Where does the current flow in. There are also other ways to create an internal current in

electric shock called the orderly movement of electric charges. The directed movement of electric charges in a conductor under the action of electric field forces is called conduction current. For the appearance and existence of the conduction current, two conditions are necessary:

1. The presence of electric charges in a given medium. In metals, they are the conduction electrons; in liquid conductors (electrolytes) - positive and negative ions; in gases - positive ions and electrons.

2. The presence of an electric field, the energy of which would be spent on the movement of electric charges.

The direction of movement of positive charges is conditionally taken as the direction of electric current. The quantitative characteristic of the electric current is current strength- the charge flowing through the cross section of the conductor per unit time:

The current strength can be related to the average speed υ orderly movement of charges. During dt through the cross section of the conductor with area dS charge will leak dq, enclosed in the volume of a conductor of length dl= u . dt, (fig.5.1)

dq=q0 . n. dS . dl,

where q0 is the charge of each particle, n is the concentration of particles.

Then the current

. (5.2)

current densityj- vector physical quantity, numerically equal to the strength of the current passing through the unit area of the cross section of the conductor, drawn perpendicular to the direction of the current, and coinciding with the direction of the current

In order for the current to be continuous, a device is needed in which some form of energy is continuously converted into the energy of an electric field. Such a device is called current source. In the current source, the movement of carriers occurs against the forces of the field, and this is possible only due to the forces of non-electrostatic origin, called outside forces.

The value equal to the work of external forces in moving a unit positive charge along a closed circuit is called electromotive force (EMF) x ,

An external force acting on a charge can be represented through the field strength of external forces

then the EMF for a closed circuit is determined by the expression

Therefore, the EMF acting in a closed circuit is equal to the circulation of the field strength vector of external forces.

The value numerically equal to the work performed by electric and external forces when moving a single positive h charge on a given section of the chain is called voltage:

dS tga=1/R

Fig.5.1 Fig.5.2

5.2 Generalized Ohm's law. Differential form of Ohm's law

For each conductor - solid, liquid and gaseous - there is a certain dependence of the current strength on the applied voltage - volt - ampere characteristic (VAC). It has the simplest form for metallic conductors and electrolyte solutions (Fig. 5.2) and is determined by Ohm's law.

According to the law Ohma for a homogeneous (not containing external forces) section of the circuit, the current strength is directly proportional to the applied voltage U and inversely proportional to the conductor resistance R

The unit of resistance is Ohm ([R ] = 1 Ohm). Ohm is the resistance of such a conductor, in which at a voltage of 1 AT flowing current 1 BUT.

The resistance depends on the properties of the conductor, the shape and its geometric dimensions. For a homogeneous cylindrical conductor

where l- the length of the conductor, S- cross-sectional area,

r- resistivity (resistance of a conductor 1 m long and with a cross-sectional area of 1 m 2)depends on the nature of the conductor and temperature ([ r] = Ohm. m).

The reciprocal of resistivity is called electrical conductivity: s = 1/r.

For an inhomogeneous section of the chain, i.e. section containing EMF (Fig. 5.3), taking into account (5.7) and (5.8), we obtain

. (5.10)

This expression is called generalized Ohm's law in integral form.

We obtain Ohm's law for a homogeneous section of the circuit in differential form. To do this, in the vicinity of a certain point inside the conductor, we select an elementary cylindrical volume with generators parallel to the current density vector j at this point (Fig. 5.4).

- + dS

R x 12J

Rice. 5.3 Fig. 5.4

A current flows through the cross section of the cylinder I=jdS. The voltage applied to the cylinder is

where E is the field strength at a given point.

cylinder resistance. Substituting I, U and R

into formula (5.8) and taking into account that the directions of the vectors coincide, we obtain Ohm's law for a homogeneous section of a circuit in differential form

. (5.11)

Ohm's law for an inhomogeneous section of a chain in differential form will be written as follows:

, (5.12)

where is the field strength of external forces.

