Electric current includes the following processes. Electric current: main characteristics and conditions of its existence

2. Under what conditions does electric current occur?

3. Why is the movement of charged particles in a conductor in the absence of an external electric field chaotic?

4. How does the movement of charged particles in a conductor differ in the absence and presence of an external electric field?

5. How is the direction of electric current selected? In what direction do electrons move in a metal conductor carrying electric current?

Current and voltage are quantitative parameters used in electrical circuits. 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 in moving a unit of charge from a point of low potential to a point of high potential is the voltage between those 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 coulomb is equal to the charge of 6x10 18 electrons.

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

• Constant pressure . 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 active work performed in the 1st half-cycle;
  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.

Electricity

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 considered that the current flows in the direction from positive potential to 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 in the circuit, or through its element. Therefore, if someone uses the expression “tension in resistance,” then this is incorrect and illiterate. But often we are talking about 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. A corollary of this law is the following statement: in a series electrical circuit of a group of elements, the current value is the same for all points.
• In a parallel circuit of elements, the voltage across 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, spent on performing mechanical work (in electric motors), converted into radiation of various types, and accumulated in containers or batteries. When designing complex electrical systems, one of the challenges 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 break the electrical circuit with movable mechanical contacts, and thereby stop 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 differences to charge carriers.
• A chemical reaction that releases new substances.
• The effect of a magnetic field on a conductor.

Current Waveforms

• Straight line.
• Variable harmonic sine wave.
• A meander, similar to a sine wave, but with sharp corners (sometimes the corners can be smoothed).
• A pulsating form of one direction, with an amplitude varying from zero to the 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.
• The occurrence of eddy currents in the cores of electrical devices.
• Electromagnetic radiation into the external environment.

When designing, creators of electrical devices and various circuits must take into account the above properties of electric current in their designs. For example, the harmful effects of eddy currents in electric motors, transformers and generators are reduced by fusion of the cores used to pass magnetic fluxes. 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 are used to operate microwave ovens and 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, the industrial frequency of electrical current 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 move, for example, raindrops.

Electric current in metals

The movement of electrons when subjected to a constant force is compared to a parachutist descending to the ground. In these two cases, uniform motion occurs. 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. The average speed of electrons reaches a constant value, just like the speed of a 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 of 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, the electrical conductivity increases. By reducing the temperature of a metal to absolute zero, the effect of superconductivity can be achieved.

In conductors, under certain conditions, continuous ordered movement of free electric charge carriers can occur. This movement is called electric shock. The direction of movement of positive free charges is taken as the direction of electric current, although in most cases electrons - negatively charged particles - move.

The quantitative measure of electric current is current strength I– scalar physical quantity equal to the charge ratio q, transferred through the cross section of the conductor over a time interval t, to this time interval:

If the current is not constant, then to find the amount of charge passed through the conductor, calculate the area of ​​the figure under the graph of the current versus time.

If the current strength and its direction do not change with time, then such a current is called permanent. The current strength is measured by an ammeter, which is connected in series to the circuit. In the International System of Units (SI) current is measured in amperes [A]. 1 A = 1 C/s.

It is found as the ratio of the total charge to the entire time (i.e., according to the same principle as the average speed or any other average value in physics):

If the current varies uniformly over time from the value I 1 to value I 2, then the average current value can be found as the arithmetic mean of the extreme values:

Current Density– current per unit cross-section of the conductor is calculated by the formula:

When current passes through a conductor, the current experiences resistance from the conductor. The reason for resistance is the interaction of charges with atoms of the conductor substance and with each other. The unit of resistance is 1 ohm. Conductor resistance R determined by the formula:

Where: l– length of the conductor, S– its cross-sectional area, ρ – specific resistance of the conductor material (be careful not to confuse the latter value with the density of the substance), which characterizes the ability of the conductor material to resist the passage of current. That is, this is the same characteristic of a substance as many others: specific heat, density, melting point, etc. The unit of measurement for resistivity is 1 ohm m. The specific resistance of a substance is a tabular value.

