Electromagnetic forces play a huge role in nature due to the fact that all bodies contain electrically charged particles. The constituent parts of atoms, nuclei and electrons, have an electrical charge
The electromagnetic forces existing between charged particles are enormous. However, the action of electromagnetic forces between bodies is not directly detectable, since the bodies in their normal state are 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 macroscopic body is electrically charged if it contains an excess amount 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 a lack of electrons.
In order to obtain an electrically charged macroscopic body, that is, to electrify it, it is necessary to separate part of the negative charge from the positive charge associated with it. This can be done using friction. If you run a comb through dry 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 be charged positively.
Equality of charges during electrification. With the help of experiment, it can be proven that during electrification by friction, both bodies acquire charges of opposite sign, but equal in magnitude. Let us take an electrometer with a
a metal sphere with a hole and two plates on long handles: one made of ebonite and the other of plexiglass. When rubbing against each other, the plates become electrified. Let's bring one of the plates inside the sphere without touching its walls. If the plate is positively charged, then some of the electrons from the arrow and rod of the electrometer will be attracted to the plate and collected on inner surface spheres. At the same time, the arrow will be charged positively and will be repelled from the rod (Fig. 92, a). If you bring another plate inside the sphere, having first removed the first one, then the electrons of the sphere and the rod will be repelled from the plate and will accumulate in excess on the arrow. This will cause the arrow to deflect at the same angle as in the first experiment. Having lowered both plates inside the sphere, we will not detect 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 does electrification of bodies occur? 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 substance. The electron movements do not exceed the 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 bodies necessary for the transfer of electrons is established only on 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.
Electrification of bodies and its application in technology. Significant electrification occurs during friction of synthetic fabrics. When removing a nylon shirt in dry air, you can hear a characteristic crackling sound. Small sparks jump between the charged areas of the rubbing surfaces. Such phenomena have to be taken into account in production. Thus, threads of yarn in textile factories are electrified due to friction, attracted to spindles and rollers and torn. Yarn attracts dust and becomes dirty.
We have to apply special measures against electrification of threads.
The electrification of bodies through close contact is used in electric 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 - negatively. 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 influence of light, the plate is discharged and the positive charge remains only in areas corresponding to the dark areas of the image. After this, 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 to the plate, an imprint is made on it. The print is fixed using heat.
Electrostatics studies the properties and interactions of charges that are stationary in the frame of reference in which they are considered.
In nature there are only two types of electrical charges - negative and positive. A positive charge can appear on a glass rod rubbed with leather, and a negative charge can appear on amber rubbed with wool.
It is known that all bodies are made of atoms. In turn, an atom consists of a positively charged nucleus and electrons that revolve around it. Since electrons have a negative charge and the nucleus has a positive charge, 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. If 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 is negative, then it has an excess.
The electrical properties of any substance are determined by its atomic structure. Atoms can even lose several 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 the charge 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, electrification of the body cannot occur continuously, but only in steps (discretely), according to the amount of charge of the electron.
If a positively charged body begins to recharge (charge 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 “uncharged” bodies always contain charges of positive and negative signs, which are contained in such quantities that their action completely compensates for each other.
During electrification, friction separates the negative and positive “elements” contained in the “uncharged body”. As a result of the movement of negative elements of the body (electrons), both bodies are electrified, one of them is negative, and the second is positive. The amount of charges “flowing” from one element to another remains constant throughout the entire process.
From this we can conclude that charges are not are created and do not disappear, but simply “flow” from one body to another or move within it. This is the essence of the law of conservation of electric charges. When friction occurs, many materials are subject to electrification - ebonite, glass and many others. In many industries (textile, paper and others), the presence of static electricity represents a serious engineering problem, since electrification of elements caused by friction of paper, fabric or other industrial products against machine parts can cause fires and explosions.
1. what energy transformations occur when a body rises and when it falls?
2.what happens to the mechanical energy when a lead weight hits a lead plate?
3.what energy is called the internal energy of the system?
