All bodies of the world around us consist of two types of stable particles - positively charged protons and electrons with the same negative charge e. The number of electrons is equal to the number of protons. Therefore, the universe is electrically neutral.

Since the electron and proton never ( at least for the last 14 billion years) do not decay, then the Universe cannot violate its neutrality by any human influences. All bodies are also usually electrically neutral, that is, they contain the same number of electrons and protons.

In order to make a body charged, it is necessary to remove from it, transferring it to another body, or add to it, taking from another body, a certain number N of electrons or protons. The charge of the body will become equal to Ne. At the same time, it is necessary to remember what is usually forgotten) that the same charge of the opposite sign (Ne) is inevitably formed on another body (or bodies). By rubbing an ebonite rod with wool, we charge not only ebonite, but also wool, transferring part of the electrons from one to another.

The statement about the attraction of two bodies with the same opposite charges according to the principles of verification and falsification is scientific, since it can in principle be confirmed or refuted experimentally. Here the experiment can be carried out purely, without involving third bodies, by simply transferring part of the electrons or protons from one experimental body to another.

There is a completely different picture with the statement about the repulsion of like charges. The fact is that only two, for example, positive, charge q1, q2 for the experiment cannot be created, since when trying to create them, it is always inevitable a third appears, negative charge q3 = -(qi + q2). Therefore, not two, and three charges. In principle, it is impossible to conduct an experiment with two similar charges.

Therefore, Coulomb's statement about the repulsion of like charges according to the mentioned principles is unscientific.

For the same reason, the experiment with two charges of different signs q1, - q2 is also impossible, if these charges are not equal to each other. Here, the third charge q3 = q1 - q2 inevitably appears, which participates in the interaction and affects the resulting force.

The presence of the third charge is forgotten and not taken into account by the blind supporters of Coulomb. Two bodies with identical charges different signs can be created by breaking atoms into two charged parts and transferring these parts from one body to another. With such a gap, it is necessary to do work and expend energy. Naturally, the charged parts will tend to return to their original state with less energy and combine, that is, they must be attracted to each other.

From the point of view of short-range interaction, any interaction assumes the existence of an exchange between interacting bodies with something material, and instantaneous action at a distance and telekinesis are impossible. Electrostatic interactions between charges are carried out by a constant electric field. We do not know what it is, but we can say with confidence that the field is material, since it has energy, mass, momentum and a finite propagation velocity.

taken for picture electric field lines of force come out of one charge (positive) and cannot break off in a void, but always enter another (negative) charge. They are like tentacles stretching from one charge to another, connecting them. To reduce the energy of the system of charges, the volume occupied by the field tends to a minimum. Therefore, the outstretched "tentacles" of the electric field always tend to contract like elastic bands stretched during charging. It is due to this contraction that the attraction of opposite charges is carried out. The force of attraction can be measured experimentally. She gives Coulomb's law.

It is a completely different matter in the case of similar charges. The total electric field of two charges comes out of each of them and goes to infinity, and the contact of the fields of one and the other charges is not achieved. Elastic "tentacles" of one charge do not reach another. Therefore, there is no direct material effect of one charge on another, they have nothing to interact with. Since we do not recognize telekinesis, therefore, there can be no repulsion.

But how, then, to explain the divergence of the petals of the eleroscope and the repulsion of charges observed in Coulomb's experiments? Let us recall that when we create two positive charges for our experience, we inevitably form a negative charge in the surrounding space as well.

Here attraction to him is mistaken and is taken for repulsion.

Definition 1

Many of those around us physical phenomena occurring in nature, do not find explanation in the laws of mechanics, thermodynamics and molecular-kinetic theory. Such phenomena are based on the influence of forces acting between bodies at a distance and independent of the masses of interacting bodies, which immediately denies their possible gravitational nature. These forces are called electromagnetic.

Even the ancient Greeks had some idea of ​​electromagnetic forces. However, it was only at the end of the 18th century that a systematic, quantitative study physical phenomena associated with the electromagnetic interaction of bodies.

Definition 2

Thanks to the painstaking work of a large number of scientists in the 19th century, the creation of an absolutely new harmonious science, which studies magnetic and electrical phenomena, was completed. So one of the most important branches of physics was called electrodynamics.

Created by electric charges and currents, electrical and magnetic fields became her main subjects of study.

The concept of charge in electrodynamics plays the same role as the gravitational mass in Newtonian mechanics. It is included in the foundation of the section and is primary for it.

Definition 3

Electric charge is a physical quantity that characterizes the property of particles or bodies to enter into electromagnetic force interactions.

The letters q or Q in electrodynamics usually denote an electric charge.

Together, all known experimentally proven facts allow us to draw the following conclusions:

Definition 4

There are two kinds of electric charges. These are conventionally named positive and negative charges .

Definition 5

Charges can transfer (for example, by direct contact) between bodies. Electric charge, unlike body mass, is not its integral characteristic. One particular body various conditions can take on different charge values.

Definition 6

Like charges repel, unlike charges attract. This fact reveals another fundamental difference between electromagnetic and gravitational forces. Gravitational forces are always forces of attraction.

conservation law electric charge is one of the fundamental laws of nature.

AT isolated system the algebraic sum of the charges of all bodies is unchanged:

q 1 + q 2 + q 3 + . . . + qn = c o n s t.

Definition 7

The law of conservation of electric charge states that in a closed system of bodies processes of the birth or disappearance of charges of only one sign cannot be observed.

From point of view modern science, charge carriers are elementary particles. Any ordinary object is made up of atoms. They are composed of positively charged protons, negatively charged electrons and neutral particles - neutrons. Protons and neutrons are integral part atomic nuclei, while electrons form electron shell atoms. By modulus, the electric charges of the proton and electron are equivalent and equal to the value of the elementary charge e.

In a neutral atom, the number of electrons in the shell and protons in the nucleus is the same. The number of any of the given particles is called the atomic number.

Such an atom has the ability to both lose and gain one or more electrons. When this happens, the neutral atom becomes a positively or negatively charged ion.

A charge can pass from one body to another only in portions, which contain an integer number of elementary charges. It turns out that the electric charge of the body is a discrete quantity:

q = ±n e (n = 0 , 1 , 2 , . . .).

Definition 8

Physical quantities that have the ability to take an exclusively discrete series of values ​​are called quantized.

Definition 9

elementary charge e represents a quantum, that is, the smallest possible portion of electric charge.

Definition 10

The fact of the existence in modern elementary particle physics of the so-called quarks– particles with fractional charge ± 1 3 e and ± 2 3 e .

However, scientists have never been able to observe quarks in a free state.