The resistance of a conductor also depends on its temperature:

Where: R 0 – conductor resistance at 0°C, t– temperature expressed in degrees Celsius, α – temperature coefficient of resistance. It is equal to the relative change in resistance with an increase in temperature by 1°C. For metals it is always greater than zero, for electrolytes, on the contrary, it is always less than zero.

Diode in DC circuit

Diode is a nonlinear circuit element whose resistance depends on the direction of current flow. The diode is designated as follows:

The arrow in the schematic symbol of a diode shows in which direction it passes current. In this case, its resistance is zero, and the diode can be replaced simply with a conductor with zero resistance. If current flows through the diode in the opposite direction, then the diode has an infinitely large resistance, that is, it does not pass current at all, and is an open circuit. Then the section of the circuit with the diode can simply be crossed out, since no current flows through it.

Ohm's law. Series and parallel connection of conductors

The German physicist G. Ohm in 1826 experimentally established that the current strength I, flowing along a homogeneous metal conductor (that is, a conductor in which no external forces act) with resistance R, proportional to voltage U at the ends of the conductor:

Size R usually called electrical resistance. A conductor with electrical resistance is called resistor. This ratio expresses Ohm's law for a homogeneous section of a chain: The current in a conductor is directly proportional to the applied voltage and inversely proportional to the resistance of the conductor.

Conductors that obey Ohm's law are called linear. Graphical dependence of current strength I from voltage U(such graphs are called current-voltage characteristics, abbreviated as VAC) is depicted by a straight line passing through the origin of coordinates. It should be noted that there are many materials and devices that do not obey Ohm's law, for example, a semiconductor diode or a gas-discharge lamp. Even for metal conductors, at sufficiently high currents, a deviation from Ohm’s linear law is observed, since the electrical resistance of metal conductors increases with increasing temperature.

Conductors in electrical circuits can be connected in two ways: series and parallel. Each method has its own rules.

1. Regularities of serial connection:

The formula for the total resistance of resistors connected in series is valid for any number of conductors. If the circuit is connected in series n identical resistances R, then the total resistance R 0 is found by the formula:

2. Patterns of parallel connection:

The formula for the total resistance of resistors connected in parallel is valid for any number of conductors. If the circuit is connected in parallel n identical resistances R, then the total resistance R 0 is found by the formula:

Electrical measuring instruments

To measure voltages and currents in DC electrical circuits, special instruments are used - voltmeters And ammeters.

Voltmeter designed to measure the potential difference applied to its terminals. It is connected in parallel to the section of the circuit on which the potential difference is measured. Any voltmeter has some internal resistance R B. In order for the voltmeter not to introduce a noticeable redistribution of currents when connected to the circuit being measured, its internal resistance must be large compared to the resistance of the section of the circuit to which it is connected.

Ammeter designed to measure current in a circuit. The ammeter is connected in series to the open circuit of the electrical circuit so that the entire measured current passes through it. The ammeter also has some internal resistance R A. Unlike a voltmeter, the internal resistance of an ammeter must be quite small compared to the total resistance of the entire circuit.

EMF. Ohm's law for a complete circuit

For the existence of direct current, it is necessary to have a device in an electrical closed circuit that is capable of creating and maintaining potential differences in sections of the circuit due to the work of forces of non-electrostatic origin. Such devices are called DC sources. Forces of non-electrostatic origin acting on free charge carriers from current sources are called outside forces.

The nature of external forces may vary. In galvanic cells or batteries they arise as a result of electrochemical processes; in direct current generators, external forces arise when conductors move in a magnetic field. Under the influence of external forces, electric charges move inside the current source against the forces of the electrostatic field, due to which a constant electric current can be maintained in a closed circuit.

When electric charges move along a direct current circuit, external forces acting inside the current sources perform work. Physical quantity equal to the work ratio A st external forces when moving a charge q from the negative pole of the current source to the positive pole to the magnitude of this charge is called source electromotive force (EMF):

Thus, the EMF is determined by the work done by external forces when moving a single positive charge. Electromotive force, like potential difference, is measured in volts (V).