Conductors and current sources in electrical circuits can be connected in series and in parallel.

Consistent such a connection of conductors is called when the end of one conductor is connected to the beginning of another (Fig. 5.5). In this case, the following relations are satisfied:

i=const;

U=U 1 +U 2 +…+U n ;

R=R 1 +R 2 +…+R n. (5.13)

Parallel such a connection is called when one ends of the conductors are connected to one node, and the other ends - to another (Fig. 5.6). In this case, the following relations are fulfilled:

I=I 1 +I 2 +…+I n ;

U=const;

. (5.14)

R 1 I 1

I U 1 U 2 U 3 I R 2 I 2 I

R 1 R 2 R 3

Rice. 5.5 Fig. 5.6

When several identical current sources are connected in series (Fig. 5.7), the total EMF of the battery is equal to the algebraic sum of the EMF of all sources, and the total resistance is equal to the sum of internal resistances:

x b \u003d x 1 + x 2 + ... + x n, r b \u003d r 1 + r 2 + ... + r n.

When connected in parallel n sources with the same EMF - x and internal resistances r(Fig. 5.8) The EMF of the battery is equal to the EMF of one source (x b = x), and the internal resistance of the battery r b \u003d r / n.

Content:

Every layman is familiar with electrical quantities - current, voltage - the operation of household appliances depends on them, but few people have a complete understanding of the definition of electric current. It is significant to compare the electric current with the flow of the river, only in it particles with a charge move, and in the river - water. It must be understood that the current moves in only one direction, conditions must be created for its existence, we will consider these processes in more detail.

Basic definitions

Electricity surrounds us every day, but not every person understands what an electric current and the quantities associated with it are, but they are important for everyday life. There are several interpretations of the concept of electric current:

The definition accepted in a school textbook that an electric current is the movement of particles that have a charge due to the action of an electric field on them. Particles are: protons, holes, electrons, ions.
In the electrical literature of higher educational institutions, it is written that electric current is the rate at which a charge changes over time. The negative charge of electrons is assumed, positive for protons and neutral for neutrons.

In electrical engineering, experts note the importance of such a concept as current strength - this is the number of particles that have a charge that pass through the cross section of a conductor over time. The movement of current in a conductor can be described as follows: “... All conductive materials have an internal structure (molecules, atoms, nuclei with rotating electrons), when a chemical reaction affects the material, electrons from one atom run to another. A situation is created in which some atoms lack electrons, while others experience an excess of them, which shows the opposite charge. Electrons tend to move from one substance to another, this movement is the electric current.

Specialists focus on the fact that in this case the current flows only until the moment when the charges in the two substances are equalized.

To understand the movement of current, it is important to know the definition of voltage - this is the potential difference that is taken at two points in the electric field, measured in volts.

Electric Energy

In different regions, in particular, in Ukraine, a simple man in the street is interested in: “What is an electric strum?”, for what purpose it is used, what it comes from. Everyday we use electrical energy, which is represented by alternating current in electrical networks.

An alternating current in a conductor is when particles that have a charge over a certain period of time change it in direction, as well as in magnitude. Graphically, alternating current is represented by a sinusoid. It is created by generators in which coils with wires rotate and, in the process of rotation, cross the magnetic field. During the period of rotation, the coils can open and close in relation to the magnetic field, which creates an electric current that changes direction in the conductors, and a full cycle takes place in one minute.

The rotation of the generators comes from steam turbines with different power sources: coal, gas, nuclear reactor, oil. Further, through the system of transformers, the voltage rises, through the conductors of the desired diameter, it is transferred without loss for a long distance. The diameter of the wire through which the current flows determines its strength and magnitude, hot lines in the energy industry are called main energy transmission lines, there are also grounded options when electricity is transmitted underground.

Where is electricity applied?

It is the current that makes life much easier for us, creating comfort in the house. It is used for lighting rooms, streets, for drying things, in heating elements of electric stoves, in other household appliances and devices, performs the work of lifting garage doors, etc.

