Appearance of current. The concept of electric current and how it is measured

An electric current is an ordered flow of negatively charged elementary particles– electrons. Electric current necessary for lighting houses and streets, ensuring the functionality of household and industrial equipment, the movement of city and mainline electric transport, etc.

Electric current

• R n – load resistance
• A – indicator
• K – circuit switch

Current– the number of charges passing per unit time through the cross section of the conductor.

• I – current strength
• q – amount of electricity
• t – time

The unit of current is called ampere A, named after the French scientist Ampere.

1A = 10 3 mA = 10 6 µA

Electric current density

Electric current has a number of physical characteristics that have quantitative values, expressed in certain units. Main physical characteristics Electric current is its strength and power. Current strength It is quantitatively expressed in amperes, and current power is expressed in watts. An equally important physical quantity is the vector characteristic of electric current, or current density. In particular, the concept of current density is used when designing power lines.

• J – electric current density A / MM 2
• S – area cross section
• I – current

Direct and alternating current

All electrical devices are powered permanent or alternating current.

Electric current, the direction and value of which do not change is called permanent.

Electric current, the direction and value of which can change is called variables.

The power supply for many electrical devices is carried out alternating current, the change of which is graphically represented as a sinusoid.

Use of Electric Current

It can be stated with confidence that the greatest achievement of mankind is the discovery electric current and its use. From electric current depend on heat and light in houses, the flow of information from outside world, communication between people located in different parts of the planet, and much more.

Modern life cannot be imagined without the widespread availability of electricity. Electricity present in absolutely all spheres of people’s life: in industry and agriculture, in science and space.

Electricity is also a constant component everyday life person. Such widespread distribution of electricity was made possible due to its unique properties. Electrical energy can be instantly transmitted to huge distances and be transformed into various types of energies of a different genesis.

Main consumers electrical energy are industrial and manufacturing sectors. With the help of electricity, various mechanisms and devices are activated, and multi-stage technological processes are carried out.

It is impossible to overestimate the role of electricity in ensuring the operation of transport. Railway transport is almost completely electrified. Electrification railway transport played a significant role in ensuring road capacity, increasing travel speed, reducing the cost of passenger transportation, and solving the problem of fuel economy.

The availability of electricity is an indispensable condition for ensuring comfortable living conditions for people. All household appliances: televisions, washing machines, microwave ovens, heating devices - found their place in human life only thanks to the development of electrical production.

The leading role of electricity in the development of civilization is undeniable. There is no area in the life of mankind that can do without the consumption of electrical energy and which could be replaced by muscular strength.

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

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

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

Usually the direction of the electric charge is taken to be the direction of the positive charge.

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

Current power.

Electric current power is called the ratio of the work performed by the current to the time during which this work was performed.

Power developed electric current on a section of the circuit, is directly proportional to the magnitude of the current and voltage in this section. Power (electric and mechanical) measured in Watts (W).

Current power does not depend on the time of the pro-te-ka-tion of the electric current in the circuit, but is defined as the pro-from-ve-de voltage on current strength.

Voltage.

Electric voltage is a quantity that shows how much work is done by the electric field when moving a charge from one point to another. The voltage in different parts of the circuit will be different.

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

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

Current strength.

Current strength refers to the number of charged particles that flow through the cross section of a conductor.

By definition current strength directly proportional to voltage and inversely proportional to resistance.

Electric current strength measured by an instrument called an Ammeter. 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 values ​​in everyday life are not used. The same can be said about nanoamps. Usually they say and write 1×10-9 Amperes.

Current resistance.

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

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

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

1) the presence of free charged particles;

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

Types of effects of electric current on conductive material.

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

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

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

Main characteristics of current.

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

I=Q/t

The current strength is determined by an ammeter.

The voltage is determined by a voltmeter.

3. Resistance R of the conductive material.

Resistance depends:

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

R=pl/S

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

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

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

Current characteristics table.

