An electric current represents an electric current. What is electric current in simple words. What is the unit of electricity

The use of electric current is diverse, since it is impossible to imagine the life of mankind without it. One should understand its nature of occurrence in order to direct the energy for good, and not for harm. Electric current obeys the laws of physics, which are used to make various devices. For its proper use, you need to know the basic electrical quantities.

electric shock called the ordered movement of charged particles, due to which an electromagnetic field can be generated. Charged particles include the following: electrons, protons, neutrons, holes and ions. AT scientific literature The neutron has no charge, but participates in the formation of electricity magnetic field.

Besides, some don't know why electric current is a vector quantity. This statement follows from its definition, since it has a direction. In some sources, you can find the following definition: electric current - the speed at which charges change elementary particles at a certain point in time. Current is characterized by strength and voltage (potential difference). The properties that an electric current has: thermal, mechanical, chemical and the creation of an electromagnetic field.

Strength and type of current

Current strength- the number of charged particles passing through the conductor per unit of time equal to one second. Conductivity materials are divided into three groups: conductors, semiconductors and dielectrics. Conductors are substances that are capable of conducting current because they contain free electrons. Their presence can be determined from the table of D. I. Mendeleev, using the electronic configuration of a chemical element.

Semiconductors can conduct a stream of charged particles under certain conditions. A simple example is a semiconductor diode that conducts current in only one direction. Charge carriers are electrons and holes. In dielectrics, there are no charge carriers at all, therefore, this fact excludes the conduction of electricity in general.

The current strength is indicated by the letter I and is measured in amperes (A). 1 A is a unit of measurement for the strength of an unchanging current that passes through two conductors of infinite length and a very small cross-sectional area, which are parallel to each other and located in vacuum space at a distance of one meter from each other, and each meter of such a conductor can cause an interaction force, equal to 2*10^(-7) N.

A simplified version of the wording is as follows: the electric current strength at which the amount of electricity Q passes through the cross-sectional area of ​​\u200b\u200bthe conductor per unit time t is called an ampere. The definition is written as a formula and has the following form: I = Q / t.

There are auxiliary units of measurement, which include mA (0.001 A), kA (1000 A), etc.

The current value is measured using an ammeter, which is connected in series in the circuit. There are only two types of electric current: direct and alternating. If the current remains constant or changes in magnitude without changing direction, then it is called constant.

Alternating current changes in amplitude value and direction of flow according to some law. Its main characteristic is the frequency. According to the law of amplitude change, they can be divided into the following types: sinusoidal and non-sinusoidal. The first change according to the harmonic law and its graph is a sinusoid. Formula sinusoidal current includes the maximum value of the power characteristic Im, time t and angular frequency w = 2 * 3.1416 * f (power supply current frequency): i = Im * sin (w * t). Another value that characterizes the electric current is voltage or potential difference.

Potential difference

Any substance consists of atoms, consisting of elementary particles. The nucleus has a positive charge, and electrons with a negative charge revolve around it in their orbits. Atoms are neutral because the number of electrons is equal to the number of protons in the nucleus.

When electrons are lost by atoms, an electromagnetic field is created by protons, as they seek to return the missing negatively charged particles. If for some reason there is an excess of electrons, then an electromagnetic field with a negative component is formed. In the first and second cases, positive and negative potentials are formed, respectively. The difference between them is called voltage or potential difference.

The magnitude of the difference is directly proportional to the voltage value: As the difference increases, the voltage value increases. When potentials are connected to various signs an electric current arises, which seeks to eliminate the cause of the difference and return the atom to its original state.

Electrical voltage is the work done by an electromagnetic field to move a point charge. The unit of voltage is the volt (V), and its value can be measured with a voltmeter. It is connected in parallel to the site or electrical appliance, where it is necessary to measure the potential difference. 1 V is the potential difference between two points with a charge of 1 C, at which the force of the electromagnetic field does work equal to 1 J.

Conditions for obtaining and laws

Electric current occurs when an electromagnetic field is applied to a conductor. But the converse statement is also true, proving the occurrence of an electric field as a result of the flow of current. Important conditions for its production are the following factors: the presence of free electrons and a voltage source. The presence of charge carriers affects the conductivity, and the voltage is an external force that contributes to the "pulling out" of these particles from the crystal lattice.

