Lesson plan. Topic Electric current Electrical conductivity. Varieties of electric current: electronic theory of structure, electric current in a conductor, conduction current, current density, electric voltage, magnitude, units

The speed of propagation of an electric current .. The speed of movement of charge carriers in an electric field .. What determines the drift speed of charge carriers? .. Thermal effect of the current ..

When studying electric current, difficulties often arise in understanding the processes that occur at the atomic level and are inaccessible to our senses - electricity cannot be seen, heard or touched. This raises a number of questions, in particular: why do the conductors get hot? What is the speed of electrons in a conductor and what does it depend on? Why, when we press the switch, the light comes on almost instantly? Let's try to figure it out together and answer these and other questions of interest to you.

Why does the bulb light up almost instantly?

First of all, you need to distinguish and not confuse concepts « electric current propagation speed"And" carrier velocity"Are not the same thing.

When we talk about the speed of propagation of an electric current in a conductor, it means the speed of propagation along the conductor electric fieldwhich is equal to the speed of light (≈ 300,000 km / s).
However, this does not mean that the movement of charge carriers in a conductor occurs at this tremendous speed. Not at all.

The movement of charge carriers (in the conductor these are free electrons) is always rather slow, with the speed of directional drift from fractions of a millimeter before several millimeters per second, since electric charges, colliding with the atoms of a substance, overcome more or less resistance to their movement in an electric field.

But the point isthat there are very, very many free electrons in the conductor (if each copper atom has one free electron, then there are as many mobile electrons in the conductor as there are copper atoms). Free electrons are available everywhere in electrical circuit, including but not limited to the filament of the light bulb that is part of this circuit.
When connecting a conductor to a source electrical energy an electric field (at the speed of light) propagates in it, which begins to act on ALL free electrons almost simultaneously.

Therefore, we do not observe any lag between the closure of the switch contacts and the beginning of the glow of a light bulb located tens or hundreds of kilometers from the power plant. They turned on the voltage, free electrons began to move (in the entire circuit at the same time), transferred the charge, transferred kinetic energy to the tungsten atoms (incandescent filament), the latter heated up to glow - and the light is on.

When alternating current to obtain the required heat (power dissipation of the filament), the direction of the current does not matter. Free electrons oscillate in accordance with changes in the electric field and carry charge back and forth. In this case, the electrons collide with the atoms of the crystal lattice of tungsten, transferring their energy to them. This causes the light bulb's filament to heat up and glow.

What determines the drift velocity of charge carriers?

Directional drift speed charge carriers in an electric field proportional to the electric current : small current means slow flow rate of charges, large current means b about higher speed.

On the speed of charge carriers also affects conductor resistance ... The thin conductor has more resistance, a large diameter conductor has less resistance. Accordingly, in a thin conductor, the flow rate of free electrons will be greater than in a thick conductor (at the same current).

The material of the conductor also matters:in an aluminum conductor, the electron flow rate will be greater than in a copper conductor of the same cross section. This means, among other things, that the same current will heat up the aluminum conductor more than the copper one.

Thermal effect of current

Consider the nature of the thermal effect of current in more detail.
In the absence of an electric field, free electrons move randomly in the metal crystal. Under the action of an electric field, free electrons, in addition to chaotic movement, acquire an ordered movement in one direction, and an electric current arises in the conductor.

Free electrons collide with the ions of the crystal lattice, giving them at each collision the kinetic energy acquired during free path under the action of an electric field. As a result, the ordered motion of electrons in a metal can be regarded as uniform motion with a certain constant speed.
Since the kinetic energy of electrons acquired under the action of an electric field transferred to lattice ions in the event of a collision, the conductor heats up during the passage of direct current.

In the case of alternating current the same effect takes place. The only difference is that the electrons do not move in one direction, but under the action of an alternating electric field, they oscillate back and forth with the mains frequency (50/60 Hz), remaining practically in place.
In this case, the electrons also collide with the atoms of the crystal lattice of the metal, transfer their kinetic energy, and this leads to heating of the crystal lattice. With enough large values current, a strongly heated lattice may even lose permanent bonds (the metal will begin to melt).

alternating current, AC), constant (eng. direct current, DC) and pulsating electric currents, as well as their various combinations. In such concepts, the word "electric" is often omitted.

Direct current is a current whose direction and magnitude do not change over time.

