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Electric charge and its types. The physical essence of electric charge

Electric charge and its types. The physical essence of electric charge

Electric charge is a physical quantity that characterizes the intensity of electromagnetic interaction between bodies. Electric charge itself does not exist; its carrier can only be a particle of matter.

Basic properties

1. Duality: in nature there are charges of two signs, like charges repel, opposite charges attract. In this regard, the conditional charges are divided into positive and negative.

The charge possessed by a glass rod rubbed against silk or paper is called positive.

Negative - the charge possessed by an amber or ebonite stick rubbed against fur or wool.

2. Quantization: If a physical quantity takes only certain discrete values, it is said to be quantized (discrete). Experience shows that any electric charge is quantized, i.e. consists of an integer number of elementary charges.

where =1,2,...integer; e =1.6·1 -19 C - elementary charge.

The electron has the smallest (elementary) negative charge, the proton has the positive charge.

1 coulomb is the charge passing through the cross-section of a conductor in one second when a direct current of one ampere flows through the conductor.

3. Charge conservation.

Electric charges can disappear and reappear only in pairs. In each such pair, the charges are equal in magnitude and opposite in sign. For example, an electron and a positron annihilate when they meet, i.e. turn into neutral g - photons, and the charges –e and +e disappear. During a process called pair production, a g photon, entering the field of an atomic nucleus, turns into a pair of particles, an electron and a positron, and charges +e and –e arise.

Law of conservation of charge: in an isolated system, the algebraic sum of charges remains constant for all changes within the system.

Isolated is a system of bodies that does not exchange charges with the external environment.

4. Invariance charge to various inertial frames of reference.

Experience shows that the magnitude of the charge does not depend on the speed of movement of the charged body. The same charge measured in different inertial reporting frames is the same.

5. Additivity: .

Classification of charges.

Depending on the size of the charged body, charges are divided into point and extended.

· A point charge is a charged body whose dimensions can be neglected in the conditions of this problem.

· Extended is the charge of a body whose dimensions cannot be neglected in the conditions of this problem. Extended charges are divided into linear, surface and volume.

By the ability to shift relative to the equilibrium position under the influence of external electricity. fields, charges are conventionally divided into free, bound and extraneous.

Free are called charges that can move freely in a body under the influence of external electricity. fields.

Related are called the charges that are part of the dielectric molecules, which under the influence of electricity. fields can only shift from their equilibrium position, but cannot leave the molecule.

Third party are called charges located on the dielectric, but not part of its molecules.

The law governing the force of interaction between point charges was established experimentally in 1785. Pendant.

Coulomb's law: the force of interaction between two stationary point charges is directly proportional to the charges, inversely proportional to the square of the distance between them, directed along the straight line connecting the charges, and depends on the environment in which they are located.

where q 1, q 2 - charge values; r is the distance between charges;

8.85 1 -12 C 2 / (N m 2) - electrical constant,

e is the dielectric constant of the medium.

The dielectric constant of a substance shows how many times the force of interaction between charges in a given dielectric is less than in a vacuum, vacuum = 1, is a dimensionless quantity.

Let us explain the reason for this weakening by considering a charged ball surrounded by a dielectric. The field of the ball orients the molecules of the dielectric, and negative bound charges appear on the surface of the dielectric adjacent to the ball.

The field at any point of the dielectric will be created by two oppositely charged spheres: the surface of the ball, positively charged, and the negatively charged surface of the dielectric adjacent to it, while the field of bound charges is subtracted from the field of free charges, and the total field will be weaker than the field of one ball.

1. Electrostatic field strength. The principle of superposition of electric fields. Vector flow.

Any charge changes the properties of the surrounding space - it creates an electric field in it.

An electric field is one of the forms of existence of matter surrounding electric charges. This field manifests itself in the fact that an electric charge placed at any point is under the influence of force.

The concept of an electric field was introduced into science in the 30s of the 19th century by English scientists Michael Faraday.

According to Faraday, every electric charge is surrounded by the electric field it creates, so such a charge is sometimes called a source charge. The charge with which the source charge field is studied is called a test charge.

In order for the force acting on the test charge to characterize the field at a given point; The test charge must be a point charge.

Point charge called a charged body, the dimensions of which can be neglected in the conditions of this problem, i.e. whose dimensions are small compared to the distances to other bodies with which it interacts. In this case, the own electric field of the test charge must be so small that it does not change the field of the source charge. The smaller the size of the charged body and the weaker its own field compared to the field of the source charge, the more accurately this charged body satisfies the test charge condition.

The electric field propagates in a vacuum with a speed c = 3·1 8 .

The field of stationary electric charges is electrostatic.