Conductivity of substances

The charge carriers in metals are electrons. At high temperature conductor, the movement of atoms occurs, some of them decay and new free electrons are formed. Charged particles interact with atoms and nodes of the crystal lattice, and part of the energy is converted into heat. This process is called the electrical resistance of the conductor. It depends on the following components:

As the temperature of a substance decreases, its resistance decreases. Dependence on the type of substance is explained by the fact that each substance consists of atoms. They form a crystal lattice among themselves, and for each substance it is different. Each atom has a certain electronic configuration, and therefore differs from others in the presence of charge carriers.

In addition, it is more difficult for a stream of charged particles to pass through a long conductor with a small value of its cross-sectional area.

An electrolyte or liquid that conducts electricity is also a conductor. Charge carriers in liquids are ions, which are positively (anions) and negatively (cations) charged. An electrode with a positive potential is called an anode, and one with a negative potential is called a cathode. The movement occurs when voltage is applied to the electrodes. Cations move to the anode, and anions move to the cathode.

When current flows through the electrolyte, it is heated, as a result of which the resistance of the liquid increases. Some gases are capable of conducting electricity too. The charge carriers in them are ions and electrons, and the “charged gas” itself is called plasma.

Electricity in semiconductors obeys the same laws as in conductors, but there are some differences. Electrons and holes can represent charge carriers in them. As the temperature decreases, its resistance increases. With an external action on the semiconductor, the bonds in the crystal lattice weaken and free electrons appear, and a hole is formed in the place where they were. However, it attracts another electron that is nearby. This is how holes move. Consequently, the sum of the hole and electron electromagnetic fields forms an electric current.

Basic ratios

All phenomena obey physical laws, and electricity is no exception. The basic relationships of the dependence of one quantity on others are described in the laws that are used to calculate various schemes for simple and complex devices. In addition, the rules help to avoid various emergencies, since electricity can also serve to harm humanity, causing fires, injuries and even death.

The basic law used in electrical engineering is Ohm's law for a section and a complete circuit. For a circuit section, it shows the dependence of the current I on the voltage U and electrical resistance R and its formulation is as follows: the current flowing in the circuit section is directly proportional to the voltage value and inversely proportional to the resistance of this section (I \u003d U / R).

For a complete circuit in which there is an electromotive force (e) and the internal resistance of the power supply: the formulation is as follows: the current flowing in the complete circuit is directly proportional to the electromotive force (EMF) and inversely proportional to the impedance of the circuit, taking into account the internal resistance of the power supply ( i = e / (R + Rin)).

From these laws, you can get the consequences that are needed to find the values ​​​​of voltage, emf and resistance. Consequences from Ohm's laws:

An electric current, when passing through a conductor or semiconductor, performs work in which thermal energy is released. This is one of its properties. Its numerical value is determined using the Joule-Lenz law.

The law shows the dependence of the amount of heat on the magnitude of voltage and current strength, as well as the time of flow of electric current.

Its formulation is as follows: the amount of heat Q released by current when flowing through a conductor per unit time is directly proportional to voltage and current strength (Q = U * I * t). The implications of this law are:

• • t = Q / (sqr (U) / R).

• Q = P * t.
• P = Q / t.
• t = Q / P.

The value of P is power and is calculated by the formula: P \u003d U * I. If the electric current in the circuit does not perform mechanical work and does not produce any action, then all electrical energy is converted into heat, i.e. A \u003d Q.

It was experimentally established that when the lines of electromagnetic induction are crossed by a conductor of a closed type, an electric current appears in it. The law on the influence of an electromagnetic field on the occurrence of current is called Faraday's law. It says: the negative value of the EMF of electromagnetic induction in a circuit that is closed is equal to the change in magnetic flux over time. It follows from Faraday's law that when a conductor moves in a constant magnetic field, a potential difference arises at the ends of the first. This principle is used to make generators, transformers, etc.

Thus, electric current, like all phenomena and processes, is subject to certain laws that allow not only to control, but also to avoid the negative consequences associated with its work. It is also necessary to make calculations to save time, since the selection of the value of any element of the circuit can lead to the failure of the device.

Electrons or holes (electron-hole conduction). Sometimes electric current is also called displacement current, resulting from a change in time of the electric field.

Electric current has the following manifestations:

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Classification

If charged particles move inside macroscopic bodies relative to a particular medium, then such a current is called electric conduction current. If macroscopic charged bodies are moving (for example, charged raindrops), then this current is called convection .

There are direct and alternating electric currents, as well as all kinds of alternating current. In such terms, the word "electric" is often omitted.

• DC Current - current, the direction and magnitude of which do not change with time.