Alternating current is a current whose magnitude and direction change over time. In a broad sense, alternating current is understood as any current that is not constant. Among the alternating currents, the main one is the current, the value of which changes according to a sinusoidal law. In this case, the potential of each end of the conductor changes with respect to the potential of the other end of the conductor alternately from positive to negative and vice versa, passing through all intermediate potentials (including zero potential). As a result, a current arises that continuously changes direction: when moving in one direction, it increases, reaching a maximum, called the amplitude value, then decreases, for a moment it becomes equal to zero, then increases again, but in the other direction and also reaches the maximum value , falls off, to then pass through zero again, after which the cycle of all changes resumes.

Quasi-stationary current - “relatively slowly changing alternating current, for instantaneous values \u200b\u200bof which the laws of direct currents are satisfied with sufficient accuracy” (TSB). These laws are Ohm's law, Kirchhoff's rules and others. Quasi-stationary current, as well as direct current, has the same current strength in all sections of an unbranched circuit. When calculating the circuits of a quasi-stationary current due to the emerging e. etc. with. capacitance and inductance inductions are taken into account as lumped parameters. Ordinary industrial currents are quasi-stationary, except for currents in long-distance transmission lines, in which the condition of quasi-stationarity along the line is not satisfied.

The speed of directed movement of particles in conductors depends on the material of the conductor, the mass and charge of the particles, the ambient temperature, the applied potential difference and is much less than the speed of light. In 1 second, the electrons in the conductor move due to ordered movement by less than 0.1 mm. Despite this, the speed of propagation of the electric current itself is equal to the speed of light (the speed of propagation of the front of an electromagnetic wave). That is, the place where the electrons change their speed of movement after changing the voltage moves with the propagation speed electromagnetic waves.

Strength and current density

Electric current has quantitative characteristics: scalar - current strength, and vector - current density.

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

R \u003d 3200 (L λ) (\\ displaystyle R \u003d 3200 \\ left ((\\ frac (L) (\\ lambda)) \\ right))

The most commonly 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 negligible compared to the heat loss power. However, with an increase in the frequency of the current, the length of the radiated wave decreases, and the radiation power increases accordingly. A conductor capable of emitting noticeable 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 sinusoidal current.

An alternating current period is the smallest time interval (expressed in seconds) after which changes in current (and voltage) are repeated. The number of periods performed by the current per unit of time is called 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. Intensity magnetic field depends on the total electric current equal to the sum of the conduction current and the displacement current. By definition, the bias current density j D → (\\ displaystyle (\\ vec (j_ (D)))) is a vector quantity proportional to the rate of change of the electric field E → (\\ displaystyle (\\ vec (E))) in time:

j D → \u003d ∂ E → ∂ t (\\ displaystyle (\\ vec (j_ (D))) \u003d (\\ frac (\\ partial (\\ vec (E))) (\\ partial t)))

The fact is that when the electric field changes, as well as when current flows, 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:

ID \u003d d Q dt \u003d - C d U dt (\\ displaystyle I_ (D) \u003d (\\ frac ((\\ rm (d)) Q) ((\\ rm (d)) t)) \u003d - C (\\ frac ( (\\ rm (d)) U) ((\\ rm (d)) t))),

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

The displacement current is not an electric current, since it is not associated with the movement of electric charge.

Basic types of conductors

In contrast to dielectrics, conductors have free carriers of uncompensated charges, which, under the action of a force, as a rule, by a difference in electrical potentials, set in motion and create an electric current. Volt-ampere characteristic (dependence of current strength on voltage) is the most important characteristic of a conductor. For metal conductors and electrolytes, it has the simplest form: the current strength is directly proportional to the voltage (Ohm's law).

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

Plasma is an ionized gas. The electric charge is carried by ions (positive and negative) and free electrons, which are formed under the action of radiation (ultraviolet, X-ray and others) and (or) heating.

Electrolytes are "liquid or solid substances and systems in which ions are present in any noticeable concentration, which cause the passage of electric current." Ions are formed in the process electrolytic dissociation... When heated, the resistance of electrolytes decreases due to an increase in the number of molecules decomposed into ions. As a result of the passage of current through the electrolyte, the ions approach the electrodes and are neutralized, settling on them. Faraday's laws of electrolysis determine the mass of the substance released on the electrodes.

There is also an electrical current of electrons in a vacuum, which is used in cathode ray devices.

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 electrical currents occur, such as stray currents or short-circuit currents.

Using electric current as a carrier of energy

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

The use of electric current in medicine

• diagnostics - biocurrents of healthy and diseased organs are different, while it is possible to determine the disease, its causes and prescribe treatment. Physiology section studying 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 arising 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 an impulse current is used for bradycardia and other cardiac arrhythmias.

electrical safety

It includes legal, socio-economic, organizational and technical, sanitary and hygienic, treatment and prophylactic, rehabilitation and other measures. Electrical safety rules are regulated by legal and technical documents, regulatory and technical base. 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 kΩ.