4. how it changes internal energy gas during its expansion; when it is compressed? give examples
5. Does a body whose temperature is 0 degrees Celsius have internal energy?
6. the same substance can be in a solid, liquid, or gaseous state. In which state is the internal energy of the body greater? less?
accompanied by the transfer of matter? A. Thermal conductivity. B. Radiation. B. Convection. 3. Which of the following substances has the highest thermal conductivity?A. Fur. B. Tree. B. Steel. 4. Which of the following substances has the lowest thermal conductivity? A. Sawdust. B. Lead. B. Copper.5. Name a possible method of heat transfer between bodies separated by airless space. A. Thermal conductivity. B. Convection. B. Radiation. 6. Metal handle and wooden door will feel equally warm to the touch at a temperature...A.above body temperature.B. below body temperature. B. equal to body temperature. 7. What happens to the temperature of a body if it absorbs as much energy as it emits? A. The body heats up. B. The body cools down.B. Body temperature does not change.8. In what way does heat transfer occur in liquids? A. Thermal conductivity. B. Convection. B. Radiation.9. Which of the following substances has the least A. Air. B. Cast iron. B. Aluminum10. The specific heat capacity of water is 4200 (J/kg*0С). This means that...A.to heat water weighing 4200 kg by 1 ° C, an amount of heat equal to 1 J.B is required. to heat water weighing 1 kg by 4200 ° C, an amount of heat equal to 1 J.B is required. To heat water weighing 1 kg by 1 ° C, it requires 11. The specific heat of combustion of a fuel shows what coA. combustion of the fuel. B. complete combustion of fuel.B. with complete combustion of fuel weighing 1 kg.12. Evaporation occurs...A.at any temperature.B. at the boiling point. B. at a certain temperature for each liquid.13. In the presence of wind, evaporation occurs...A.faster.B. slower.B. at the same speed as in its absence.14. Can the efficiency of a heat engine become 100% if friction between the moving parts of this machine is reduced to zero?A. Yes. B. No.15. From which pole of the magnet do the magnetic field lines emerge?A. From the north. B. From the south. B. From both poles.16. A body charged with a negative charge is brought to the ball of an uncharged electroscope without touching it. What charge will the leaves of the electroscope acquire? A. Negative. B. Positive. B. None.17. Can an atom of hydrogen or any other substance change its charge by 1.5 times the charge of an electron?A. Yes. B. No.18. What image is produced on the human retina?A. Magnified, real, inverted.B. Diminished, real, inverted.V. Enlarged, imaginary, direct.G. Diminished, imaginary, direct.19. What does an ammeter measure?A) Electrical resistance of conductorsB) Voltage at the poles of a current source or at some section of the circuitC) Current strength in the circuitD) Electric current power20. Diffusion is: A) The process of increasing temperature B) The phenomenon in which mutual penetration of molecules of one substance occurs between the molecules of another C) The phenomenon in which a body passes from a solid state to a liquid state D) The process of increasing the density of a body 21. Efficiency formula:A) ŋ= Аn* 100%АɜB) ŋ= Аɜ * 100%АnВ) ŋ= Аn * Аɜ100%D) ŋ= Аn * Аɜ * 100%22. What does Archimedes' law say?A) The buoyancy force acting on a body immersed in a liquid is equal to the weight of the liquid displaced by this bodyB) The buoyancy force acting on a body immersed in a liquid is equal to the speed of immersion of this body in the liquidC) The buoyancy force acting on a body immersed in a liquid , is equal to the density of this body D) The buoyant force acting on a body immersed in a liquid is equal to the weight of this body23. What deyA)tep24. InnerA) only B) only C) only D) from topics25. Which of the following substances are conductors? a) rubber; b) copper, c) plastic; d) glass.26. The body is electrified only when it...... charge.a) acquires; b) loses; c) gains or loses.27. Which of the following substances are dielectrics? a) rubber; b) copper; c) sulfuric acid solution; d) steel.28. Likely charged bodies ......., and oppositely charged ones - ......a) ...repel, ...attract, b) ...attract, ...repel.29. Electric shock called...A. Movement of electrons along a conductor.B. Ordered movement of electrons along a conductor.V. Ordered movement of protons along a conductor.G. Ordered movement of charged particles.D. Movement of electric charges along a conductor.30. What energy transformation occurs when an electric coffee grinder operates? Electrical energy is converted...A. To the chemical department. B. To mechanical. B. Into the light room. G. Internal
1) Two problems are solved: a) the diving speed of the submarine is calculated b) the time it takes for the boat to move from one military base is calculatedto another.
In what case submarine can be considered as a material point?
2) Two pulleys of different radii are connected by a belt drive and driven rotational movement. How do physical quantities (linear speed, period of rotation, angular speed) change when moving from point B to point A if the belt does not slip?
Electrification of bodies
If a macroscopic body contains an excess number of elementary particles with any one sign, then it is electrically charged. Thus, the negative charge is due to the excess number of electrons compared to the number of protons, and the positive charge is due to the lack of electrons. In order to obtain an electrically charged macroscopic body, that is, to electrify it, it is necessary to separate part of the negative charge from the positive charge associated with it. This can be done, for example, using friction. Let's say, if you run a comb through dry hair, then 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 be charged positively.
