What is electric current, types and conditions of its existence

Home | Physics 11th grade | Conditions for the existence of electric current

The further development of the science of electricity is associated with the study of processes observed during the movement of charged particles. The first works in this direction are associated with the names of Italian scientists Luigi Galvani (1737-1798) and Alessandro Volta (1745-1827). Galvani discovered the so-called “animal electricity”, and Volta correctly interpreted his experiments and invented the first direct current source in the history of science. At the beginning of the 19th century. electricity and magnetism were considered as different physical phenomena, although the idea of ​​their interrelation was repeatedly expressed. In 1820, the Danish scientist Hans Oersted (1777-1851) discovered the effect of electric current on a magnetic needle. In the same year, the French physicist André Marie Ampère (1775–1836) experimentally discovered the magnetic interaction of conductors with currents. The results of the experiments of Oersted and Ampere clearly demonstrated the connection between electrical and magnetic phenomena.

Michael Faraday (1791–1867) made a fundamental contribution to the development of electrodynamics In 1831, he experimentally discovered the phenomenon of electromagnetic induction. In addition, Faraday proposed the concept of the field, discovered the laws of electrolysis, investigated the magnetic properties of matter, etc. Summarizing Faraday's experimental research on electromagnetic induction, the British physicist James Clerk Maxwell (1831-1879) created the theory of the electromagnetic field. Within its framework, a changing magnetic field generates an electric field, and a changing electric field, in turn, generates a magnetic field. These changing fields exist inseparably and represent a single electromagnetic field. Disturbances of the electromagnetic field (electromagnetic oscillations) propagate in space in the form of electromagnetic waves.

Continuation of the study of electrodynamics is associated with the consideration of the laws of direct current, the flow of electric current in various media, magnetic phenomena, the phenomenon of electromagnetic induction, electromagnetic oscillations and waves.

Electrical hazards and other hazardous properties of electricity and safety precautions

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

1.

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

2.

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

3.

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

4.

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

5.

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

How to protect yourself from electric shock

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

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

Conditions for the existence of direct electric current

Electricity. Ohm's law

If an insulated conductor is placed in an electric field, then the free charges q

a force will act in the conductor. As a result, a short-term movement of free charges occurs in the conductor. This process will end when the own electric field of the charges arising on the surface of the conductor completely compensates for the external field. The resulting electrostatic field inside the conductor will be zero (see § 1.5).

However, in conductors, under certain conditions, continuous ordered movement of free electric charge carriers can occur. This movement is called electric current . The direction of the electric current is taken to be the direction of movement of positive free charges. For an electric current to exist in a conductor, an electric field must be created in it.

A quantitative measure of electric current is the current strength I

–
scalar physical quantity equal to the ratio of the charge Δ
q transferred through the cross section of the conductor (Fig. 1.8.1) over the time interval Δ
t
to this time interval:

If the strength of the current and its direction do not change over time, then such a current is called constant .

In the International System of Units (SI) current is measured in amperes (A). The current unit of 1 A is established by the magnetic interaction of two parallel conductors with current (see § 1.16).

A direct electric current can only be created in a closed circuit in which free charge carriers circulate along closed trajectories. The electric field at different points of such a circuit is constant over time. Consequently, the electric field in a direct current circuit has the character of a frozen electrostatic field. But when an electric charge moves in an electrostatic field along a closed path, the work done by electric forces is zero (see § 1.4). Therefore, for the existence of direct current, it is necessary to have a device in the electrical circuit that is capable of creating and maintaining potential differences in sections of the circuit due to the work of forces of non-electrostatic origin . Such devices are called constant current sources . Forces of non-electrostatic origin acting on free charge carriers from current sources are called extraneous forces .

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

When electric charges move along a direct current circuit, external forces acting inside the current sources perform work.

Physical quantity equal to the work ratio A

st of external forces when moving a charge
q
from the negative pole of a current source to the positive pole to the value of this charge is called
the electromotive force of the source
(EMF):

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

When a single positive charge moves along a closed direct current circuit, the work done by external forces is equal to the sum of the emf acting in this circuit, and the work done by the electrostatic field is zero.

A DC circuit can be divided into separate sections. Those areas where no external forces act (i.e. areas that do not contain current sources) are called homogeneous . Areas that include current sources are called inhomogeneous .

When a single positive charge moves along a certain section of the circuit, work is performed by both electrostatic (Coulomb) and external forces. The work of electrostatic forces is equal to the potential difference Δφ12 = φ1 – φ2 between the initial (1) and final (2) points of the inhomogeneous section. The work of external forces is equal, by definition, to the electromotive force 12 acting in a given area. Therefore the total work is equal to

Electric current: conditions for the existence of electric current

What is electric current?

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

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

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

Potential difference (voltage). This is one of the mandatory conditions. There must be a potential difference between the 2 points, meaning that the repulsive force that is created by the charged particles at one place must be greater than their force at another point. Voltage sources, as a rule, do not occur in nature, and electrons are distributed fairly evenly in the environment. 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 that is necessary for the existence of 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 humanity.

Electric current and conditions of its existence.

17th century O. Roemer, using an astronomical method, obtained the speed of light approximately 214.3 km/s

19th century . Physical speed of light is approximately 313 thousand km/s

The nature of light is quantum.

approximately 500 BC Pythagoras: light is a stream of particles.

17th century Isaac Newton adhered to the same theory. Carpuscula (from Latin) – particle.

Newton's carpuscular theory: 1) rectilinear propagation of light 2) law of reflection 3) formation of shadows from objects

19. Heinrich Hertz discovered the phenomenon of the photoelectric effect.

20th century. Light has a duality

nature - has wave-particle
duality
: when propagating - like a wave, and when emitting and absorbing - like a stream of particles.

relationship between lambda wavelength and nu frequency

lambda = s/nu s - speed of light in vacuum [m/s] lambda [m] nu [Hz]

Laws of reflection

1. The incident ray, the reflecting ray and the perpendicular to the interface between the two media, reconstructed at the point of incidence of the ray, lie in the same plane.

2The angle of reflection γ is equal to the angle of incidence α: γ = α

Specular reflection - if the roughness is less than lambda and diffuse roughness is comparable to lambda

Diffuse reflection of light. Specular reflection of light.

Laws of light refraction.

The law of light refraction: the incident and refracted rays, as well as the perpendicular to the interface between two media, restored at the point of incidence of the ray, lie in the same plane. The ratio of the sine of the angle of incidence α to the sine of the angle of refraction γ is a constant value for two given media:

The constant value n is called the relative refractive index of the second medium relative to the first. The refractive index of a medium relative to vacuum is called the absolute refractive index.

The relative refractive index of two media is equal to the ratio of their absolute refractive indices:

n = n2/n1.

The physical meaning of the refractive index is the ratio of the speed of propagation of waves in the first medium υ1 to the speed of their propagation in the second medium υ2:

The absolute refractive index is equal to the ratio of the speed of light c

in vacuum to the speed of light υ in the medium:

Nature of light from 26.

Wave interference is the phenomenon of superposition of coherent waves; characteristic of waves of any nature (mechanical, electromagnetic, etc.)

Coherent waves are waves emitted by sources that have the same frequency and constant phase difference.

When coherent waves are superimposed at any point in space, the amplitude of oscillations (displacement) of this point will depend on the difference in distances from the sources to the point in question. This distance difference is called the stroke difference. When superposing coherent waves, two limiting cases are possible:

Maximum condition:

Where

The wave path difference is equal to an integer number of wavelengths (otherwise an even number of half-wavelengths).

In this case, the waves at the point under consideration arrive with the same phases and reinforce each other - the amplitude of the oscillations of this point is maximum and equal to double the amplitude.