Using a test charge, we investigate the field created by a stationary charge - the source.

The force acting on the test charge at a given point in the field depends on the size of the test charge. If we take different test charges, then the force acting on them at a given point in the field will be different.

However, the ratio of the force to the magnitude of the test charge remains constant and characterizes the field itself. This ratio is called the electric field strength at a given point.

Electric field strength is a vector quantity numerically equal to the force with which the field acts on a unit positive test charge at a given point in the field and codirectional with this force.

Strength is the main characteristic of the field and completely characterizes the field at each point in magnitude and direction.

Field strength of a point charge.

According to Coulomb's law

is the electric field strength of a point charge at a distance r from this charge.

It is convenient to graphically depict the electric field using a picture of the so-called lines of force, or lines of tension.

Tension line is a line whose tangent at each point coincides in direction with the tension vector at that point.

The field strength lines created by stationary charges always begin and end at the charges (or at infinity) and are never closed. A stronger field is represented by more densely spaced tension lines. The density of the lines is chosen so that the number of lines piercing a unit surface of the site perpendicular to the lines is equal to the numerical value of the vector. Lines of tension never intersect, because... their intersection would mean two different directions of the field strength vector at the same point, which does not make sense.

A field in which the intensity at all points has the same magnitude and the same direction is called homogeneous. In such a field, the lines of force are parallel and their density is the same everywhere, i.e. they are located at the same distance from each other.

Superposition principle.

If the electric field at a given point is created by several charges, then the strength of the resulting field is equal to the vector sum of the field strengths created by each charge separately.

The principle of superposition is an experimental fact that is valid up to very strong fields. According to the same law, not only static, but also rapidly changing electromagnetic fields are formed

Let us select in the vector field a certain volume limited by the surface S. Let us divide this surface into elementary areas of size .

A directed surface element can be introduced into consideration. A directed element of a surface is a vector whose length is equal to the area of the element, and the direction coincides with the direction of the normal to this element. For a closed surface, the outer normal to the surface is taken. Since the choice of direction is arbitrary (conditional), it can be directed either in one direction from the site or in the other; it is not a true vector, but a pseudo-vector.

Directional surface element,

Elementary surface.

The flow of the tension vector through an elementary surface dS called the scalar product

where a is the angle between vectors and ,

E n - projection onto the normal direction.

Having summed up the flows through all the elementary areas into which the surface S was divided, we obtain the vector flow through the surface S.

The flow of a vector through the surface S is the integral

For a closed surface.

Vector flux is an algebraic quantity:

For a uniform field

The flow of the tension vector can be given a clear geometric interpretation: it is numerically equal to the number of tension lines crossing a given surface.

2. Gauss's theorem for vector flux and its application to calculate the fields of extended charges in vacuum.

Knowing the field strength of a point charge, and using the principle of superposition, it is possible to calculate the field strength created by several point charges. However, for extended charges the application of the superposition principle is difficult. A method for calculating fields created by extended charges was proposed by the German scientist Gauss at the beginning of the 19th century.

Gauss's theorem for the electrostatic field in vacuum.

Let us consider the field of a point charge in vacuum and calculate the radius of the sphere through the surface

Field strength at any point on the surface of the sphere

Electricity surrounds us on all sides. But once upon a time this was not the case. Because the word itself comes from the Greek name for a specific material: “electron”, in Greek, “amber”. They conducted interesting experiments with him, similar to magic tricks. People have always loved miracles, but here all sorts of specks of dust, villi, threads, hairs began to be attracted to a piece of amber, as soon as it was rubbed with a piece of cloth. That is, this golden stone doesn’t have any small “handles”, but it can pick up fluff.

In contact with

Classmates

Accumulation of electricity and knowledge about it

Visible accumulation of electricity also occurred when they put on crafts made of amber: amber beads, amber hair clips. There are no explanations other than obvious magic, there couldn't be any. After all, for the trick to be successful, it was necessary to sort the beads exclusively with clean, dry hands and while sitting in clean clothes. And clean hair, well rubbed with a hairpin, gives something beautiful and terrifying: a halo of hair sticking up. And even crackling. And even in the darkness there are flashes. This is the action of a spirit that is demanding and capricious, as well as scary and incomprehensible. But the time has come, and electrical phenomena have ceased to be the territory of the spirit.

They began to call everything simply “interaction.” That's when we started experimenting. They came up with a special machine for this (electrophoric machine), and a jar for storing electricity (Leyden jar). And a device that could already show some “equal-more-less” in relation to electricity (electroscope). All that remains is to explain it all with the help of the increasingly powerful language of formulas.

