize him in his homeland. In 1849 he received a professorship at the University of Munich.

Ohm discovered a simple law establishing the relationship between current and voltage for a piece of wire (for part of a circuit, for the entire circuit). In addition, he compiled 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 a 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 does some work. For example, let's take any section of the circuit between the ends of which there is a voltage (U). By 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 equal to i, then in time t the charge it will pass, and therefore the work of the electric current in this section will be:

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

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 stationary 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:

where r is the conductor resistance. In this case:

This law was first experimentally deduced by E. Lenz and, independently of him, by Joule.

It should be noted that heating conductors has numerous applications in technology. 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 induction. Due to the fact that an electric current arises in a circuit only when free charges are exposed to external forces, then with a changing magnetic flux passing along the surface of a closed circuit, these same external forces appear in it. The action of external forces in physics is called electromotive force or induced emf.

Electromagnetic induction also appears in open conductors. In the case when a conductor crosses magnetic power lines, tension arises at its ends. The reason for the appearance of such voltage is the induced emf. If the magnetic flux passing through a closed loop does not change, induced current doesn't appear.

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

Lenz's rule. As we already know, an induced current arises in a 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 induced current arising 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, so-called self-induction can appear in the coil. Its essence is as follows. If a current arises in the coil or its strength changes, a changing magnetic field appears. And if the magnetic flux passing through the coil changes, then an electromotive force appears in it, which is called Self-induced emf.

According to Lenz's rule, the self-inductive emf when closing a circuit interferes with the current strength and prevents it from increasing. When the circuit is turned off, the self-inductive emf 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.

Electric current is charged particles capable of moving in an orderly manner in any conductor. This movement occurs under the influence of an electric field. The appearance of electrical charges occurs almost constantly. This is especially pronounced when various substances come into 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 conductivity, as well as salt 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 according to its basic parameters. Based on the measurement results, 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, changes occur in these parameters over a certain period of time.

Directed movement of charged particles in an electric field.

Charged particles can be electrons or ions (charged atoms).

An atom that has lost one or more electrons acquires a positive charge. - Anion (positive ion).
An atom that has gained one or more electrons acquires a negative charge. - Cation (negative ion).
Ions are considered as mobile charged particles in liquids and gases.

In metals, charge carriers are free electrons, like negatively charged particles.

In semiconductors, we consider the movement (movement) of negatively charged electrons from one atom to another and, as a result, the movement between the atoms of the resulting positively charged vacancies - holes.

Behind direction of electric current the direction of movement of positive charges is conventionally accepted. This rule was established long before the study of the electron and remains true to this day. The electric field strength is also determined for a positive test charge.

For any single charge q in an electric field of intensity E force acts F = qE, which moves the charge in the direction of the vector of this force.

The figure shows that the force vector F - = -qE, acting on a negative charge -q, is directed in the direction opposite to the field strength vector, as the product of the vector E to a negative value. Consequently, negatively charged electrons, which are charge carriers in metal conductors, actually have a direction of movement opposite to the field strength vector and the generally accepted direction of electric current.

Charge amount Q= 1 Coulomb moved through the cross section of the conductor in time t= 1 second, determined by current value I= 1 Ampere from the ratio:

I = Q/t.

Current ratio I= 1 Ampere in conductor to its cross-sectional area S= 1 m 2 will determine the current density j= 1 A/m2:

Job A= 1 Joule spent on transporting charge Q= 1 Coulomb from point 1 to point 2 will determine the value of the electrical voltage U= 1 Volt, as potential difference φ 1 and φ 2 between these points from the calculation:

U = A/Q = φ 1 - φ 2

Electric current can be direct or alternating.

Direct current is an electric current whose direction and magnitude do not change over time.

Alternating current is an electric current whose magnitude and direction changes over time.

Back in 1826, the German physicist Georg Ohm discovered an important law of electricity, which determines the quantitative relationship between electric current and the properties of a conductor, characterizing their ability to withstand electric current.
These properties later became known as electrical resistance, denoted by a letter R and measured in Ohms in honor of the discoverer.
Ohm's law modern interpretation the classic U/R ratio determines the amount of electric current in a conductor based on voltage U at the ends of this conductor and its resistance R:

Electric current in conductors

Conductors contain free charge carriers, which, under the influence of an electric field, move and create an electric current.

In metal conductors, charge carriers are free electrons.
As the temperature rises, the chaotic thermal movement of atoms interferes with the directional movement of electrons and the resistance of the conductor increases.
When cooling and the temperature approaches absolute zero, when thermal movement stops, the resistance of the metal tends to zero.

Electric current in liquids (electrolytes) exists as the directed movement of charged atoms (ions), which are formed in the process of electrolytic dissociation.
The ions move towards electrodes opposite in sign and are neutralized, settling on them. - Electrolysis.
Anions are positive ions. They move to the negative electrode - the cathode.
Cations are negative ions. They move to the positive electrode - the anode.
Faraday's laws of electrolysis determine the mass of a substance released on the electrodes.
When heated, the resistance of the electrolyte decreases due to an increase in the number of molecules decomposed into ions.

Electric current in gases - plasma. Electric charge is carried by positive or negative ions and free electrons, which are formed under the influence of radiation.

There is an electric current in a vacuum as a flow of electrons from the cathode to the anode. Used in electron beam devices - lamps.

Electric current in semiconductors

Semiconductors occupy an intermediate position between conductors and dielectrics in terms of their resistivity.
A significant difference between semiconductors and metals can be considered the dependence of their resistivity on temperature.
As the temperature decreases, the resistance of metals decreases, while for semiconductors, on the contrary, it increases.
As the temperature approaches absolute zero, metals tend to become superconductors, and semiconductors - insulators.
The fact is that at absolute zero, electrons in semiconductors will be busy creating covalent bonds between the atoms of the crystal lattice and, ideally, there will be no free electrons.
As the temperature increases, some of the valence electrons can receive energy sufficient to break covalent bonds and free electrons will appear in the crystal, and vacancies are formed at the sites of the break, which are called holes.
The vacant place can be occupied by a valence electron from a neighboring pair and the hole will move to a new place in the crystal.
When a free electron meets a hole, the electronic bond between the atoms of the semiconductor is restored and the reverse process occurs - recombination.
Electron-hole pairs can appear and recombine when a semiconductor is illuminated due to the energy of electromagnetic radiation.
In the absence of an electric field, electrons and holes participate in chaotic thermal motion.
Not only the resulting free electrons, but also holes, which are considered as positively charged particles, participate in the electric field in ordered motion. Current I in a semiconductor it consists of electron I n and hole Ip currents

Semiconductors include: chemical elements, such as germanium, silicon, selenium, tellurium, arsenic, etc. The most common semiconductor in nature is silicon.

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