Minimum condition:

, Where

The wave path difference is equal to an odd number of half-wave lengths.

The waves arrive at the point in question in antiphase and cancel each other out. The amplitude of oscillations of a given point is zero.

As a result of the superposition of coherent waves (wave interference), an interference pattern is formed.

With wave interference, the amplitude of the oscillations of each point does not change over time and remains constant.

When incoherent waves are superimposed, there is no interference pattern, because the amplitude of oscillations of each point changes over time.

Interference of light

1802 English physicist Thomas Young conducted an experiment in which the interference of light was observed.

Thomas Young's experience

Two beams of light were formed from one source through slit A (through slits B and C), then the light beams fell on screen E. Since the beams from slits B and C were coherent, an interference pattern could be observed on the screen: alternating light and dark stripes .

Light stripes – the waves reinforced each other (the maximum condition was met). Dark stripes – the waves were added in antiphase and canceled each other out (minimum condition).

If Young’s experiment used a source of monochromatic light (one wavelength), then only light and dark stripes of a given color were observed on the screen.

If the source produced white light (i.e., complex in its composition), then rainbow stripes were observed on the screen in the area of ​​​​light stripes. The iridescence was explained by the fact that the conditions of maximums and minimums depend on the wavelengths.

Interference in thin films

The phenomenon of interference can be observed, for example:

— rainbow stains on the surface of a liquid during an oil spill, kerosene, or in soap bubbles;

The thickness of the film must be greater than the wavelength of light.

During his experiment, Young was able to measure the wavelength of light for the first time.

As a result of the experiment, Jung proved that light has wave properties.

Application of interference: - interferometers - instruments for measuring the wavelength of light - clearing of optics (in optical instruments, when light passes through the lens, light loss is up to 50%) - all glass parts are covered with a thin film with a refractive index slightly lower than that of glass; interference maxima and minima are redistributed and light losses are reduced.

Nature of light from 26.

DIFFRACTION OF LIGHT

Diffraction is a phenomenon inherent in wave processes for any kind of waves.

Diffraction of light is the deflection of light rays from straight propagation when passing through narrow slits, small holes, or when bending around small obstacles.

The phenomenon of light diffraction proves that light has wave properties.

To observe diffraction you can:

- pass light from the source through a very small hole or place the screen at a large distance from the hole. Then a complex pattern of light and dark concentric rings is observed on the screen. - or direct the light onto a thin wire, then light and dark stripes will be observed on the screen, and in the case of white light, a rainbow stripe.

Diffraction grating

is an optical instrument for measuring the wavelength of light.

A diffraction grating is a collection of a large number of very narrow slits separated by opaque spaces.

If a monochromatic wave is incident on the grating. then the slits (secondary sources) create coherent waves. A collecting lens is placed behind the grille, followed by a screen. As a result of the interference of light from various slits of the grating, a system of maxima and minima is observed on the screen.

The path difference between the waves from the edges of adjacent slits is equal to the length of the segment AC. If this segment contains an integer number of wavelengths, then the waves from all slits will reinforce each other. When using white light, all maxima (except the central one) have a rainbow color.

So, the maximum condition is:

where k is the order (or number) of the diffraction spectrum

The more lines are applied to the grating, the farther the diffraction spectra are from each other and the smaller the width of each line on the screen, so the maxima are visible as separate lines, i.e. the resolving power of the grating increases.

The more lines there are per unit length of the grating, the greater the accuracy of wavelength measurement.

POLARIZATION OF LIGHT

Wave polarization

The property of transverse waves is polarization.

A polarized wave is a transverse wave in which all particles oscillate in the same plane. Polarization of light

The experiment with tourmaline is proof of the transverse nature of light waves.

A tourmaline crystal is a transparent, green mineral with an axis of symmetry.

In a beam of light from a conventional source, there are fluctuations in the vectors of electric field strength E and magnetic induction B in all possible directions perpendicular to the direction of propagation of the light wave. Such a wave is called a natural wave.

When light passes through a tourmaline crystal, it becomes polarized. In polarized light, oscillations of the intensity vector E occur only in one plane, which coincides with the symmetry axis of the crystal.

The polarization of light after passing through tourmaline is detected if a second tourmaline crystal (analyzer) is placed behind the first crystal (polarizer). If the axes of two crystals are identically directed, the light beam will pass through both and will only be slightly weakened due to partial absorption of light by the crystals.

Scheme of operation of the polarizer and the analyzer behind it:

If the second crystal begins to rotate, i.e. shift the position of the symmetry axis of the second crystal relative to the first, then the beam will gradually go out and go out completely when the position of the symmetry axes of both crystals becomes mutually perpendicular.

Application of polarized light:

— smooth adjustment of illumination using two polaroids — to suppress glare when photographing (glare is suppressed by placing a polaroid between the light source and the reflective surface)

— to eliminate the glare of the headlights of oncoming cars.

Polaroid, polarizing filter, one of the main types of optical linear polarizers; It is a thin polarizing film, sealed to protect against mechanical damage and moisture between two transparent plates (films).

DISPERSION

A ray of white light passing through a triangular prism is not only deflected, but also decomposed into component colored rays. This phenomenon was discovered by Isaac Newton through a series of experiments.

Newton's experiments

Experience in decomposing white light into a spectrum:

or

Newton directed a beam of sunlight through a small hole onto a glass prism. When hitting the prism, the beam was refracted and on the opposite wall gave an elongated image with a rainbow alternation of colors - a spectrum.

Experience in the synthesis (production) of white light:

First, Newton directed a ray of sunlight onto a prism. Then, having collected the colored rays emerging from the prism using a collecting lens, Newton received a white image of a hole on a white wall instead of a colored stripe.

Newton's conclusions:

- a prism does not change the light, but only decomposes it into its components - light rays that differ in color differ in the degree of refraction; Violet rays are refracted most strongly, red ones less strongly

- red light, which refracts less, has the highest speed, and violet light has the lowest, which is why the prism decomposes the light. The dependence of the refractive index of light on its color is called dispersion.

Remember the phrase, the initial letters of the words give the sequence of colors of the spectrum:

"Every Hunter Wants to Know Where the Pheasant Sits."

White light spectrum:

Conclusions:

- a prism decomposes light - white light is complex (composite) - violet rays are refracted more strongly than red ones.

The color of a light beam is determined by its vibration frequency.

When moving from one medium to another, the speed of light and wavelength change, but the frequency that determines the color remains constant.

The boundaries of the ranges of white light and its components are usually characterized by their wavelengths in vacuum. White light is a collection of waves with lengths from 380 to 760 nm.

Where can you observe the phenomenon of dispersion?

- when light passes through a prism - refraction of light in water drops, for example, on grass or in the atmosphere when a rainbow is formed - around lanterns in fog.

How to explain the color of any object?

- white paper reflects all rays of different colors falling on it - a red object reflects only rays of red color, and absorbs rays of other colors - The eye perceives rays of a certain wavelength reflected from an object and thus perceives the color of the object.

Spectral analysis is a set of methods for qualitative and quantitative determination of the composition of an object, based on the study of the spectra of interaction of matter with radiation, including the spectra of electromagnetic radiation, acoustic waves, mass and energy distributions of elementary particles, etc.

Electric current and conditions of its existence.

Electric current is the ordered, directed movement of free charges in a conductor.

Direct current is an electric current whose characteristics do not change over time.

Conditions for the existence of electric current For the emergence and maintenance of current in any medium, two conditions must be met: - the presence of free electric charges in the medium - the creation of an electric field in the medium. In different environments, the carriers of electric current are different charged particles.