Thus, humanity has come up with the need to recognize the presence of a certain electric charge in nature. Actually, the title does not contain any discovery. Electric means associated with phenomena the study of which began with the magic of amber. The word “charge” speaks only of vague possibilities embedded in an object, like a cannonball. It’s just clear that electricity can be somehow produced and somehow stored. And somehow it has to be measured. The same as an ordinary substance, for example, oil.

And, by analogy with substances, the smallest particles of which (atoms) were spoken confidently since the time of Democritus, and decided that the charge must certainly consist of similar very small “corpuscles” - bodies. The number of which in a large charged body will give the amount of electric charge.

Electric charge - law of conservation of charge

Of course, at that time they could not even approximately imagine how many such electrical “corpuscles” could appear in even a very small charged body. But a practical unit of electric charge was still needed. And they began to invent it. The pendant, after whom such a unit was later named, apparently measured the magnitude of charges using metal balls with which he conducted experiments, but somehow relatively. Opened my famous Coulomb's law, in which he wrote algebraically that the force acting between two charges q1 and q2 separated by a distance R is proportional to their product and inversely proportional to the square of the distance between them.

Coefficient k depends on the medium in which the interaction occurs, but in a vacuum it is equal to unity.

Probably, after Kepler and Newton, doing such things was not so difficult. Distance is easy to measure. He divided the charges physically, touching one ball to another. It turned out that on two identical balls, if one is charged and the other is not, upon contact the charge is divided in half - it scatters across both balls. Thus, he received fractional values of the original unknown quantity q.

Studying interaction of electric charges, he took measurements at different distances between the balls, recorded the deviations on his torsion balances, which are obtained when charged balls repel each other. Apparently, his law was a pure victory for algebra, since Coulomb himself did not know the unit of measurement of charge “coulomb” and simply could not know it.

Another victory was the discovery of the fact that the total amount of this same quantity q in the balls that he was able to charge in this way always remained unchanged. That is why he called the open law the law of conservation of charge.

Q = q 1 + q 2 + q 3 + … + q n

We must pay tribute to the accuracy and patience of the scientist, as well as the courage with which he proclaimed his laws, without having a unit of the amount of what he studied.

A particle of electricity - minimum charge

It was only later that they realized that the elementary, that is, the smallest, electric charge is... an electron. Only not a small piece of amber, but an inexpressibly small particle that is not even a substance (almost), but which is necessarily present in any material body. And even in every atom of every substance. And not only in atoms, but also around them. And those:

that are found in atoms are called bound electrons.
and those around are free electrons.

Electrons are bound in an atom because the atomic nucleus also contains particles of charge - protons, and each proton will certainly attract an electron to itself. Just according to Coulomb's law.

And the charge that you can see or feel results from:

friction,
savings, accumulation
chemical reaction,
electromagnetic induction,

consist only of free electrons that were ejected from atoms due to various misunderstandings:

from being hit by another atom (thermal emission)
quantum of light (photoemission) and for other reasons

and wandering inside huge macroscopic bodies (for example, hairs).

For electrons, the bodies of our objects are truly huge. One unit of charge (coulomb) contains approximately this amount of electrons: a little over 624,150,912,514,351,000. It sounds like this: 624 quadrillion 150 trillion 912 billion 514 million 351 thousand electrons in one coulomb of electric charge.

And the pendant is a very simple quantity and close to us. A coulomb is the same charge that flows in one second through the cross-section of a conductor if the current in it has a force of one ampere. That is, at 1 ampere, for every second, just these 624 quadrillion ... electrons will flicker through the cross section of the wire.

Electrons are so mobile and move so quickly inside physical bodies that they turn on our light bulb in an instant, as soon as we press the switch. And that’s why our electrical interaction is so fast that events called “recombination” occur every second. The escaped electron finds the atom from which the electron just escaped and takes up a free space in it.

The number of such events per second is also of the order of... well, everyone already imagines this. And these events are continuously repeated when electrons leave atoms and then return to atoms. They run away and come back. This is their life, without it they simply cannot exist. And only thanks to this, electricity exists - that system that has become part of our life, our comfort, our nutrition and preservation.

Current direction. Who is in charge of our charge?

That’s the only thing left is one small curiosity that everyone knows, but none of the physicists wants to correct.

When Coulomb played tricks with his balls, they saw that there were two types of charges. And charges of the same type repel each other, and charges of different types attract each other. It was natural to name some of them positive and others negative. And assume that electric current flows from where there is more to where there is less. That is, from plus to minus. So it stuck in the minds of physicists for many generations.

But then it was not electrons, but ions that were discovered first. These are precisely those inconsolable atoms that have lost their electron. In the nucleus of which there is an “extra” proton, and therefore they are charged. Well, when they discovered this, they immediately sighed and said - here it is, you are our positive charge. And the proton gained the reputation of a positively charged particle.

