What currents (electric) exist? The main types of electric current (direct and alternating), their features and differences

What is electric current

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

In the theory of electrical circuits, current is considered to be the directional movement of charge carriers in a conducting medium under the influence 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 a conductor: i=q/t, where i is current. A; q = 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)= dq/dt.

The first condition for the long-term existence of an electric current of the type under consideration is the presence of a source or generator that maintains a potential difference between charge carriers. The second condition is the closedness of the path. In particular, for direct current to exist, there must be a closed path along which charges can move within the circuit without changing their value.

As is known, in accordance with the law of conservation of electric charges, they cannot be created or disappeared. Therefore, if any volume of space where electric currents flow is surrounded by a closed surface, then the current flowing into this volume must be equal to the current flowing out of it.

A closed path along which electric current flows is called an electric current circuit, or electrical circuit. An electrical circuit is divided into two parts: an internal part, in which electrically charged particles move against the direction of electrostatic forces, and an external part, in which these particles move in the direction of electrostatic forces. The ends of the electrodes to which the external circuit is connected are called clamps.

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

Instead of the term “current” (“current magnitude”), 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 electrical charges in the conductor, the amount of electricity passing per unit time through the cross-sectional area of ​​the conductor.
Current is characterized by , which in the SI system is measured in amperes (A), and by current density, which in the SI system is measured in amperes per square meter.

1A = 1C/s.

In the general case, denoting the current by the letter i and the charge by q, we obtain:

i = dq / dt.

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

If a voltage is applied along a conductor, an electric field arises inside the conductor. At field strength E, electrons with charge e are acted upon by a force f = Ee. The quantities f and E are vector. During the free path time, electrons acquire directional motion along with chaotic motion. Each electron has a negative charge and receives a velocity component directed opposite to vector E (Fig. 1). Ordered motion, characterized by a certain average electron speed vcp, determines the flow of electric current.

Electrons can 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 pole to the negative, historically the direction of current was taken to be opposite to the direction of electron movement.

The direction of the current 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 current in a passive circuit (outside energy sources) is taken to be the direction of movement of positively charged particles from a higher potential to a lower one. This direction was adopted at the very beginning of the development of electrical engineering and contradicts the true direction of movement of charge carriers - electrons moving in conducting media from minus to plus.

The value equal to the ratio of current to cross-sectional area S is called current density: I/S

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

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

Electric current transfer call the phenomenon of transfer of electric charges by charged particles or bodies moving in free space. The main type of electric transfer current 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 type of current can be observed in dielectrics.

Total electric current- a scalar quantity equal to the sum of the electric conduction current, the electric transfer current and the electric displacement current through the surface under consideration.

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

Magnetization current - constant microscopic (ampere) current, which is the reason for the existence of the own magnetic field of magnetized substances.

An alternating current is a current that periodically changes both in magnitude and sign.The quantity characterizing alternating current is frequency (measured in hertz in the SI system), in the case when its strength changes periodically.

High frequency alternating current is forced onto the surface of the conductor. High frequency currents are used in mechanical engineering for heat treatment of surfaces of parts and welding, and in metallurgy for melting metals.Alternating currents are divided into sinusoidal and non-sinusoidal. A current that varies according to a harmonic law is called sinusoidal:

i = Im sin wt,

where Im, - , A,

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

w = 2pif

Steady (fixed) values ​​of direct and alternating currents denote by the capital letter I unsteady (instantaneous) values ​​- the letter i. Conventionally, the positive direction of current is considered to be the direction of movement of positive charges.

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

Alternating current also refers to current in conventional single- and three-phase networks. In this case, the alternating current parameters change according to a 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 undergo oscillatory motion - flow from one place in the circuit to another and back. In this case, unlike direct current circuits, the currents in series-connected conductors may not be the same.

Capacitances present in AC circuits enhance this effect. In addition, when the current changes, self-induction effects occur, which become significant even at low frequencies if coils with high inductance 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 treated as if it consists of a generalized resistor, capacitor, and inductor connected in series.