Definition 11

To detect and measure electric charges in the laboratory, an electrometer is usually used - a device consisting of a metal rod and an arrow that can rotate around a horizontal axis (Fig. 1. 1. 1).

The arrowhead is insulated from the metal case. In contact with the rod of the electrometer, the charged body provokes the distribution of electric charges of the same sign along the rod and the needle. The impact of electrical repulsion forces causes the needle to deviate at a certain angle, by which it is possible to determine the charge transferred to the electrometer rod.

Picture 1 . one . one . Transfer of charge from a charged body to an electrometer.

An electrometer is a fairly crude instrument. Its sensitivity does not allow to investigate the forces of interaction of charges. In 1785, the law of interaction of fixed charges was first discovered. The French physicist Ch. Coulomb became the discoverer. In his experiments, he measured the forces of attraction and repulsion of charged balls using a device he designed for measuring electric charge - a torsion balance (Fig. 1. 1. 2), which has an extremely high sensitivity. The rocker of the scales rotated 1 ° under the action of a force of approximately 10 - 9 N.

The idea of ​​measurements was based on the physicist's guess that when a charged ball comes into contact with the same uncharged one, the existing charge of the first one will be divided into equal parts between the bodies. Thus, a method was obtained to change the charge of the ball by two or more times.

Definition 12

Coulomb in his experiments measured the interaction between the balls, the dimensions of which were much smaller than the distance separating them, because of which they could be neglected. Such charged bodies are called point charges.

Picture 1 . one . 2. Coulomb device.

Picture 1 . one . 3 . Interaction forces of like and unlike charges.

Based on many experiments, Coulomb established the following law:

Definition 13

The forces of interaction of fixed charges are directly proportional to the product of charge modules and inversely proportional to the square of the distance between them: F = k q 1 · q 2 r 2 .

Interaction forces are repulsive forces with the same signs of charges and attractive forces with different signs (Fig. 1.1.3), and also obey Newton's third law:
F 1 → = - F 2 →.

Definition 14

Coulomb or electrostatic interaction is the effect of stationary electric charges on each other.

Definition 15

The section of electrodynamics devoted to the study of the Coulomb interaction is called electrostatics.

Coulomb's law can be applied to charged point bodies. In practice, it is fully fulfilled if the dimensions of the charged bodies can be neglected due to the distance between the objects of interaction that is much greater than them.

The coefficient of proportionality k in Coulomb's law depends on the choice of the system of units.

AT international system C And the unit of measurement of electric charge is the pendant (K l).

Definition 16

Pendant- this is a charge passing in 1 s through the cross section of the conductor at a current strength of 1 A. The unit of current strength (amperes) in C And is, along with units of length, time and mass, the main unit of measurement.

The coefficient k in the C system And in most cases is written as the following expression:

k = 1 4 π ε 0 .

In which ε 0 \u003d 8, 85 10 - 12 K l 2 N m 2 is an electrical constant.

In the C AND system, the elementary charge e is:

e \u003d 1.602177 10 - 19 K l ≈ 1.6 10 - 19 K l.

Based on experience, we can say that the forces of the Coulomb interaction obey the principle of superposition.

Theorem 1

If a charged body interacts simultaneously with several charged bodies, then the resulting force acting on this body is equal to the vector sum of the forces acting on this body from all other charged bodies.

Figure 1. one . 4, using the example of the electrostatic interaction of three charged bodies, the principle of superposition is explained.

Picture 1 . one . four . The principle of superposition of electrostatic forces F → = F 21 → + F 31 → ; F 2 → = F 12 → + F 32 →; F 3 → = F 13 → + F 23 →.

Picture 1 . one . 5 . Interaction model point charges.

Although the principle of superposition is a fundamental law of nature, its use requires some care when applied to the interaction of charged bodies of finite size. Two conductive charged balls 1 and 2 can serve as an example of such. If another charged ball is brought to such a system consisting of two charged balls, then the interaction between 1 and 2 will change due to the redistribution of charges.

The principle of superposition assumes that the forces of electrostatic interaction between any two bodies do not depend on the presence of other bodies with a charge, provided that the distribution of charges is fixed (given).

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Electric charge- a physical quantity characterizing the ability of bodies to enter into electromagnetic interactions. Measured in Coulomb.

elementary electric charge- the minimum charge that elementary particles have (the charge of a proton and an electron).

e= Cl

The body has a charge, means it has extra or missing electrons. This charge is denoted q = ne. (it is equal to the number of elementary charges).

electrify the body- to create an excess and a shortage of electrons. Ways: electrification by friction and electrification by contact.

pinpoint dawn e - the charge of the body, which can be taken as a material point.

trial charge () - a point, small charge, necessarily positive - is used to study the electric field.

Law of conservation of charge: in an isolated system, the algebraic sum of the charges of all bodies remains constant for any interactions of these bodies with each other.

Coulomb's Law: the interaction forces of two point charges are proportional to the product of these charges, inversely proportional to the square of the distance between them, depend on the properties of the medium and are directed along the straight line connecting their centers.

, where
F / m, C 2 / nm 2 - dielectric. fast. vacuum

- relates. dielectric constant (>1)

- absolute dielectric permeability. environments

Electric field- the material medium through which the interaction of electric charges occurs.

Electric field properties:

Characteristics of the electric field:

tension (E) is a vector quantity, equal to strength acting on a unit test charge placed at a given point.

Measured in N/C.

Direction is the same as for the active force.

tension does not depend neither on strength nor on the magnitude of the trial charge.

Superposition of electric fields: the strength of the field created by several charges is equal to the vector sum of the field strengths of each charge:

Graphically The electronic field is depicted using lines of tension.

tension line- a line, the tangent to which at each point coincides with the direction of the tension vector.

Stress Line Properties: they do not intersect, only one line can be drawn through each point; they are not closed, leave a positive charge and enter a negative one, or dissipate to infinity.

Field types:

Uniform electric field- a field, the intensity vector of which at each point is the same in absolute value and direction.

Non-uniform electric field- a field, the intensity vector of which at each point is not the same in absolute value and direction.

Constant electric field– the tension vector does not change.

Non-constant electric field- the tension vector changes.

The work of the electric field to move the charge.

, where F is force, S is displacement, - angle between F and S.

For uniform field: force is constant.

The work does not depend on the shape of the trajectory; the work done to move along a closed path is zero.

For an inhomogeneous field:

Electric field potential- the ratio of the work that the field does, moving the trial electric charge to infinity, to the magnitude of this charge.

- potential is the energy characteristic of the field. Measured in Volts

Potential difference:

If a
, then

, means

- potential gradient.

For a homogeneous field: potential difference - voltage:

. It is measured in Volts, devices - voltmeters.