Conditions necessary to receive electricity

For the existence of an electric current, the following conditions are necessary: the presence of particles with a charge, an electrically conductive material along which the particles will move, and a voltage source. An important condition for obtaining electric current is the presence of voltage, which is determined by the potential difference. In other words, the force created by the charged particles of repulsion is greater at one point than at another.

There are no natural sources of voltage, for this reason electrons are evenly distributed around us, but inventions such as batteries made it possible to accumulate electrical energy in them.

Another important condition is the electrical resistance, or conductor, along which the charged particles will move. Materials in which this action is possible are called electrically conductive, and those in which there is no free movement of electrons are called insulators. An ordinary wire has a conductive metal core and an insulating sheath.

Electric current in conductors

In any conductor there are carriers of electric charge, which are set in motion under the influence of the force of the field created by the electric machine.

Metal conductors carry charge with the help of electrons. The higher the temperature of the conductor and the heating of the wire, the worse the current flows, since the chaotic movement of atoms begins in it from thermal exposure, and the resistance of the conductive material increases. The lower the temperature of the conductor (ideally, tending to zero), the lower its resistance.

Liquids can conduct electricity using ions (electrolytes). The movement occurs to the electrode, which has the opposite sign with the ion, and, settling on it, the ions carry out the electrolysis process. Anions are positively charged ions moving towards the cathode. Cations - ions having a negative charge move towards the anode. In the process of heating the electrolyte, its resistance decreases.

The gas also has conductivity, the electric current in it is plasma. Movement occurs with the help of charged ions or free electrons, which are obtained in the process of radiation.

A cathode ray tube is an example of electric current in a vacuum from a cathode rod to an anode rod.

Electric current in semiconductors

To understand the passage of current in this material, let's give it a definition. Semiconductor - an intermediate material between a conductor and an insulator, depends on the specific conductivity, the presence of impurities in it, the temperature state and the radiation acting on it. The lower the temperature, the greater the resistance of the semiconductor, its properties affect the measurement of characteristics. The electric current in a semiconductor is the sum of the electron and hole currents.

When the temperature of the semiconductor rises, covalent bonds are broken due to the action of thermal energy on valence electrons, free electrons are formed, and a hole is obtained at the break point. It is engaged in the valence electron of another pair, and itself moves further in the crystal. When a free electron meets a hole, recombination occurs between them, the restoration of electronic bonds. When a semiconductor is exposed to the energy of electromagnetic radiation, electron-hole pairs appear in it.

Electric current laws

In electrical engineering, the basic laws that define the electric current are applied. One of the most important is Ohm's law, a feature of which is the speed of energy transfer without changing its shape from one point to another.

This law shows the relationship between voltage and current strength, as well as the resistance of a conductor or circuit section. Resistance is measured in ohms.

The work of an electric current is determined by the Joule-Lenz law, which says that at any point in the circuit, the current does work.

Faraday discovered magnetic induction, and also experimentally established that when the line of magnetic induction crosses the surface of a closed conductor, an electric current appears in it. He derived the law of electromagnetic induction:

Non-closed conductors crossing the lines of the magnetic field receive voltage at the ends, which indicates the appearance of an EMF of induction. If the magnetic flux is unchanged and crosses a closed circuit, then no electric current arises in it. The EMF of induction of a closed circuit, when the magnetic flux changes, is equal to the modulus of its rate of change.

Conclusion

When an electric current flows through the conductor, it heats it up, for this reason it is necessary to observe safety measures when working with electrical appliances and devices. The power transmission line must not be overloaded, it may become hot and cause a fire. Electric current always follows the path of least resistance.

At the moment of the appearance of a short circuit (short circuit), the current increases several times, an instantaneous release of a huge thermal value occurs, which melts the metal. Electric current can cause burns on the body of a person or animal, but is used in intensive care units, for depressive solutions and the treatment of diseases.