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

• thermal (burns, heating and damage to blood vessels);
• electrolytic (decomposition of blood, violation of the physical and chemical composition);
• biological (irritation and excitement 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. For safety reasons, electric current is classified as follows:

• safe a current is considered, the prolonged 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 AC is about 0.6-1.5mA (AC 50Hz) and 5-7mA DC;
• threshold not letting go is called the minimum current of such a force at which a person is no longer able to tear his hands from the live part by an effort of will. For alternating current it is about 10-15 mA, for constant - 50-80 mA;
• fibrillation threshold is called an alternating current (50 Hz) of about 100 mA and 300 mA of direct current, the impact of which for more than 0.5 s is likely to cause fibrillation of the heart muscles. This threshold is simultaneously considered conditionally fatal 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.

see also

• Leakage current

Notes

Literature

• Baumgart K.K.,. // Encyclopedic Dictionary of Brockhaus and Efron: in 86 volumes (82 volumes and 4 additional). - SPb. , 1890-1907.

". Today I want to touch on such a topic as 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 appears), you need to create an electric field. 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 electric charges, and the bodies themselves are called charged. We know from the school curriculum that all bodies are composed of the smallest particles (molecules). A molecule is a particle of matter that can be separated from the body and it will have all the properties inherent in this body. The 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, the 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 the nucleus of 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 electric charges and electrons. But protons, unlike electrons, are inactive, but their mass is many times greater than the mass of an electron. The particle neutron, which is part of the atom, has no electric charge, is neutral. The electrons that revolve around the nucleus of the atom and the protons that make up the nucleus are carriers of equal electric charges. A force of mutual attraction always acts between an electron and a proton, and between the electrons themselves and between the protons there is a force of mutual repulsion. Because of this, the electron has a negative electric charge, and the proton is positive. 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 forces of mutual attraction act between the positively charged nucleus of the atom and the electrons revolving around it, holding the atom together. Atoms differ from each other in the number of neutrons and protons in nuclei, which is why the positive charge of the nuclei of atoms of different substances is not the same. For 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 firmly bound to the nucleus, while in others this bond may be much weaker. This explains the different strength of bodies. Steel wire is much stronger than copper, which means that steel particles are more attracted to each other than copper particles. The attraction between molecules is especially noticeable when they are close to each other. Most vivid example - two drops of water merge into one on contact.

Electric charge

In the atom of any substance, the number of electrons revolving 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 electrons is equal to the positive charge of the nucleus. These charges mutually balance each other, and the atom remains neutral. In an atom, electrons create an electron shell around the nucleus. The electron shell and the nucleus of the atom are in continuous oscillatory motion. As the atoms move, they collide with each other and one or more electrons are emitted from them. The atom ceases to be neutral, becomes positively charged. Since its positive charge has become more negative (weak bond between the electron and the nucleus - metal and coal). Other bodies (wood and glass) have a violation electronic shells not happening. Having torn off atoms, free electrons move randomly and can be captured by other atoms. The process of appearing and disappearing in the body occurs continuously. With increasing temperature, the speed of vibrational motion of atoms increases, collisions become more frequent, become stronger, 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, then the plus charge becomes greater than the total charge. The body will be charged positively and vice versa. If a lack of electrons is created in the body, then it is charged additionally. If the excess is negative. The more this deficiency or excess, the more electric charge... In the first case (more positively charged particles), bodies are called conductors (metals, aqueous solutions salts and acids), and in the second (lack of electrons, negatively charged particles) dielectrics or insulators (amber, quartz, ebonite). For the continuous existence of an electric current, a potential difference must be constantly maintained in a conductor.

Well, here's a little physics course over. I think you, with my help, remembered the school curriculum for grade 7, and what is the potential difference we will analyze in my next article. Until next time on the site.

And others. For the direction of the current take the direction of motion of positively charged particles; if the current is created by negatively charged particles (for example, electrons), then the direction of the current is considered opposite to the direction of motion of the particles.

Distinguish between electric conduction current associated with the movement of charged particles relative to a particular medium (i.e., inside macroscopic bodies), and convection current - the movement of macroscopic charged bodies as a whole (for example, charged rain drops).

If an electric current is established in the circuit, this means that an electric charge is constantly transferred through the cross section of the conductor. The charge transferred per unit of time serves as the main quantitative characteristic of the current, called the current strength. The strength of the current is equal to the ratio of the amount of charge transferred through the cross-section of the conductor for a certain interval of time to the duration of this interval. If the current strength and its direction do not change over time, then the current is called direct current.