Electrical capacity- the ability of bodies to accumulate an electric charge; the ratio of charge to potential, which is always constant for a given conductor.

.

Does not depend on charge and does not depend on potential. But it depends on the size and shape of the conductor; on the dielectric properties of the medium.

, where r is the size,
- permeability of the medium around the body.

The electrical capacity increases if any bodies are nearby - conductors or dielectrics.

Capacitor- a device for accumulating a charge. Electrical capacity:

Flat capacitor- two metal plates with a dielectric between them. Capacitance of a flat capacitor:

, where S is the area of ​​the plates, d is the distance between the plates.

Energy of a charged capacitor is equal to the work done by the electric field in transferring charge from one plate to another.

Small charge transfer
, the voltage will change to
, work will be done
. Because
, and С = const,
. Then
. We integrate:

Electric field energy:
, where V=Sl is the volume occupied by the electric field

For an inhomogeneous field:
.

Volumetric electric field density:
. Measured in J / m 3.

electric dipole- a system consisting of two equal, but opposite in sign, point electric charges located at some distance from each other (dipole arm - l).

The main characteristic of a dipole is dipole moment is a vector equal to the product of the charge and the arm of the dipole, directed from a negative charge to a positive one. Denoted
. Measured in coulomb meters.

Dipole in a uniform electric field.

The forces acting on each of the charges of the dipole are:
and
. These forces are oppositely directed and create a moment of a pair of forces - torque: , where

M - torque F - forces acting on the dipole

d – force arm l – dipole arm

p - dipole moment E - intensity

- angle between p and E q - charge

Under the action of a torque, the dipole will turn and settle in the direction of the lines of tension. Vectors p and E will be parallel and unidirectional.

Dipole in an inhomogeneous electric field.

There is a torque, so the dipole will turn. But the forces will be unequal, and the dipole will move to where the force is greater.

- strength gradient. The higher the tension gradient, the higher the lateral force that pulls the dipole off. The dipole is oriented along the lines of force.

Dipole's own field.

But . Then:

.

Let the dipole be at point O and its arm be small. Then:

.

The formula was obtained taking into account:

Thus, the potential difference depends on the sine of the half-angle at which the dipole points are visible, and the projection of the dipole moment onto the straight line connecting these points.

Dielectrics in an electric field.

Dielectric- a substance that has no free charges, and therefore does not conduct electricity. However, in fact, conductivity exists, but it is negligible.

Dielectric classes:

with polar molecules (water, nitrobenzene): the molecules are not symmetrical, the centers of mass of positive and negative charges do not coincide, which means that they have a dipole moment even in the case when there is no electric field.

with non-polar molecules (hydrogen, oxygen): the molecules are symmetrical, the centers of mass of positive and negative charges coincide, which means that they do not have a dipole moment in the absence of an electric field.

crystalline (sodium chloride): a combination of two sublattices, one of which is positively charged and the other is negatively charged; in the absence of an electric field, the total dipole moment is zero.

Polarization- the process of spatial separation of charges, the appearance of bound charges on the surface of the dielectric, which leads to a weakening of the field inside the dielectric.

Polarization ways:

1 way - electrochemical polarization:

On the electrodes - the movement of cations and anions towards them, the neutralization of substances; areas of positive and negative charges are formed. The current gradually decreases. The rate of establishment of the neutralization mechanism is characterized by the relaxation time - this is the time during which the polarization EMF will increase from 0 to the maximum from the moment the field is applied. = 10 -3 -10 -2 s.

Method 2 - orientational polarization:

On the surface of the dielectric, uncompensated polar ones are formed, i.e. polarization occurs. The tension inside the dielectric is less than the external tension. Relaxation time: = 10 -13 -10 -7 s. Frequency 10 MHz.

3 way - electronic polarization:

Characteristic of non-polar molecules that become dipoles. Relaxation time: = 10 -16 -10 -14 s. Frequency 10 8 MHz.

4 way - ionic polarization:

Two lattices (Na and Cl) are displaced relative to each other.

Relaxation time:

Method 5 - microstructural polarization:

It is typical for biological structures when charged and uncharged layers alternate. There is a redistribution of ions on semi-permeable or ion-impermeable partitions.

Relaxation time: \u003d 10 -8 -10 -3 s. Frequency 1 kHz

Numerical characteristics of the degree of polarization:

Electricity is the ordered movement of free charges in matter or in vacuum.

Conditions for the existence of an electric current:

presence of free charges

the presence of an electric field, i.e. forces acting on these charges

Current strength- a value equal to the charge that passes through any cross section of the conductor per unit time (1 second)

Measured in amperes.

n is the concentration of charges

q is the amount of charge

S is the cross-sectional area of ​​the conductor

- speed of the directed movement of particles.

The speed of movement of charged particles in an electric field is small - 7 * 10 -5 m / s, the speed of propagation of the electric field is 3 * 10 8 m / s.

current density- the amount of charge passing in 1 second through a section of 1 m 2.

. Measured in A / m 2.

- the force acting on the ion from the side of the electric field is equal to the friction force

- ion mobility

- speed of directed movement of ions = mobility, field strength

The specific conductivity of the electrolyte is the greater, the greater the concentration of ions, their charge and mobility. As the temperature rises, the mobility of the ions increases and the electrical conductivity increases.

Essay on electrical engineering

Completed by: Roman Agafonov

Luga Agro-Industrial College

It is impossible to give a short definition of charge that is satisfactory in all respects. We are accustomed to finding understandable explanations for very complex formations and processes, such as the atom, liquid crystals, the distribution of molecules over velocities, and so on. But the most basic, fundamental concepts, indivisible into simpler ones, devoid, according to science today, of any internal mechanism, cannot be briefly explained in a satisfactory way. Especially if the objects are not directly perceived by our senses. It is to such fundamental concepts that the electric charge belongs.

Let us first try to find out not what an electric charge is, but what is hidden behind the statement, a given body or particle has an electric charge.

You know that all bodies are built from the smallest, indivisible into simpler (as far as science is now known) particles, which are therefore called elementary. All elementary particles have mass and due to this they are attracted to each other. According to the law gravity the force of attraction decreases relatively slowly as the distance between them increases: inversely proportional to the square of the distance. In addition, most elementary particles, although not all, have the ability to interact with each other with a force that also decreases inversely with the square of the distance, but this force is a huge number, times greater than the force of gravity. So, in the hydrogen atom, shown schematically in Figure 1, the electron is attracted to the nucleus (proton) with a force 1039 times greater than the force of gravitational attraction.

If particles interact with each other with forces that slowly decrease with distance and are many times greater than the forces of universal gravitation, then these particles are said to have an electric charge. The particles themselves are called charged. There are particles without electric charge, but there is no electric charge without a particle.