According to the rules of electrical safety, a current perceptible by a person comes from a value of one milliampere, and a current with 0.01 amperes is considered hazardous to health, a current of 0.1 amperes is defined as a lethal value. Safe voltage for humans is 12-24-32-42 volts.

Without electricity, it is impossible to imagine the life of a modern person. Volts, Amps, Watts - these words are heard in a conversation about devices that run on electricity. But what is this electric current and what are the conditions for its existence? We will talk about this further, providing a brief explanation for beginner electricians.

Definition

An electric current is a directed movement of charge carriers - this is a standard formulation from a physics textbook. In turn, certain particles of matter are called charge carriers. They may be:

Electrons are negative charge carriers.
Ions are positive charge carriers.

But where do charge carriers come from? To answer this question, you need to remember the basic knowledge about the structure of matter. Everything that surrounds us is matter, it consists of molecules, its smallest particles. Molecules are made up of atoms. An atom consists of a nucleus around which electrons move in given orbits. Molecules also move randomly. The movement and structure of each of these particles depends on the substance itself and the influence of the environment on it, such as temperature, stress, and so on.

An ion is an atom in which the ratio of electrons and protons has changed. If the atom is initially neutral, then the ions, in turn, are divided into:

Anions are the positive ion of an atom that has lost electrons.
Cations are an atom with "extra" electrons attached to the atom.

The unit of current is Ampere, according to it is calculated by the formula:

where U is voltage [V] and R is resistance [Ohm].

Or directly proportional to the amount of charge transferred per unit of time:

where Q is the charge, [C], t is the time, [s].

Conditions for the existence of an electric current

We figured out what electric current is, now let's talk about how to ensure its flow. For electric current to flow, two conditions must be met:

The presence of free charge carriers.
Electric field.

The first condition for the existence and flow of electricity depends on the substance in which the current flows (or does not flow), as well as its state. The second condition is also feasible: for the existence of an electric field, the presence of different potentials is necessary, between which there is a medium in which charge carriers will flow.

Recall: Voltage, EMF is a potential difference. It follows that in order to fulfill the conditions for the existence of current - the presence of an electric field and an electric current, voltage is needed. These can be plates of a charged capacitor, a galvanic cell, an EMF that has arisen under the influence of a magnetic field (generator).

We figured out how it arises, let's talk about where it is directed. The current, in its usual use, moves in conductors (wiring in an apartment, incandescent bulbs) or in semiconductors (LEDs, your smartphone's processor and other electronics), less often in gases (fluorescent lamps).

So, in most cases, the main charge carriers are electrons, they move from minus (a point with a negative potential) to a plus (a point with a positive potential, you will learn more about this below).

But an interesting fact is that the direction of current movement was taken to be the movement of positive charges - from plus to minus. Although in fact the opposite is happening. The fact is that the decision on the direction of the current was made before studying its nature, and also before it was determined due to which the current flows and exists.

Electric current in different environments

We have already mentioned that in different media the electric current can differ in the type of charge carriers. Media can be divided according to the nature of conductivity (in descending order of conductivity):

Conductor (metals).
Semiconductor (silicon, germanium, gallium arsenide, etc.).
Dielectric (vacuum, air, distilled water).

in metals

Metals contain free charge carriers and are sometimes referred to as "electric gas". Where do free charge carriers come from? The fact is that metal, like any substance, consists of atoms. Atoms somehow move or oscillate. The higher the temperature of the metal, the stronger this movement. At the same time, the atoms themselves generally remain in their places, actually forming the structure of the metal.

In the electron shells of an atom, there are usually several electrons that have a rather weak bond with the nucleus. Under the influence of temperatures, chemical reactions and the interaction of impurities, which in any case are in the metal, electrons are detached from their atoms, positively charged ions are formed. The detached electrons are called free and move randomly.