Current and voltage are quantitative parameters used in electrical diagrams. Most often, these quantities change over time, otherwise there would be no point in the operation of the electrical circuit.

Voltage

Conventionally, voltage is indicated by the letter "U". The work expended on moving a unit of charge from a point of low potential to a point with great potential, is the voltage between these two points. In other words, it is the energy released after a unit of charge moves from high to low potential.

Voltage can also be called potential difference, as well as electromotive force. This parameter is measured in volts. To move 1 coulomb of charge between two points that have a voltage of 1 volt, 1 joule of work must be done. Coulombs measure electrical charges. 1 pendant equal to charge 6x10 18 electrons.

Voltage is divided into several types, depending on the types of current.

• Constant voltage . It is present in electrostatic and direct current circuits.
• AC voltage . This type of voltage is found in circuits with sinusoidal and alternating currents. In the case of sinusoidal current, the following voltage characteristics are considered:
  amplitude of voltage fluctuations– this is its maximum deviation from the x-axis;
  instantaneous voltage, which is expressed at a certain point in time;
  effective voltage , is determined by the execution active work 1st half period;
  average rectified voltage, determined by the magnitude of the rectified voltage over one harmonic period.

When transmitting electricity through overhead lines, the design of supports and their dimensions depend on the magnitude of the applied voltage. The voltage between phases is called line voltage , and the voltage between the ground and each phase is phase voltage . This rule applies to all types of overhead lines. In Russia, in household electrical networks, the standard is three-phase voltage with a linear voltage of 380 volts and a phase voltage of 220 volts.

Electric current

Current in an electrical circuit is the speed of movement of electrons at a certain point, measured in amperes, and denoted in diagrams by the letter “ I" Derived units of ampere with the corresponding prefixes milli-, micro-, nano, etc. are also used. A current of 1 ampere is generated by moving a unit of charge of 1 coulomb in 1 second.

It is conventionally assumed that the current flows in the direction from positive potential to the negative. However, from the physics course we know that the electron moves in the opposite direction.

You need to know that voltage is measured between 2 points on the circuit, and current flows through one specific point circuit, or through its element. Therefore, if someone uses the expression “tension in resistance,” then this is incorrect and illiterate. But often we're talking about about the voltage at a certain point in the circuit. This refers to the voltage between the ground and this point.

Voltage is generated from exposure to electrical charges in generators and other devices. Current is created by applying a voltage to two points on a circuit.

To understand what current and voltage are, it would be more correct to use. On it you can see the current and voltage, which change their values ​​over time. In practice, the elements of an electrical circuit are connected by conductors. At certain points, the elements of the circuit have their own voltage value.

Current and voltage obey the rules:

• The sum of currents entering a point is equal to the sum of currents leaving the point (charge conservation rule). This rule is Kirchhoff's law for current. The point of entry and exit of the current in this case is called a node. The corollary of this law is next statement: in a series electrical circuit of a group of elements, the current value is the same for all points.
• IN parallel circuit elements, the voltage on all elements is the same. In other words, the sum of the voltage drops in a closed circuit is zero. This Kirchhoff law applies to stresses.
• The work done per unit time by a circuit (power) is expressed as follows: P = U*I. Power is measured in watts. 1 joule of work done in 1 second is equal to 1 watt. Power is distributed in the form of heat and is spent to perform mechanical work(in electric motors), converted into radiation various types, accumulates in containers or batteries. When designing complex electrical systems,One of the problems is the thermal load of the system.

Characteristics of electric current

A prerequisite for the existence of current in an electrical circuit is a closed circuit. If the circuit is broken, the current stops.

Everyone in electrical engineering operates on this principle. They're tearing apart electrical circuit movable mechanical contacts, and this stops the flow of current, turning off the device.

In the energy industry, electric current occurs inside current conductors, which are made in the form of busbars and other parts that conduct current.

There are also other ways to create internal current in:

• Liquids and gases due to the movement of charged ions.
• Vacuum, gas and air using thermionic emission.
• , due to the movement of charge carriers.