Interactions between charged particles are called electromagnetic. When we say that electrons and protons are electrically charged, this means that they are capable of interactions of a certain type (electromagnetic), and nothing more. The absence of a charge on the particles means that it does not detect such interactions. Electric charge determines the intensity electromagnetic interactions, just as mass determines the intensity of gravitational interactions. Electric charge is the second most important characteristic of elementary particles (after mass), which determines their behavior in the surrounding world.

In this way

Electric charge is a physical scalar quantity that characterizes the property of particles or bodies to enter into electromagnetic force interactions.

Electric charge is denoted by the letters q or Q.

Just as in mechanics the concept of a material point is often used, which makes it possible to significantly simplify the solution of many problems, when studying the interaction of charges, the concept of a point charge turns out to be effective. A point charge is a charged body whose dimensions are much smaller than the distance from this body to the point of observation and other charged bodies. In particular, if we talk about the interaction of two point charges, then we thereby assume that the distance between the two charged bodies under consideration is much greater than their linear dimensions.

The electric charge of an elementary particle is not a special “mechanism” in a particle that could be removed from it, decomposed into its component parts and reassembled. The presence of an electric charge in an electron and other particles means only the existence of certain interactions between them.

In nature, there are particles with charges of opposite signs. The charge of a proton is called positive, and that of an electron is called negative. The positive sign of the charge of a particle does not mean, of course, that it has special advantages. The introduction of charges of two signs simply expresses the fact that charged particles can both attract and repel. Particles with the same sign of charge repel each other, and with different signs they attract.

There is no explanation of the reasons for the existence of two types of electric charges now. In any case, no fundamental differences between positive and negative charges are found. If the signs of the electric charges of the particles were reversed, then the nature of electromagnetic interactions in nature would not change.

Positive and negative charges are very well compensated in the universe. And if the Universe is finite, then its total electric charge, in all probability, is equal to zero.

The most remarkable thing is that the electric charge of all elementary particles is strictly the same in absolute value. There is a minimum charge, called elementary, which all charged elementary particles possess. The charge can be positive, like a proton, or negative, like an electron, but the charge modulus is the same in all cases.

It is impossible to separate part of the charge, for example, from an electron. This is perhaps the most amazing thing. No modern theory can explain why the charges of all particles are the same, and cannot calculate the value of the minimum electric charge. It is determined experimentally with the help of various experiments.

In the 1960s, after the number of newly discovered elementary particles began to grow menacingly, a hypothesis was put forward that all strongly interacting particles are composite. The more fundamental particles were called quarks. It turned out to be striking that quarks should have a fractional electric charge: 1/3 and 2/3 of the elementary charge. To construct protons and neutrons, two kinds of quarks are sufficient. And their maximum number, apparently, does not exceed six.

It is impossible to create a macroscopic standard of the unit of electric charge, similar to the standard of length - a meter, because of the inevitable charge leakage. It would be natural to take the electron charge as a unit (this is now done in atomic physics). But at the time of Coulomb, the existence of an electron in nature was not yet known. In addition, the electron charge is too small and therefore difficult to use as a reference.

There are two kinds of electric charges, conventionally called positive and negative. Positively charged bodies are those that act on other charged bodies in the same way as glass electrified by friction against silk. Negatively charged bodies are those that act in the same way as ebonite electrified by friction with wool. The choice of the name "positive" for charges arising on glass and "negative" for charges on ebonite is completely accidental.

Charges can be transferred (for example, by direct contact) from one body to another. Unlike body mass, electric charge is not an inherent characteristic of a given body. The same body in different conditions can have a different charge.

Like charges repel, unlike charges attract. This also shows the fundamental difference between electromagnetic forces and gravitational ones. Gravitational forces are always forces of attraction.

An important property of an electric charge is its discreteness. This means that there is some smallest, universal, further indivisible elementary charge, so that the charge q of any body is a multiple of this elementary charge:

where N is an integer, e is the value of the elementary charge. According to modern concepts, this charge is numerically equal to the electron charge e = 1.6∙10-19 C. Since the magnitude of the elementary charge is very small, for the majority of charged bodies observed and used in practice, the number N is very large, and the discrete nature of the charge change does not manifest itself. Therefore, it is believed that under normal conditions the electric charge of bodies changes almost continuously.

The law of conservation of electric charge.

Inside a closed system, for any interactions, the algebraic sum of electric charges remains constant:

An isolated (or closed) system we will call a system of bodies into which no electric charges are introduced from the outside and are not removed from it.

Nowhere and never in nature does an electric charge of the same sign arise and disappear. The appearance of a positive electric charge is always accompanied by the appearance of a negative charge equal in absolute value. Neither a positive nor a negative charge can disappear separately, they can only mutually neutralize each other if they are equal in absolute value.

So elementary particles are able to transform into each other. But always at the birth of charged particles, the appearance of a pair of particles with charges of the opposite sign is observed. The simultaneous birth of several such pairs can also be observed. Charged particles disappear, turning into neutral ones, also only in pairs. All these facts leave no doubt about the strict implementation of the law of conservation of electric charge.

The reason for the conservation of electric charge is still unknown.

Electrification of the body

Macroscopic bodies are, as a rule, electrically neutral. An atom of any substance is neutral, since the number of electrons in it is equal to the number of protons in the nucleus. Positively and negatively charged particles bond with each other electrical forces and form neutral systems.

A large body is charged when it contains an excess of elementary particles with the same charge sign. The negative charge of the body is due to an excess of electrons compared to protons, and the positive charge is due to their lack.

In order to obtain an electrically charged macroscopic body, or, as they say, to electrify it, it is necessary to separate part of the negative charge from the positive charge associated with it.

The easiest way to do this is with friction. If you run a comb through your hair, then a small part of the most mobile charged particles - electrons - will pass from the hair to the comb and charge it negatively, and the hair will be charged positively. When electrified by friction, both bodies acquire charges opposite in sign, but identical in magnitude.

It is very easy to electrify bodies by means of friction. But to explain how this happens, it turned out to be a very difficult task.

1 version. When electrifying bodies, close contact between them is important. Electrical forces hold the electrons inside the body. But for different substances these forces are different. In close contact, a small part of the electrons of the substance, in which the connection of electrons with the body is relatively weak, passes to another body. In this case, the displacements of electrons do not exceed the sizes of interatomic distances (10-8 cm). But if the bodies are separated, then both of them will be charged. Since the surfaces of bodies are never perfectly smooth, the close contact between the bodies necessary for the transition is established only in small areas of the surfaces. When bodies rub against each other, the number of areas with close contact increases, and thereby the total number of charged particles passing from one body to another increases. But it is not clear how electrons can move in such non-conductive substances (insulators) as ebonite, plexiglass and others. They are bound in neutral molecules.