Eddy currents

Eddy currents (Foucault currents) are “closed electric currents in a massive conductor that arise when the magnetic flux penetrating it changes,” therefore, eddy currents are induction currents. The faster the magnetic flux changes, the stronger the eddy currents. Eddy currents do not flow along certain paths in the wires, but, closing in the conductor, form vortex-like contours.

The existence of eddy currents leads to the skin effect, that is, to the fact that the alternating electric current and magnetic flux propagate mainly in the surface layer of the conductor. Eddy current heating of conductors leads to energy losses, especially in the cores of AC coils. To reduce energy losses due to eddy currents, the division of alternating current magnetic circuits into separate plates, isolated from each other and located perpendicular to the direction of eddy currents, is used, which limits the possible contours of their paths and greatly reduces the magnitude of these currents. At very high frequencies, instead of ferromagnets, magnetodielectrics are used for magnetic circuits, in which, due to the very high resistance, eddy currents practically do not occur.

Characteristics

It is historically accepted that current direction coincides with the direction of movement of positive charges in the conductor. In this case, if the only current carriers are negatively charged particles (for example, electrons in a metal), then the direction of the current is opposite to the direction of movement of charged particles. .

Drift velocity of electrons

Radiation resistance is caused by the formation of electromagnetic waves around the conductor. This resistance is in complex dependence on the shape and dimensions of the conductor, on the wavelength of the emitted wave. For a single rectilinear conductor, in which the current of the same direction and strength is everywhere, and the length of which L is much less than the length radiated by it electromagnetic wave λ (\displaystyle \lambda ), the dependence of resistance on wavelength and conductor is relatively simple:

R = 3200 (L λ) (\displaystyle R=3200\left((\frac (L)(\lambda ))\right))

The most used electric current with a standard frequency of 50 Hz corresponds to a wave with a length of about 6 thousand kilometers, which is why the radiation power is usually negligibly small compared to the heat loss power. However, as the frequency of the current increases, the length of the emitted wave decreases, and the radiation power increases accordingly. A conductor capable of radiating appreciable energy is called an antenna.

Frequency

Frequency refers to an alternating current that periodically changes strength and/or direction. This also includes the most commonly used current, which varies according to a sinusoidal law.

An alternating current period is the shortest period of time (expressed in seconds) after which changes in current (and voltage) are repeated. The number of periods completed by the current per unit of time is called the frequency. Frequency is measured in hertz, one hertz (Hz) corresponds to one cycle per second.

Bias current

Sometimes, for convenience, the concept of displacement current is introduced. In Maxwell's equations, the displacement current is present on an equal footing with the current caused by the movement of charges. The intensity of the magnetic field depends on the total electric current, which is equal to the sum of the conduction current and the displacement current. By definition, the bias current density j D → (\displaystyle (\vec (j_(D))))- vector quantity proportional to the rate of change of the electric field E → (\displaystyle (\vec (E))) in time:

j D → = ∂ E → ∂ t (\displaystyle (\vec (j_(D)))=(\frac (\partial (\vec (E)))(\partial t)))

The fact is that with a change in the electric field, as well as with the flow of current, a magnetic field is generated, which makes these two processes similar to each other. In addition, a change in the electric field is usually accompanied by energy transfer. For example, when charging and discharging a capacitor, despite the fact that there is no movement of charged particles between its plates, they speak of a displacement current flowing through it, carrying some energy and closing the electrical circuit in a peculiar way. Bias current I D (\displaystyle I_(D)) in the capacitor is determined by the formula:

I D = d Q d t = − C d U d t (\displaystyle I_(D)=(\frac ((\rm (d))Q)((\rm (d))t))=-C(\frac ( (\rm (d))U)((\rm (d))t))),

where Q (\displaystyle Q)- charge on the capacitor plates, U (\displaystyle U)- potential difference between the plates, C (\displaystyle C) is the capacitance of the capacitor.

Displacement current is not an electric current because it is not related to displacement electric charge.

Main types of conductors

Unlike dielectrics, conductors contain free carriers of uncompensated charges, which, under the action of a force, usually a difference in electrical potentials, set in motion and create an electric current. The current-voltage characteristic (dependence of current strength on voltage) is the most important characteristic of a conductor. For metallic conductors and electrolytes, it has simplest form: current is directly proportional to voltage (ohm's law).

Metals - here the current carriers are conduction electrons, which are usually considered as an electron gas, clearly showing the quantum properties of a degenerate gas.

Electric currents in nature

Electric current is used as a carrier of signals of varying complexity and types in different areas (telephone, radio, control panel, door lock button, and so on).