Ohm's law for a complete (closed) circuit: The current strength in a closed circuit is equal to the electromotive force of the source divided by the total (internal + external) resistance of the circuit:

Resistance r– internal (own) resistance of the current source (depends on the internal structure of the source). Resistance R– load resistance (external circuit resistance).

Voltage drop in external circuit in this case it is equal (it is also called voltage at the source terminals):

It is important to understand and remember: the EMF and internal resistance of the current source do not change when different loads are connected.

If the load resistance is zero (the source closes on itself) or is much less than the source resistance, then the circuit will flow short circuit current:

Short circuit current - the maximum current that can be obtained from a given source of electromotive force ε and internal resistance r. For sources with low internal resistance, the short circuit current can be very large and cause destruction of the electrical circuit or source. For example, lead-acid batteries used in automobiles can have short-circuit currents of several hundred amperes. Short circuits in lighting networks powered from substations (thousands of amperes) are especially dangerous. To avoid the destructive effects of such large currents, fuses or special circuit breakers are included in the circuit.

Several sources of EMF in the circuit

If there is a several emfs connected in series, That:

1. With the correct connection (the positive pole of one source is connected to the negative of another) the sources are connected, the total EMF of all sources and their internal resistance can be found using the formulas:

For example, such connection of sources is carried out in remote controls, cameras and other household appliances that operate on several batteries.

2. If the sources are connected incorrectly (the sources are connected by the same poles), their total EMF and resistance are calculated using the formulas:

In both cases, the total resistance of the sources increases.

At parallel connection It makes sense to connect sources only with the same EMF, otherwise the sources will discharge towards each other. Thus, the total EMF will be the same as the EMF of each source, that is, with a parallel connection we will not get a battery with a large EMF. At the same time, the internal resistance of the source battery decreases, which allows you to obtain greater current and power in the circuit:

This is the meaning of parallel connection of sources. In any case, when solving problems, you first need to find the total EMF and the total internal resistance of the resulting source, and then write Ohm’s law for the complete circuit.

Work and current power. Joule-Lenz law

Job A electric current I flowing through a stationary conductor with resistance R, is converted into heat Q, standing out on the conductor. This work can be calculated using one of the formulas (taking into account Ohm’s law, they all follow from each other):

The law of converting the work of current into heat was experimentally established independently of each other by J. Joule and E. Lenz and is called Joule–Lenz law. Electric current power equal to the ratio of current work A to the time interval Δ t, for which this work was done, so it can be calculated using the following formulas:

The work of electric current in SI, as usual, is expressed in joules (J), power - in watts (W).

Closed circuit energy balance

Let us now consider a complete direct current circuit consisting of a source with an electromotive force ε and internal resistance r and an external homogeneous area with resistance R. In this case, the useful power or power released in the external circuit:

The maximum possible useful power of the source is achieved if R = r and is equal to:

If, when connected to the same current source with different resistances R 1 and R 2 equal powers are allocated to them, then the internal resistance of this current source can be found by the formula:

Power loss or power inside the current source:

Total power developed by the current source:

Current source efficiency:

Electrolysis

Electrolytes It is customary to call conducting media in which the flow of electric current is accompanied by the transfer of matter. The carriers of free charges in electrolytes are positively and negatively charged ions. Electrolytes include many metal compounds with metalloids in the molten state, as well as some solids. However, the main representatives of electrolytes widely used in technology are aqueous solutions of inorganic acids, salts and bases.

The passage of electric current through the electrolyte is accompanied by the release of a substance on the electrodes. This phenomenon is called electrolysis.

Electric current in electrolytes represents the movement of ions of both signs in opposite directions. Positive ions move towards the negative electrode ( cathode), negative ions – to the positive electrode ( anode). Ions of both signs appear in aqueous solutions of salts, acids and alkalis as a result of the splitting of some neutral molecules. This phenomenon is called electrolytic dissociation.