2 version. On the example of an ionic crystal LiF (insulator), this explanation looks like this. During the formation of a crystal, various kinds of defects arise, in particular vacancies - unfilled places in the nodes of the crystal lattice. If the number of vacancies for positive ions lithium and negative - fluorine is not the same, then the crystal will be charged in volume during formation. But the charge as a whole cannot be stored in the crystal for a long time. There is always a certain amount of ions in the air, and the crystal will draw them out of the air until the charge of the crystal is neutralized by the layer of ions on its surface. Different insulators have different space charges, and therefore the charges of the surface layers of ions are different. During friction, the surface layers of the ions are mixed, and when the insulators are separated, each of them becomes charged.

And can two identical insulators become electrified during friction, for example, the same LiF crystals? If they have the same intrinsic space charges, then no. But they can also have different intrinsic charges if the crystallization conditions were different and a different number of vacancies appeared. As experience has shown, electrification during friction of identical crystals of ruby, amber, etc. can indeed occur. However, this explanation is hardly correct in all cases. If the bodies consist, for example, of molecular crystals, then the appearance of vacancies in them should not lead to the charging of the body.

Another method of electrification of bodies is the impact on them of various radiations (in particular, ultraviolet, x-ray and γ-radiation). This method is most effective for the electrization of metals, when electrons are knocked out from the surface of the metal under the action of radiation, and the conductor acquires a positive charge.

Electrification through influence. The conductor is charged not only upon contact with a charged body, but also when it is at some distance. Let's explore this phenomenon in more detail. We hang light sheets of paper on an insulated conductor (Fig. 3). If the conductor is not initially charged, the leaves will be in the undeflected position. Let us now approach the conductor with an insulated metal ball, strongly charged, for example, with a glass rod. We will see that the sheets suspended at the ends of the body, at points a and b, are deflected, although the charged body does not touch the conductor. The conductor was charged through influence, which is why the phenomenon itself was called "electrification through influence" or "electrical induction." Charges obtained by electrical induction are called induced or induced. Leaves suspended near the middle of the body, at points a' and b', do not deviate. This means that induced charges arise only at the ends of the body, while its middle remains neutral, or uncharged. By bringing an electrified glass rod to the sheets suspended at points a and b, it is easy to make sure that the sheets at point b are repelled from it, and the sheets at point a are attracted. This means that at the remote end of the conductor a charge of the same sign arises as on the ball, and charges of a different sign arise on nearby parts. After removing the charged ball, we will see that the sheets will fall. The phenomenon proceeds in a completely analogous way if the experiment is repeated by charging the ball negatively (for example, with the help of sealing wax).

From the point of view of electronic theory, these phenomena are easily explained by the existence of free electrons in a conductor. When a positive charge is applied to a conductor, electrons are attracted to it and accumulate at the nearest end of the conductor. On it is a certain number of "excess" electrons, and this part of the conductor is charged negatively. At the far end, there is a shortage of electrons and, consequently, an excess of positive ions: here a positive charge appears.

When a negatively charged body is brought to the conductor, electrons accumulate at the remote end, and an excess of positive ions is obtained at the near end. After the removal of the charge, which causes the movement of electrons, they are again distributed over the conductor, so that all sections of it are still uncharged.

The movement of charges along the conductor and their accumulation at its ends will continue until the effect of excess charges formed at the ends of the conductor balances those electric forces emanating from the ball, under the influence of which the redistribution of electrons occurs. The absence of a charge at the middle of the body shows that the forces emanating from the ball are balanced here, and the forces with which the excess charges accumulated at the ends of the conductor act on free electrons.

The induced charges can be separated if, in the presence of a charged body, the conductor is divided into parts. Such an experience is shown in Fig. 4. In this case, the displaced electrons can no longer return back after the removal of the charged ball; since there is a dielectric (air) between both parts of the conductor. Excess electrons are distributed over the entire left side; the lack of electrons at point b is partially replenished from the region of point b ', so that each part of the conductor turns out to be charged: the left - with a charge opposite in sign to the charge of the ball, the right - with a charge of the same name as the charge of the ball. Not only do the leaves diverge at points a and b, but also the sheets that previously remained motionless at points a’ and b’.

Burov L.I., Strelchenya V.M. Physics from A to Z: for students, applicants, tutors. - Minsk: Paradox, 2000. - 560 p.

Myakishev G.Ya. Physics: Electrodynamics. 10-11 cells: textbook. For in-depth study physics /G.Ya. Myakishev, A.Z. Sinyakov, B.A. Slobodskov. - M.Zh Drofa, 2005. - 476 p.

Physics: Proc. allowance for 10 cells. school and classes with deepening. study physicists / O. F. Kabardin, V. A. Orlov, E. E. Evenchik and others; Ed. A. A. Pinsky. - 2nd ed. – M.: Enlightenment, 1995. – 415 p.

Elementary Textbook of Physics: A Study Guide. In 3 volumes / Ed. G.S. Landsberg: T. 2. Electricity and magnetism. - M: FIZMATLIT, 2003. - 480 p.

If you rub a glass rod on a sheet of paper, then the stick will acquire the ability to attract leaves of the "sultan", fluffs, thin streams of water. When combing dry hair with a plastic comb, the hair is attracted to the comb. In these simple examples, we meet with the manifestation of forces that are called electrical.

Bodies or particles that act on surrounding objects by electric forces are called charged or electrified. For example, the glass rod mentioned above, after being rubbed against a sheet of paper, becomes electrified.

Particles have an electrical charge if they interact with each other through electrical forces. Electric forces decrease as the distance between particles increases. Electric forces are many times greater than the forces of universal gravitation.

Electric charge is a physical quantity that determines the intensity of electromagnetic interactions.

Electromagnetic interactions are interactions between charged particles or bodies.

Electric charges are divided into positive and negative. Stable elementary particles - protons and positrons, as well as ions of metal atoms, etc. have a positive charge. The stable negative charge carriers are the electron and the antiproton.

There are electrically uncharged particles, that is, neutral: neutron, neutrino. These particles do not participate in electrical interactions, since their electric charge is zero. There are particles without electric charge, but there is no electric charge without a particle.

On glass rubbed with silk, positive charges arise. On ebonite, shabby on fur - negative charges. Particles repel with charges of the same sign (like charges), and with different signs (opposite charges), particles attract.

All bodies are made up of atoms. Atoms are made up of positively charged atomic nucleus and negatively charged electrons that move around the nucleus of an atom. The atomic nucleus consists of positively charged protons and neutral particles - neutrons. The charges in an atom are distributed in such a way that the atom as a whole is neutral, that is, the sum of the positive and negative charges in the atom is zero.

