Electric charge – positive and negative. Law of conservation of electric charge

3.1. Electric charge

Even in ancient times, people noticed that a piece of amber worn with wool began to attract various small objects: specks of dust, threads, and the like. You can easily see for yourself that a plastic comb, rubbed against your hair, begins to attract small pieces of paper. This phenomenon is called electrification, and the forces acting in this case are electrical forces. Both names come from the Greek word electron, meaning amber.
When rubbing a comb on hair or an ebonite stick on wool objects charging, they form electric charges. Charged bodies interact with each other and electrical forces arise between them.
Not only solids, but also liquids and even gases can be electrified by friction.
When bodies are electrified, the substances that make up the electrified bodies do not transform into other substances. Thus, electrification is a physical phenomenon.
There are two different kinds of electrical charges. Quite arbitrarily they are named " positive" charge and " negative" charge (and one could call them “black” and “white”, or “beautiful” and “terrible”, or something else).
Positively charged call bodies that act on other charged objects in the same way as glass electrified by friction with silk.
Negatively charged call bodies that act on other charged objects in the same way as sealing wax electrified by friction on wool.
The main property of charged bodies and particles: Likely charged bodies and particles repel, and oppositely charged bodies attract. In experiments with sources of electric charges, you will become familiar with some other properties of these charges: charges can “flow” from one object to another, accumulate, an electric discharge can occur between charged bodies, and so on. You will study these properties in detail in a physics course.

3.2. Coulomb's law

Electric charge ( Q or q) is a physical quantity, it can be larger or smaller, and therefore can be measured. But physicists are not yet able to directly compare charges with each other, so they compare not the charges themselves, but the effect that charged bodies have on each other, or on other bodies, for example, the force with which one charged body acts on another.

The forces (F) acting on each of the two point charged bodies are oppositely directed along the straight line connecting these bodies. Their values are equal to each other, directly proportional to the product of the charges of these bodies (q 1 ) and (q 2 ) and are inversely proportional to the square of the distance (l) between them.

This relationship is called "Coulomb's law" in honor of the French physicist Charles Coulomb (1763-1806) who discovered it in 1785. The dependence of Coulomb forces on the sign of the charge and the distance between charged bodies, which is most important for chemistry, is clearly shown in Fig. 3.1.

The unit of measurement of electric charge is the coulomb (definition in a physics course). A charge of 1 C flows through a 100-watt light bulb in about 2 seconds (at a voltage of 220 V).

3.3. Elementary electric charge

Until the end of the 19th century, the nature of electricity remained unclear, but numerous experiments led scientists to the conclusion that the magnitude of the electric charge cannot change continuously. It was found that there is a smallest, further indivisible portion of electricity. The charge of this portion is called "elementary electric charge" (denoted by the letter e). It turned out to be 1.6. 10–19 Grades This is a very small value - almost 3 billion billion elementary electrical charges pass through the filament of the same light bulb in 1 second.
Any charge is a multiple of the elementary electric charge, so it is convenient to use the elementary electric charge as a unit of measurement for small charges. Thus,

1e= 1.6. 10–19 Grades

At the turn of the 19th and 20th centuries, physicists realized that the carrier of an elementary negative electric charge is a microparticle, called electron(Joseph John Thomson, 1897). The carrier of an elementary positive charge is a microparticle called proton- was discovered a little later (Ernest Rutherford, 1919). At the same time it was proven that positive and negative elementary electric charges are equal in absolute value

Thus, the elementary electric charge is the charge of a proton.
You will learn about other characteristics of the electron and proton in the next chapter.

Despite the fact that the composition of physical bodies includes charged particles, in the normal state the bodies are uncharged, or electrically neutral. Many complex particles, such as atoms or molecules, are also electrically neutral. The total charge of such a particle or such a body turns out to be zero because the number of electrons and the number of protons included in the composition of the particle or body are equal.

Bodies or particles become charged if electric charges are separated: on one body (or particle) there is an excess of electric charges of one sign, and on the other - of another. In chemical phenomena, an electric charge of any one sign (positive or negative) can neither appear nor disappear, since carriers of elementary electric charges of only one sign cannot appear or disappear.