If they are affected by an electric field, for example, if you connect a battery to a piece of metal, the chaotic movement of electrons will become ordered. Electrons from a point to which a negative potential is connected (the cathode of a galvanic cell, for example) will begin to move towards a point with a positive potential.

in semiconductors

Semiconductors are materials in which there are no free charge carriers in the normal state. They are in the so-called forbidden zone. But if external forces are applied, such as an electric field, heat, various radiations (light, radiation, etc.), they overcome the band gap and pass into the free band or conduction band. Electrons break away from their atoms and become free, forming ions - positive charge carriers.

Positive carriers in semiconductors are called holes.

If you simply transfer energy to a semiconductor, for example, heat it, a chaotic movement of charge carriers will begin. But if we are talking about semiconductor elements, such as a diode or a transistor, then at the opposite ends of the crystal (a metallized layer is applied to them and the leads are soldered), an EMF will appear, but this does not apply to the topic of today's article.

If you apply an EMF source to a semiconductor, then charge carriers will also move into the conduction band, and their directed movement will also begin - holes will go to the side with a lower electric potential, and electrons - to the side with a larger one.

In vacuum and gas

A vacuum is a medium with a complete (ideal case) absence of gases or a minimized (in reality) its amount. Since there is no matter in vacuum, there is no source for charge carriers. However, the flow of current in a vacuum marked the beginning of electronics and a whole era of electronic elements - vacuum tubes. They were used in the first half of the last century, and in the 50s they began to gradually give way to transistors (depending on the specific field of electronics).

Let's assume that we have a vessel from which all the gas has been pumped out, i.e. it is a complete vacuum. Two electrodes are placed in the vessel, let's call them an anode and a cathode. If we connect the negative potential of the EMF source to the cathode, and positive to the anode, nothing will happen and no current will flow. But if we start heating the cathode, the current will start to flow. This process is called thermionic emission - the emission of electrons from a heated surface of an electron.

The figure shows the process of current flow in a vacuum lamp. In vacuum tubes, the cathode is heated by a nearby filament in Fig. (H), such as that found in a lighting lamp.

At the same time, if you change the polarity of the supply - apply a minus to the anode, and apply a plus to the cathode - the current will not flow. This will prove that the current in vacuum flows due to the movement of electrons from the CATHODE to the ANODE.

A gas, like any substance, consists of molecules and atoms, which means that if the gas is under the influence of an electric field, then at a certain strength (ionization voltage), the electrons will come off the atom, then both conditions for the flow of electric current will be met - the field and free media.

As already mentioned, this process is called ionization. It can occur not only from the applied voltage, but also when the gas is heated, x-rays, under the influence of ultraviolet and other things.

Current will flow through the air, even if a burner is installed between the electrodes.

The flow of current in inert gases is accompanied by gas luminescence; this phenomenon is actively used in fluorescent lamps. The flow of electric current in a gaseous medium is called a gas discharge.

in liquid

Let's say that we have a vessel with water in which two electrodes are placed, to which a power source is connected. If the water is distilled, that is, pure and does not contain impurities, then it is a dielectric. But if we add a little salt, sulfuric acid, or any other substance to the water, an electrolyte is formed and a current begins to flow through it.

An electrolyte is a substance that conducts electricity by dissociating into ions.

If copper sulfate is added to water, then a layer of copper will settle on one of the electrodes (cathode) - this is called electrolysis, which proves that the electric current in the liquid is carried out due to the movement of ions - positive and negative charge carriers.

Electrolysis is a physical and chemical process, which consists in the separation of the components that make up the electrolyte on the electrodes.

Thus, copper plating, gilding and coating with other metals occur.

Conclusion

To summarize, for the flow of electric current, free charge carriers are needed:

electrons in conductors (metals) and vacuum;
electrons and holes in semiconductors;
ions (anions and cations) in liquids and gases.

In order for the movement of these carriers to become ordered, an electric field is needed. In simple terms, apply a voltage at the ends of the body or install two electrodes in an environment where an electric current is expected to flow.