Electrons and protons are part of any substance and are the smallest stable elementary particles. These particles can exist indefinitely in a free state. The electric charge of the electron and proton is called the elementary charge.

The elementary charge is the minimum charge that all charged elementary particles possess. The electric charge of the proton is equal in absolute value to the charge of the electron:

e \u003d 1.6021892 (46) * 10-19 C

The value of any charge is a multiple of the absolute value elementary charge, that is, the charge of the electron. Electron in translation from the Greek electron - amber, proton - from the Greek protos - the first, neutron from the Latin neutrum - neither one nor the other.

Simple experiments on the electrification of various bodies illustrate the following points.

1. There are two types of charges: positive (+) and negative (-). A positive charge arises when glass is rubbed against skin or silk, and a negative charge occurs when amber (or ebonite) is rubbed against wool.

2. Charges (or charged bodies) interact with each other. Charges of the same name repel, and unlike charges are attracted.

3. The state of electrification can be transferred from one body to another, which is associated with the transfer of electric charge. In this case, a larger or smaller charge can be transferred to the body, i.e., the charge has a value. When electrified by friction, both bodies acquire a charge, one being positive and the other negative. It should be emphasized that the absolute values ​​of the charges of bodies electrified by friction are equal, which is confirmed by numerous measurements of charges using electrometers.

It became possible to explain why bodies are electrified (i.e., charged) during friction after the discovery of the electron and the study of the structure of the atom. As you know, all substances are composed of atoms; atoms, in turn, consist of elementary particles - negatively charged electrons, positively charged protons and neutral particles - neutrons. Electrons and protons are carriers of elementary (minimal) electric charges.

elementary electric charge ( e) - this is the smallest electric charge, positive or negative, equal to the magnitude of the electron charge:

e = 1.6021892(46) 10 -19 C.

There are many charged elementary particles, and almost all of them have a charge. +e or -e, however, these particles are very short-lived. They live less than a millionth of a second. Only electrons and protons exist in a free state indefinitely.

Protons and neutrons (nucleons) make up the positively charged nucleus of the atom, around which negatively charged electrons revolve, the number of which is equal to the number of protons, so that the atom as a whole is a power plant.

Under normal conditions, bodies consisting of atoms (or molecules) are electrically neutral. However, in the process of friction, some of the electrons that have left their atoms can move from one body to another. In this case, the displacements of electrons do not exceed the sizes of interatomic distances. But if the bodies are separated after friction, then they will be charged; the body that has donated some of its electrons will be positively charged, and the body that has acquired them will be negatively charged.

So, bodies become electrified, that is, they receive an electric charge when they lose or gain electrons. In some cases, electrification is due to the movement of ions. New electric charges do not arise in this case. There is only a division of the available charges between the electrified bodies: part of the negative charges passes from one body to another.

Charge definition.

It should be emphasized that the charge is an inherent property of the particle. It is possible to imagine a particle without a charge, but it is impossible to imagine a charge without a particle.

Charged particles manifest themselves in attraction (opposite charges) or in repulsion (charges of the same name) with forces that are many orders of magnitude greater than gravitational ones. Thus, the force of electric attraction of an electron to the nucleus in a hydrogen atom is 10 39 times greater than the force of gravitational attraction of these particles. The interaction between charged particles is called electromagnetic interaction, and the electric charge determines the intensity of electromagnetic interactions.

In modern physics, charge is defined as follows:

Electric charge- this is physical quantity, which is the source of the electric field, through which the interaction of particles with a charge is carried out.

Electric charge- a physical quantity characterizing the ability of bodies to enter into electromagnetic interactions. Measured in Coulomb.

elementary electric charge- the minimum charge that elementary particles have (the charge of a proton and an electron).

The body has a charge, means it has extra or missing electrons. This charge is denoted q=ne. (it is equal to the number of elementary charges).

electrify the body- to create an excess and a shortage of electrons. Ways: electrification by friction and electrification by contact.

pinpoint dawn e - the charge of the body, which can be taken as a material point.

trial charge() - a point, small charge, necessarily positive - is used to study the electric field.

Law of conservation of charge:in an isolated system, the algebraic sum of the charges of all bodies remains constant for any interactions of these bodies with each other.

Coulomb's Law:the interaction forces of two point charges are proportional to the product of these charges, inversely proportional to the square of the distance between them, depend on the properties of the medium and are directed along the straight line connecting their centers.

, where

F / m, C 2 / nm 2 - dielectric. fast. vacuum

- relates. dielectric constant (>1)

- absolute dielectric permeability. environments

Electric field- the material medium through which the interaction of electric charges occurs.

Electric field properties:

Characteristics of the electric field:

tension(E) is a vector quantity equal to the force acting on a unit test charge placed at a given point.

Measured in N/C.

Direction is the same as for the active force.

tension does not depend neither on strength nor on the magnitude of the trial charge.

Superposition of electric fields: the strength of the field created by several charges is equal to the vector sum of the field strengths of each charge:

Graphically The electronic field is depicted using lines of tension.

tension line- a line, the tangent to which at each point coincides with the direction of the tension vector.

Stress Line Properties: they do not intersect, only one line can be drawn through each point; they are not closed, leave a positive charge and enter a negative one, or dissipate to infinity.

Field types:

Uniform electric field- a field, the intensity vector of which at each point is the same in absolute value and direction.

Non-uniform electric field- a field, the intensity vector of which at each point is not the same in absolute value and direction.

Constant electric field– the tension vector does not change.

Non-constant electric field- the tension vector changes.

The work of the electric field to move the charge.

, where F is force, S is displacement, - angle between F and S.

For a uniform field: the force is constant.

The work does not depend on the shape of the trajectory; the work done to move along a closed path is zero.

For an inhomogeneous field:

Electric field potential- the ratio of the work that the field does, moving the trial electric charge to infinity, to the magnitude of this charge.

-potential is the energy characteristic of the field. Measured in Volts

Potential difference:

, then

, means

-potential gradient.

For a homogeneous field: potential difference - voltage:

. It is measured in Volts, devices - voltmeters.

Electrical capacity- the ability of bodies to accumulate an electric charge; the ratio of charge to potential, which is always constant for a given conductor.

.

Does not depend on charge and does not depend on potential. But it depends on the size and shape of the conductor; on the dielectric properties of the medium.

, where r is the size,

- permeability of the medium around the body.

The electrical capacity increases if any bodies are nearby - conductors or dielectrics.

Capacitor- a device for accumulating a charge. Electrical capacity:

Flat capacitor- two metal plates with a dielectric between them. Capacitance of a flat capacitor:

, where S is the area of ​​the plates, d is the distance between the plates.