POSITIVE ELECTRIC CHARGE, NEGATIVE ELECTRIC CHARGE, BASIC PROPERTIES OF CHARGED BODIES AND PARTICLES, COULLOMB'S LAW, ELEMENTARY ELECTRIC CHARGE
1.How is silk charged when rubbed against glass? What about wool when rubbed against sealing wax?
2.What number of elementary electric charges makes up 1 coulomb?
3. Determine the force with which two bodies with charges +2 C and –3 C, located at a distance of 0.15 m from each other, are attracted to each other.
4. Two bodies with charges +0.2 C and –0.2 C are at a distance of 1 cm from each other. Determine the force with which they attract.
5. With what force do two particles carrying the same charge equal to +3 repel each other? e, and located at a distance of 2 E? The value of the constant in the equation of Coulomb's law k= 9. 10 9 N. m 2 / Cl 2.
6. With what force is an electron attracted to a proton if the distance between them is 0.53 E? What about proton to electron?
7.Two like and identically charged balls are connected by a non-conducting thread. The middle of the thread is fixedly fixed. Draw how these balls will be located in space under conditions where the force of gravity can be neglected.
8. Under the same conditions, how will three identical balls, tied by threads of equal length to one support, be located in space? How about four?
Experiments on attraction and repulsion of charged bodies.

Associated with a material carrier; internal characteristic of an elementary particle that determines its electromagnetic interactions.

Electric charge is a physical quantity that characterizes the property of bodies or particles to enter into electromagnetic interactions, and determines the values of forces and energies during such interactions. Electric charge is one of the basic concepts in the study of electricity. The entire set of electrical phenomena is a manifestation of the existence, movement and interaction of electric charges. Electric charge is an inherent property of some elementary particles.

There are two types of electrical charges, conventionally called positive and negative. Charges of the same sign repel, charges of different signs attract each other. The charge of an electrified glass rod was conventionally considered positive, and that of a resin rod (in particular, an amber rod) was considered negative. In accordance with this condition, the electric charge of an electron is negative (Greek “electron” - amber).

The charge of a macroscopic body is determined by the total charge of the elementary particles that make up this body. To charge a macroscopic body, you need to change the number of charged elementary particles it contains, that is, transfer to or remove from it a certain number of charges of the same sign. In real conditions, such a process is usually associated with the movement of electrons. A body is considered charged only if it contains an excess of charges of the same sign, constituting the charge of the body, usually denoted by the letter q or Q If charges are placed on point bodies, then the force of interaction between them can be determined by Coulomb's law. The SI unit of charge is the coulomb - Cl.

Electric charge q of any body is discrete, there is a minimal, elementary electric charge - e, to which all electric charges of bodies are multiples:

\(q = n e\)

The minimum charge that exists in nature is the charge of elementary particles. In SI units, the modulus of this charge is equal to: e= 1, 6.10 -19 Cl. Any electric charges are an integer number of times greater than the elementary ones. All charged elementary particles have an elementary electric charge. At the end of the 19th century. the electron, a carrier of a negative electric charge, was discovered, and at the beginning of the 20th century, a proton, which has the same positive charge, was discovered; Thus, it was proven that electric charges do not exist on their own, but are associated with particles and are an internal property of particles (other elementary particles carrying a positive or negative charge of the same magnitude were later discovered). The charge of all elementary particles (if it is not zero) is the same in absolute value. Elementary hypothetical particles - quarks, whose charge is 2/3 e or +1/3 e, have not been observed, but their existence is assumed in the theory of elementary particles.

The invariance of the electric charge has been established experimentally: the magnitude of the charge does not depend on the speed at which it moves (i.e., the magnitude of the charge is invariant with respect to inertial frames of reference, and does not depend on whether it is moving or at rest).

Electric charge is additive, that is, the charge of any system of bodies (particles) is equal to the sum of the charges of bodies (particles) included in the system.