Let's consider the properties of such a circuit connected to a sinusoidal alternating current generator. To formulate rules for calculating AC circuits, you need to find the relationship between voltage drop and current for each of the components of such a circuit.

Plays completely different roles in AC and DC circuits. If, for example, an electrochemical element is connected to the circuit, then until the voltage on it becomes equal to the EMF of the element. Then charging will stop and the current will drop to zero.

If the circuit is connected to an alternating current generator, then in one half-cycle electrons will flow out of the left plate of the capacitor and accumulate on the right, and in the other - vice versa.

These moving electrons represent 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 inductor is also the same.

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

Electric current in all its manifestations is a kinetic phenomenon similar to the flow of fluid in closed hydraulic systems. By analogy, the process of current movement is called “flow” (current flows).

The material in which current flows is called. Some materials become superconducting 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 the circuit section and Ohm's law for the complete circuit.

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

How does current affect the human body?

Current passed through the body of a person or animal can cause electrical burns, fibrillation or death. On the other hand, electric current is used in intensive care to treat 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 regulations, the minimum human-perceivable current is 1 mA. The current becomes dangerous to human life starting from a force of approximately 0.01 A. The current becomes lethal for a person starting from a force of approximately 0.1 A. A voltage of less than 42 V is considered safe.

Electric current is the directional movement of charged particles that occurs under the influence of electricity.

How is current generated?

An electric current appears in a substance provided that there are free (unbound) charged particles. Charge carriers can be present in the medium initially, or formed with the assistance of external factors (ionizers, electromagnetic field, temperature).

In the absence of an electric field, their movements are chaotic, but when connected to two points, the substances become directed - from one potential to another.

The number of such particles affects - distinguish between conductors, semiconductors, dielectrics,...

Where does the current occur?

The processes of electric current formation in various environments have their own characteristics:

1. In metals The charge is moved by free negatively charged particles - electrons. The transfer of the substance itself does not occur - the metal ions remain in their nodes of the crystal lattice. When heated, the chaotic vibrations of ions near the equilibrium position intensify, which interferes with the ordered movement of electrons—the conductivity of the metal decreases.
2. In liquids(electrolytes) charge carriers are ions - charged atoms and decayed molecules, the formation of which is caused by electrolytic dissociation. Ordered movement in this case represents their movement towards oppositely charged electrodes, on which they are neutralized and deposited.
   

   Cations (positive ions) move towards the cathode (negative electrode), anions (negative ions) move towards the anode (positive electrode). As the temperature rises, the conductivity of the electrolyte increases, as the number of molecules decomposed into ions increases.

3. In gases plasma is formed under the influence of a potential difference. Charged particles are ions, positive and negative, and free electrons formed under the influence of an ionizer.
4. In a vacuum Electric exists in the form of a flow of electrons that move from the cathode to the anode.
5. In semiconductors Directed movement involves electrons moving from one atom to another, and the resulting vacant spaces - holes, which are conventionally considered positive.
   

   At low temperatures, semiconductors have properties similar to insulators, since electrons are occupied by covalent bonds of atoms in the crystal lattice.

   As the temperature increases, the valence electrons receive enough energy to break bonds and become free. Accordingly, the higher the temperature, the better the conductivity of the semiconductor.

Watch the video below for a detailed explanation of electric current:

Https:="">magnetic field, ionizing radiation.

Https:="">ammeter.

Current strength is measured in Amperes(A) and represents the amount of charge that passes through a cross section of a conductive material per unit time. The unit of current is called Ampere (A). One ampere is equal to the ratio of one Coulomb (C) to one second.

Current density is the ratio of current strength to the area of ​​this section. The unit of measurement is Amperes per square meter (A/m2).

Below is a video about the strength of electric current as part of the school curriculum:

It is impossible to imagine the life of a modern person without electricity. Volts, Amps, Watts - these words are heard when talking about devices that operate on electricity. But what is electric current and what are the conditions for its existence? We will talk about this further, providing a brief explanation for novice electricians.

Definition

Electric current is the directed movement of charge carriers - this is a standard formulation from a physics textbook. In turn, charge carriers are called certain particles of matter. 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 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 others.