In some cases, unwanted electric currents appear, such as stray currents or short circuit current.

The use of electric current as a carrier of energy

• obtaining mechanical energy in various electric motors,
• obtaining thermal energy in heating devices, electric furnaces, during electric welding,
• obtaining light energy in lighting and signaling devices,
• excitation of electromagnetic oscillations of high frequency, ultrahigh frequency and radio waves,
• receiving sound,
• obtaining various substances by electrolysis, charging electric batteries. This is where electromagnetic energy is converted into chemical energy.
• creating a magnetic field (in electromagnets).

The use of electric current in medicine

• diagnostics - the biocurrents of healthy and diseased organs are different, while it is possible to determine the disease, its causes and prescribe treatment. The branch of physiology that studies electrical phenomena in the body is called electrophysiology.
  • Electroencephalography is a method for studying the functional state of the brain.
  • Electrocardiography is a technique for recording and studying electric fields during the work of the heart.
  • Electrogastrography is a method for studying the motor activity of the stomach.
  • Electromyography is a method for studying bioelectric potentials that occur in skeletal muscles.
• Treatment and resuscitation: electrical stimulation of certain areas of the brain; treatment of Parkinson's disease and epilepsy, also for electrophoresis. A pacemaker that stimulates the heart muscle with a pulsed current is used for bradycardia and other cardiac arrhythmias.

electrical safety

It includes legal, socio-economic, organizational and technical, sanitary and hygienic, medical and preventive, rehabilitation and other measures. Electrical safety rules are regulated by legal and technical documents, regulatory and technical framework. Knowledge of the basics of electrical safety is mandatory for personnel servicing electrical installations and electrical equipment. The human body is a conductor of electric current. Human resistance with dry and intact skin ranges from 3 to 100 kOhm.

The current passed through the human or animal body produces the following actions:

• thermal (burns, heating and damage to blood vessels);
• electrolytic (blood decomposition, violation of the physico-chemical composition);
• biological (irritation and excitation of body tissues, convulsions)
• mechanical (rupture of blood vessels under the action of steam pressure obtained by heating with blood flow)

The main factor determining the outcome of electric shock is the amount of current passing through the human body. According to safety engineering, electric current is classified as follows:

• safe a current is considered, the long passage of which through the human body does not harm him and does not cause any sensations, its value does not exceed 50 μA (alternating current 50 Hz) and 100 μA direct current;
• minimally perceptible human alternating current is about 0.6-1.5 mA (alternating current 50 Hz) and 5-7 mA direct current;
• threshold relentless called the minimum current of such a force at which a person is no longer able to tear his hands away from the current-carrying part by an effort of will. For alternating current, this is about 10-15 mA, for direct current - 50-80 mA;
• fibrillation threshold is called an alternating current (50 Hz) of about 100 mA and 300 mA of direct current, the effect of which is longer than 0.5 s with a high probability of causing fibrillation of the heart muscles. This threshold is simultaneously considered conditionally lethal for humans.

In Russia, in accordance with the Rules for the technical operation of electrical installations of consumers and the Rules for labor protection during the operation of electrical installations, 5 qualification groups for electrical safety have been established, depending on the qualifications and experience of the employee and the voltage of electrical installations.

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

Electric Energy

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

Generators have spinning coils of wire that pass through magnetic fields as they spin. As the coils rotate, they open and close with respect to the magnetic field and create an electrical current that changes direction with each turn. The current goes through a full cycle back and forth 60 times per second.

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

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

Why is electricity needed?

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

When connected to the Internet, a computer uses only a small part of the electrical current, but this is something without which modern man does not represent his life.

The concept of electric current

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

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

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

Curious about electric current

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

Direct and alternating current

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

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

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

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

How electric current works

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

It is the latter that plays an important role in Everyday life any person, as it represents an electric current that passes through the circuit. We use it daily to power our homes and more.

What is electric current?

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

Current power

On the conditions for the existence of an electric current in a conductor big influence provides such a voltage characteristic as power. This is a measure of how much energy is being used over a given period of time.

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

Electric charge in motion

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

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

If we draw an analogy with water, then take, for example, a pipe. When we open a valve at one end to let water enter the pipe, we don't have to wait for that water to work its way all the way to the end of the pipe. We get water at the other end almost instantly because the incoming water pushes the water that is already in the pipe. This is what happens in the case of an electric current in a wire.