Electric charge obeys the conservation law, which was established after many experiments. In an electrically closed system, the total total charge is conserved and remains constant during any physical processes occurring in the system. This law is valid for isolated electrical closed systems into which charges are not introduced or removed. This law also applies to elementary particles, which are born and annihilate in pairs, the total charge of which is zero.

The word electricity comes from the Greek name for amber - ελεκτρον .
Amber is the fossilized resin of coniferous trees. The ancients noticed that if you rub amber with a piece of cloth, it will attract light objects or dust. This phenomenon, which we today call static electricity, can be observed by rubbing an ebonite or glass rod or simply a plastic ruler with a cloth.

A plastic ruler, which has been thoroughly rubbed with a paper napkin, attracts small pieces of paper (Fig. 22.1). You may have seen discharges of static electricity while combing your hair or taking off your nylon blouse or shirt. You may have experienced an electrical shock when you touched a metal door handle after standing up from a car seat or walking on synthetic carpet. In all these cases, the object acquires an electrical charge through friction; they say that electrification occurs by friction.

Are all electric charges the same or are there different types? It turns out that there are two types of electric charges, which can be proven by the following simple experiment. Hang a plastic ruler by the middle on a thread and rub it thoroughly with a piece of cloth. If we now bring another electrified ruler to it, we will find that the rulers repel each other (Fig. 22.2, a).
In the same way, bringing another electrified glass rod to one, we will observe their repulsion (Fig. 22.2,6). If a charged glass rod is brought to an electrified plastic ruler, they will be attracted (Fig. 22.2, c). The ruler appears to have a different kind of charge than the glass rod.
It has been experimentally established that all charged objects are divided into two categories: either they are attracted by plastic and repelled by glass, or, conversely, repelled by plastic and attracted by glass. There appear to be two kinds of charges, charges of the same kind repel, and charges of different kinds attract. We say that like charges repel, and unlike charges attract.

The American statesman, philosopher and scientist Benjamin Franklin (1706-1790) called these two types of charges positive and negative. It made absolutely no difference what charge to call;
Franklin proposed that the charge of an electrified glass rod be considered positive. In this case, the charge appearing on the plastic ruler (or amber) will be negative. This agreement is still followed today.

Franklin's theory of electricity was in effect a "one fluid" concept: a positive charge was seen as an excess of the "electrical fluid" over its normal content in a given object, and a negative charge as a deficiency. Franklin argued that when, as a result of some process, a certain charge arises in one body, the same amount of charge of the opposite kind simultaneously arises in another body. The names “positive” and “negative” should therefore be understood in an algebraic sense, so that the total charge acquired by bodies in any process is always equal to zero.

For example, when a plastic ruler is rubbed with a paper napkin, the ruler acquires a negative charge, and the napkin acquires an equal positive charge. There is a separation of charges, but their sum is zero.
This example illustrates the firmly established law of conservation of electric charge, which reads:

The total electric charge resulting from any process is zero.

Deviations from this law have never been observed, therefore we can consider that it is as firmly established as the laws of conservation of energy and momentum.

Electric charges in atoms

Only in the last century did it become clear that the reason for the existence of electric charge lies in the atoms themselves. Later we will discuss the structure of the atom and the development of ideas about it in more detail. Here we will briefly discuss the main ideas that will help us better understand the nature of electricity.

According to modern concepts, an atom (somewhat simplified) consists of a heavy positively charged nucleus surrounded by one or more negatively charged electrons.
In the normal state, the positive and negative charges in an atom are equal in magnitude, and the atom as a whole is electrically neutral. However, an atom can lose or gain one or more electrons. Then its charge will be positive or negative, and such an atom is called an ion.

In a solid, nuclei can vibrate, remaining near fixed positions, while some electrons move completely freely. Electrification by friction can be explained by the fact that in different substances the nuclei hold electrons with different strengths.
When a plastic ruler that is rubbed with a paper napkin acquires a negative charge, this means that the electrons in the paper napkin are held less tightly than in the plastic, and some of them transfer from the napkin to the ruler. The positive charge of the napkin is equal in magnitude to the negative charge acquired by the ruler.