For the emergence and existence of an electric current, it is necessary to have free positively or negatively charged particles that are not connected into a single electrically neutral system, and a force that creates and maintains their ordered movement. Typically, the force causing this movement is the force from the electric field within the conductor, which is determined by the electrical voltage at the ends of the conductor.

The most important characteristic of a conductor is the dependence of the current strength on the voltage - the current-voltage characteristic. It has the simplest form for metal conductors and electrolytes: the current is directly proportional to the voltage (Ohm's law).

Flowing through a substance, an electric current can provide magnetic, thermal, chemical attack... The magnetic action consists in the appearance of a magnetic field, this action is universal, manifests itself in all conductors without exception. The thermal effect of an electric current consists in heating the substance through which the current flows (with the exception of superconductors , in which there is no heat release). The chemical action is observed mainly in electrolytes and consists in the occurrence of chemical reactions under the influence of an electric current (for example, during electrolysis).

Maxwell introduced the concept of total current, which, in accordance with his theory, is always closed: only the conduction current breaks off at the ends of the conductor, and in the dielectric (vacuum) there is a displacement current between the ends of the conductor, which closes the conduction current. Therefore, the density of the total electric current j is equal to the sum of the conduction current density j and bias current density j cm, and determines the magnetic field created by it.

J full \u003d j +? D /? T

The ability of substances to conduct electric current varies greatly for different materials and is characterized by electrical conductivity. Conductors, due to the presence in them of a large number of mobile charged particles - charge carriers, conduct electric current well. The concentration of charge carriers in dielectrics is extremely low, and even at high voltages they serve as good insulators. In metals, free charged particles - current carriers - are conduction electrons, the concentration of which is practically independent of temperature and is 10 22 -10 23 cm -3. In electrolytes, the electric current is caused by the directional movement of positive and negative ions formed as a result of electrolytic dissociation.

Gases from neutral molecules are dielectrics. Only ionized gases - plasma - conduct electric current. The carriers of current in plasma are positive and negative ions (as in electrolytes) and free electrons (as in metals).

Chapter V1. DC ELECTRIC CURRENT

In the 7th grade, we fantasized about what would happen if the friction force disappears, which, by the way, is due to electrical interactions between molecules and atoms of different bodies.

Now imagine that all devices that use electrical interactions are down for a few minutes. It would be a disaster. The elevator, subway trains, electric trains will stop, water supply to houses and air to mines will stop. Airplanes will not be able to land, radio and television will stop working, telephones will be silenced, etc.

And the creation of all these devices began with the simplest observation of how small particles adhered to amber, worn on silk. Modern society cannot imagine life without electricity.

§53 Transformation of energy when a charge moves in an electric field.

Due to electrical interactions, a body with a charge can change its energy. Consider a few thought experiments. Let there be a charge on the top of the mountain that creates an electric field

At the foot of the mountain we will put a cart with a charge of the opposite sign. The electric field will drive the cart up the hill. We will assume that the field, charge and mass of the cart are such that this movement is possible.

What are we seeing? As a result of the work done by the electric field, the potential energy of the cart increases.

Let's come up with a thought experiment that would demonstrate the transformation of the energy of an electric field into the kinetic energy of a charged cart.

When a negatively charged cart moves along a horizontal surface, its speed will increase under the influence of an electric field. (We can mentally make the friction very small.)

This increases the kinetic energy of the cart E k \u003d mv 2/2. If friction is created so that the trolley moves on a horizontal surface without increasing its speed, the trolley and the surface on which it moves will heat up, i.e. the energy of the electric field can be converted into internal energy.

In the three considered cases, the energy of the electric field decreased, since as the charged cart approached the charge creating the field, the strength behind the cart decreased. Let's imagine that a drop of water with the same modulus but opposite in sign is attracted to a positively charged ball. Between them there is a field, the view of which is shown in

As the drop approaches the ball, its speed, and hence kinetic energy, increases. At the moment of contact of the drop with the ball, their charge will be equal to zero. The field will disappear, that is, all the energy of the field will turn into mechanical and thermal.

We emphasize once again that work by an electric field is performed only when a charged body moves. Moreover, the larger the charge has moved, the more work is done by the field. In this case, the field itself changes and its energy decreases if the total energy of the moving charge of the charge increases.

The shorter the charge travel time, the greater the field power. For example, when lightning occurs between two clouds

An electric field can do work when a charged body moves.

Due to the performance of work by an electric field, the potential, kinetic and internal energy of the body can change.

Exercise 53.

The cart is in a field with a voltage of 10,000 V / m. What is the force acting on the cart if its charge is 5 μC? What kind of work will the electric field do when the cart moves 10 cm along the lines of force?

