Electromagnetic forces play a huge role in nature due to the fact that the composition of all bodies includes electrically charged particles. The constituent parts of atoms of the nucleus and electrons have an electric charge
The electromagnetic forces that exist between charged particles are enormous. However, the action of electromagnetic forces between bodies is not directly detected, since the bodies in the normal state are electrically neutral. An atom of any substance is neutral, since the number of electrons in it is equal to the number of protons in the nucleus. Positively and negatively charged particles are connected to each other by electrical forces and form neutral systems.
A macroscopic body is electrically charged if it contains an excess of elementary particles with the same charge sign. The negative charge of the body is due to an excess of electrons compared to protons, and the positive charge is due to a lack of electrons.
In order to obtain an electrically charged macroscopic body, i.e., to electrify it, it is necessary to separate part of the negative charge from the positive charge associated with it. This can be done with friction. If you run a comb over dry hair, then a small part of the most mobile charged particles - electrons - will pass from the hair to the comb and charge it negatively, and the hair will be charged positively.
Equality of charges during electrization. With the help of experience, it can be proved that when electrified by friction, both bodies acquire charges opposite in sign, but identical in absolute value. Let us take an electrometer with a
a metal sphere with a hole and two plates on long handles: one of ebonite and the other of plexiglass. When rubbing against each other, the plates are electrified. Let us bring one of the plates inside the sphere without touching its walls. If the plate is positively charged, then some of the electrons from the needle and the electrometer rod will be attracted to the plate and collect on the inner surface of the sphere. In this case, the arrow will be positively charged and repelled from the rod (Fig. 92, a). If another plate is introduced inside the sphere, having previously removed the first one, then the electrons of the sphere and the rod will be repelled from the plate and accumulate in excess on the arrow. This will cause the arrow to deviate, and at the same angle as in the first experiment. Having lowered both plates inside the sphere, we will not find the deviation of the arrow (Fig. 92, b). This proves that the charges of the plates are equal in magnitude and opposite in sign.
How is the electrification of bodies? When electrifying bodies, close contact between them is important. Electrical forces hold the electrons inside the body. But for different substances these forces are different. In close contact, a small part of the electrons of that substance, in which the connection of electrons with the body is relatively weak, passes to another substance. In this case, the displacements of electrons do not exceed the sizes of interatomic distances (cm). But if the bodies are separated, then both of them will be charged.
Since the surfaces of bodies are never perfectly smooth, the close contact between the bodies necessary for the transfer of electrons is established only in small areas of the surfaces (Fig. 93). When bodies rub against each other, the number of areas with close contact increases and thereby increases the total number of charged particles passing from one body to another.
Electrization of bodies and its application in technology. Significant electrification occurs during friction of synthetic fabrics. When taking off a nylon shirt in dry air, a characteristic crackle can be heard. Small sparks jump between charged areas of rubbing surfaces. Such phenomena have to be considered in production. Thus, threads of yarn in textile factories are electrified by friction, are attracted to spindles and rollers and break. Yarn attracts dust and gets dirty.
It is necessary to apply special measures against the electrification of threads.
The electrification of bodies in close contact is used in electro-copying machines (such as "Era", "Xerox", etc.).
So, in one of these installations, black resin powder is mixed with tiny glass beads. In this case, the balls are charged positively, and the powder particles are negatively charged. Due to attraction, they cover the surface of the balls with a thin layer.
The copied text or drawing is projected onto a thin selenium plate, the surface of which is positively charged. The plate rests on a negatively charged metal surface. Under the action of light, the plate is discharged and a positive charge remains only in areas corresponding to dark areas of the image. After that, the plate is covered with a thin layer of balls. Due to the attraction of opposite charges, the resin powder is attracted to the positively charged areas of the plate. Then the balls are shaken off and, pressing a sheet of paper tightly against the plate, an imprint is obtained on it. The imprint is fixed by heating.
Many physical phenomena observed in nature and the life around us cannot be explained only on the basis of the laws of mechanics, molecular-kinetic theory and thermodynamics. These phenomena manifest forces acting between bodies at a distance, and these forces do not depend on the masses of the interacting bodies and, therefore, are not gravitational. These forces are called electromagnetic forces.
The law of conservation of electric charge
Under normal conditions, microscopic bodies are electrically neutral because the positively and negatively charged particles that form atoms are connected to each other by electrical forces and form neutral systems. If the electrical neutrality of the body is violated, then such a body is called electrified body. To electrify a body, it is necessary that an excess or deficiency of electrons or ions of the same sign be created on it.
Methods of electrification of bodies, which represent the interaction of charged bodies, can be as follows:
- Electrification of bodies upon contact . In this case, with close contact, a small part of the electrons passes from one substance, in which the bond with the electron is relatively weak, to another substance.