Energy of a charged capacitor is equal to the work done by the electric field in transferring charge from one plate to another.

Small charge transfer

, the voltage will change to

, work will be done

. Because

, and C \u003d const,

. Then

. We integrate:

Electric field energy:

, where V=Sl is the volume occupied by the electric field

For an inhomogeneous field:

.

Volumetric electric field density:

. Measured in J / m 3.

electric dipole- a system consisting of two equal, but opposite in sign, point electric charges located at some distance from each other (dipole arm -l).

The main characteristic of a dipole is dipole moment is a vector equal to the product of the charge and the arm of the dipole, directed from a negative charge to a positive one. Denoted

. Measured in coulomb meters.

Dipole in a uniform electric field.

The forces acting on each of the charges of the dipole are:

and

. These forces are oppositely directed and create a moment of a pair of forces - torque:, where

M - torque F - forces acting on the dipole

d– arm arm l– arm of the dipole

p– dipole moment E– intensity

- angle between p Eq - charge

Under the action of a torque, the dipole will turn and settle in the direction of the lines of tension. The vectors pi and E will be parallel and unidirectional.

Dipole in an inhomogeneous electric field.

There is a torque, so the dipole will turn. But the forces will be unequal, and the dipole will move to where the force is greater.

-strength gradient. The higher the tension gradient, the higher the lateral force that pulls the dipole off. The dipole is oriented along the lines of force.

Dipole's own field.

But. Then:

.

Let the dipole be at point O and its arm be small. Then:

.

The formula was obtained taking into account:

Thus, the potential difference depends on the sine of the half-angle at which the dipole points are visible, and the projection of the dipole moment onto the straight line connecting these points.

Dielectrics in an electric field.

Dielectric- a substance that does not have free charges, and therefore does not conduct electric current. However, in fact, conductivity exists, but it is negligible.

Dielectric classes:

with polar molecules (water, nitrobenzene): the molecules are not symmetrical, the centers of mass of positive and negative charges do not coincide, which means that they have a dipole moment even in the case when there is no electric field.

with non-polar molecules (hydrogen, oxygen): the molecules are symmetrical, the centers of mass of positive and negative charges coincide, which means that they do not have a dipole moment in the absence of an electric field.

crystalline (sodium chloride): a combination of two sublattices, one of which is positively charged and the other is negatively charged; in the absence of an electric field, the total dipole moment is zero.

Polarization- the process of spatial separation of charges, the appearance of bound charges on the surface of the dielectric, which leads to a weakening of the field inside the dielectric.

Polarization ways:

1 way - electrochemical polarization:

On the electrodes - the movement of cations and anions towards them, the neutralization of substances; areas of positive and negative charges are formed. The current gradually decreases. The rate of establishment of the neutralization mechanism is characterized by the relaxation time - this is the time during which the polarization EMF will increase from 0 to the maximum from the moment the field is applied. = 10 -3 -10 -2 s.

Method 2 - orientational polarization:

On the surface of the dielectric, uncompensated polar ones are formed, i.e. polarization occurs. The tension inside the dielectric is less than the external tension. Relaxation time: = 10 -13 -10 -7 s. Frequency 10 MHz.

3 way - electronic polarization:

Characteristic of non-polar molecules that become dipoles. Relaxation time: = 10 -16 -10 -14 s. Frequency 10 8 MHz.

4 way - ionic polarization:

Two lattices (Na and Cl) are displaced relative to each other.

Relaxation time:

Method 5 - microstructural polarization:

It is typical for biological structures when charged and uncharged layers alternate. There is a redistribution of ions on semi-permeable or ion-impermeable partitions.

Relaxation time: \u003d 10 -8 -10 -3 s. Frequency 1 kHz

Numerical characteristics of the degree of polarization:

Electricity is the ordered movement of free charges in matter or in vacuum.

Conditions for the existence of an electric current:

presence of free charges

the presence of an electric field, i.e. forces acting on these charges

Current strength- a value equal to the charge that passes through any cross section of the conductor per unit time (1 second)

Measured in amperes.

n is the concentration of charges

q is the amount of charge

S- cross-sectional area of ​​the conductor

- speed of the directed movement of particles.

The speed of movement of charged particles in an electric field is small - 7 * 10 -5 m / s, the speed of propagation of the electric field is 3 * 10 8 m / s.

current density- the amount of charge passing in 1 second through a section of 1 m 2.

. Measured in A / m 2.

- the force acting on the ion from the side of the electric field is equal to the friction force

- ion mobility

- speed of directed movement of ions = mobility, field strength

The specific conductivity of the electrolyte is the greater, the greater the concentration of ions, their charge and mobility. As the temperature rises, the mobility of the ions increases and the electrical conductivity increases.

Based on observations of the interaction of electrically charged bodies, the American physicist Benjamin Franklin called some bodies positively charged, while others negatively. Accordingly, and electric charges called positive and negative.

Bodies with like charges repel each other. Bodies with opposite charges attract.

These names of charges are quite arbitrary, and their only meaning is that bodies that have electric charges can either attract or repel.

The sign of the electric charge of the body is determined by the interaction with the conditional standard of the sign of the charge.

As one of these standards, the charge of an ebonite stick worn with fur was taken. It is believed that an ebonite stick after being rubbed with fur always has a negative charge.

If it is necessary to determine what sign of the charge of a given body, it is brought to an ebonite rod, worn with fur, fixed in a light suspension, and the interaction is observed. If the stick is repelled, then the body has a negative charge.

After the discovery and study of elementary particles, it turned out that negative charge always has an elementary part-ca - electron.

Electron (from Greek - amber) - a stable elementary particle with a negative electric chargee = 1.6021892(46) . 10 -19 C, rest massme =9.1095. 10 -19 kg. Discovered in 1897 by the English physicist J. J. Thomson.

Like a standard positive charge a charge of a glass rod worn with natural silk is taken. If the stick repels from an electrified body, then this body has a positive charge.

positive charge always has proton, which is part of the atomic nucleus. material from the site

Using the above rules to determine the sign of the charge of a body, one must remember that it depends on the substance of the interacting bodies. So, an ebonite stick can have a positive charge if it is rubbed with a cloth made of synthetic materials. A glass rod will have a negative charge if it is rubbed with fur. Therefore, when planning to get a negative charge on an ebonite stick, you should definitely use fur or woolen cloth when rubbing. The same applies to the electrification of a glass rod, which is rubbed with a cloth made of natural silk to obtain a positive charge. Only the electron and proton always and uniquely have negative and positive charges, respectively.

This page contains material on topics.

« Physics - Grade 10 "

Let us first consider the simplest case, when electrically charged bodies are at rest.