Typically, objects electrified by friction only hold a charge for a while and eventually return to an electrically neutral state. Where does the charge go? It “drains” onto the water molecules contained in the air.
The fact is that water molecules are polar: although in general they are electrically neutral, the charge in them is not uniformly distributed (Fig. 22.3). Therefore, excess electrons from the electrified ruler will “drain” into the air, being attracted to the positively charged region of the water molecule.
On the other hand, the positive charge of the object will be neutralized by electrons, which are weakly held by water molecules in the air. In dry weather, the influence of static electricity is much more noticeable: there are fewer water molecules in the air and the charge does not flow off as quickly. In damp, rainy weather, the item is unable to hold its charge for long.

Insulators and conductors

Let there be two metal balls, one of which is highly charged and the other is electrically neutral. If we connect them with, say, an iron nail, the uncharged ball will quickly acquire an electric charge. If we simultaneously touch both balls with a wooden stick or a piece of rubber, then the ball, which had no charge, will remain uncharged. Substances such as iron are called conductors of electricity; wood and rubber are called non-conductors, or insulators.

Metals are generally good conductors; Most other substances are insulators (however, insulators conduct electricity a little). Interestingly, almost all natural materials fall into one of these two sharply different categories.
There are, however, substances (among which silicon, germanium and carbon should be mentioned) that belong to an intermediate (but also sharply separated) category. They are called semiconductors.

From the point of view of atomic theory, electrons in insulators are bound to nuclei very tightly, while in conductors many electrons are bound very weakly and can move freely within the substance.
When a positively charged object is brought close to or touches a conductor, free electrons quickly move toward the positive charge. If an object is negatively charged, then electrons, on the contrary, tend to move away from it. In semiconductors there are very few free electrons, and in insulators they are practically absent.

Induced charge. Electroscope

Let's bring a positively charged metal object to another (neutral) metal object.

Upon contact, free electrons of a neutral object will be attracted to a positively charged one and some of them will transfer to it. Since the second object now lacks a certain number of negatively charged electrons, it acquires a positive charge. This process is called electrification due to electrical conductivity.

Let us now bring the positively charged object closer to the neutral metal rod, but so that they do not touch. Although the electrons will not leave the metal rod, they will nevertheless move towards the charged object; a positive charge will arise at the opposite end of the rod (Fig. 22.4). In this case, it is said that a charge is induced (or induced) at the ends of the metal rod. Of course, no new charges arise: the charges simply separated, but on the whole the rod remained electrically neutral. However, if we were now to cut the rod crosswise in the middle, we would get two charged objects - one with a negative charge, the other with a positive charge.

You can also impart a charge to a metal object by connecting it with a wire to the ground (or, for example, to a water pipe going into the ground), as shown in Fig. 22.5, a. The subject is said to be grounded. Due to its enormous size, the earth accepts and gives up electrons; it acts as a charge reservoir. If you bring a charged, say, negatively, object close to the metal, then the free electrons of the metal will be repelled and many will go along the wire into the ground (Fig. 22.5,6). The metal will be positively charged. If you now disconnect the wire, a positive induced charge will remain on the metal. But if you do this after the negatively charged object is removed from the metal, then all the electrons will have time to return back and the metal will remain electrically neutral.

An electroscope (or simple electrometer) is used to detect electrical charge.

As can be seen from Fig. 22.6, it consists of a body, inside of which there are two movable leaves, often made of gold. (Sometimes only one leaf is made movable.) The leaves are mounted on a metal rod, which is insulated from the body and ends on the outside with a metal ball. If you bring a charged object close to the ball, a separation of charges occurs in the rod (Fig. 22.7, a), the leaves turn out to be similarly charged and repel each other, as shown in the figure.

You can completely charge the rod due to electrical conductivity (Fig. 22.7, b). In any case, the greater the charge, the more the leaves diverge.

Note, however, that the sign of the charge cannot be determined in this way: a negative charge will separate the leaves exactly the same distance as an equal positive charge. And yet, an electroscope can be used to determine the sign of the charge; for this, the rod must first be given, say, a negative charge (Fig. 22.8, a). If you now bring a negatively charged object to the electroscope ball (Fig. 22.8,6), then additional electrons will move to the leaves and they will move apart further. On the contrary, if a positive charge is brought to the ball, then the electrons will move away from the leaves and they will come closer (Fig. 22.8, c), since their negative charge will decrease.