Current strength I is a scalar quantity characterizing the charge Q passing through the cross section of the conductor per unit time. Q=q*NI=Q/t

Current is measured in amperes and charge in coulombs. I=[A], Q=[Cl]

Current density – j vector quantity j Vq, shows the current strength per unit Ssec.

j=I/Ssection Sectional area Ssection. measured in square meters

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Definition

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

• Electrons are negative charge carriers.
• Ions are positive charge carriers.

But where do charge carriers come from? To answer this question, you need to remember basic knowledge about the structure of matter. Everything that surrounds us is matter; it consists of molecules, its smallest particles. Molecules are made up of atoms. An atom consists of a nucleus around which electrons move in given orbits. Molecules also move randomly. The movement and structure of each of these particles depends on the substance itself and the influence of the environment on it, such as temperature, stress, and others.

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

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

The unit of current measurement is Ampere, according to Ohm's law it is calculated by the formula:

I=U/R,

where U is voltage, , and R is resistance, .

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

I=Q/t,

where Q – charge, , t – time, .

Electric charge in motion

Electricity

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

Electrons in a conducting material such as a metal are largely free to move from one atom to another along their conduction bands, which are the highest electron orbits. Sufficient electromotive force or voltage creates a charge imbalance that can cause electrons to flow 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 the valve at one end to allow water to flow into the pipe, we do not have to wait for that water to make its way all the way to the end. 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 when there is an electric current in a wire.

“Conditions for the existence of direct current” (grade 10)

Lesson No. Date:

Topic: Conditions for the existence of direct current. Electromotive force of a source of electrical energy.

The purpose of the lesson:

organizing student activities to generalize knowledge about direct current; about the electromotive force of the source of electrical energy; and improving methods of problem solving activities.

Lesson objectives:

Educational:

repeat, generalize and consolidate knowledge about direct current; about the electromotive force of the source of electrical energy; and improving methods of problem solving activities.

Developmental:

development of attention, logical thinking, intelligence, interest in physics.

Educational:

education of hard work; friendly attitude towards each other; culture of communication.

Lesson type:

generalization and systematization of knowledge and methods of activity.

Form of lesson organization:

verbal-visual, problem solving.

Didactic support of the lesson:

“Physics 10th grade”: N.Koyshybaev, B.Krongart, V.Kem, “Didactic materials”: ​​G.Z.Baizhasarova, U.K.Tokbergenova, A.A.Medetbekova, M.Zh.Zhubanov, “Collection tasks": S.T.Tuyakbaev, Sh.B.Tyntaeva, Zh.O.Bakynov.

Plan:

1.
Organizational moment:
greeting, preparing students for the lesson.

2.
Updating new knowledge: (
checking homework
)
frontal survey:

A) why does a charged capacitor have energy? What is it called? What formula can be used to calculate this energy?

B) what is the difference between a capacitor as a source of electrical energy and a conventional current source?

C) what is the volumetric energy density of the electric field? What formula is it calculated by?

D) what are the units of measurement for the volumetric energy density of the electric field?

3.
Generalization and systematization of knowledge and methods of activity
We have discussed in sufficient detail how a charged particle behaves in an electric field. Under the influence of electric field forces, a charged particle will move in a certain direction.

Let us consider the behavior of a large number of free charged particles in a uniform electric field. Under the influence of the forces of this field, they will begin to move in one direction, maintaining thermal motion, which is chaotic and depends on temperature. This direction, or also called the ordered movement of electrically charged particles, is called electric current.

The electric current that arises in conducting media as a result of the ordered movement of free charges under the influence of an electric field created in these media is called conduction current.

Examples of conduction current are: current in metals and semiconductors, associated with the ordered movement of free electrons; current in electrolytes, which is the ordered movement of ions of opposite signs.

Conditions necessary for the appearance and existence of electric current

1)

The presence
in a given medium of free current carriers - charged particles that could move in an orderly manner.
Such particles in metals are conduction electrons; in semiconductors – conduction electrons and holes; in liquid conductors (electrolytes) – positive and negative ions; in gases - positive and negative ions and electrons.

2) The existence in a given environment of an external electric field, the energy of which must be spent on the ordered movement of electric charges. To maintain an electric current, the energy of the electric field must be continuously replenished, i.e. is needed - a device in which any type of energy is converted into the energy of an electric field.

In order to fulfill these two conditions, an electrical circuit is assembled. The main components of an electrical circuit are : current source

– a device that converts any type of energy into electrical energy;
connecting wires;
elements that close and open the circuit; consumer current - devices that convert electrical energy into thermal, mechanical, etc.

The current, passing through the elements of the circuit, has a thermal, chemical and magnetic effect, and it always has a magnetic effect.

In order to evaluate the effect of current, a special physical quantity is quantitatively introduced - current strength. Current strength is a physical quantity that shows how much charge passes through the cross section of a conductor per unit time, i.e.

Electric current is called constant if the current strength does not change either in magnitude or direction. For DC

where is the electric charge transferred through the cross section of the conductor in time.

To fully characterize the electric current, another physical quantity is introduced: current density, which shows how much current passes in a unit section of the conductor:

The direction of electric current is the direction of the ordered movement of positive electrical charges.

However, in reality, in metal conductors, current is carried out by the ordered movement of electrons, which move in the direction opposite to the direction of the current.

In the previous chapter, we established that the work of an electric field to move a charge can be calculated using the formula , where is the amount of charge moved; - voltage (potential difference). From here we can define electric voltage: electric voltage is a physical quantity determined by the work of the electric field to move a unit electric charge, i.e.

At the same time, the work done by the electric field forces when moving a charge along a closed circuit is zero. This means that if in a closed circuit only electric forces act on the charges, then work cannot be obtained using current. Consequently, in the electrical circuit there must be at least one such section in which, in addition to the forces of the electric field, some other forces would act on the mobile charge carriers, capable of doing work to move these charges in the direction opposite to the action of the electric forces. Such forces are called third-party (consider Fig. 9.1).

A quantity characterizing the dependence of the electrical energy acquired by the charge in the source on the internal structure of the latter. It is called the electromotive force of the source (EMF) and is designated . The electromotive force of the source is characterized by the work of external forces performed when moving a single positive charge:

From the above it follows that external forces must be of a non-electric nature. In current sources, external forces can be forces arising from the energy of a chemical reaction (batteries, galvanic cells), or thermal (thermocouples),

or light
(solar batteries),
etc. (consider Fig. 9.2).

4.
Consolidation:
A) what is called electric current? What conditions must be met for electric current to flow in the circuit?

B) what is the direction of the electric current? what is an electrical circuit called? What are its components?

C) what effects does electric current have?

D) what physical quantity is called current strength? What physical quantity is called current density?

D) what forces are called external? What is a current source?

E) explain the principle of operation of the current source? What types of current sources do you know?

G) what do you mean by internal resistance of a current source?

5.
House. ass
§9.1 (1, 3, 5, 7) 9.2 (1, 3, 5, 7)

6.
Lesson summary
(grading)

7.
Reflection:
“Today in class I understood (a). . . Today in class I didn't understand. . ."

5

Necessary conditions for the existence of electric current in metals

Current formula

Metals are the most common substances used as conductors of electricity. A special feature of these materials is the crystal lattice inherent in their solid state. This structure consists of atoms with a certain number of valence electrons on the outer shell.

When a crystal lattice is formed, atoms with positive ions remain at its nodes, and some electrons lose their connection and begin to move chaotically inside the material. They will be the charge carriers that will allow the existence of electric current if the required conditions for its occurrence are created.

Thus, charge carriers are observed that move chaotically and unsystematically, which in itself cannot guarantee the occurrence of an electric current. The second condition for the existence of which is the presence of some additional force that can lead to the ordered movement of electrons. The appearance of such a force is caused by the electric field that arises when a conductor is connected to a source of electricity. As long as such a field exists, the condition for the existence of an electric current will be satisfied.