It is also worth noting that the current in a certain way affects the substance, there are three types of exposure:

thermal;
chemical;
physical.

Useful

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 consider the very concept of "electric current" and the conditions for the existence of an 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 spinning coils of wire that pass through magnetic fields as they spin. As the coils rotate, they open and close relative to the magnetic field and create an electrical current that changes direction with each turn. The current goes 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 strength of current they can carry without overheating and wasting power, and voltage is only limited by how well the lines are insulated from ground.

It is interesting to note that the current is carried by only one wire, 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 grounded (passing underground to complete the circuit).

Why is electricity needed?

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

When connected to the Internet, a computer uses only a small part of the electric current, but this is something without which a modern person cannot imagine his life.

The concept of electric current

Like a river current, a stream of water molecules, an electric current is a stream 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 flow, what do you think of? Perhaps it will be a river. It's a good association, because that's the reason the electric current got its name. It is very similar to the flow of water, only instead of water molecules moving along the channel, charged particles move along the conductor.

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

The 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

Interestingly, when electrical energy is transmitted through a conductor at the speed of light, the electrons themselves move much more slowly. In fact, if you walked leisurely next to a conductive wire, your speed would be 100 times faster than the electrons are moving. 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, two different types of current are widely used - direct and alternating. In the first, the electrons move in one direction, from the "negative" side to the "positive" side. The alternating current pushes the electrons back and forth, changing the direction of the flow several times per second.

Generators used in power plants to produce electricity are designed to produce alternating current. You probably never noticed that the light in your house is actually flickering as the current direction changes, but it happens too fast for the eyes to recognize.

What are the conditions for the existence of direct electric current? Why do we need both types and which one is better? These are good questions. The fact that we still use both types of current suggests that they both serve specific purposes. As far back as the 19th century, it was clear that efficient transmission of power over long distances between a power plant and a house was possible only 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 stress outside the home before sending it inside. To this day, DC electric current is used for long distance transmission, mainly because of its ability to be easily converted to other voltages.

How electric current works

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

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

What is electric current?

When electric charges circulate in a circuit from one place to another, an electric current is generated. The conditions for the existence of an electric current include, in addition to charged particles, the presence of a conductor. Most often it is a wire. Its circuit is a closed circuit in which current flows from a 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 an electric current in a conductor are greatly influenced by such a voltage characteristic as power. This is a measure of how much energy is being used over a given 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 an electric current? It can take the form of a sudden discharge of static electricity, such as lightning or a spark from friction with a woolen cloth. More often, however, when we talk about electric current, we mean a more controlled form of electricity that makes lights and appliances work. Most of the electrical charge is carried by the negative electrons and positive protons within the atom. However, the latter are mostly immobilized inside atomic nuclei, so the work of transferring charge from one place to another is done by electrons.

Electrons in a conductive material such as a metal are largely free to move from one atom to another along their conduction bands, which are the higher electron orbits. A sufficient electromotive force or voltage creates a charge imbalance that can cause electrons to move 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 a valve at one end to let water enter the pipe, we don't have to wait for that water to work its way all the way to the end of the pipe. 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 in the case of an electric current in a wire.

Electric current: conditions for the existence of an electric current

Electric current is usually viewed as a flow of electrons. When the two ends of the battery are connected to each other with 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 call the conditions for the existence of an electric current:

charged particles.
Conductor.
Voltage source.

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

Potential difference (voltage). This is one of the prerequisites. Between the 2 points there must be a potential difference, meaning that the repulsive force that is created by charged particles in one place must be greater than their force at another point. Voltage sources, as a rule, do not occur in nature, and electrons are distributed fairly evenly in the environment. 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 that is necessary for the existence of an 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 mankind.