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

• Anion is a positive ion of an atom that has lost electrons.
• Cations are an atom with “extra” electrons attached to the atom.

The unit of current measurement is Ampere, according to which it is calculated using the formula:

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

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

where Q – charge, [C], t – time, [s].

Conditions for the existence of 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:

1. Presence of free charge carriers.
2. 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 required, between which there is a medium in which charge carriers will flow.

Let us remind you: Voltage, EMF is the potential difference. It follows that in order to fulfill the conditions for the existence of current - the presence of an electric field and electric current, voltage is needed. These can be the plates of a charged capacitor, a galvanic element, or an EMF generated under the influence of a magnetic field (generator).

We have figured out how it arises, let’s talk about where it is directed. Current, mainly in our usual use, moves in conductors (electrical wiring in an apartment, incandescent light bulbs) or in semiconductors (LEDs, the processor of your smartphone and other electronics), less often in gases (fluorescent lamps).

So, the main charge carriers in most cases 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 everything happens the other way around. The fact is that the decision on the direction of the current was made before studying its nature, and also before it was determined how the current flows and exists.

Electric current in different environments

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

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

In metals

Metals contain free charge carriers, they are sometimes called "electric gas". Where do free charge carriers come from? The fact is that metal, like any substance, consists of atoms. Atoms move or vibrate one way or another. 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 whose connection with the nucleus is rather weak. Under the influence of temperatures, chemical reactions and the interaction of impurities, which are in any case in the metal, electrons are torn away from their atoms, and positively charged ions are formed. The detached electrons are called free and move chaotically.

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 orderly. Electrons from a point at 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 in the normal state there are no free charge carriers. 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 move into the free zone 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 transistor, then an EMF will arise at the opposite ends of the crystal (a metallized layer is applied to them and the leads are soldered), but this does not relate to the topic of today’s article.

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

In vacuum and gas

A vacuum is a medium with a complete (ideal case) absence of gases or a minimized (in reality) amount of gas. Since there is no matter in a vacuum, there is no place for charge carriers to come from. 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 us assume that we have a vessel from which all the gas has been pumped out, i.e. there is a complete vacuum in it. Two electrodes are placed in the vessel, let's call them anode and cathode. If we connect the negative potential of the EMF source to the cathode and the positive potential to the anode, nothing will happen and no current will flow. But if we start heating the cathode, current will begin to flow. This process is called thermionic emission - the emission of electrons from a heated electron surface.

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

At the same time, if you change the polarity of the power supply - apply minus to the anode, and apply plus to the cathode - no current will flow. This will prove that current in a vacuum flows due to the movement of electrons from the CATHODE to the ANODE.

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) electrons will break away from the atom, then both conditions for the flow of electric current will be satisfied - field and free media.

As already mentioned, this process is called ionization. It can occur not only from applied voltage, but also from heating the gas, X-ray radiation, under the influence of ultraviolet radiation, 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 luminescence of the gas; 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 water, an electrolyte is formed and current begins to flow through it.

An electrolyte is a substance that conducts electric current due to dissociation into ions.

If you add copper sulfate to water, a layer of copper will deposit 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 that involves the separation of the components that make up the electrolyte on the electrodes.

This is how copper plating, gilding and coating with other metals occurs.

Conclusion

To summarize, for electric current to flow, 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 words, apply a voltage to the ends of a body or install two electrodes in an environment where electric current is expected to flow.

It is also worth noting that current influences a substance in a certain way; there are three types of influence:

• thermal;
• chemical;
• physical.

Useful

Today it is difficult to imagine life without such a phenomenon as electricity, but humanity learned to use it for its own purposes not so long ago. The study of the essence and characteristics of this special type of matter took several centuries, but even now we cannot say with confidence that we know absolutely everything about it.

The concept and essence of electric current

Electric current, as is known from school physics courses, is nothing more than the ordered movement of any charged particles. The latter can be either negatively charged electrons or ions. It is believed that this type of matter can only arise in so-called conductors, but this is far from true. The thing is that when any bodies come into contact, a certain number of oppositely charged particles always arise, which can begin to move. In dielectrics, the free movement of the same electrons is very difficult and requires enormous external forces, which is why they say that they do not conduct electric current.