- Electrization of bodies during friction . This increases the contact area of the bodies, which leads to increased electrization.
- Influence. Influence is based phenomenon of electrostatic induction, that is, the induction of an electric charge in a substance placed in a constant electric field.
- Electrification of bodies under the action of light . This is based on photoelectric effect, or photoelectric effect when, under the action of light, electrons can fly out of the conductor into the surrounding space, as a result of which the conductor is charged.
Numerous experiments show that when body electrification, then electric charges appear on the bodies, equal in magnitude and opposite in sign.
negative charge body is due to an excess of electrons on the body compared to protons, and positive charge due to a lack of electrons.
When the electrification of the body occurs, that is, when the negative charge is partially separated from the positive charge associated with it, law of conservation of electric charge. The law of conservation of charge is valid for a closed system, which does not enter from the outside and from which charged particles do not go out.
The law of conservation of electric charge is formulated as follows:
In a closed system, the algebraic sum of the charges of all particles remains unchanged:
q 1 + q 2 + q 3 + ... + q n = const
where
q 1 , q 2 etc. are the particle charges.
Definitions
Elementary particles may have email charge, then they are called charged;
Elementary particles - interact with each other with forces that depend on the distance between the particles, but many times exceed the forces of mutual gravitation (this interaction is called electromagnetic).
Electric charge- physical quantity, determines the intensity of electromagnetic interactions.
There are 2 signs of electric charges:
- positive
- negative
Particles with the same charges repel, with opposite names - are attracted. The proton has positive charge, electron negative, neutron - electrically neutral.
elementary charge- the minimum charge that cannot be divided.
How to explain the presence of electromagnetic forces in nature? All bodies contain charged particles.
In the normal state, bodies are electrically neutral (because the atom is neutral), and electromagnetic forces do not appear.
Body charged, if it has an excess of charges of any sign:
- negatively charged - if there is an excess of electrons;
- positively charged - if the lack of electrons.
Electrification of bodies- this is one of the ways to obtain charged bodies, for example, by contact).
In this case, both bodies are charged, and the charges are opposite in sign, but equal in magnitude.
Interaction of bodies, having charges of the same or different signs, can be demonstrated in the following experiments. We electrify the ebonite stick by rubbing against the fur and touch it to a metal sleeve suspended on a silk thread.
Charges of the same sign (negative charges) are distributed on the sleeve and ebonite stick. Bringing a negatively charged ebonite rod closer to a charged cartridge case, one can see that the cartridge case will be repelled from the stick (Fig. 1.1).
If we now bring a glass rod rubbed on silk (positively charged) to the charged sleeve, then the sleeve will be attracted to it (Fig. 1.2).
Let's take two identical electrometers and charge one of them (Fig. 2.1). Its charge corresponds to 6 divisions of the scale.
If you connect these electrometers with a glass rod, then no change will occur. This confirms the fact that glass is a dielectric. If, however, to connect the electrometers, use a metal rod A (Fig. 2.2), holding it by a non-conductive handle B, then you can see that the initial charge is divided into two equal parts: half of the charge will transfer from the first ball to the second. Now the charge of each electrometer corresponds to 3 divisions of the scale. Thus, the original charge has not changed, it has only split into two parts.
If the charge is transferred from a charged body to an uncharged body of the same size, then the charge is divided in half between these two bodies. But if the second, uncharged body is larger than the first, then more than half of the charge will transfer to the second. The larger the body to which the charge is transferred, the greater part of the charge will transfer to it.
But the total amount of charge will not change. Thus, it can be argued that the charge is conserved. Those. the law of conservation of electric charge is satisfied.
Electric charges do not exist by themselves, but are internal properties of elementary particles - electrons, protons, etc.
Empirically in 1914, the American physicist R. Milliken showed that electric charge is discrete . The charge of any body is an integer multiple of elementary electric charge e = 1.6 × 10 -19 C .
In the reaction of formation of an electron-positron pair, law of conservation of charge.
q electron +q positron = 0.
Positron- an elementary particle having a mass approximately equal to the mass of an electron; The charge of the positron is positive and equal to the charge of the electron.
Based law of conservation of electric charge explains the electrification of macroscopic bodies.
As you know, all bodies are made up of atoms, which include electrons and protons. The number of electrons and protons in an uncharged body is the same. Therefore, such a body does not exhibit electrical action on other bodies. If two bodies are in close contact (during rubbing, compression, impact, etc.), then the electrons associated with atoms are much weaker than protons, they pass from one body to another.
The body to which the electrons have passed will have an excess of them. According to the law of conservation, the electric charge of this body will be equal to the algebraic sum of the positive charges of all protons and the charges of all electrons. This charge will be negative and equal in value to the sum of the charges of excess electrons.
A body with an excess of electrons has a negative charge.