Electric current: conditions for the existence of an electric current

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

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

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

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

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

The first discoveries related to the work of electricity began in the 7th century BC. Philosopher Ancient Greece Thales of Miletus revealed that when amber is rubbed against wool, it is subsequently able to attract lightweight objects. From Greek "electricity" is translated as "amber". In 1820, André-Marie Ampère established the law of direct current. In the future, the magnitude of the current, or what the electric current is measured in, began to be denoted in amperes.

Term meaning

The concept of electric current can be found in any physics textbook. electric current- this is an ordered movement of electrically charged particles in a direction. To understand to a simple layman what an electric current is, you should use the dictionary of an electrician. In it, the term stands for the movement of electrons through a conductor or ions through an electrolyte.

Depending on the movement of electrons or ions inside the conductor, the following are distinguished: types of currents:

• constant;
• variable;
• intermittent or pulsating.

Basic measurements

The strength of the electric current- the main indicator used by electricians in their work. The strength of the electric current depends on the magnitude of the charge that flows through the electrical circuit for a set period of time. How large quantity electrons flowed from one beginning of the source to the end, the greater will be the charge transferred by the electrons.

A quantity that is measured by the ratio of the electric charge flowing through the cross section of particles in a conductor to the time it passes. Charge is measured in coulombs, time is measured in seconds, and one unit of electricity flow is determined by the ratio of charge to time (coulomb to second) or amperes. The determination of the electric current (its strength) occurs by connecting two terminals in series to the electrical circuit.

When the electric current is working, the movement of charged particles is carried out with the help of an electric field and depends on the strength of the movement of electrons. The value on which the work of the electric current depends is called voltage and is determined by the ratio of the work of the current in a particular part of the circuit and the charge passing through the same part. The volt unit is measured with a voltmeter when the two terminals of the instrument are connected in parallel to the circuit.

The value of electrical resistance is directly dependent on the type of conductor used, its length and cross section. It is measured in ohms.

Power is determined by the ratio of the work of the movement of currents to the time when this work occurred. Measure power in watts.

Such physical quantity, as a capacitance, is determined by the ratio of the charge of one conductor to the potential difference between the same conductor and the neighboring one. The lower the voltage when the conductors receive an electric charge, the greater their capacitance. It is measured in farads.

The value of the work of electricity at a certain interval of the chain is found using the product of the current strength, voltage and the time period at which the work was carried out. The latter is measured in joules. The determination of the work of the electric current occurs with the help of a meter that connects the readings of all quantities, namely voltage, force and time.

Electrical safety engineering

Knowing the rules of electrical safety will help prevent an emergency and protect human health and life. Since electricity tends to heat the conductor, there is always the possibility of a situation dangerous to health and life. For home security must adhere following simple but important rules:

1. Network insulation must always be in good working order to avoid overloads or the possibility of short circuits.
2. Moisture should not get on electrical appliances, wires, shields, etc. Also, a humid environment provokes short circuits.
3. Be sure to make grounding for all electrical devices.
4. It is necessary to avoid overloading the electrical wiring, as there is a risk of ignition of the wires.

Safety precautions when working with electricity involves the use of rubberized gloves, mittens, rugs, discharge devices, grounding devices for work areas, circuit breakers or fuses with thermal and current protection.

Experienced electricians, when there is a possibility of electric shock, work with one hand, and the other is in their pocket. Thus, the hand-to-hand circuit is interrupted in case of involuntary contact with the shield or other grounded equipment. In case of ignition of equipment connected to the network, extinguish the fire exclusively with powder or carbon dioxide extinguishers.

Application of electric current

Electric current has many properties that allow it to be used in almost all areas. human activity. Ways to use electric current:

Electricity is the most environmentally friendly form of energy today. In the conditions of the modern economy, the development of the electric power industry is of planetary importance. In the future, if there is a shortage of raw materials, electricity will take a leading position as an inexhaustible source of energy.

Electricity

First of all, it is worth finding out what constitutes an electric current. Electric current is the ordered movement of charged particles in a conductor. In order for it to arise, an electric field must first be created, under the influence of which the above-mentioned charged particles will begin to move.

The first information about electricity, which appeared many centuries ago, related to electrical "charges" obtained through friction. Already in ancient times, people knew that amber, worn on wool, acquires the ability to attract light objects. But only at the end of the 16th century, the English physician Gilbert studied this phenomenon in detail and found out that many other substances have exactly the same properties. Bodies capable, like amber, after being rubbed to attract light objects, he called electrified. This word is derived from the Greek electron - "amber". At present, we say that there are electric charges on bodies in this state, and the bodies themselves are called "charged."