The charged ball, moving under the action of the forces of the electric field, increased the kinetic energy by 3 J. What work did the electric field do?

When a charge of 6 μC moved in a uniform electric field at a distance of 0.5 m, its velocity increased from 0 to 2 m / s. What is the strength of the electric field if the mass of the charge is 4g?

In a heat-insulated tube with distilled water, a light ball with a charge of 10 μC moves at a distance of 1 m under the action of an electric field with a strength of 10,000 V / m. How much will the water temperature increase if its mass is 20 g? The mass of the ball can be neglected. Consider that all work is aimed at increasing the internal energy of water. Specific heat water 4200 J / kg deg.

§54 Electric current. Current strength.

Imagine a beam of protons, i.e. a huge number of protons flying in one direction. (Such beams are created by scientists in laboratories and in manufacturing.)

In the same way as the movement of cars on a highway in one direction is called a cargo flow. If you are standing on the side of the road, you can calculate what is the mass of cargo passing by you in one hour. This value characterizes the intensity of the movement of the load. You can compare on which road more goods are moving.

So, if 10 19 protons flew past the observer in one direction in one second, then their total charge is 1.6 C, and amperage 1.6 Cl / s.

Electric current has a direction. The direction of movement is taken as the direction of the electric current positive charges, in our case - protons. If negatively charged particles move directionally, then the direction of the electric current is opposite to the movement of these particles.

In the movement of charges, not only positively and negatively charged particles (for example, electrons) can participate simultaneously. If positive and negative charges move in one direction, their charges are subtracted; if in the opposite, then their charges add up.

For example, if 12 particles with a charge of +2 C each flew past the observer into the basket in 1 second, and 4 particles with a charge of –1 C each flew into the same basket, then amperage 20 C / s. (The charge of the basket, into which these particles fell, increases by 20 C per second).

If positive particles fly into the basket, and negative ones fly out of it, then the basket charge increases by 28 C per 1 second. For example, 4 particles with a charge of –1 C, emitted from the basket, increase its charge by 4 C. The current strength is 28 C / s.

The direction of the current coincides with the speed of positive particles and againstthe opposite speed is negative.

Current strength is measured in amperes (A). This unit is named after the French physicist A. Ampere (1775 - 1836). With a current of one ampere, a charge of one coulomb passes through the cross section of the conductor every second.

Fractional and multiple units of mA, μA, kA are used.

The filament of an electric bulb heats up due to the fact that a directional movement of electrons occurs in it under the action of an electric field. Wherein amperage in a thread about 0.5 A. What does this mean? If we mentally cross the thread with a plane

and we calculate what charge passed through this plane (it is called a cross section), then we find that in one second this charge is 0.5 C, in 5 seconds - 2.5 C.

I

I = q/ t

To measure the current strength, devices called ammeters are used.

To measure the current strength, devices called an ammeter are used.

The directional movement of charged particles is called electric current.

The magnitude of the charge of all particles flying past the observer in 1 second is called the current strength.

Electric current has a direction.

The direction of movement of positive charges is taken as the direction of the electric current.

Current strength is measured in amperes (1 A)

With a current of one ampere, a charge of one coulomb passes through the cross section of the conductor every second.

The current strength is indicated by the letterI

Exercise 54.

The electron gun "shoots" 10 18 electrons per second. What is the current strength of the electron beam?

In molten table salt, chlorine ions move with a charge of -1.6 10 -19 C and sodium ions with a charge of +1.6 10 -19 C. 10 19 sodium ions passed through the cross section from right to left in 1 second, and 2 10 19 chlorine ions from left to right in the same second. What is the current strength?

An electron beam with a current of 1 μA is directed at an uncharged metal ball. What will the ball's charge be in 5 s?

What will be the field strength at the surface of the ball (see problem 3) if its radius is 1 cm?

Drops with a charge of 0.1 μC fall from the pipette. 10 drops fell in 10 seconds. How much has the charge of the pipette changed with the remaining liquid? What is the average current strength (the ratio of the total charge passed to the time during which this charge passed)?

Tasks

1. Take a plastic bottle, fill it with water. Make a hole at the cover. The hole must be large enough to allow air bubbles to enter the bottle. Cork the bottle. Flip the bottle upside down over a sink or bathtub.

What needs to be done to keep the bubbles from moving? Watch for the rise of air bubbles. Imagine that each bubble carries a charge of -10 mC. Determine the average strength of the "current". What is the direction of the current?