And then they realized that atoms are most often neutral because the electric charge of the nucleus is balanced by the charge of the electron shells rotating around the nucleus. That is, they built a planetary model of the atom. And only then did they understand that atoms make up all (almost) matter, its solid crystal lattice, or the entire mass of its liquid body. That is, protons with neutrons sit solidly in the nuclei of atoms. And not at your beck and call, like light and mobile electrons. Consequently, the current does not flow from plus to minus, but, on the contrary, from minus to plus.

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!

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 concept 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, they thereby 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 with 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.

Electric charge is a physical quantity that characterizes the property of particles or bodies to enter into electromagnetic force interactions. El z. usually denoted by the letters q or Q. The totality of all known experimental facts allows us to draw the following conclusions:

There are two types of electric charges, conventionally called positive and negative.

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

One of the fundamental laws of nature is the experimentally established law of conservation of electric charge .

In an isolated system, the algebraic sum of the charges of all bodies remains constant:

q 1 + q 2 + q 3 + ... +qn= const.

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

From a modern point of view, charge carriers are elementary particles. All ordinary bodies are composed of atoms, which include positively charged protons, negatively charged electrons and neutral particles - neutrons. Protons and neutrons are part of atomic nuclei, electrons form the electron shell of atoms. The electric charges of a proton and an electron are exactly the same in magnitude and equal to the elementary charge e.

In a neutral atom, the number of protons in the nucleus is equal to the number of electrons in the shell. This number is called atomic number . An atom of a given substance may lose one or more electrons or gain an extra electron. In these cases, the neutral atom turns into a positively or negatively charged ion.

Charge can be transferred from one body to another only in portions containing an integer number of elementary charges. Thus, the electric charge of the body is discrete quantity:

Physical quantities that can only take a discrete series of values are called quantized . Elementary charge e is a quantum (smallest portion) of electric charge. It should be noted that in modern physics of elementary particles the existence of so-called quarks is assumed - particles with a fractional charge and However, quarks have not yet been observed in a free state.

In common laboratory experiments, a electrometer - a device consisting of a metal rod and a pointer that can rotate around a horizontal axis.

The electrometer is a rather crude instrument; it does not allow one to study the forces of interaction between charges. The law of interaction of stationary charges was first discovered by the French physicist C. Coulomb in 1785. In his experiments, Coulomb measured the forces of attraction and repulsion of charged balls using a device he designed - a torsion balance (Fig. 1.1.2), which was distinguished by extremely high sensitivity. For example, the balance beam was rotated by 1° under the influence of a force of the order of 10 –9 N.

The idea of the measurements was based on Coulomb's brilliant guess that if a charged ball is brought into contact with exactly the same uncharged one, then the charge of the first will be divided equally between them. Thus, a way was indicated to change the charge of the ball by two, three, etc. times. In Coulomb's experiments, the interaction between balls whose dimensions were much smaller than the distance between them was measured. Such charged bodies are usually called point charges.

A point charge is a charged body whose dimensions can be neglected in the conditions of this problem.

There are also: linear charge t(tau)=dq/dl, l-length, dq-charge of the thread

Surface charge: σ =dq/ds s-surface area (cell/m 2)

Volume charge p(ro)=dq/dv (cell/m3)

Interaction forces obey Newton's third law: They are repulsive forces with the same signs of charges and attractive forces with different signs (Fig. 1.1.3). The interaction of stationary electric charges is called electrostatic or Coulomb interaction. The branch of electrodynamics that studies the Coulomb interaction is called electrostatics .

Coulomb's law is valid for point charged bodies. In practice, Coulomb's law is well satisfied if the sizes of charged bodies are much smaller than the distance between them.

Proportionality factor k in Coulomb's law depends on the choice of system of units. In the International SI System, the unit of charge is taken to be pendant(Cl).

Pendant is a charge passing through the cross-section of a conductor in 1 s at a current of 1 A. The unit of current (ampere) in SI is, along with units of length, time and mass basic unit of measurement.

Coefficient k in the SI system it is usually written as:

Experience shows that the Coulomb interaction forces obey the principle of superposition.

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

The principle of superposition is a fundamental law of nature. However, its use requires some caution when we are talking about the interaction of charged bodies of finite sizes (for example, two conducting charged balls 1 and 2). If a third charged ball is brought to a system of two charged balls, then the interaction between 1 and 2 will change due to charge redistribution.

The principle of superposition states that when given (fixed) charge distribution on all bodies, the forces of electrostatic interaction between any two bodies do not depend on the presence of other charged bodies.