When the chain is broken, the electrons stop moving in an orderly manner, although their chaotic movement inside the metal will continue. But in this case, the electric field will be zero and the existence of current will become impossible in principle. It should be noted that in modulus the total charge of free electrons in this case is equal to the total charge of positive ions, but has the opposite sign. This explains the fact that a metal conductor, in the absence of a field, remains in an electrically neutral state.

If we slightly generalize and expand the above, then to the necessary conditions for the existence of electric current in metals, in addition to free electrons and a field, we can add the presence of a source of electricity and the requirement of continuity of connection in the circuit.

Lesson 4. Conditions for the existence of electric current

Home » Electronics for Beginners » Lesson 4. Conditions for the existence of electric current

And again, good day to you, dear ones. Without further preludes, let's begin our conversation today. It would seem that we have long ago figured out the reasons for the occurrence of current in a conductor. We placed a conductor in the field - electrons began to flow and a current arose. What else does. But it turns out that in order for this current to exist in the conductor constantly, certain conditions must be met. For a clearer understanding of the physics of the process of electric current flow in a conductor, let's consider an example. Let's assume that we have some conductor, which we will place in an electric field as shown in Figure 4.1.

Figure 4.1 – Conductor in an electric field

Conventionally, we denote the voltage value at the ends of the conductor as E1 and E2, with E1>E2. As we found out earlier, free electrons in the conductor will begin to move towards a higher field strength, that is, to point A. However, over time, the potential formed by the accumulation of electrons at point A will become such that the own electromagnetic field E0 created by it will be equal in magnitude to the external field , and the directions of the fields will be opposite, since the potential of point B is more positive (lack of electrons caused by the influence of an external field).

Since the resulting action of two identical opposite forces is zero: |E|+|(E0)|=0, the electrons stop their ordered motion and the electric current stops. In order for the flow of electrons to be continuous, it is necessary: ​​firstly, to apply an additional force of a non-potential nature, which would compensate for the influence of the conductor’s own electric field and, secondly, to create a closed loop, since the movement of electrons can only occur in conductors (previously we indicated that dielectrics, although they have some electrical conductivity, do not transmit electric current) and to ensure the constancy of the compensating force, the constancy of the fields is necessary: ​​both external and intrinsic.

Let's start with the second point. We will consider a conductor placed in a field, as shown in Figure 4.2. Let us assume that after the interaction of the external and internal electromagnetic fields has been compensated, we additionally apply another similar field to the external field. The total effect of the external field will be 2•|E|. The current in the conductor will continue to flow in the same direction, but exactly until the moment 2•|E|>|E0|, after which the electric current will stop again. That is, the external influence must increase continuously to ensure the flow of current in an open conductor, which is impossible. If you close the conductor so that one part of it lies outside the field, then due to the work of an additional force in addition to the external field (this force in this case should not be potential, since the work of a potential force in a closed loop is zero and does not depend on the shape of the trajectory), then an electric current will arise in the conductor due to the influence of only the external field, since the conductor field itself will be completely compensated. That is why any electrical circuit must always be closed.

You can try to explain the need to introduce additional force from the following consideration: if we could partially transfer charges from end B of the conductor to end A of the conductor, the electric current would also not stop. However, such a “landing” also requires energy. This means that the introduction of additional force is still necessary. Non-potential forces are also called third-party forces. And their sources are sources or generators of current.

Figure 4.2 – Emergence of its own electromagnetic field in a conductor

So where can we get additional force, which, moreover, should not be created by the field, because without it we will not get current? It turns out that during a chemical redox reaction, for example, the interaction of lead dioxide and dilute sulfuric acid, free electrons are released: In order to “attract” all the electrons released during the reaction to one point in space, a solution of sulfuric acid is placed in several lead grids called electrodes. One part of the electrodes is made of lead and is called the cathode, the other - the anode - is made of lead dioxide. The cathode is a source of free electrodes for the external circuit, and the anode is a receiver.

The given example corresponds to a device known to all motorists (and not only) - a lead-acid battery. Of course, the example given has little overlap with what happens inside the battery in reality, however, the essence of the occurrence of current reflects well. Thus, an electric field arises between the positive anode (few electrons) and the negative cathode (many electrons), which generates external forces and creates a current in the conductor. This force depends only on the course of the chemical reaction; it is practically constant until the elements of this reaction - acid and lead oxide - exist. Therefore, if we remove the electric field and connect a conductor to the anode and cathode, electric current will still flow due to the battery creating an external force. The conductor will have its own electric field around it, which the battery must overcome in order to transfer an electron from the cathode to the anode. This is the essence of outside force.

Now consider the situation with a battery and a conductor connected to it. The electric field does positive work to move a positive charge (we are talking about positive charges, since the direction of their movement corresponds to the direction of the current) in the direction of decreasing the field potential. The current source separates electrical charges - positive charges accumulate on one pole and negative charges on the other. The electric field strength in the source is directed from the positive pole to the negative pole, so the work of the electric field to move a positive charge will be positive when it moves from “plus” to “minus”. The work of external forces, on the contrary, is positive if positive charges move from the negative pole to the positive, that is, from “minus” to “plus.” This is the fundamental difference between the concepts of potential difference and EMF, which must always be remembered.

Figure 4.3 shows the direction of current flow I in the conductor connected to the battery - from the positive anode to the negative cathode, however, inside the battery, external chemical reaction forces produce “dropping” of electrons that came from the external circuit from the anode to the cathode and positive ions from the cathode to the anode, that is, they act against the direction of current movement and the direction of the field.

Figure 4.3 – Demonstration of third-party forces when an electric current occurs

From the above considerations, we can draw the following conclusion: the forces acting on the charge inside the current source are different from the forces acting inside the conductor. Accordingly, it is necessary to distinguish these forces from each other. To characterize external forces, the value of electromotive force (EMF) was introduced - the work done by external forces to move a single positive charge. It is denoted by the Latin letter ε (“epsilon”) and is measured in the same way as the potential difference - in volts. Since the potential difference and EMF are forces of different types, we can say that the EMF outside the source terminals is zero. Although in ordinary life these subtleties are neglected and they say: “The voltage on the battery is 1.5V,” although strictly speaking, the voltage on a section of the circuit is the total work of electrostatic and third-party forces to move a single positive charge. In the future, we will still encounter these concepts and they will be useful to us when calculating complex electrical circuits.

That's probably all, because the lesson turned out to be too loaded... But you need to be able to distinguish the concepts of voltage and EMF.

• For the existence of electric current, two conditions are necessary: ​​1) a closed electrical circuit; 2) the presence of a source of third-party non-potential forces.
• Electromotive force (EMF) is the work done by external forces to move a single positive charge.
• Sources of external forces in an electrical circuit are also called current sources.
• The positive terminal of the battery is called the anode, the negative terminal is called the cathode.

There will be no problems this time; it’s better to repeat this lesson again in order to understand the entire physics of current flow in a conductor. As always, you can leave any questions, suggestions and wishes in the comments below! See you again!

← Lesson 3: Ohm's Law | Contents | Lesson 5: Power Supplies →

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 rotating coils of wire that pass through magnetic fields as they rotate. As the coils rotate, they open and close relative to the magnetic field and create an electric current that changes direction with each turn. The current passes 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 a 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 intensity of current they can carry without overheating and losing energy, and the voltage is limited only by how well the lines are insulated from ground.

Electricity.