Conditions for the occurrence of electric current

• Heating of conductors (not superconductors).
• Application of potential difference to charge carriers.
• A chemical reaction that releases new substances.
• Impact magnetic field to the conductor.

Current Waveforms

• Straight line.
• Variable harmonic sine wave.
• A meander, similar to a sine wave, but having sharp corners(sometimes the corners may be smoothed).
• A pulsating shape in one direction, with an amplitude varying from zero to greatest value according to a certain law.

Types of work of electric current

• Light radiation created by lighting devices.
• Generating heat using heating elements.
• Mechanical work (rotation of electric motors, operation of other electrical devices).
• Creation of electromagnetic radiation.

Negative phenomena caused by electric current

• Overheating of contacts and live parts.
• Emergence eddy currents in the cores of electrical devices.
• Electromagnetic radiation into the external environment.

Creators of electrical devices and various schemes when designing, they must take into account the above properties of electric current in their designs. For example, harmful influence eddy currents in electric motors, transformers and generators are reduced by fusion of the cores used for transmission magnetic flux. Lamination of the core is its production not from a single piece of metal, but from a set of individual thin plates of special electrical steel.

But, on the other hand, eddy currents used for work microwave ovens, ovens operating on the principle of magnetic induction. Therefore, we can say that eddy currents are not only harmful, but also beneficial.

Alternating current with a signal in the form of a sinusoid can differ in frequency of oscillations per unit time. In our country power frequency current of electrical devices is standard and equal to 50 hertz. In some countries, a current frequency of 60 hertz is used.

For various purposes in electrical engineering and radio engineering, other frequency values ​​are used:

• Low frequency signals with a lower current frequency.
• High frequency signals that are much higher than the frequency of industrial current.

It is believed that electric current arises from the movement of electrons within a conductor, which is why it is called conduction current. But there is another type of electric current, which is called convection. It occurs when charged macrobodies, such as raindrops, move.

Electric current in metals

The movement of electrons when exposed to them constant force compared to a parachutist who is falling to the ground. In these two cases it happens uniform motion. The force of gravity acts on the skydiver, and the force of air resistance opposes it. The movement of electrons is affected by the force of the electric field, and the ions of the crystal lattices resist this movement. Average speed electrons reaches constant value, as well as the speed of the parachutist.

In a metal conductor, the speed of movement of one electron is 0.1 mm per second, and the speed of electric current is about 300 thousand km per second. This is because electric current only flows where voltage is applied to charged particles. Therefore, a high current flow rate is achieved.

When electrons move in a crystal lattice, the following pattern exists. Electrons do not collide with all oncoming ions, but only with every tenth of them. This is explained by the laws quantum mechanics, which can be simplified as follows.

The movement of electrons is hampered by large ions that offer resistance. This is especially noticeable when metals are heated, when heavy ions “sway”, increase in size and reduce the electrical conductivity of the conductor crystal lattices. Therefore, when metals are heated, their resistance always increases. As the temperature decreases, it increases electrical conductivity. By reducing the temperature of a metal to absolute zero, the effect of superconductivity can be achieved.

Conditions for the appearance of current

Modern science has created theories to explain natural processes. Many processes are based on one of the models of atomic structure, the so-called planetary model. According to this model, an atom consists of a positively charged nucleus and a negatively charged cloud of electrons surrounding the nucleus. Various substances, consisting of atoms, are mostly stable and unchanged in their properties under constant conditions environment. But in nature there are processes that can change the stable state of substances and cause in these substances a phenomenon called electric current.

Such a fundamental process for nature is friction. Many people know that if you comb your hair with a comb made of certain types of plastic, or wear clothes made of certain types of fabric, a sticking effect occurs. Hair is attracted and sticks to the comb, and the same thing happens with clothes. This effect is explained by friction, which disrupts the stability of the comb material or fabric. The electron cloud can shift relative to the nucleus or be partially destroyed. And as a result, the substance acquires an electric charge, the sign of which is determined by the structure of this substance. Electric charge resulting from friction is called electrostatic.