2. Dip twenty small (smaller than a match head) pieces of plasticine of an irregular shape into the same bottle. Turn the bottle upside down sharply. Measure the time it takes for all the pieces to get to the cork. Determine the average amperage, assuming that each piece carries a charge of + 30 mC. How will the current strength change if instead of water there was jelly in the bottle?

*** For those interested

During experiments with an electric machine, physicists noticed that electricity was transferred from a rubbed glass circle to a conductor. Many times they tried to discharge the Leyden Jar through a long line of people holding hands. But no one expressed a clear thought about the possibility of a long flow of electricity through conductors. The concept of electric current was introduced into science later by the Italian physicist Volta.

The discovery of the current was preceded by the experiments of the Italian anatomist Luigi Galvani (1737-1798), who investigated the effect of an electric discharge on the muscles and nerves of a dead frog. Unloading the conductor electric machine through the nerve of a frog's leg, connected with a metal wire to the ground, he observed convulsive muscle contractions. There was nothing unexpected in this yet. Convulsive muscle contractions were also observed when discharging leiden jar"Through a chain of people holding hands. But chance allowed the experimenter to observe an amazing phenomenon.

A frog leg lay on the metal plastic. In order not to take the foot with his hands, Galvani hooked it with a copper hook. Accidentally touching the end of the hook to the iron plate, Galvani was surprised to see that the muscles of the frog's leg contracted as if from an electric discharge. Reflecting on the cause of this phenomenon, Galvani decided that "animal electricity" is contained in the muscles of the frog. Therefore, when the nerve is connected to the muscles by conductors (a copper hook and an iron plate), a discharge occurs.

Galvani's discovery interested the Italian physicist Volta, who began testing these experiments to see if "animal electricity" really existed. Putting a piece of metal foil to the tip of his tongue, and a silver coin to the top of his tongue and connecting them with a thin wire, he tasted sour. Volta suggested that the cause of the phenomenon observed by Galvani was the presence of two metals (a copper hook and iron). Guided by this thought, he set up many experiments and, finally, made an important discovery, as reported in 1800 to London Royal Society... Volta wrote that he had found a new source of electricity. The device charges by itself and discharges continuously. At the same time, he also gave a description of his device, arranged as follows.

Volta took several dozen zinc and copper mugs. He put the mugs in a column, alternating copper and zinc, and arranged them with cardboard mugs soaked in a solution of sodium chloride. When Volta touched the lower copper mug with one hand and the upper zinc mug with the other, he experienced a strong electric shock. At the same time, the device was not discharged, and no matter how many times the experimenter touched the circles, the impact was repeated, that is, the charge of electricity appeared continuously.

This was a new source of electricity - the "volt pole", which physicists immediately began to use.

The Russian physicist V.V. Petrov (1761-1834) built for his experiments a battery of 4200 copper and zinc circles, packed in four wooden boxes. For isolation, he covered the inner walls of the boxes with wax varnish. Having connected two carbon rods (electrodes) with a copper wire (to the upper and lower circles) of the battery poles and brought their ends closer, V.V.Petrov saw a bright arc appear between them. She lit up the laboratory, and when the physicist began to introduce pieces of metals into it, they melted very quickly. This was the so-called voltaic arc.

So a new phenomenon was discovered - electric current.

§55 Thermal effect of electric current

What distinguishes conductors from dielectrics is that charged particles can move directionally in them.

In metals, such particles are electrons, in conducting liquids (electrolytes) - ions, in plasma - ions and electrons.

In the absence of an electric field, all particles move chaotically. The average kinetic energy of all particles is the same.

When an electric field appears inside a conductor, charged particles begin to move along the lines of force.

Keeping chaotic thermal motion. The figure shows the rates of thermal motion of particles.

Positively charged particles move in the direction of the field strength, negatively - towards it, an electric current arises. Particles, moving, acquire additional energy, which is transferred to uncharged particles due to chaotic collisions. As a result, the kinetic energy of all particles increases, i.e. the temperature and internal energy of the body increases.

Consequently, the electric current in the substance causes it to heat up. This phenomenon is called the thermal effect of an electric current.

Thermal effect of electric current used in electric heaters.

In everyday life, many different electric heating devices are used. These include: an electric fireplace, which provides additional heat in the place of the room where you need it; electric kettles, coffee pots are used to heat water; food is prepared quickly on electric stoves; wet hair can be quickly dried with a stream of dry hot air created by an electric hairdryer; Housewives iron the washed linen with an electric iron. This listing can be continued. Let us dwell on individual devices in detail.