Law of Electrolysis was experimentally established by the English physicist M. Faraday in 1833. Faraday's law determines the amount of primary products released on the electrodes during electrolysis. So, the mass m substance released on the electrode is directly proportional to the charge Q passed through the electrolyte:

Size k called electrochemical equivalent. It can be calculated using the formula:

Where: n– valency of the substance, N A – Avogadro’s constant, M– molar mass of the substance, e– elementary charge. Sometimes the following notation for Faraday's constant is also introduced:

Electric current in gases and vacuum

Electric current in gases

Under normal conditions, gases do not conduct electricity. This is explained by the electrical neutrality of gas molecules and, therefore, the absence of electric charge carriers. In order for a gas to become a conductor, one or more electrons must be removed from the molecules. Then free charge carriers will appear - electrons and positive ions. This process is called ionization of gases.

Gas molecules can be ionized by external influence - ionizer. Ionizers can be: a stream of light, X-rays, a stream of electrons or α -particles Gas molecules also become ionized at high temperatures. Ionization leads to the appearance of free charge carriers in gases - electrons, positive ions, negative ions (an electron combined with a neutral molecule).

If you create an electric field in the space occupied by an ionized gas, then the electric charge carriers will come into ordered motion - this is how an electric current arises in gases. If the ionizer stops working, the gas becomes neutral again, as it recombination– formation of neutral atoms by ions and electrons.

Electric current in a vacuum

Vacuum is the degree of rarefaction of a gas at which we can neglect the collision between its molecules and assume that the mean free path exceeds the linear dimensions of the vessel in which the gas is located.

Electric current in a vacuum is the conductivity of the interelectrode gap in a vacuum state. There are so few gas molecules that their ionization processes cannot provide the number of electrons and ions that are necessary for ionization. The conductivity of the interelectrode gap in a vacuum can be ensured only with the help of charged particles arising due to emission phenomena on the electrodes.

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How to successfully prepare for the CT in physics and mathematics?

In order to successfully prepare for the CT in physics and mathematics, among other things, it is necessary to fulfill three most important conditions:

1. Study all topics and complete all tests and assignments given in the educational materials on this site. To do this, you need nothing at all, namely: devote three to four hours every day to preparing for the CT in physics and mathematics, studying theory and solving problems. The fact is that the CT is an exam where it is not enough just to know physics or mathematics, you also need to be able to quickly and without failures solve a large number of problems on different topics and of varying complexity. The latter can only be learned by solving thousands of problems.
2. Learn all the formulas and laws in physics, and formulas and methods in mathematics. In fact, this is also very simple to do; there are only about 200 necessary formulas in physics, and even a little less in mathematics. In each of these subjects there are about a dozen standard methods for solving problems of a basic level of complexity, which can also be learned, and thus, completely automatically and without difficulty solving most of the CT at the right time. After this, you will only have to think about the most difficult tasks.
3. Attend all three stages of rehearsal testing in physics and mathematics. Each RT can be visited twice to decide on both options. Again, on the CT, in addition to the ability to quickly and efficiently solve problems, and knowledge of formulas and methods, you must also be able to properly plan time, distribute forces, and most importantly, correctly fill out the answer form, without confusing the numbers of answers and problems, or your own last name. Also, during RT, it is important to get used to the style of asking questions in problems, which may seem very unusual to an unprepared person at the DT.

Successful, diligent and responsible implementation of these three points will allow you to show an excellent result at the CT, the maximum of what you are capable of.

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Electric current is charged particles capable of moving in an orderly manner in any conductor. This movement occurs under the influence of an electric field. The emergence of electrical charges occurs almost constantly. This is especially pronounced when various substances come into contact with each other.

If complete free movement of charges relative to each other is possible, then these substances are conductors. When such movement is not possible, this category of substances is considered insulators. Conductors include all metals with varying degrees of conductivity, as well as salt and acid solutions. Insulators can be natural substances in the form of ebonite, amber, various gases and quartz. They can be of artificial origin, for example, PVC, polyethylene and others.

Electric current values

As a physical quantity, current can be measured according to its basic parameters. Based on the measurement results, the possibility of using electricity in a particular area is determined.

There are two types of electric current - direct and alternating. The first one always remains unchanged in time and direction, and in the second case, changes occur in these parameters over a certain period of time.

" Today I want to touch on the topic of electric current. What is it? Let's try to remember the school curriculum.