Electric charges always arise when different substances are in close contact. If the bodies are solid, then their close contact is prevented by microscopic protrusions and irregularities that exist on their surface. By squeezing such bodies and rubbing them together, we bring their surfaces together, which without pressure would touch only at a few points. In some bodies, electric charges can move freely between various parts while in others it is not possible. In the first case, the bodies are called "conductors", and in the second - "dielectrics, or insulators." Conductors are all metals, aqueous solutions of salts and acids, etc. Examples of insulators are amber, quartz, ebonite and all gases that are under normal conditions.

Nevertheless, it should be noted that the division of bodies into conductors and dielectrics is very arbitrary. All substances conduct electricity to a greater or lesser extent. Electric charges are either positive or negative. This kind of current will not last long, because the electrified body will run out of charge. For the continuous existence of an electric current in a conductor, it is necessary to maintain an electric field. For these purposes, electric current sources are used. The simplest case of the occurrence of an electric current is when one end of the wire is connected to an electrified body, and the other to the ground.

Electric circuits supplying current to lighting bulbs and electric motors did not appear until after the invention of batteries, which dates back to about 1800. After that, the development of the doctrine of electricity went so fast that in less than a century it became not just a part of physics, but formed the basis of a new electrical civilization.

The main quantities of electric current

The amount of electricity and current strength. The effects of electric current can be strong or weak. The strength of the electric current depends on the amount of charge that flows through the circuit in a certain unit of time. The more electrons moved from one pole of the source to the other, the greater the total charge carried by the electrons. This total charge is called the amount of electricity passing through the conductor.

The amount of electricity depends, in particular, on the chemical effect of the electric current, i.e., the greater the charge passed through the electrolyte solution, the more the substance will settle on the cathode and anode. In this regard, the amount of electricity can be calculated by weighing the mass of the substance deposited on the electrode and knowing the mass and charge of one ion of this substance.

The current strength is a quantity that is equal to the ratio of the electric charge that has passed through the cross section of the conductor to the time of its flow. The unit of charge is the coulomb (C), time is measured in seconds (s). In this case, the unit of current strength is expressed in C/s. This unit is called the ampere (A). In order to measure the current strength in a circuit, an electrical measuring device called an ammeter is used. For inclusion in the circuit, the ammeter is equipped with two terminals. It is included in the circuit in series.

electrical voltage. We already know that electric current is an ordered movement of charged particles - electrons. This movement is created with the help of an electric field, which does a certain amount of work. This phenomenon is called the work of an electric current. In order to move more charge through an electric circuit in 1 second, the electric field must do more work. Based on this, it turns out that the work of an electric current should depend on the strength of the current. But there is another value on which the work of the current depends. This value is called voltage.

Voltage is the ratio of the work of the current in a certain section of the electrical circuit to the charge flowing through the same section of the circuit. The current work is measured in joules (J), the charge is measured in pendants (C). In this regard, the unit of voltage measurement will be 1 J/C. This unit is called the volt (V).

In order for a voltage to appear in an electrical circuit, a current source is needed. In an open circuit, voltage is present only at the current source terminals. If this current source is included in the circuit, voltage will also appear in certain sections of the circuit. In this regard, there will also be a current in the circuit. That is, briefly we can say the following: if there is no voltage in the circuit, there is no current. In order to measure voltage, an electrical measuring device called a voltmeter is used. His appearance it resembles the previously mentioned ammeter, with the only difference that the letter V is on the scale of the voltmeter (instead of A on the ammeter). The voltmeter has two terminals, with the help of which it is connected in parallel to the electrical circuit.

Electrical resistance. After connecting all kinds of conductors and an ammeter to an electrical circuit, you can notice that when using different conductors, the ammeter gives different readings, that is, in this case, the current strength available in the electrical circuit is different. This phenomenon can be explained by the fact that different conductors have different electrical resistance, which is a physical quantity. In honor of the German physicist, she was named Ohm. As a rule, larger units are used in physics: kiloohm, megaohm, etc. The conductor resistance is usually denoted by the letter R, the conductor length is L, the cross-sectional area is S. In this case, the resistance can be written as a formula:

where the coefficient p is called resistivity. This coefficient expresses the resistance of a conductor 1 m long with a cross-sectional area equal to 1 m2. Resistivity is expressed in Ohm x m. Since wires, as a rule, have a rather small cross section, their areas are usually expressed in square millimeters. In this case, the unit of resistivity will be Ohm x mm2/m. In the table below. 1 shows the resistivity of some materials.