Electric current is the directed movement of charged particles. Charge carriers can be electrons, ions, protons and holes. For the occurrence and existence of an electric current, the presence of free charged particles and the presence of an electric field are necessary. Depending on the presence or absence of charged particles in substances, they can be conductors, semiconductors and dielectrics. Conventionally, the direction of current movement is considered to be the direction from the positively charged pole to the negative one. In practice, the direction of movement of infected particles depends on the sign of their charge: negatively charged electrons move from minus to plus, positively charged ions move from plus to minus.

A quantitative characteristic of electric current is current strength. Current strength is designated by the letter I and is measured in Amperes (A). A current of 1 A occurs when a charge of 1 K passes through the cross-section of a conductor in 1 second.

Let's go back to the example of water in a container. Let's take two tanks with the same water level, but different diameters of the outlet pipes.

Let's compare the nature of water flowing out of both tanks: the water level in the left tank decreases faster than in the right. That is, the intensity of water flow depends on the diameter of the pipe. Let's try to equalize the two flows: add water to the right tank, thus increasing the height of the liquid column. This will increase the pressure in the right tank and, accordingly, increase the intensity of water flow. It’s similar in electrical circuits: as the current voltage increases, its strength also increases. An analogue of the pipe diameter in a circuit is the electrical resistance of the conductor.

The examples given with water clearly demonstrate the relationship between electric current, voltage and resistance.

Electricity. Conditions for the existence of direct current. EMF

Electric current is the ordered (directed) movement of charged particles. Electric current arises from the ordered movement of free electrons or ions.

Convection current is the current of movement of a charged body. Conduction current is the current of movement of free carriers in a conductor under the influence of an electric field. The total charge transferred through any cross section of a conductor is zero, since charges of different signs move with the same average speed. Electric current has a certain direction. The direction of current is taken to be the direction of movement of positively charged particles.

Conditions for the existence of direct current:

1) Availability of free charge carriers

2) the direct current conduction circuit must be closed;

3) Presence of outside forces

Third-party forces are any forces acting on charges of a non-electrostatic nature.

The charge transferred per unit time serves as the main quantitative characteristic of current, called current strength. If a charge Δq is transferred through the cross section of a conductor during a time Δt, then the current strength is equal to: I=. The current strength is equal to the ratio of the charge Δq transferred through the cross section of the conductor during the time interval Δt to this time interval. If the current strength does not change over time, then the current is called constant . I=q0nVS

The current strength depends on:

1. charge carried by each particle (q0);

2. particle concentration (n);

3. speed of directional movement of particles (v);

4. cross-sectional area of ​​the conductor (S).

The electromotive force of a current source is the ratio of the work of external forces to the amount of positive charge transferred from the negative pole of the current source to the positive. . Thus, the EMF is determined by the work done by external forces when moving a single positive charge. Electromotive force, like potential difference, is measured in volts (V). When a single positive charge moves along a closed direct current circuit, the work done by external forces is equal to the sum of the emf acting in this circuit, and the work done by the electrostatic field is zero.

Electric current is the ordered (directed) movement of charged particles. Electric current arises from the ordered movement of free electrons or ions.

Convection current is the current of movement of a charged body. Conduction current is the current of movement of free carriers in a conductor under the influence of an electric field. The total charge transferred through any cross section of a conductor is zero, since charges of different signs move with the same average speed. Electric current has a certain direction. The direction of current is taken to be the direction of movement of positively charged particles.

Conditions for the existence of direct current:

1) Availability of free charge carriers

2) the direct current conduction circuit must be closed;

3) Presence of outside forces

Third-party forces are any forces acting on charges of a non-electrostatic nature.

The charge transferred per unit time serves as the main quantitative characteristic of current, called current strength. If a charge Δq is transferred through the cross section of a conductor during a time Δt, then the current strength is equal to: I=. The current strength is equal to the ratio of the charge Δq transferred through the cross section of the conductor during the time interval Δt to this time interval. If the current strength does not change over time, then the current is called constant . I=q0nVS

The current strength depends on:

1. charge carried by each particle (q0);

2. particle concentration (n);

3. speed of directional movement of particles (v);

4. cross-sectional area of ​​the conductor (S).

The electromotive force of a current source is the ratio of the work of external forces to the amount of positive charge transferred from the negative pole of the current source to the positive. . Thus, the EMF is determined by the work done by external forces when moving a single positive charge. Electromotive force, like potential difference, is measured in volts (V). When a single positive charge moves along a closed direct current circuit, the work done by external forces is equal to the sum of the emf acting in this circuit, and the work done by the electrostatic field is zero.

Conditions for the existence of electric current in liquids

In these substances the situation will be somewhat different from the above conditions. It is necessary to make a reservation that we will talk about so-called conductor liquids. of the second kind. These are substances that have ionic conductivity. These do not include metal melts, which are characterized by an electronic version.

Liquid conductors of the second type are solutions of salts, bases and acids. Please note that this list does not include water. The fact is that in their pure form, molecules in water have polarity, which is inherent in dielectrics. Thus, to create conditions for the existence of an electric current in a liquid, it is necessary to introduce a substance from the outside, which will provide free carriers for the movement of charge.

Let's look at a simple practical example. If you close an electrical circuit with a built-in light bulb through a container of distilled water, nothing will happen. The lamp will not light up. But it is enough to add a whisper of table salt (NaCl) to the liquid and you will be able to observe the operation of the light source as usual. This is explained as follows - when salt is added, water interacts with the NaCl molecule and separates it into pairs of oppositely charged ions.

Thus, one of the main conditions for the existence of electric current in liquids is created, i.e. the presence of free charge carriers is ensured. Chlorine ions have a negative charge, while sodium ions have a positive charge. It is these ions that will move between the electrodes under the influence of an electric field.

Let's summarize. For the occurrence and existence of electric current in metals and liquids, a prerequisite is the presence of free charge carriers. Electrons or ions – it doesn’t change the essence. The second point is that an electric field is needed, with the help of which a force is generated that ensures the movement of charge carriers between the cathode and the anode.

§ 1. Conditions for the existence of electric current. Electric current in conductors

Actions of electric current.

We do not observe the movement of charged particles in the conductor. However, the existence of an electric current can be judged by the various phenomena that it causes. Such phenomena are called the effects of electric current.

1. The conductor through which electric current flows heats up . This is the thermal effect of current. It is thanks to the thermal effect of current that the coils in an electric stove or iron heat up, and the tungsten filament in a light bulb becomes white hot.

2. Electric current can change the chemical composition of the conductor . This shows the chemical effect of current. For example, when a current passes through a solution of copper sulfate, copper is released from the solution, and when a current passes through acidified water, it decomposes into hydrogen and oxygen. Chemical action occurs only when current passes through solutions or melts of electrolytes.

3. Electric current has a magnetic effect . A magnetic needle located along a current-carrying conductor turns perpendicular to the conductor (Fig. 1.1).

Rice. 1.1

This phenomenon was discovered by Oersted in 1820. If an insulated wire is wound around an iron nail, it becomes a magnet and attracts iron filings (Fig. 1.2).

Rice. 1.2

The magnetic effect of the current, in contrast to the thermal and chemical effects, is the main one, since it always accompanies the current.

What is electric current?

Let us give a strict definition of what is called electric current.

The ordered (directed) movement of charged particles is called electric current.

Electric current exists only when electrical charges are transferred from one place to another. If charged particles undergo random thermal motion, such as free electrons in a piece of metal, then charge transfer does not occur (Fig. 1.3, a).

Rice. 1.3

An electric charge moves through the cross section of a conductor in a certain direction if, along with the random movement, electrons participate in the ordered movement of charged particles (Fig. 1.3, b). In this case, an electric current is established in the conductor.