The electroscope was widely used at the dawn of electrical engineering. Very sensitive modern electrometers operate on the same principle when using electronic circuits.

This publication is based on materials from the book by D. Giancoli. "Physics in two volumes" 1984 Volume 2.

To be continued. Briefly about the following publication:

Force F, with which one charged body acts on another charged body, is proportional to the product of their charges Q 1 and Q 2 and inversely proportional to the square of the distance r between them.

Comments and suggestions are accepted and welcome!

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 occurs when glass rubs against leather or silk, and a negative charge occurs when amber (or ebonite) rubs against wool.

2. Charges (or charged bodies) interact with each other. Same charges push away, 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 magnitude. 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 become 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 consist 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) electrical charges.

Elementary electric charge ( e) is the smallest electric charge, positive or negative, equal to 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 an atom, around which negatively charged electrons rotate, the number of which is equal to the number of protons, so that the atom as a whole is a powerhouse.

Under normal conditions, bodies consisting of atoms (or molecules) are electrically neutral. However, during the process of friction, some of the electrons that have left their atoms can move from one body to another. The electron movements do not exceed the interatomic distances. But if the bodies are separated after friction, they will turn out to be charged; the body that gave up some of its electrons will be charged positively, and the body that 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 caused by the movement of ions. In this case, no new electrical charges arise. There is only a division of the existing charges between the electrifying bodies: part of the negative charges passes from one body to another.

Determination of charge.

It should be especially emphasized that charge is an integral property of the particle. You can imagine a particle without a charge, but you cannot imagine a charge without a particle.

Charged particles manifest themselves in attraction (opposite charges) or repulsion (like charges) with forces that are many orders of magnitude greater than gravitational forces. Thus, the force of electrical 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 is a physical quantity that is the source of the electric field through which the interaction of particles with a charge occurs.

Abstract on electrical engineering

Completed by: Agafonov Roman

Luga Agro-Industrial College

It is impossible to give a brief 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 by speed, etc. But the most basic, fundamental concepts, indivisible into simpler ones, devoid, according to science today, of any internal mechanism, can no longer be briefly explained in a satisfactory way. Especially if objects are not directly perceived by our senses. It is these fundamental concepts that electric charge refers to.

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

You know that all bodies are built from tiny particles, indivisible into simpler (as far as science now knows) 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 of universal gravitation, 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 in inverse proportion to the square of the distance, but this force is a huge number of times greater than the force of gravity. Thus, in the hydrogen atom, schematically shown 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 increasing distance and are many times greater than the forces of gravity, then these particles are said to have an electric charge. The particles themselves are called charged. There are particles without an 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 lack of charge on the particles means that it does not detect such interactions. Electric charge determines the intensity of electromagnetic interactions, just as mass determines the intensity of gravitational interactions. Electric charge is the second (after mass) most important characteristic of elementary particles, which determines their behavior in the surrounding world.

Thus

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

Electric charge is symbolized 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 idea of a point charge is effective. A point charge is a charged body whose dimensions are significantly less than the distance from this body to the point of observation and other charged bodies. In particular, if they talk about the interaction of two point charges, then they assume that the distance between the two charged bodies under consideration is significantly greater than their linear dimensions.

The electric charge of an elementary particle is not a special “mechanism” in the particle that could be removed from it, decomposed into its component parts and reassembled. The presence of an electric charge on an electron and other particles only means 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 the charge of an electron is called negative. The positive sign of a charge on a particle does not mean, of course, that it has any special advantages. The introduction of charges of two signs simply expresses the fact that charged particles can both attract and repel. If the charge signs are the same, the particles repel, and if the charge signs are different, they attract.

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

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

The most remarkable thing is that the electric charge of all elementary particles is strictly the same in magnitude. There is a minimum charge, called elementary, that 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 surprising thing. No modern theory can explain why the charges of all particles are the same, and is not able to calculate the value of the minimum electric charge. It is determined experimentally using various experiments.