Table 1. Electrical resistivity of some materials
Material	p, Ohm x m2/m	Material	p, Ohm x m2/m
		Platinum iridium alloy
Metal or Alloy
		Manganin (alloy)
Aluminum		Constantan (alloy)
Tungsten
		Nichrome (alloy)
Nickel (alloy)		Fechral (alloy)
		Chromel (alloy)

According to Table. 1, it becomes clear that copper has the smallest electrical resistivity, and an alloy of metals has the largest. In addition, dielectrics (insulators) have high resistivity.

Electrical capacitance. We already know that two conductors isolated from each other can accumulate electric charges. This phenomenon is characterized by a physical quantity, which is called electrical capacitance. The electrical capacitance of two conductors is nothing more than the ratio of the charge of one of them to the potential difference between this conductor and the neighboring one. The lower the voltage when the conductors receive a charge, the greater their capacitance. The farad (F) is taken as the unit of electrical capacitance. In practice, fractions of this unit are used: microfarad (µF) and picofarad (pF).

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If you take two conductors isolated from each other, place them at a small distance from one another, you get a capacitor. The capacitance of a capacitor depends on the thickness of its plates and the thickness of the dielectric and its permeability. By reducing the thickness of the dielectric between the plates of the capacitor, it is possible to greatly increase the capacitance of the latter. On all capacitors, in addition to their capacitance, the voltage for which these devices are designed must be indicated.

Work and power of electric current. From the foregoing, it is clear that the electric current does a certain amount of work. When electric motors are connected, the electric current makes all kinds of equipment work, moves trains along the rails, illuminates the streets, heats the home, and also produces a chemical effect, that is, it allows electrolysis, etc. We can say that the work of the current in a certain section of the circuit is equal to the product current, voltage and time during which the work was done. Work is measured in joules, voltage in volts, current in amperes, and time in seconds. In this regard, 1 J = 1V x 1A x 1s. From this it turns out that in order to measure the work of an electric current, three devices should be used at once: an ammeter, a voltmeter and a clock. But this is cumbersome and inefficient. Therefore, usually, the work of electric current is measured by electric meters. The device of this device contains all of the above devices.

The power of an electric current is equal to the ratio of the work of the current to the time during which it was performed. Power is denoted by the letter "P" and is expressed in watts (W). In practice, kilowatts, megawatts, hectowatts, etc. are used. In order to measure the power of the circuit, you need to take a wattmeter. Electrical work is expressed in kilowatt-hours (kWh).

Basic laws of electric current

Ohm's law. Voltage and current are considered the most convenient characteristics of electrical circuits. One of the main features of the use of electricity is the rapid transportation of energy from one place to another and its transfer to the consumer in desired form. The product of the potential difference and the current strength gives power, i.e., the amount of energy given off in the circuit per unit time. As mentioned above, to measure the power in an electrical circuit, it would take 3 devices. Is it possible to do with one and calculate the power from its readings and some characteristic of the circuit, such as its resistance? Many people liked this idea, they considered it fruitful.

So, what is the resistance of a wire or a circuit as a whole? Does a wire, like water pipes or pipes in a vacuum system, have a constant property that might be called resistance? For example, in pipes, the ratio of the pressure difference creating flow divided by the flow rate is usually a constant characteristic of the pipe. In the same way, the heat flow in a wire is subject to a simple relationship, which includes the temperature difference, the cross-sectional area of ​​the wire, and its length. The discovery of such a relationship for electrical circuits was the result of a successful search.

In the 1820s, the German schoolteacher Georg Ohm was the first to start looking for the above ratio. First of all, he aspired to fame and fame, which would allow him to teach at the university. That was the only reason he chose a field of study that offered particular advantages.

Om was the son of a locksmith, so he knew how to draw metal wire of different thicknesses, which he needed for experiments. Since in those days it was impossible to buy a suitable wire, Om made it with his own hands. During the experiments, he tried different lengths, different thicknesses, different metals and even different temperatures. All these factors he varied in turn. In Ohm's time, batteries were still weak, giving a current of variable magnitude. In this regard, the researcher used a thermocouple as a generator, the hot junction of which was placed in a flame. In addition, he used a crude magnetic ammeter, and measured potential differences (Ohm called them "voltages") by changing the temperature or the number of thermal junctions.

The doctrine of electrical circuits has just received its development. After the invention of batteries around 1800, it began to develop much faster. Various devices were designed and manufactured (quite often by hand), new laws were discovered, concepts and terms appeared, etc. All this led to a deeper understanding of electrical phenomena and factors.