An electric current occurs during the ordered movement of free electrons in a metal, positive and negative ions in aqueous solutions and melts of electrolytes (salts, acids, alkalis), ions and electrons in gases, during the fall of charged raindrops, during the movement of a charged ebonite rod, etc. .

Electric current has a certain direction. The direction of current is taken to be the direction of movement of positively charged particles. Therefore, if the current is formed by the movement of negatively charged particles, then the direction of the current is considered opposite to the direction of movement of the particles.

Current strength.

Electric current in a conductor is characterized by a physical quantity - current strength.

Current strength is a scalar physical quantity equal to the ratio of the charge Δq transferred through the cross section of the conductor over a period of time Δt to the value of this gap.

Formula (1) expresses the average current value Δt If at any equal intervals of time equal charges pass through any cross-section of a conductor, that is, if the strength of the current and its direction do not change over time, then the electric current is called constant. The strength of direct current is numerically equal to the charge passing through the cross section of the conductor in 1 s:

It is convenient to sometimes consider the current strength as a positive or negative value, depending on the choice of the positive direction along the conductor. If the direction of the current coincides with the conditionally chosen positive direction, then I > 0 , otherwise I < 0 . Often, current strength is understood as its absolute value, additionally indicating the direction of the current.

The SI unit of current, the ampere ( A ), is the base unit. It is installed on the basis of the magnetic interaction of two conductors with currents. According to formula (1) we can write: 1A = 1 C/1 s .

Conditions for the occurrence and existence of electric current.

Let us consider the conditions that are necessary for the occurrence and existence of electric current.

1. The presence of free charged particles (charge carriers). Such charge carriers in metals and semiconductors are electrons, in electrolyte solutions - positive and negative ions, in gases - electrons and ions.

2. The presence of a force acting on charged particles (charge carriers) in a certain direction. Charged particles, as we know, are acted upon by an electric field with force = q . Usually it is the electric field inside the conductor that causes and maintains the ordered movement of charged particles.

Only in the static case, when the charges are at rest, is there no electric field inside the conductor.

If there is an electric field inside a conductor, then there is a potential difference (voltage) between the ends of the conductor. If it does not change over time, then a constant current is established in the conductor.

In order for the current to exist continuously in the conductor AB (Fig. 1.4), it is necessary to maintain different potentials at its ends.

Rice. 1.4

This can be done in different ways. For example, one could continuously charge body A and discharge body B. You can charge body A from an electrophore machine, and ground B But it is possible to maintain a continuous current in a conductor by transferring charges back from body B to body A through another conductor, forming a closed circuit for this (Fig. 1.4, b).

, such charge transfer is impossible, since the potential of body B is less than the potential of body A. The transfer of charges from body B to body A can be accomplished only with the help of forces of non-electric origin - external forces.

Any forces acting on electrically charged particles, with the exception of electrostatic (Coulomb) forces, are called extraneous forces.

The presence of such forces is provided by a current source included in the electrical circuit. The forces acting in the current source transfer charge from a body with a lower potential to a body with a higher potential, i.e. the current source has energy. Current sources are electrical machines, galvanic cells, batteries, generators, etc. A number of interconnected conductors, together with the current source, form a closed electrical circuit.

Figure 1.4, c shows a diagram of the electrical circuit in which the current source is located. Terminals A and B of the source have excess charges - positive and negative . In the outer section of the circuit, positive charges move under the influence of electric field forces from points with high potential to points with lower potential. In the internal section of the circuit BA, the transfer of charges from B to A is carried out by external forces acting in the current source.

How does an electric field arise inside a conductor in the presence of a current source? When a conductor is connected to the terminals of a source, the free charges of the conductor located near the terminals are displaced and act with their electric field on neighboring charges. At a speed close to the speed of light, this interaction is transmitted throughout the circuit, as a result of which charges appear along the surface of the conductor, creating an electric field inside it, ensuring the existence of direct current. This field is potential, just like an electrostatic field.

The speed of ordered movement of electrons in a metal conductor.

Let us consider how the current strength in a homogeneous conductor is related to the quantities characterizing the movement of charged particles. In a medium in which electric current exists, let us select a very small volume in the form of a straight cylinder with a cross-sectional area S (Fig. 1.5).

Rice. 1.5

The cylinder is oriented so that its bases are perpendicular to the speed of ordered particle motion. By the speed of ordered movement of particles 1 in a small volume Δ V (but containing many particles), we understand the ratio of the geometric sum of particle velocities to the number of them in this volume:

1 This speed is also called particle drift speed.

The average speed of chaotically moving particles is zero.

Let the height of the cylinder be equal to the path υ Δ t traversed by the particles during the time Δ t . Here υ is the modulus of the speed of ordered particle motion. Then all charged particles located inside the cylinder will cross the cross section of the cylinder with area B Δt . If the concentration of charged particles in the medium is n , then during the time Δ t through a section with area S where q 0 is the charge of an individual particle.

Using formula (1), we find the current strength in the conductor:

Thus, the current strength in a conductor is directly proportional to the modulus of the charge carried by each particle, the concentration of particles, the modulus of the speed of their ordered movement and the cross-sectional area of ​​the conductor.

From formula (2) it follows that the speed of ordered movement of particles in a conductor is equal to

For a metal conductor, the charge q 0 carried by each particle is the charge of the electron: q 0 = e . Hence,

The speed of ordered movement of electrons in a conductor is quite low. Calculations show that in a copper conductor with a cross-sectional area of ​​1 mm2, with a current strength of 10A, this speed is approximately 7 ∙ 10 -4 m/s. It is hundreds of millions of times less than the average speed of their thermal motion.

Questions:

1. Give examples of the effects of electric current.

2. What is electric current?

3. What is called current strength?

4. What conditions are necessary for the occurrence and existence of electric current?

5. What physical quantities does the speed of ordered motion of electrons in a metal conductor depend on?

Issues for discussion:

1. Electrons in metals move under the influence of an electric field, the intensity of which is equal to . In this case, it acts on electrons with a force = q . Why don't electrons move uniformly accelerated?

2. Electric current flows in a conductor of variable cross-section ( S1 > S2 ). The current strength is I. Is the electric field strength the same in sections of the conductor 1 - 2 and 2 - 3 (Fig. 1.6)?

Rice. 1.6

Is the current strength the same in these areas?

3. Why, when stepping on a tram rail along which current flows, do we not expose ourselves to the danger of electric shock?

Example of problem solution

The current strength in a homogeneous metal conductor varies according to the law I = kt , where the proportionality coefficient k = 10 A/s. Determine the modulus of the charge passing through the cross section of the conductor in the time interval from 2 to 5 s.

The area of ​​the figure under the graph (in this case, a trapezoid) is numerically equal to the modulus of the charge passing through the cross section of the conductor.

Substituting numerical data, we get

Answer: q = 105 Cl.

Direct and alternating current

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

Generators used in power plants to produce electricity are designed to produce alternating current

You've probably never noticed that the lights in your home actually flicker because the current direction changes, but it happens too quickly for your eyes to detect.

What are the conditions for the existence of direct electric current? Why do we need both types and which one is better? These are good questions. The fact that we still use both types of current suggests that they both serve specific purposes. Back in the 19th century, it was clear that efficient transmission of power over long distances between a power plant and a home was only possible 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 tension outside the home before sending it inside. To this day, direct electric current is used for long distance transmission, mainly due to its ability to be easily converted into other voltages.

Direct and alternating current

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

Generators used in power plants to produce electricity are designed to produce alternating current

You've probably never noticed that the lights in your home actually flicker because the current direction changes, but it happens too quickly for your eyes to detect.