Electric current is the ordered movement of charged particles in a conductor

If you remember, in order for charged particles to move (an electric current arises), an electric field must be created. To create an electric field, you can carry out such elementary experiments as rubbing a plastic handle on wool and it will attract light objects for some time. Bodies capable of attracting objects after rubbing are called electrified. We can say that a body in this state has electrical charges, and the bodies themselves are called charged. From the school curriculum we know that all bodies consist of tiny particles (molecules). A molecule is a particle of a substance that can be separated from a body and it will have all the properties inherent in this body. Molecules of complex bodies are formed from various combinations of atoms of simple bodies. For example, a water molecule consists of two simple ones: an oxygen atom and one hydrogen atom.

Atoms, neutrons, protons and electrons - what are they?

In turn, an atom consists of a nucleus and revolving around it electrons. Each electron in an atom has a small electrical charge. For example, a hydrogen atom consists of a nucleus with an electron rotating around it. The nucleus of an atom consists, in turn, of protons and neutrons. The nucleus of an atom, in turn, has an electrical charge. The protons that make up the nucleus have the same electrical charges and electrons. But protons, unlike electrons, are inactive, but their mass is many times greater than the mass of the electron. The neutron particle that is part of the atom has no electrical charge and is neutral. The electrons that rotate around the nucleus of an atom and the protons that make up the nucleus are carriers of electric charges of equal magnitude. There is always a force of mutual attraction between an electron and a proton, and a force of mutual repulsion between the electrons themselves and between protons. Because of this, the electron has a negative electrical charge, and the proton has a positive charge. From this we can conclude that there are 2 types of electricity: positive and negative. The presence of equally charged particles in an atom leads to the fact that mutual attractive forces act between the positively charged nucleus of the atom and the electrons rotating around it, holding the atom together. Atoms differ from each other in the number of neutrons and protons in their nuclei, which is why the positive charge of the nuclei of atoms of different substances is not the same. In atoms of different substances, the number of rotating electrons is not the same and is determined by the magnitude of the positive charge of the nucleus. The atoms of some substances are strongly bonded to the nucleus, while in others this bond may be much weaker. This explains the different strengths of bodies. Steel wire is much stronger than copper wire, which means that steel particles are more strongly attracted to each other than copper particles. The attraction between molecules is especially noticeable when they are close to each other. The most striking example is that two drops of water merge into one upon contact.

Electric charge

In an atom of any substance, the number of electrons rotating around the nucleus is equal to the number of protons contained in the nucleus. The electric charge of an electron and a proton are equal in magnitude, which means that the negative charge of the electrons is equal to the positive charge of the nucleus. These charges cancel each other out, and the atom remains neutral. In an atom, electrons create an electron shell around the nucleus. The electron shell and nucleus of the atom are in continuous oscillatory motion. When moving, atoms collide with each other and one or more electrons are emitted from them. The atom ceases to be neutral and becomes positively charged. Since its positive charge has become greater than its negative charge (weak connection between the electron and the nucleus - metal and coal). In other bodies (wood and glass), the electron shells are not damaged. Once separated from atoms, free electrons move randomly and can be captured by other atoms. The process of appearances and disappearances in the body occurs continuously. With increasing temperature, the speed of vibrational motion of atoms increases, collisions become more frequent and stronger, and the number of free electrons increases. However, the body remains electrically neutral, since the number of electrons and protons in the body does not change. If a certain amount of free electrons is removed from the body, the positive charge becomes greater than the total charge. The body will be positively charged and vice versa. If a lack of electrons is created in the body, then it is charged additionally. If there is an excess, it is negative. The greater this deficiency or excess, the greater the electrical charge. In the first case (more positively charged particles), the bodies are called conductors (metals, aqueous solutions of salts and acids), and in the second (lack of electrons, negatively charged particles) dielectrics or insulators (amber, quartz, ebonite). For the continued existence of electric current, a potential difference must be constantly maintained in the conductor.

Well, the short physics course is over. I think, with my help, you remembered the school curriculum for the 7th grade, and we will look at what potential difference is in my next article. See you again on the pages of the site.