The renewal of knowledge about electricity, on the one hand, caused the emergence of a new field of physics, on the other hand, was the basis for the rapid development of electrical engineering, i.e., batteries, generators, power supply systems for lighting and electric drive, electric furnaces, electric motors, etc. were invented , other.

Ohm's discoveries were of great importance both for the development of the theory of electricity and for the development of applied electrical engineering. They made it easy to predict the properties of electrical circuits for direct current, and later for alternating current. In 1826, Ohm published a book in which he outlined the theoretical conclusions and experimental results. But his hopes were not justified, the book was met with ridicule. This happened because the method of rough experimentation seemed little attractive in an era when many people were fond of philosophy.

Omu had no choice but to leave his position as a teacher. He did not achieve an appointment at the university for the same reason. Within 6 years scientist lived in poverty, without confidence in the future, experiencing a feeling of bitter disappointment.

But gradually his works gained fame first outside of Germany. Om was respected abroad, his research was used. In this regard, compatriots were forced to recognize him in their homeland. In 1849 he received a professorship at the University of Munich.

Ohm discovered a simple law that establishes a relationship between current strength and voltage for a piece of wire (for part of the circuit, for the entire circuit). In addition, he made rules that allow you to determine what will change if you take a wire of a different size. Ohm's law is formulated as follows: the current strength in a section of the circuit is directly proportional to the voltage in this section and inversely proportional to the resistance of the section.

Joule-Lenz law. Electric current in any part of the circuit performs a certain work. For example, let's take some section of the circuit, between the ends of which there is a voltage (U). By the definition of electric voltage, the work done when moving a unit of charge between two points is equal to U. If the current strength in a given section of the circuit is i, then the charge it will pass in time t, and therefore the work of the electric current in this section will be:

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

This formula is used in the SI system to determine the unit of voltage.

Let us assume that the section of the circuit is a fixed conductor. In this case, all the work will turn into heat, which will be released in this conductor. If the conductor is homogeneous and obeys Ohm's law (this includes all metals and electrolytes), then:

where r is the resistance of the conductor. In this case:

This law was first empirically derived by E. Lenz and, independently of him, by Joule.

It should be noted that the heating of conductors finds numerous applications in engineering. The most common and important among them are incandescent lighting lamps.

Law of electromagnetic induction. In the first half of the 19th century, the English physicist M. Faraday discovered the phenomenon of magnetic induction. This fact, having become the property of many researchers, gave a powerful impetus to the development of electrical and radio engineering.

In the course of experiments, Faraday found out that when the number of magnetic induction lines penetrating a surface bounded by a closed loop changes, an electric current arises in it. This is the basis of perhaps the most important law of physics - the law of electromagnetic induction. The current that occurs in the circuit is called inductive. Due to the fact that electric current occurs in the circuit only in the case of external forces acting on free charges, then with a changing magnetic flux passing over the surface of a closed circuit, these same external forces appear in it. The action of external forces in physics is called the electromotive force or induction EMF.

Electromagnetic induction also appears in open conductors. In the case when the conductor crosses the magnetic field lines, a voltage appears at its ends. The reason for the appearance of such a voltage is the induction EMF. If the magnetic flux passing through the closed circuit does not change, the inductive current does not appear.

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

Lenz's rule. As we already know, an inductive current occurs in the conductor. Depending on the conditions of its appearance, it has a different direction. On this occasion, the Russian physicist Lenz formulated the following rule: the induction current arising in closed loop, always has such a direction that the magnetic field it creates does not allow the magnetic flux to change. All this causes the appearance of an induction current.

Induction current, like any other, has energy. This means that in the event of an induction current, electrical energy appears. According to the law of conservation and transformation of energy, the above-mentioned energy can only arise due to the amount of energy of some other type of energy. Thus, Lenz's rule fully corresponds to the law of conservation and transformation of energy.

In addition to induction, the so-called self-induction can appear in the coil. Its essence is as follows. If a current appears in the coil or its strength changes, then a changing magnetic field appears. And if the magnetic flux passing through the coil changes, then an electromotive force arises in it, which is called the EMF of self-induction.

According to Lenz's rule, the EMF of self-induction when the circuit is closed interferes with the current strength and does not allow it to increase. When the EMF circuit is turned off, self-induction reduces the current strength. In the case when the current strength in the coil reaches a certain value, the magnetic field stops changing and the self-induction EMF becomes zero.