What is electric current

Directional movement of electrically charged particles under the influence of . Such particles can be: in conductors - electrons, in electrolytes - ions (cations and anions), in semiconductors - electrons and so-called "holes" ("electron-hole conductivity"). There is also a "bias current", the flow of which is due to the process of charging the capacitance, i.e. change in the potential difference between the plates. Between the plates, no movement of particles occurs, but the current flows through the capacitor.

In the theory of electrical circuits, current is considered to be the directed movement of charge carriers in a conducting medium under the action of an electric field.

Conduction current (simply current) in the theory of electrical circuits is the amount of electricity flowing per unit time through the cross section of the conductor: i \u003d q / t, where i is the current. BUT; q \u003d 1.6 10 9 - electron charge, C; t - time, s.

This expression is valid for DC circuits. For alternating current circuits, the so-called instantaneous current value is used, equal to the rate of change of charge over time: i (t) \u003d dq / dt.

An electric current occurs when an electric field appears in a section of an electrical circuit, or a potential difference between two points of a conductor. The potential difference between two points is called voltage or voltage drop in this section of the circuit.

Instead of the term "current" ("current value"), the term "current strength" is often used. However, the latter cannot be called successful, since the current strength is not any force in the literal sense of the word, but only the intensity of the movement of electric charges in the conductor, the amount of electricity passing per unit time through the cross-sectional area of \u200b\u200bthe conductor.
The current is characterized, which in the SI system is measured in amperes (A), and current density, which in the SI system is measured in amperes per square meter.
One ampere corresponds to the movement through the cross section of the conductor for one second (s) of a charge of electricity of one pendant (C):

1A = 1C/s.

In the general case, denoting the current with the letter i, and the charge with q, we get:

i = dq / dt.

The unit of current is called the ampere (A). The current in the conductor is 1 A if an electric charge equal to 1 pendant passes through the cross section of the conductor in 1 second.

If a voltage acts along the conductor, then an electric field arises inside the conductor. When the field strength E, the electrons with charge e are affected by the force f = Ee. The values f and E are vector. During the free path time, the electrons acquire a directed motion along with a chaotic one. Each electron has a negative charge and receives a velocity component directed opposite to the vector E (Fig. 1). Ordered motion, characterized by some average electron velocity vcp, determines the flow of electric current.

Electrons can also have directed motion in rarefied gases. In electrolytes and ionized gases, the flow of current is mainly due to the movement of ions. In accordance with the fact that in electrolytes positively charged ions move from the positive to the negative pole, historically the direction of the current was taken to be the opposite of the direction of electrons.

The current direction is taken to be the direction in which positively charged particles move, i.e. the direction opposite to the movement of electrons.
In the theory of electrical circuits, the direction of movement of positively charged particles from a higher potential to a lower one is taken as the direction of current in a passive circuit (outside energy sources). This direction was taken at the very beginning of the development of electrical engineering and contradicts the true direction of movement of charge carriers - electrons moving in conductive media from minus to plus.

The value equal to the ratio of the current to the cross-sectional area S is called the current density (denoted δ): δ= I/S

It is assumed that the current is uniformly distributed over the cross section of the conductor. Current density in wires is usually measured in A/mm2.

According to the type of carriers of electric charges and the medium of their movement, they are distinguished conduction currents and displacement currents. Conductivity is divided into electronic and ionic. For steady modes, two types of currents are distinguished: direct and alternating.

Electric current transfer called the phenomenon of the transfer of electric charges by charged particles or bodies moving in free space. The main type of electric current transfer is the movement in the void of elementary particles with a charge (the movement of free electrons in electron tubes), the movement of free ions in gas-discharge devices.

Electric displacement current (polarization current) called the ordered movement of bound carriers of electric charges. This kind of current can be observed in dielectrics.
Full electric current is a scalar value equal to the sum of the electrical conduction current, the electrical transfer current and the electrical displacement current through the considered surface.