What are the conditions for the existence of direct electric current? Why do we need both types and which one is better? These are good questions. The fact that we still use both types of current suggests that they both serve specific purposes. Back in the 19th century, it was clear that efficient transmission of power over long distances between a power plant and a home was only possible 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 tension outside the home before sending it inside. To this day, direct electric current is used for long distance transmission, mainly due to its ability to be easily converted into other voltages.

Transcript

1 I option Test on the topic “Direct electric current”. 1. For a current to arise in a conductor, it is necessary that... A - a force acts on its free charges in a certain direction. B - forces acted on its free charges. B - a constant force acted on its free charges. 2. A force acts on charges at each point of a conductor if it... A - there are electric dipoles. B - there is an electric field. 3. A circuit is assembled from a current source, an ammeter and a lamp. Will the ammeter reading change if another lamp of the same type is connected in series? A - It will not change, since with a series connection the current strength in all parts of the circuit is the same. B - It will increase because the circuit resistance has decreased. B - Will decrease because the circuit resistance has decreased. D - Will decrease as the circuit resistance has increased. 4. In a circuit of a current source, an ammeter and a lamp, another one with the same resistance is connected parallel to the lamp. Will this change the ammeter reading? A - Will double in size. B - The reading will not change. B - Will decrease by half. 5. What is the total resistance of the electrical circuit if the resistance of each resistor is 4 ohms? A - 10 Ohm, B - 16 Ohm,

2 V - 12 Ohm, G Ohm 6. What letter indicates current strength and in what units is it measured? A - I; volt (V). B - U; ampere (A). B - I; ampere (A). 7. What device can measure the current strength of a section of an electrical circuit and how is this device connected to the electrical circuit? A - Ammeter, in series. B - Ammeter, parallel. B - Voltmeter, in series. 8. Under the influence of what forces do electric charges move inside a current source? A - Under the influence of electrical forces. B - Under the influence of non-electric forces. 9. Express the value of the current 2 ka in microamperes A 0, ka B mka V mka 10. Conducting materials are used for the manufacture of A - housings of household appliances B - wires C - armatures of electrical machines D - contact clamps

3 Option II Test on the topic “Direct electric current”. 1. What letter denotes potential difference (voltage) and in what units is it measured? A - U; volt (V). B - I; volt (V). B - R; om (Ohm). 2. What device can be used to measure the potential difference in an electrical circuit and how is this device connected to the electrical circuit? A - Ammeter, in series. B - Voltmeter, in parallel. B - Voltmeter, in series. 3. As the temperature of a metal conductor increases, its resistance to electric current... A increases. B - decreases. B - does not change. 4. Under the influence of what forces do electric charges move in an external electrical circuit? A - Under the influence of external forces. B - Under the influence of magnetic forces. B - Under the influence of electric field forces.

4 5. How to practically determine the EMF of a current source? A - Using a voltmeter connected to the poles of the current source with the external circuit open. B - Using a voltmeter connected in parallel with a resistor in the external circuit. B - Using a voltmeter and an ammeter connected to a resistor in an external circuit. 6. Select the definition of direct current: A is a current that does not change in magnitude over time B is a current that always flows in an electrical circuit C is a current that does not change in magnitude or direction over time 7. Select the definition of parallel connection resistors: A is a connection in which the same voltage is applied to all resistors B is a connection in which the same voltage is applied to all resistors C is a connection in which the resistors are connected on top of each other 8. What is equal to the current flowing through resistor R1, if its resistance is R1 = 100 Ohms, the resistance of resistor R2 = 500 Ohms, if the current flowing through resistor R2 is I2 = 0.1 A. A 0.5 A B- 0, 1 A V 0.02 A 9. Two series-connected resistors were connected to a network with a voltage of U = 24 V. In this case, the current became equal to I 1 = 0.6 A.

5 When the resistors were connected in parallel, the total current became equal to I 2 = 3.2 A. Determine the resistance of the resistors. 10. If the network voltage is 220 V, the lamp resistance is 20 ohms, then the current in the circuit is ... A A B - 11 A B A

Abstract on the topic “Conditions for the occurrence of electric current”

No. 40. Conditions necessary for the occurrence and maintenance of electric current. Current strength and current density.

We know about charge carriers, so we define electric current as the movement of charged particles.

You remember from the molecular kinetic theory that the particles that make up matter, including electrons, are constantly in thermal chaotic motion, but this is not an electric current, just as the thermal movement of water molecules does not create a flow. All directions of such movement are equally probable, and the total movement is equal to zero (Fig. 107 a). Flow occurs when movement is directed. In this case, the chaotic movement does not stop, but it adds up to the directed one, and the total movement is no longer equal to zero, the system of particles as a whole moves. An electric charge moves through the cross section of a conductor only if, along with random movement, electrons participate in ordered movement (Fig. 106, b). In this case, they say that an electric current is established in the conductor.

Electric current is the ordered movement of charged particles.

It is impossible to see, for example, electrons in a conductor.

How can you detect electric current? Current is detected by the action it produces:

a) thermal (the conductor through which current flows heats up).

b) magnetic (the current exerts a force on neighboring currents and magnetized bodies);

c) chemical (electric current can change the chemical composition of the conductor);

d) physiological (current, when passing through a living organism, causes muscle contraction).

Page 133

Page 134 current strength

I =

Δt =

I = = = qnvS

For the emergence and existence of a constant electric current in a substance, it is necessary, firstly, the presence of free charged particles.

If positive and negative charges are bonded to each other in atoms or molecules, their movement will not produce an electric current.
But the presence of free charges is not yet sufficient for the generation of current. To create and maintain the ordered movement of charged particles, a force is required that acts on them in a certain direction.
If this force ceases to act, then the ordered movement of charged particles will cease due to the electrical resistance exerted to their movement by ions of the crystal lattice of metals or neutral molecules of electrolytes. Charged particles, as we know, are acted upon by an electric field with a force

Typically, it is the electric field inside the conductor that causes and maintains the ordered movement of charged particles.

Only in the static case, when the charges are at rest, the electric field inside the conductor is zero.
If there is an electric field inside the conductor, then between the ends of the conductor, in accordance with the formula ( E = ), there is a potential difference.
When the potential difference does not change over time, a constant electric current is established in the conductor. Along the conductor, the potential decreases from the maximum value at one end of the conductor to the minimum at the other, since the positive charge, under the influence of field forces, moves in the direction of decreasing potential. Electric current, as is known, can be obtained only in a substance in which there are free charged particles. In order for these particles to come into orderly motion, an electric field must be created in the conductor.

Page 136

Homework

§ 45-46, questions.

Tasks:

1. Find the speed of ordered movement of electrons in a wire with a cross section of 5 mm2 at a current strength of 10A, if the concentration of conduction electrons is 5 .10 28 m 3

Given:

S=5mm2 =5·10 -6m2 Solution: I=q0nSv; v= I/q0nS; v=2.5 .10 -4m/s

I=2A

n= 5 .1028 m 3

q0= 1.6. 10 -19 classes

v=?

Answer: v=0.5. 10 -4m/s

2. How many electrons pass through the cross section of a conductor in 1 s at a current of 32 μA.

Given:

q0= 1.6.10 -19 C

t=1s

I=32μA=32. 10 -6 A

n=?

Solution:

I = q
/ t . Let's express the charge: q = I * t .
Charge = charge of one electron (
e ) multiplied by their number ( N ) : q = e * N. e * N = I * t .
N = I * t
/ e . ( e =1.6*10^(-19) C) (convert current and time values ​​to the SI system). N =2*10^5.
(200,000 electrons). Answer: N=2. 10 14

The concept of electric current

Like a river flow, a flow of water molecules, an electric current is a flow 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 "flowing", what do you think of? Perhaps it will be a river. This is a good association because it is for this reason that electric current gets its name. It is very similar to the flow of water, but instead of water molecules moving along a channel, charged particles move along a conductor.