The section of electrodynamics devoted to the study of the equilibrium conditions for electrically charged bodies is called electrostatics.

What is an electric charge?
What are the charges?

With words electricity, electric charge, electric current you met many times and managed to get used to them. But try to answer the question: “What is an electric charge?” The concept itself charge- this is the main, primary concept, which at the present level of development of our knowledge cannot be reduced to any simpler, elementary concepts.

Let us first try to find out what is meant by the statement: "A given body or particle has an electric charge."

All bodies are built from the smallest particles, which are indivisible into simpler ones and therefore are called elementary.

Elementary particles have mass and due to this they are attracted to each other according to the law of universal gravitation. As the distance between particles increases, the gravitational force decreases in inverse proportion to the square of this distance. Most elementary particles, although not all, also have the ability to interact with each other with a force that also decreases inversely with the square of the distance, but this force is many times greater than the force of gravity.

So in the hydrogen atom, shown schematically in Figure 14.1, the electron is attracted to the nucleus (proton) with a force 10 39 times greater than the force of gravitational attraction.

If particles interact with each other with forces that decrease with increasing distance in the same way as the forces of universal gravitation, but exceed the forces of gravity many times over, then these particles are said to have an electric charge. The particles themselves are called charged.

There are particles without electric charge, but there is no electric charge without a particle.

The interaction of charged particles is called electromagnetic.

Electric charge determines the intensity of electromagnetic interactions, just as mass determines the intensity of gravitational interactions.

The electric charge of an elementary particle is not a special mechanism in a particle that could be removed from it, decomposed into its component parts and reassembled. The presence of an electric charge in an electron and other particles means only the existence of certain force interactions between them.

We, in essence, know nothing about the charge, if we do not know the laws of these interactions. Knowledge of the laws of interactions should be included in our understanding of the charge. These laws are not simple, and it is impossible to state them in a few words. Therefore, it is impossible to give a sufficiently satisfactory concise definition of the concept electric charge.

Two signs of electric charges.

All bodies have mass and therefore attract each other. Charged bodies can both attract and repel each other. This most important fact, familiar to you, means that in nature there are particles with electric charges of opposite signs; In the case of charges of the same sign, the particles repel, and in the case of different signs, they attract.

Charge of elementary particles - protons, which are part of all atomic nuclei, is called positive, and the charge electrons- negative. There are no internal differences between positive and negative charges. If the signs of the particle charges were reversed, then the nature of electromagnetic interactions would not change at all.

elemental charge.

In addition to electrons and protons, there are several more types of charged elementary particles. But only electrons and protons can exist indefinitely in a free state. The rest of the charged particles live less than millionths of a second. They are born during collisions of fast elementary particles and, having existed for a negligible time, decay, turning into other particles. You will get acquainted with these particles in the 11th grade.

Particles that do not have an electrical charge include neutron. Its mass only slightly exceeds the mass of a proton. Neutrons, along with protons, are part of the atomic nucleus. If an elementary particle has a charge, then its value is strictly defined.

charged bodies Electromagnetic forces in nature play a huge role due to the fact that the composition of all bodies includes electrically charged particles. The constituent parts of atoms - nuclei and electrons - have an electric charge.

The direct action of electromagnetic forces between bodies is not detected, since the bodies in the normal state are electrically neutral.

An atom of any substance is neutral, since the number of electrons in it is equal to the number of protons in the nucleus. Positively and negatively charged particles are connected to each other by electrical forces and form neutral systems.

A macroscopic body is electrically charged if it contains an excess number of elementary particles with any one charge sign. So, the negative charge of the body is due to an excess of the number of electrons in comparison with the number of protons, and the positive charge is due to the lack of electrons.

In order to obtain an electrically charged macroscopic body, i.e., to electrify it, it is necessary to separate part of the negative charge from the positive charge associated with it, or to transfer a negative charge to a neutral body.

This can be done with friction. If you run a comb over dry hair, then a small part of the most mobile charged particles - electrons will pass from the hair to the comb and charge it negatively, and the hair will be charged positively.

Equality of charges during electrization

With the help of experience, it can be proved that when electrified by friction, both bodies acquire charges that are opposite in sign, but identical in magnitude.

Let's take an electrometer, on the rod of which a metal sphere with a hole is fixed, and two plates on long handles: one of ebonite, and the other of plexiglass. When rubbing against each other, the plates become electrified.

Let's bring one of the plates inside the sphere without touching its walls. If the plate is positively charged, then some of the electrons from the needle and the electrometer rod will be attracted to the plate and collect on the inner surface of the sphere. In this case, the arrow will be positively charged and repelled from the electrometer rod (Fig. 14.2, a).

If another plate is introduced inside the sphere, having previously removed the first one, then the electrons of the sphere and the rod will be repelled from the plate and accumulate in excess on the arrow. This will cause the arrow to deviate from the rod, moreover, by the same angle as in the first experiment.

Having lowered both plates inside the sphere, we will not find any deflection of the arrow at all (Fig. 14.2, b). This proves that the charges of the plates are equal in magnitude and opposite in sign.

Electrification of bodies and its manifestations. Significant electrification occurs during friction of synthetic fabrics. When taking off a shirt made of synthetic material in dry air, you can hear a characteristic crackle. Small sparks jump between charged areas of rubbing surfaces.

In printing houses, the paper becomes electrified during printing, and the sheets stick together. To prevent this from happening, special devices are used to drain the charge. However, the electrification of bodies in close contact is sometimes used, for example, in various electrocopying machines, etc.

The law of conservation of electric charge.

Experience with the electrification of plates proves that when electrified by friction, the existing charges are redistributed between bodies that were previously neutral. A small part of the electrons passes from one body to another. In this case, new particles do not appear, and the previously existing ones do not disappear.

When electrifying bodies, law of conservation of electric charge. This law is valid for a system that does not enter from the outside and from which charged particles do not exit, i.e., for isolated system.

In an isolated system, the algebraic sum of the charges of all bodies is conserved.

q 1 + q 2 + q 3 + ... + q n = const. (14.1)

where q 1, q 2, etc. are the charges of individual charged bodies.

The law of conservation of charge has a deep meaning. If the number of charged elementary particles does not change, then the law of charge conservation is obvious. But elementary particles can transform into each other, be born and disappear, giving life to new particles.

However, in all cases, charged particles are produced only in pairs with charges of the same modulus and opposite in sign; charged particles also disappear only in pairs, turning into neutral ones. And in all these cases, the algebraic sum of the charges remains the same.

The validity of the law of conservation of charge is confirmed by observations of a huge number of transformations of elementary particles. This law expresses one of the most fundamental properties of electric charge. The reason for the conservation of charge is still unknown.