A body that has lost electrons will have a positive charge, the modulus of which will be equal to the sum of the electron charges lost by the body.
A positively charged body has fewer electrons than protons.
The electric charge does not change when the body moves to another frame of reference.
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Electrostatics studies the properties and interactions of charges that are stationary in the reference frame in which they are considered.
In nature, there are only two types of electric charges - negative and positive. A positive charge can occur on a glass rod rubbed with skin, and a negative charge can occur on amber rubbed with a woolen cloth.
We know that all bodies are made up of atoms. In turn, an atom consists of a positively charged nucleus and electrons that revolve around it. Since the electrons have a negative charge and the nucleus is positive, the atom as a whole is electrically neutral. When exposed to it from the outside, it can lose one or more electrons and turn into a positively charged ion. In the event that an atom (or molecule) attaches an additional electron to itself, it will turn into a negative ion.
Thus, electric charge can exist in the form of negative or positive ions and electrons. There is one kind of "free electricity" - negative electrons. Therefore, if a body has a positive charge, it does not have enough electrons, and if it has a negative charge, then it has an excess.
The electrical properties of any substance are determined by its atomic structure. Atoms can lose even a few electrons, in which case they are called multiply ionized. The nucleus of an atom is made up of protons and neutrons. Each proton carries a charge that is equal to that of the electron, but opposite in sign. Neutrons are electrically neutral particles (have no electrical charge).
In addition to protons and electrons, other elementary particles also have an electric charge. Electric charge is an integral part of elementary particles.
The smallest charge is considered to be the charge equal to the charge of the electron. It is also called the elementary charge, which is equal to 1.6 10 -19 C. Any charge is a multiple of an integer number of electron charges. Therefore, the electrification of the body cannot occur continuously, but only in steps (discretely), by the value of the electron charge.
If a positively charged body begins to be recharged (charged with negative electricity), then its charge will not change instantly, but will first decrease to zero, and only then acquire a negative potential. From this we can conclude that they compensate each other. This fact led scientists to the conclusion that in "uncharged" bodies there are always charges of positive and negative signs, which are contained in such quantities that their action completely compensates for each other.
When electrified by friction, the negative and positive "elements" contained in the "uncharged body" are separated. As a result of the movement of the negative elements of the body (electrons), both bodies are electrified, and one of them is negative, and the second is positive. The amount of "flow" from one element to another charges remains constant throughout the entire process.
From this it can be concluded that charges are not are created and do not disappear, but only “flow” from one body to another or move inside it. This is the essence of the law of conservation of electric charges. During friction, many materials are subject to electrification - ebonite, glass and many others. In many industries (textile, paper and others), the presence of static electricity is a serious engineering problem, since the electrification of elements caused by the friction of paper, fabric or other production products on machine parts can cause fires and explosions.
Topics of the USE codifier: electrization of bodies, interaction of charges, two types of charge, law of conservation of electric charge.
Electromagnetic interactions are among the most fundamental interactions in nature. Forces of elasticity and friction, gas pressure and much more can be reduced to electromagnetic forces between particles of matter. The electromagnetic interactions themselves are no longer reduced to other, deeper types of interactions.
An equally fundamental type of interaction is gravity - the gravitational attraction of any two bodies. However, there are several important differences between electromagnetic and gravitational interactions.
1. Not everyone can participate in electromagnetic interactions, but only charged bodies (having electric charge).
2. Gravitational interaction is always the attraction of one body to another. Electromagnetic interactions can be both attraction and repulsion.
3. The electromagnetic interaction is much more intense than the gravitational one. For example, the electric repulsion force of two electrons is several times greater than the force of their gravitational attraction to each other.
Every charged body has some amount of electric charge. Electric charge is a physical quantity that determines the strength of the electromagnetic interaction between objects of nature. The unit of charge is pendant(CL).
Two types of charge
Since the gravitational interaction is always an attraction, the masses of all bodies are non-negative. But this is not the case for charges. Two types of electromagnetic interaction - attraction and repulsion - are conveniently described by introducing two types of electric charges: positive and negative.
Charges of different signs attract each other, and charges of different signs repel each other. This is illustrated in fig. one ; balls suspended on threads are given charges of one sign or another.
Rice. 1. Interaction of two types of charges
The ubiquitous manifestation of electromagnetic forces is explained by the fact that charged particles are present in the atoms of any substance: positively charged protons are part of the atomic nucleus, and negatively charged electrons move in orbits around the nucleus.
The charges of a proton and an electron are equal in absolute value, and the number of protons in the nucleus is equal to the number of electrons in orbits, and therefore it turns out that the atom as a whole is electrically neutral. That is why, under normal conditions, we do not notice the electromagnetic effect from the surrounding bodies: the total charge of each of them is zero, and the charged particles are evenly distributed throughout the volume of the body. But if electrical neutrality is violated (for example, as a result of electrification) the body immediately begins to act on the surrounding charged particles.