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

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.

Physics - answers to exam 1-29 / Electric current, conditions of its existence

Electricity

- directionally ordered movement of electric charges.
The direction of current is taken to be the direction of movement of positive
charges.

The passage of current through a conductor is accompanied by the following actions:

CONDITIONS FOR THE EXISTENCE OF ELECTRIC CURRENT

For the occurrence and maintenance of current in any environment, two conditions must be met:

In different environments, the carriers of electric current are different charged particles.

To maintain current in an electrical circuit

In addition to Coulomb forces, charges must be acted upon by forces
of a non-electrical
nature (external forces).

A device that creates external forces, maintains a potential difference in a circuit and converts various types of energy into electrical energy is called a current source.

For the existence of electric current in a closed circuit, it is necessary to include a current source in it

Main characteristics

1.

Current strength - I, unit of measurement - 1 A (Ampere).

Current strength is a quantity equal to the charge flowing through the cross section of a conductor per unit time.

I = q/t.(1)

Formula (1) is valid for direct current,

in which the current strength and its direction do not change over time.
If the strength of the current and its direction change over time, then such a current is called alternating.
For AC:

I = lim q/t , (*) t — 0

those. I = q', where q' is the time derivative of the charge.

2.

Current density is j, unit of measurement is 1 A/m2.

Current density is a value equal to the strength of the current flowing through a unit cross-section of a conductor:

j = I/S.(2)

3.

Electromotive force of the current source - emf. (  ), unit of measurement is 1 V (Volt). Emf is a physical quantity equal to the work done by external forces when moving a single positive charge along an electrical circuit:

 = Ast./q .(3)

4.

Conductor resistance is R, unit of measurement is 1 Ohm.

Under the influence of an electric field in a vacuum, free charges would move accelerated. In matter they move uniformly on average, because part of the energy is given to particles of matter during collisions.

The theory states that the energy of the ordered movement of charges is dissipated by distortions of the crystal lattice. Based on the nature of electrical resistance, it follows that

R = *l/S , (4)

Where

l is the length of the conductor,
S is the cross-sectional area,  is a proportionality coefficient called the resistivity of the material.
5.

Voltage - U, unit of measurement - 1 V. Voltage is a physical quantity equal to the work done by external and electrical forces when moving a single positive charge.

U = (Ast.+ Ael.)/q.(8)

Since Ast./q = , and Ael./q = , then

U =  + (



The concept of electric current

Like a river flow, a flow of water molecules, an electric current is a flow 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 "flowing", what do you think of? Perhaps it will be a river. This is a good association because it is for this reason that electric current gets its name. It is very similar to the flow of water, but instead of water molecules moving along a channel, charged particles move along a conductor.

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

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.

Electricity. Conditions necessary for the occurrence and existence of electric current.

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If an electric field is applied to an insulated conductor, then a force =q will act on the free charges q in the conductor. As a result, an ordered movement of free charges occurs in the conductor, and an electric current arises.

The continuous ordered movement of free electric charge carriers is called electric current. The direction of the electric current is taken to be the direction of movement of positive free charges.

Conditions necessary for the existence of electric current:

—the presence of free charged particles;

— presence of an electric field;

– closed circuit.

The action of current accompanying its flow:

1) Thermal. The conductor through which current flows heats up. Thermal effects are almost always present. An exception is the phenomenon of superconductivity; the thermal effect of current does not appear when current flows in a vacuum.

2) Chemical . Electric current changes the chemical composition of the conductor. Observed when current flows in electrolytes.

3) Magnetic. The current exerts a force on neighboring currents and on magnetic bodies. Magnetic influence on neighboring points and on magnetic bodies. The magnetic effect, in contrast to the chemical and thermal phenomena, is the main one, since it manifests itself in all conductors without exception (it is always observed).

4)

Electric current in conductors (metals) is always due to the presence of free electrons.

Positively charged metal ions form a crystal lattice. A “free electron gas” is formed by one or more electrons given up by each atom. Free electrons are able to wander throughout the entire volume of the crystal.

Current strength is a scalar physical quantity that is numerically equal to the electric charge passing through the cross section of a conductor per unit time:

I=.

If the magnitude of the current and its direction do not change over time, then the current is called constant and I=const=.

The unit of current is 1 Ampere. The ampere in the SI system is the basic unit and is determined from the magnetic interaction of two parallel straight conductors carrying a current of 1 A in one direction, located at a distance of 1 m from each other in a vacuum, causing an interaction force between these conductors equal to 2 * 10 -7 N per meter of length.

The current strength depends on the charge of the particle e, the concentration n, the speed of the particles v and the cross-sectional area of ​​the conductor S:

I===, where q=eN; n-particle concentration; V=vtS contains N=nV particles.

Current density is a vector quantity that is numerically equal to the current per unit area oriented perpendicular to the current: .

Vector j is directed along the current along the vector of the electric field strength in the conductor. In the SI system, current density is measured in A/m2. For DC

Ticket 25.1

The first law of thermodynamics , one of the three fundamental laws of thermodynamics, is the law of conservation of energy for thermodynamic systems.

The first law of thermodynamics was formulated in the middle of the 19th century as a result of the work of the German scientist J. R. Mayer, the English physicist J. P. Joule and the German physicist G. Helmholtz [1]. According to the first law of thermodynamics, a thermodynamic system can perform work only due to its internal energy or any external energy sources. The first law of thermodynamics is often formulated as the impossibility of the existence of a perpetual motion machine of the first kind, which would do work without drawing energy from any source.

In thermodynamics, the concepts of molar heat capacity at constant volume CV and molar heat capacity at constant pressure Cp are widely used. In an ideal gas they satisfy the Mayer equation:

The heat capacity of one mole of a monatomic ideal gas at constant volume is equal to , for a diatomic one – , and for a polyatomic one – 3R.

The internal energy of an ideal gas is directly proportional to its absolute temperature:

The work ΔA performed by the gas is determined by the gas pressure and the change in its volume:

The first law of thermodynamics. The amount of heat received by the system goes to change its internal energy and to perform work on external bodies:

In an isochoric process, the gas does no work, and ΔU = Q. In an isobaric process, A = pΔV = p (V2 – V1). In an isothermal process, ΔU = 0, and A = Q; all the heat transferred to the body goes to work on external bodies. Graphically, work is equal to the area under the process curve on the p, V plane.

Adiabatic is a quasi-static process in which no heat is transferred to the system from the environment: Q = 0. In an adiabatic process, all the work is done due to the internal energy of the gas.

Heat capacity of a body (usually denoted by the Latin letter C

) is a physical quantity that determines the ratio of the infinitesimal amount of heat δ
Q
received by the body to the corresponding increment in its temperature δ
T
:

The SI unit of heat capacity is J/K.

Specific heat

Specific heat capacity

is called the heat capacity per unit amount of a substance. The amount of a substance can be measured in kilograms, cubic meters and moles. Depending on which quantitative unit the heat capacity belongs to, mass, volumetric and molar heat capacity are distinguished.

Mass heat capacity ( C

) is the amount of heat that must be supplied to a unit mass of a substance in order to heat it by a unit temperature. In SI it is measured in joules per kilogram per kelvin (J kg−1 K−1).

Volumetric heat capacity ( C′

) is the amount of heat that must be supplied to a unit volume of a substance in order to heat it per unit temperature. In SI it is measured in joules per cubic meter per kelvin (J m−3 K−1).

Molar heat capacity ( C

μ) is the amount of heat that must be supplied to 1 mole of a substance to heat it per unit temperature. In SI it is measured in joules per mole per kelvin (J/(mol K)).

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