In modern apartments, electric stoves are installed in the kitchens. They replaced solid fuel stoves and gas stoves, since they are more environmentally friendly: there are no combustion products of solid fuel / ash, slag, smoke /, there is no environmental pollution. Electric stoves also have technical advantages: they are equipped with an automatic temperature control system, which allows them to automatically disconnect from the electrical network whole or part of the appliance / electric heating element of an oven or hotplate /. When the electric heater cools down, it is automatically connected to the network again.

Home construction electric stoves very diverse.

Figure 187 shows one of them. There are two tiles (burners) on the upper surface of the stove. The heating element of the tile, made of nichrome (nichrome is an alloy of two metals nickel and chromium) wire, is pressed into a heat-resistant ceramic base in the shape of a ring. (The choice of nichrome is determined by the fact that it has a high melting point and does not oxidize when high temperatures... In addition, the properties of nichrome are such that with a small current strength, a large amount of heat is released in it).

On the front wall of the stove there are special switches for regulating the degree of heating of the plates and the oven.

2. Thermal effect of current used not only in everyday life, but also in technology.

An example is contact electric welding. This type of electric welding is based on the use of heat released at the place of contact (contact) of two pieces of metal, at the place of their contact when an electric current passes through them.

The parts to be welded are fixed between the clamps, brought into contact and an electric current is passed through them.

At the point of contact, the most large quantity heat, as a result of which the metal becomes very hot. When it becomes plastic due to heating, the current is automatically turned off, and the machine compresses the softened parts of the parts so hard that they are firmly connected.

Contact electric welding is performed automatically by automatic machines.

3. In agriculture thermalcurrent action also found use, for example, for drying stacks of rain-soaked hay.

The jets of heated air from the fan and the heater are carried through the pipe from below to the very middle of the stack and quickly dries it out. On livestock farms, special devices are used in which electric heaters maintain the best temperature for newly born animals.

In incubators, hundreds and thousands of chicks are hatched from eggs. In these "electric hens" a certain temperature (about 38 ° C) is maintained with great precision, which is most favorable for the development of embryos in eggs. And a special mechanism turns the eggs over so that they evenly warm up from all sides.

Electric current in a substance causes it to heat up. This phenomenon is called the thermal effect of an electric current.

The more charge passes through the conductor, the more the conductor heats up, and the more its energy increases.

Exercise 55.

What is the main part of all electric heaters? What effect of current is used in them?

Why electric lamp incandescent can be used as an electric heater? How does an incandescent lamp work? What part of the incandescent lamp is the main part?

What kind of electric heaters do you use?

Why is nichrome wire used for heating elements of electric stoves?

The heating element of the electric stoves can be switched on for several degrees of heating. How is this achieved?

§56 Chemical action of current.

Under the influence of an electric field, not only a change in the internal energy of a conductor (its heating) is possible, but also other phenomena.

Let us put two carbon plates into the molten salt, one of which - the anode - is positively charged (it has a lack of electrons), and the other - the cathode - is negative (it has an excess of electrons). The anode and cathode are called electrodes.

The electric field between the plates will cause the positive sodium ions to move towards the negative plate - cathode, and negatively charged chlorine ions - to a positively charged plate - anode. Therefore, a positively charged ion moving under the action of an electric field to the cathode is called a cation, and a negatively charged ion moving to the anode is called an anion.

On the surface of the plates, sodium ions will capture an electron, since the negative plate has an excess of them, and turn into a neutral sodium atom. On the other plate, the chlorine ion will donate an electron, also turning into a neutral chlorine atom.

On the surface anodeand sodium will be released, cathode - chlorine. Under the action of an electric field, sodium chloride is converted into sodium and chlorine, i.e. a chemical reaction occurs.

More precise definition electrolysisand is given in chemistry.

The amount of substance released on the electrode, the greater, the more ions come to it. Each ion has a charge. This means that the more charge moves to the electron, the more matter is released on it.

A chemical reaction that occurs under the action of an electric field is accompanied by the directional movement of ions, i.e. electric shock. The concept is often used chemical action of current... In our opinion, it would be more correct to speak of the occurrence of a current at chemical reaction.

As a result of exposure to an electric field, chemical reactions are possible that are not associated with the receipt or release of electrons at the electrodes. So, when a spark occurs between the balls of an electrophore machine (in the lesson) or lightning (in nature), a large amount of new substances is formed: ozone, nitrogen compounds, etc.

The effect of an electric field on chemical reactions can be exploited.

For example uh electrolysis It is used to apply the thinnest layer of another metal on the metal surfaces of a product of any configuration: gold, silver, chromium, nickel, etc. The coatings are strong, uniform in thickness, and durable. They not only protect the metal from corrosion, but also give a beautiful decorative look to metal products. This industry is called electroplating.