Conditions for the existence of current in the circuit

Scientists have long noticed that this physical phenomenon cannot arise and persist for a long time on its own. The conditions for the existence of electric current include several important provisions. Firstly, this phenomenon is impossible without the presence of free electrons and ions, which act as charge transmitters. Secondly, in order for these elementary particles to begin to move in an orderly manner, it is necessary to create a field, the main feature of which is the potential difference between any points of the electrician. Finally, thirdly, an electric current cannot exist for a long time only under the influence of Coulomb forces, since the potentials will gradually equalize. That is why certain components are needed that are converters of various types of mechanical and thermal energy. They are usually called current sources.

Question about current sources

Electric current sources are special devices that generate an electric field. The most important of them include galvanic cells, solar panels, generators, and batteries. characterized by their power, productivity and operating time.

Current, voltage, resistance

Like any other physical phenomenon, electric current has a number of characteristics. The most important of these include its strength, circuit voltage and resistance. The first of them is a quantitative characteristic of the charge that passes through the cross section of a particular conductor per unit time. Voltage (also called electromotive force) is nothing more than the magnitude of the potential difference due to which a passing charge does a certain amount of work. Finally, resistance is an internal characteristic of a conductor, showing how much force a charge must expend to pass through it.

In a physics textbook there is a definition:

ELECTRICITY- this is the ordered (directed) movement of charged particles under the influence of an electric field. Particles can be: electrons, protons, ions, holes.

In academic textbooks the definition is described as follows:

ELECTRICITY is the rate of change of electric charge over time.

• • The electron charge is negative.

• • protons- particles with a positive charge;

• neutrons- with a neutral charge.

CURRENT STRENGTH is the number of charged particles (electrons, protons, ions, holes) flowing through the cross section of the conductor.

All physical substances, including metals, consist of molecules consisting of atoms, which in turn consist of nuclei and electrons rotating around them. During chemical reactions, electrons pass from one atom to another, therefore, the atoms of one substance lack electrons, and the atoms of another substance have an excess of them. This means that substances have opposite charges. If they come into contact, electrons will tend to move from one substance to another. It is this movement of electrons that is ELECTRICITY. A current that will flow until the charges of the two substances are equal. The departed electron is replaced by another. Where? From the neighboring atom, to it - from its neighbor, so to the extreme, to the extreme - from the negative pole of the current source (for example, a battery). From the other end of the conductor, electrons go to the positive pole of the current source. When all the electrons on the negative pole are gone, the current will stop (the battery is dead).

is a characteristic of the electric field and represents the potential difference between two points inside the electric field.

It seems like it’s not clear. Conductor- in the simplest case, this is a wire made of metal (copper and aluminum are more often used). The mass of the electron is 9.10938215(45)×10 -31 kg. If an electron has mass, then this means that it is material. But the conductor is made of metal, and metal is solid, so how do some electrons flow through it?

The number of electrons in a substance equal to the number of protons only ensures its neutrality, and the chemical element itself is determined by the number of protons and neutrons based on Mendeleev’s periodic law. If, purely theoretically, we subtract all its electrons from the mass of any chemical element, it will practically not approach the mass of the nearest chemical element. The difference between the masses of the electron and the nucleus is too large (the mass of only the 1st proton is approximately 1836 times greater than the mass of the electron). A decrease or increase in the number of electrons should only lead to a change in the total charge of the atom. The number of electrons in an individual atom is always variable. They either leave it due to thermal movement, or return back, having lost energy.

If electrons move in a direction, it means that they “leave” their atom, and the atomic mass will not be lost and, as a result, the chemical composition of the conductor will change? No. A chemical element is determined not by atomic mass, but by the number of PROTONS in the nucleus of an atom, and nothing else. In this case, the presence or absence of electrons or neutrons in an atom does not matter. Let's add - subtract electrons - we get an ion; add - subtract neutrons - we get an isotope. In this case, the chemical element will remain the same.