A constant current is a current that can vary in magnitude, but does not change its sign for an arbitrarily long time. Read more about this here:

An alternating current is a current that periodically changes both in magnitude and in sign.The quantity characterizing the alternating current is the frequency (in the SI system it is measured in hertz), in the case when its strength changes periodically. High frequency alternating current pushed out to the surface of the conductor. High-frequency currents are used in mechanical engineering for heat treatment of surfaces of parts and welding, in metallurgy for melting metals.Alternating currents are divided into sinusoidal and non-sinusoidal. A sinusoidal current is a current that changes according to a harmonic law:

i = Im sin ωt,

The rate of change of alternating current is characterized by it, defined as the number of complete repetitive oscillations per unit time. Frequency is denoted by the letter f and is measured in hertz (Hz). So, the frequency of the current in the network 50 Hz corresponds to 50 complete oscillations per second. The angular frequency ω is the rate of change of current in radians per second and is related to frequency by a simple relationship:

ω = 2πf

Steady (fixed) values of direct and alternating currents designate with a capital letter I unsteady (instantaneous) values - with the letter i. The conditionally positive direction of the current is considered the direction of movement of positive charges.

This is a current that changes according to the sine law over time.

Alternating current also means current in conventional single- and three-phase networks. In this case, the alternating current parameters change according to the harmonic law.

Since alternating current varies with time, simple problem solving methods suitable for direct current circuits are not directly applicable here. At very high frequencies, charges can oscillate - flow from one place in the circuit to another and back. In this case, unlike DC circuits, the currents in series-connected conductors may not be the same. Capacitances present in AC circuits amplify this effect. In addition, when the current changes, self-induction effects come into play, which become significant even at low frequencies if high inductance coils are used. At relatively low frequencies, AC circuits can still be calculated using , which, however, must be modified accordingly.

A circuit that includes various resistors, inductors, and capacitors can be considered as if it consisted of a generalized resistor, capacitor, and inductor connected in series.

Consider the properties of such a circuit connected to a sinusoidal alternator. In order to formulate rules that allow you to design AC circuits, you need to find the relationship between voltage drop and current for each of the components of such a circuit.

It plays completely different roles in AC and DC circuits. If, for example, an electrochemical element is connected to the circuit, then the capacitor will begin to charge until the voltage across it becomes equal to the EMF of the element. Then the charging will stop and the current will drop to zero. If the circuit is connected to an alternator, then in one half-cycle, the electrons will flow from the left side of the capacitor and accumulate on the right, and vice versa in the other. These moving electrons are an alternating current, the strength of which is the same on both sides of the capacitor. As long as the frequency of the alternating current is not very high, the current through the resistor and the inductor is also the same.

In AC consuming devices, AC is often rectified by rectifiers to produce DC.

Electrical conductors

The material in which current flows is called. Some materials become superconductive at low temperatures. In this state, they offer almost no resistance to current, their resistance tends to zero. In all other cases, the conductor resists the flow of current and, as a result, part of the energy of the electrical particles is converted into heat. The current strength can be calculated using for a section of the circuit and Ohm's law for a complete circuit.

The speed of particles in conductors depends on the material of the conductor, the mass and charge of the particle, the ambient temperature, the applied potential difference and is much less than the speed of light. Despite this, the speed of propagation of the actual electric current is equal to the speed of light in a given medium, that is, the speed of propagation of the front of an electromagnetic wave.

How current affects the human body

Current passed through the human or animal body can cause electrical burns, fibrillation, or death. On the other hand, electric current is used in intensive care, for the treatment of mental illness, especially depression, electrical stimulation of certain areas of the brain is used to treat diseases such as Parkinson's disease and epilepsy, a pacemaker that stimulates the heart muscle with a pulsed current is used for bradycardia. In humans and animals, current is used to transmit nerve impulses.

According to safety precautions, the minimum perceptible current is 1 mA. The current becomes dangerous for human life starting from a strength of about 0.01 A. The current becomes fatal for a person starting from a strength of about 0.1 A. A voltage of less than 42 V is considered safe.