In the 1960s, after the number of newly discovered elementary particles began to grow alarmingly, it was hypothesized that all strongly interacting particles are composite. More fundamental particles were called quarks. What was striking was that quarks should have a fractional electric charge: 1/3 and 2/3 of the elementary charge. To build protons and neutrons, two types of quarks are enough. And their maximum number, apparently, does not exceed six.

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

In the International System of Units (SI), the unit of charge, the coulomb, is established using the unit of current:

1 coulomb (C) is the charge passing through the cross-section of a conductor in 1 s at a current of 1 A.

A charge of 1 C is very large. Two such charges at a distance of 1 km would repel each other with a force slightly less than the force with which the globe attracts a load weighing 1 ton. Therefore, it is impossible to impart a charge of 1 C to a small body (about a few meters in size). Repelling from each other, charged particles would not be able to stay on such a body. No other forces exist in nature that would be capable of compensating for Coulomb repulsion under these conditions. But in a conductor that is generally neutral, it is not difficult to set a charge of 1 C in motion. Indeed, in an ordinary light bulb with a power of 100 W at a voltage of 127 V, a current is established that is slightly less than 1 A. At the same time, in 1 s a charge almost equal to 1 C passes through the cross-section of the conductor.

An electrometer is used to detect and measure electrical charges. The electrometer consists of a metal rod and a pointer that can rotate around a horizontal axis (Fig. 2). The rod with the arrow is fixed in a plexiglass sleeve and placed in a cylindrical metal case, closed with glass covers.

The principle of operation of the electrometer. Let's touch the positively charged rod to the electrometer rod. We will see that the electrometer needle deviates by a certain angle (see Fig. 2). The rotation of the arrow is explained by the fact that when a charged body comes into contact with the electrometer rod, electrical charges are distributed along the arrow and the rod. Repulsive forces acting between like electric charges on the rod and the pointer cause the pointer to rotate. Let's electrify the ebonite rod again and touch the electrometer rod with it again. Experience shows that with increasing electric charge on the rod, the angle of deviation of the arrow from the vertical position increases. Consequently, by the angle of deflection of the electrometer needle, one can judge the value of the electric charge transferred to the electrometer rod.

The totality of all known experimental facts allows us to highlight the following properties of the charge:

There are two types 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. Bodies that act in the same way as ebonite electrified by friction with wool are called negatively charged. The choice of the name “positive” for charges arising on glass, and “negative” for charges on ebonite, is completely random.

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

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

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 value of the elementary charge is very small, for most of the charged bodies observed and used in practice, the number N is very large, and the discrete nature of the charge change does not appear. Therefore, it is believed that under normal conditions the electric charge of bodies changes almost continuously.

Law of conservation of electric charge.

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

We will call an isolated (or closed) system a system of bodies into which electric charges are not introduced from the outside and are not removed from it.

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

This is how elementary particles can transform into each other. But always during 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 because 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 large body is charged when it contains an excess number of elementary particles with the same charge sign. The negative charge of a body is due to an excess of electrons compared to protons, and the positive charge is due to their deficiency.

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, a small part of the most mobile charged particles - electrons - will move from the hair to the comb and charge it negatively, and the hair will become positively charged. When electrified by friction, both bodies acquire charges of opposite sign, but equal in magnitude.

It is very simple to electrify bodies using friction. But explaining how this happens turned out to be a very difficult task.

1 version. When electrifying bodies, close contact between them is important. Electrical forces hold electrons inside the body. But for different substances these forces are different. During 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. The electron movements do not exceed the 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 bodies necessary for transition is established only on 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-conducting substances (insulators) as ebonite, plexiglass and others. They are bound in neutral molecules.