With protons it’s a different story: one proton is hydrogen, two protons are helium, three protons are lithium, etc. (see periodic table). Therefore, no matter how much current you pass through the conductor, its chemical composition will not change.

Electrolytes are another matter. This is where the CHEMICAL COMPOSITION CHANGES. Electrolyte elements are released from the solution under the influence of current. When everyone is released, the current will stop. This is because charge carriers in electrolytes are ions.

There are chemical elements without electrons:

1. Atomic cosmic hydrogen.

2. Gases in the upper layers of the atmosphere of the Earth and other planets with an atmosphere.

2. All substances are in a plasma state.

3. In accelerators, colliders.

When exposed to electric current, chemicals (conductors) can “scatter”. For example, a fuse. Moving electrons push atoms apart along their path; if the current is strong, the crystal lattice of the conductor is destroyed and the conductor melts.

Let's consider the operation of electric vacuum devices.

Let me remind you that during the action of an electric current in an ordinary conductor, an electron, leaving its place, leaves a “hole” there, which is then filled with an electron from another atom, where in turn a hole is also formed, which is subsequently filled by another electron. The entire process of electron movement occurs in one direction, and the movement of “holes” occurs in the opposite direction. That is, the hole is a temporary phenomenon; it fills up anyway. Filling is necessary to maintain charge equilibrium in the atom.

Now let's look at the operation of an electric vacuum device. For example, let's take the simplest diode - a kenotron. Electrons in the diode during the action of electric current are emitted by the cathode towards the anode. The cathode is coated with special metal oxides, which facilitate the escape of electrons from the cathode into vacuum (low work function). There is no reserve of electrons in this thin film. To ensure the release of electrons, the cathode is strongly heated with a filament. Over time, the hot film evaporates, settles on the walls of the flask, and the emissivity of the cathode decreases. And such an electronic vacuum device is simply thrown away. And if the device is expensive, it is restored. To restore it, the flask is unsoldered, the cathode is replaced with a new one, after which the flask is sealed back.

The electrons in the conductor move “carrying” the electric current, and the cathode is replenished with electrons from the conductor connected to the cathode. The electrons that leave the cathode are replaced by electrons from the current source.

The concept of “speed of movement of electric current” does not exist. At a speed close to the speed of light (300,000 km/s), an electric field propagates through the conductor, under the influence of which all electrons begin to move at a low speed, which is approximately equal to 0.007 mm/s, not forgetting to also rush chaotically in thermal motion.

Let's now understand the main characteristics of current

Let's imagine the picture: You have a standard cardboard box of 12 bottles of strong drink. And you're trying to put another bottle in there. Let's say you succeeded, but the box barely held up. You put another one in there, and suddenly the box breaks and the bottles fall out.

A box of bottles can be compared to a cross-section of a conductor:

The wider the box (thicker the wire), the greater the number of bottles (CURRENT POWER) it can accommodate (provide).

You can place from one to 12 bottles in a box (in a conductor) - it will not fall apart (the conductor will not burn), but it cannot accommodate a larger number of bottles (higher current strength) (represents resistance).
If we place another box on top of the box, then on one unit of area (conductor cross-section) we will place not 12, but 24 bottles, another one on top - 36 bottles. One of the boxes (one floor) can be taken as a unit similar to the VOLTAGE of electric current.

The wider the box (less resistance), the more bottles (CURRENT) it can supply.

By increasing the height of the boxes (voltage), we can increase the total number of bottles (POWER) without destroying the boxes (conductor).

Using our analogy we got:

The total number of bottles is POWER

The number of bottles in one box (layer) is the CURRENT POWER

The number of boxes in height (floors) is VOLTAGE

The width of the box (capacity) is the RESISTANCE of the electrical circuit section

Through the above analogies, we came to “ OMA'S LAW“, which is also called Ohm’s Law for a section of a circuit. Let's represent it as a formula:

Where I – current strength, U R - resistance.

In simple terms, it sounds like this: Current is directly proportional to voltage and inversely proportional to resistance.

In addition, we came to " WATT'S LAW". Let’s also depict it in the form of a formula:

Where I – current strength, U – voltage (potential difference), R – power.