Why there are exactly two types of electric charges, and not some other number of them, is not currently known. We can only assert that the acceptance of this fact as primary gives an adequate description of electromagnetic interactions.
The charge of a proton is Cl. The charge of an electron is opposite to it in sign and is equal to C. Value
called elementary charge. This is the minimum possible charge: free particles with a smaller charge were not found in the experiments. Physics cannot yet explain why nature has the smallest charge and why its magnitude is precisely that.
The charge of any body is always the sum of the whole number of elementary charges:
If , then the body has an excess number of electrons (compared to the number of protons). If, on the contrary, the body lacks electrons: there are more protons.
Electrification of bodies
In order for a macroscopic body to exert an electrical influence on other bodies, it must be electrified. Electrification- this is a violation of the electrical neutrality of the body or its parts. As a result of electrification, the body becomes capable of electromagnetic interactions.
One of the ways to electrify a body is to impart an electric charge to it, that is, to achieve an excess of charges of the same sign in a given body. This is easy to do with friction.
So, when rubbing a glass rod with silk, part of its negative charges goes to the silk. As a result, the stick is charged positively, and the silk is negatively charged. But when rubbing an ebonite stick with wool, part of the negative charges transfers from the wool to the stick: the stick is charged negatively, and the wool is positively charged.
This method of electrification of bodies is called electrification by friction. You encounter electrification by friction every time you take off a sweater over your head ;-)
Another type of electrification is called electrostatic induction, or electrification through influence. In this case, the total charge of the body remains equal to zero, but is redistributed so that positive charges accumulate in some parts of the body, and negative charges in others.
Rice. 2. Electrostatic induction
Let's look at fig. 2. At some distance from the metal body there is a positive charge. It attracts the negative charges of the metal (free electrons), which accumulate on the areas of the body surface closest to the charge. Uncompensated positive charges remain in the far regions.
Despite the fact that the total charge of the metallic body remained equal to zero, a spatial separation of charges occurred in the body. If we now divide the body along the dotted line, then the right half will be negatively charged, and the left half positively.
You can observe the electrification of the body using an electroscope. A simple electroscope is shown in Fig. 3 (image from en.wikipedia.org).
Rice. 3. Electroscope
What happens in this case? A positively charged rod (for example, previously rubbed) is brought to the electroscope disk and collects a negative charge on it. Below, on the moving leaves of the electroscope, uncompensated positive charges remain; pushing away from each other, the leaves diverge in different directions. If you remove the wand, then the charges will return to their place and the leaves will fall back.
The phenomenon of electrostatic induction on a grandiose scale is observed during a thunderstorm. On fig. 4 we see a thundercloud going over the earth.
Rice. 4. Electrification of the earth by a thundercloud
Inside the cloud there are ice floes of different sizes, which are mixed by ascending air currents, collide with each other and become electrified. In this case, it turns out that a negative charge accumulates in the lower part of the cloud, and a positive charge accumulates in the upper part.
The negatively charged lower part of the cloud induces positive charges on the surface of the earth. A giant capacitor appears with a colossal voltage between the cloud and the ground. If this voltage is sufficient to break through the air gap, then a discharge will occur - lightning, well known to you.
Law of conservation of charge
Let's return to the example of electrification by friction - rubbing the stick with a cloth. In this case, the stick and the piece of cloth acquire charges equal in magnitude and opposite in sign. Their total charge, as it was equal to zero before the interaction, remains equal to zero after the interaction.
We see here law of conservation of charge which reads: in a closed system of bodies, the algebraic sum of charges remains unchanged for any processes that occur with these bodies:
Closedness of a system of bodies means that these bodies can exchange charges only among themselves, but not with any other objects external to the given system.
When the stick is electrified, there is nothing surprising in the conservation of charge: how many charged particles left the stick - the same amount came to a piece of cloth (or vice versa). Surprisingly, in more complex processes, accompanied by mutual transformations elementary particles and number change charged particles in the system, the total charge is still conserved!
For example, in fig. 5 shows the process in which a portion of electromagnetic radiation (the so-called photon) turns into two charged particles - an electron and a positron. Such a process is possible under certain conditions - for example, in the electric field of the atomic nucleus.
Rice. 5. Creation of an electron–positron pair
The charge of the positron is equal in absolute value to the charge of the electron and is opposite to it in sign. The law of conservation of charge is fulfilled! Indeed, at the beginning of the process we had a photon whose charge is zero, and at the end we got two particles with zero total charge.
The law of conservation of charge (along with the existence of the smallest elementary charge) is today the primary scientific fact. Physicists have not yet succeeded in explaining why nature behaves in this way and not otherwise. We can only state that these facts are confirmed by numerous physical experiments.