Version 2. Using the example of an ionic LiF crystal (insulator), this explanation looks like this. During the formation of a crystal, various types of defects arise, in particular vacancies - unfilled spaces at the nodes of the crystal lattice. If the number of vacancies for positive lithium ions and negative fluorine ions is not the same, then the crystal will be charged in volume upon formation. But the charge as a whole cannot be retained by the crystal for long. There is always a certain amount of ions in the air, and the crystal will pull them out of the air until the charge of the crystal is neutralized by a 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 ions are mixed, and when the insulators are separated, each of them becomes charged.

Can two identical insulators, for example the same LiF crystals, be electrified by friction? If they have the same own space charges, then no. But they can also have different own 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 actually occur. However, the above explanation is unlikely to be correct in all cases. If bodies consist, for example, of molecular crystals, then the appearance of vacancies in them should not lead to charging of the body.

Another way to electrify bodies is by exposing them to various radiations (in particular, ultraviolet, x-ray and γ-radiation). This method is most effective for electrifying metals, when, under the influence of radiation, electrons are knocked out from the surface of the metal 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. Let's hang light sheets of paper on an insulated conductor (Fig. 3). If the conductor is not charged at first, the leaves will be in the non-deflected position. Let us now bring an insulated metal ball, highly charged, to the conductor, for example, using 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 through electrical induction are called induced or induced. The leaves suspended at 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, and 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 verify that the sheets at point b repel 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 appears as on the ball, and on nearby parts charges of a different sign arise. By removing the charged ball, we will see that the leaves will go down. The phenomenon proceeds in a completely similar way if we repeat the experiment by charging the ball negatively (for example, using 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. A certain number of “excess” electrons appear on it, and this part of the conductor becomes negatively charged. At the far end there is a lack of electrons and, therefore, an excess of positive ions: a positive charge appears here.

When a negatively charged body is brought close to a conductor, electrons accumulate at the far end, and an excess of positive ions is produced at the near end. After removing the charge that causes the movement of electrons, they are again distributed throughout the conductor, so that all parts of it are still uncharged.

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

Induced charges can be separated if, in the presence of a charged body, the conductor is divided into parts. Such an experience is depicted in Fig. 4. In this case, the displaced electrons can no longer return back after removing the charged ball; since there is a dielectric (air) between both parts of the conductor. Excess electrons are distributed throughout the left side; the lack of electrons at point b is partially replenished from the area 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 the leaves at points a and b diverge, but also the previously stationary leaves at points a’ and b’.

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

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

Physics: Textbook. allowance for 10th grade. school and advanced classes studied physicists/ O. F. Kabardin, V. A. Orlov, E. E. Evenchik and others; Ed. A. A. Pinsky. – 2nd ed. – M.: Education, 1995. – 415 p.

Elementary physics textbook: 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, the rod will acquire the ability to attract plume leaves, fluff, and thin streams of water. When you comb dry hair with a plastic comb, the hair is attracted to the comb. In these simple examples we encounter the manifestation of forces that are called electrical.

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

Particles have an electrical charge if they interact with each other through electrical forces. Electrical forces decrease with increasing distance between particles. Electrical forces are many times greater than the forces of universal gravity.

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. Stable negative charge carriers are the electron and antiproton.

There are electrically uncharged particles, that is, neutral ones: neutron, neutrino. These particles do not participate in electrical interactions, since their electric charge is zero. There are particles without an electric charge, but an electric charge does not exist without a particle.

Positive charges appear on glass rubbed with silk. Ebonite rubbed on fur has negative charges. Particles repel with charges of the same signs (like charges), and with different signs (opposite charges) particles attract.

All bodies are made of atoms. Atoms consist of a positively charged atomic nucleus and negatively charged electrons that move around the atomic nucleus. 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 in a free state for an unlimited time. The electric charge of an electron and a proton is called the elementary charge.

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

e = 1.6021892(46) * 10-19 C

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

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

2. Charges (or charged bodies) interact with each other. Same charges push away, and unlike charges are attracted.

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

e = 1.6021892(46) 10 -19 C.

Protons and neutrons (nucleons) make up the positively charged nucleus of an 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 powerhouse.

Determination of charge.

It should be especially emphasized that charge is an integral 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.

In modern physics, charge is defined as follows:

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

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

Elementary electric charge– the minimum charge that elementary particles have (proton and electron charge).