In simple terms, it sounds like this: Power is equal to the product of current and voltage.

Electric current strength measured by an instrument called an Ammeter. As you guessed, the amount of electric current (the amount of charge transferred) is measured in amperes. To increase the range of unit of change designations, there are multiplicity prefixes such as micro - microampere (µA), miles - milliampere (mA). Other consoles are not used in everyday use. For example: They say and write “ten thousand amperes”, but they never say or write 10 kiloamperes. Such meanings are not real in everyday life. The same can be said about nanoamps. Usually they say and write 1×10 -9 Amperes.

Electrical voltage(electric potential) is measured by a device called a Voltmeter, as you guessed it, voltage, i.e. the potential difference that causes current to flow, is measured in Volts (V). Just as for current, to increase the range of designations, there are multiple prefixes: (micro - microvolt (μV), miles - millivolt (mV), kilo - kilovolt (kV), mega - megavolt (MV). Voltage is also called EMF - electromotive force.

Electrical resistance measured by a device called an Ohmmeter, as you guessed it, the unit of resistance is Ohm (Ohm). Just as for current and voltage, there are multiplicity prefixes: kilo - kiloohm (kOhm), mega - megaohm (MOhm). Other meanings are not real in everyday life.

Earlier, you learned that the resistance of a conductor directly depends on the diameter of the conductor. To this we can add that if a large electric current is applied to a thin conductor, it will not be able to pass it, which is why it will heat up very much and, in the end, may melt. The operation of fuses is based on this principle.

The atoms of any substance are located at some distance from each other. In metals, the distances between atoms are so small that the electron shells practically touch. This allows electrons to wander freely from nucleus to nucleus, creating an electric current, which is why metals, as well as some other substances, are CONDUCTORS of electricity. Other substances, on the contrary, have widely spaced atoms, electrons tightly bound to the nucleus, which cannot move freely. Such substances are not conductors and are usually called DIELECTRICS, the most famous of which is rubber. This is the answer to the question why electrical wires are made of metal.

The presence of electric current is indicated by the following actions or phenomena that accompany it:

;1. The conductor through which current flows may become hot;

2. Electric current can change the chemical composition of a conductor;

3. The current exerts a force on neighboring currents and magnetized bodies.

When electrons are separated from the nuclei, a certain amount of energy is released, which heats the conductor. The “heating” capacity of a current is usually called power dissipation and is measured in watts. The same unit is used to measure mechanical energy converted from electrical energy.

Electrical hazards and other hazardous properties of electricity and safety precautions

Electric current heats the conductor through which it flows. That's why:

1. If a household electrical network is overloaded, the insulation gradually chars and crumbles. There is a possibility of a short circuit, which is very dangerous.

2. Electric current flowing through wires and household appliances encounters resistance, so it “chooses” the path with the least resistance.

3. If a short circuit occurs, the current increases sharply. This generates a large amount of heat that can melt the metal.

4. A short circuit can also occur due to moisture. If a fire occurs in the case of a short circuit, then in the case of exposure to moisture on electrical appliances, it is the person who suffers first.

5. Electrical shock is very dangerous and can be fatal. When electric current flows through the human body, tissue resistance decreases sharply. Processes of tissue heating, cell destruction, and death of nerve endings occur in the body.

How to protect yourself from electric shock

To protect yourself from exposure to electric current, use means of protection against electric shock: work in rubber gloves, use a rubber mat, discharge rods, grounding devices for equipment, workplaces. Automatic switches with thermal protection and current protection are also a good means of protection against electric shock that can save human life. When I am not sure that there is no danger of electric shock, when performing simple operations in electrical panels or equipment units, I usually work with one hand and put the other hand in my pocket. This eliminates the possibility of electric shock along the hand-to-hand path in case of accidental contact with the shield body or other massive grounded objects.

To extinguish a fire that occurs on electrical equipment, only powder or carbon dioxide fire extinguishers are used. Powder extinguishers are better, but after covering the equipment with dust from a fire extinguisher, it is not always possible to restore this equipment.