Another industry using electrolysis is called g alvanoplasticoth. Method electroplating make metal copies from various objects. A wax print / matrix / is made from the embossed object. In order for it to conduct current, its surface is covered with a thin layer of graphite. This graphite cathode dipped in a bath with a copper sulfate solution ; anode serves as copper. When electrolysis anodedissolves, and on cathode copper is precipitated. Thus, an exact copper copy of the item is obtained.

Electrotype was invented in 1838 by the Russian scientist B.S. Jacobi. (1801 - 1874). Using r alvanoplastyand make cliches for printing, gramophone records, etc.

Electrolytesfrom a solution of copper sulfate (copper sulfate / СuSО 4 /) is accompanied by the transfer of copper from the positive electrode / anode/ on the negative electrode / cathode/, while on k atode pure copper is released. This is widely used for purification / refining / copper from impurities. On

depicts a copper purification plant. Refining with e electrolysis salt solutions are also used to obtain silver, gold, and other metals.

Electrolysis solutions are used in the chemical industry and to obtain pure substances. For example, through electrolysissolution of sodium chloride receive chlorine, hydrogen and sodium hydroxide.

Of great importance is e electrolysis melts. Sodium is extracted from the molten caustic soda. Beryllium, magnesium and other metals are obtained in the same way. To obtain aluminum from bauxite (a special kind of clay) By processing, aluminum oxide / alumina / is obtained, which, in a mixture with low-melting salts, is melted in an electric furnace, and then in the same furnace is subjected to electrolysis. In this case, aluminum is released at the cathode / at the bottom of the electric bath /.

A positively charged plate (it has a lack of electrons) is called an anode, and a negatively charged one (it has an excess of electrons) is called a cathode.

The anode and cathode are called electrodes.

The chemical reaction that takes place on the surface of an electrode is called electrolysis.

The amount of substance released on the electrode, the greater, the more ions come to it.

Each ion has a charge. This means that the more charge moves to the electron, the more matter is released on it.

Exercise # 56

What are mobile charged particles in solutions of salts, acids and alkalis?

What is the name of a positively charged electrode? Negatively charged?

The wind is blowing north. What is the direction of the wind?

Chlorine ions move from west to east. What is the direction of the current?

* What charge should be transferred to the electrode to release one mole of sodium on it. The charge of the sodium ion is equal in modulus to the electron charge q \u003d 1.6 10 -19 C. Avagadro number N A \u003d 6 10 -23 1 / mol. Is it realistic to accumulate such a charge on an insulated electrode?

Tasks

1. Take a battery KBS or "crown". Place the battery contacts on a fresh cut of potatoes. After a few minutes, examine the contact marks on the potatoes. Describe the observed picture.

Place the battery contacts on the tongue. You will taste a sour taste. A change in taste indicates the appearance of a new substance, i.e. chemical reaction. Take two different metal objects, such as aluminum foil and a steel key, or copper wire and a silver fork. The main thing is that the metals are different. Taste each item individually. Then connect the ends of these items and taste the loose ends at the same time. Tell us about your feelings.

*** For those interested

To maintain electric current in conductors, devices called current sources are used. Consider current sources in which the chemical energy required for their action is renewed through electrolysis. Such cells are called batteries / storage units /, and the process of accumulating energy in them through electrolysis is called battery charging. When batteries are charged, current is passed through them from some foreign source in the opposite direction to the direction of the current they give.

There are two types of batteries: acidic and alkaline. Acid batteries consist of plates immersed in a sulfuric acid solution

The negative plates are made of pure lead with a highly loosened surface; the positive plates are coated with lead peroxide. When the battery is discharged, both plates are gradually covered with lead sulfate. When batteries are charged, the difference in the composition of the positive and negative plates is restored by electrolysis. In the process of charging the battery, the hydrogen ions / H / move in the same direction in which the current flows, and the ions formed as a result of the decomposition of sulfuric acid / SO 4 / go in the opposite direction.

In alkaline batteries, plates and vessels are made of iron. Iron plates are immersed in a solution of caustic potassium / KOH / or caustic sodium / NaOH / in distilled water. Both solutions are alkaline, therefore batteries are called alkaline.

Alkaline batteries are easy to transport as they are shockproof. They are distinguished by their structural strength, do not emit harmful gases during operation and when charging, they are not afraid of overload and can remain in a semi-discharged or discharged state for a long time.

Compared to alkaline batteries, acid batteries have a higher operating voltage and a higher coefficient useful action. Internal resistance more alkaline batteries than

acidic.

Batteries are most commonly used in cars, airplanes, trains with electric lights, telephone exchanges, submarines, etc.