The result is a pair of charged substances. Each substance has a specific electric potential. Electricity acts on the space between two charged substances, in this case electrostatic field. Efficiency electrostatic field depends on potential values ​​and is defined as potential difference or voltage.

• When voltage arises, a directed movement of charged particles of substances appears in the space between the potentials - an electric current.

Where does electric current flow?

In this case, the potentials will decrease if the friction stops. And, in the end, the potentials will disappear, and the substances will regain stability.

But if the process of formation of potentials and voltage continues in the direction of their increase, the current will also increase according to the properties of the substances filling the space between the potentials. The most obvious demonstration of this process is lightning. The friction of the upward and downward air flows against each other leads to the appearance of enormous tension. As a result, one potential is formed by updrafts in the sky, and the other by downdrafts in the ground. And, in the end, due to the properties of the air, an electric current appears in the form of lightning.

• The first cause of electric current is voltage.
• The second reason for the appearance of electric current is the space in which the voltage operates - its size and what it is filled with.

Tension does not only come from friction. Other physical and chemical processes, which disrupt the balance of the atoms of a substance, also lead to the appearance of tension. Tension arises only as a result of interaction or

• one substance with another substance;
• one or more substances with a field or radiation.

Voltage can come from:

• a chemical reaction that occurs in a substance, such as in all batteries and accumulators, as well as in all living things;
• electromagnetic radiation, such as in solar powered and thermal electric generators;
• electromagnetic field, such as in all dynamos.

Electric current has a nature corresponding to the substance in which it flows. Therefore it differs:

• in metals;

• in liquids and gases;

• in semiconductors

In metals, electric current consists only of electrons, in liquids and gases – of ions, in semiconductors – of electrons and “holes”.

Direct and alternating current

Voltage relative to its potentials, the signs of which remain unchanged, can only change in magnitude.

• In this case, a constant or pulsed electric current appears.

The electric current depends on the duration of this change and the properties of the space filled with matter between the potentials.

• But if the signs of the potentials change and this leads to a change in the direction of the current, it is called variable, as is the voltage that determines it.

Life and Electric Current

For quantitative and qualitative assessments electric current in modern science and technology, certain laws and quantities are used. The basic laws are:

• Coulomb's law;
• Ohm's law.

Charles Coulomb in the 80s of the 18th century determined the appearance of voltage, and Georg Ohm in the 20s of the 19th century determined the appearance of electric current.

In nature and human civilization it is used primarily as a carrier of energy and information, and the topic of its study and use is as vast as life itself. For example, studies have shown that all living organisms live because the heart muscles contract under the influence of electrical current pulses generated in the body. All other muscles work similarly. When a cell divides, it uses information based on electric current over high frequencies. The list of such facts with clarifications can be continued throughout the book.

Many discoveries related to electric current have already been made, and much more remains to be done. Therefore, with the advent of new research tools, new laws, materials and other results appear for practical use of this phenomenon.

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 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 vacant places - holes.

For 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 on 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 The pendant from point 1 to point 2 will determine the value 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 important law electricity, which determines the quantitative relationship between electric current and the properties of the conductor, characterizing their ability to withstand electric current.
These properties subsequently began to be called electrical resistance, denoted by the letter R and measured in Ohms in honor of the discoverer.
Ohm's law in its modern interpretation using the classical 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

The conductors have free media charges that, 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 - 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 that 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 intermediate position between conductors and dielectrics according to their resistivity.
A significant difference between semiconductors and metals can be considered their dependence 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 point is that when absolute zero electrons in semiconductors will be busy creating covalent bonds between atoms 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 will form at the breakpoints, which are called holes.
Vacant position may be filled valence electron from a neighboring pair and the hole will move to a new location 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 illuminating a semiconductor 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 formed 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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