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

Electrify the body– create an excess and deficiency of electrons. Methods: electrification by friction And electrification by contact.

Point dawn d is the charge of the body, which can be taken as a material point.

Test charge() – point, small charge, always positive – 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 forces of interaction between 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, Cl 2 /nm 2 – dielectric. fast. vacuum

- relates. dielectric constant (>1)

- absolute dielectric permeability. environment

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

Electric field properties:

Electric field characteristics:

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– the same as that of the acting force.

Tension does not depend neither on the strength nor on the size of the test charge.

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

Graphically The electronic field is represented using tension lines.

Tension line– a line whose tangent at each point coincides with the direction of the tension vector.

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

Types of fields:

Uniform electric field– a field whose intensity vector at each point is the same in magnitude and direction.

Non-uniform electric field– a field whose intensity vector at each point is unequal in magnitude and direction.

Constant electric field– the tension vector does not change.

Variable electric field– the tension vector changes.

Work done by an electric field to move a 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 a non-uniform field:

Electric field potential– the ratio of the work that the field does, moving a test electric charge to infinity, to the magnitude of this charge.

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

Potential difference:

, That

, Means

-potential gradient.

For a uniform field: potential difference – voltage:

. It is measured in Volts, the devices are voltmeters.

Electrical capacity– the ability of bodies to accumulate electrical 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 environment around the body.

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

Capacitor– device for accumulating charge. Electrical capacity:

Flat capacitor– two metal plates with a dielectric between them. Electric capacity of a flat capacitor:

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

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

Small charge transfer

, the voltage will change to

, the work is done

. Because

, and C =const,

. Then

. Let's integrate:

Electric field energy:

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

For a non-uniform 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– a vector equal to the product of the charge and the dipole arm, directed from the negative charge to the positive one. Designated

. Measured in Coulomb meters.

Dipole in a uniform electric field.

The following forces act on each charge of the dipole:

And

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

M – torque F – forces acting on the dipole

d – sill arm – dipole arm

p – dipole moment E – tension

- angle between p Eq – charge

Under the influence of a torque, the dipole will rotate and align itself in the direction of the tension lines. Vectors p and E will be parallel and unidirectional.

Dipole in a non-uniform electric field.

There is a torque, which means the dipole will rotate. But the forces will be unequal, and the dipole will move to where the force is greater.

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

Dipole intrinsic field.

But. Then:

Let the dipole be at point O and its arm 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 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 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 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 methods:

Method 1 – electrochemical polarization:

On the electrodes – movement of cations and anions towards them, 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 increases from 0 to a maximum from the moment the field is applied. = 10 -3 -10 -2 s.

Method 2 – orientational polarization:

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

Method 3 – electronic polarization:

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

Method 4 – ion polarization:

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

Relaxation time:

Method 5 – microstructural polarization:

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

Relaxation time: =10 -8 -10 -3 s. Frequency 1KHz

Numerical characteristics of the degree of polarization:

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

Conditions for the existence of 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 a conductor per unit of time (1 second)

Measured in Amperes.

n – charge concentration

q – charge value

S – cross-sectional area of the conductor

- speed of directional 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 through a cross section of 1 m2 in 1 second.

. Measured in A/m2.

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

- ion mobility

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

The greater the concentration of ions, their charge and mobility, the greater the specific conductivity of the electrolyte. As the temperature increases, the mobility of 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 and others negatively charged. Accordingly to this and electric charges called positive And negative.

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

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

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

The charge of an ebonite stick rubbed with fur was taken as one of these standards. 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 stick, rubbed 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 particle - electron.

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

The charge of a glass rod rubbed with natural silk was taken as a standard of positive charge. If a stick is repelled 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, you need to remember that it depends on the substance of the interacting bodies. Thus, an ebonite stick can have a positive charge if rubbed with a cloth made of synthetic materials. A glass rod will have a negative charge if rubbed with fur. Therefore, if you plan to get a negative charge on an ebonite stick, you should definitely use it when rubbing it with fur or woolen cloth. 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 unambiguously have negative and positive charges, respectively.

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