Electrification – is the process of obtaining electrically charged macroscopic bodies from electrically neutral ones .
The degree of electrification of bodies as a result of mutual friction is characterized by the value and sign of the electric charge received by the body. Rubber rubbed on fur becomes negatively charged, while glass rubbed on silk becomes positively charged. In this case, the fur receives a positive charge, and the silk – a negative one. The sign of the charge of bodies as a result of electrification is determined by the fact that some substances give up electrons during friction, while others add them. The reason for this phenomenon is the difference in the binding energy of an electron with an atom in these substances. It turns out that, depending on the binding energy, the same substance during friction with different substances can receive a charge of different sign.
Friction is just one of many ways to electrify matter. The body can be charged due to contact with a charged body, as a result of heating, light irradiation, etc. Electrification by irradiation is used, for example, in a copier and laser printer.
2. Law of conservation of charge .
We know that the mass of bodies is conserved. The electrical charge is also retained. It is the charge, not the number of charged particles. During electrification by friction, a redistribution of existing charges occurs between bodies that are neutral at the first moment. A small portion of electrons moves from one body to another. In this case, new particles do not appear, and pre-existing ones do not disappear.
When bodies are electrified, the law of conservation of electric charge is satisfied. This law is valid for a system into which charged particles do not enter from the outside and from which charged particles do not come out, that is, for closed system which is called electrically isolated .
Electrically isolated system of bodies –a system of bodies through which charges do not penetrate.
The law of conservation of charge is formulated as follows:
The algebraic sum of the charges of an electrically isolated system is constant, that is :
Q 1 + Q 2 + … + Q n = const
The law of conservation of charge has a deep meaning. If the number of charged elementary particles does not change, then the fulfillment of the charge conservation law is obvious. But elementary particles can transform into each other, be born and disappear, giving life to new particles. However, in all cases, charged particles are born only in pairs with charges of the same magnitude and opposite in sign; The particles also disappear only in pairs, turning into neutral ones. And in all cases the sum of the charges remains the same. The validity of the law of conservation of charge is confirmed by observations of a huge number of transformations of elementary particles. This law expresses one of the most fundamental properties of electric charge. The reason why the charge is retained is still unknown.
The law of conservation of charge is valid in any inertial reference frame (IFR). This means that observers measuring the charge in different ISOs will receive the same value.
Electric charge is conserved in the universe. The total electric charge of the universe is most likely zero, that is, the number of positively charged elementary particles is equal to the number of negatively charged elementary particles.
Coulomb's law.
We study electrostatics, and fundamental law of electrostatics – law of interaction of two stationary point charged bodies .
The fundamental law of electrostatics was established experimentally by the French scientist Charles Coulomb in 1785 and bears his name. Coulomb's law, like the law of universal gravitation, is also formulated for point bodies.
Point bodies do not exist in nature, but if the distance between the bodies is many times greater than their size, then neither the shape nor the size of the charged bodies significantly influence the interactions between them. In this case, the bodies can be considered as pointlike, that is, as points.
The strength of interaction between charged bodies depends on the properties of the medium between the charged bodies. For now, we will assume that the interaction occurs in a vacuum. However, experience shows that air has very little effect on the force of interaction between charged bodies; it turns out to be almost the same as in a vacuum.
The discovery of the law of interaction of electric charges was facilitated by the fact that these forces turned out to be large. Here there was no need to use particularly sensitive equipment, as when testing the law of universal gravitation under terrestrial conditions. How stationary charged bodies interact with each other was established using torsion scales .
Torsion scales consist of a glass rod suspended on a thin elastic wire. A small metal ball is attached to one end of the stick, and a counterweight is attached to the other. Another metal ball is fixedly fixed on a rod, which in turn is mounted on the lid of the scale.
When the balls are given charges of the same name, they begin to repel each other. To keep them at a fixed distance, the elastic wire must be twisted at a certain angle. The force of interaction between the balls is determined by the angle of twist of the wire. Torsion balances made it possible to study the dependence of the interaction force of charged balls on the magnitude of the charges and on the distance between them. At that time they knew how to measure force and distance. The only difficulty was with the charge, for which there were not even units to measure it. Pendant found a simple way to change the charge of the balls by 2, 4 or more times by connecting it with the same uncharged ball. In this case, the charge was distributed equally between the balls, which reduced the charge under study in a certain ratio. The new value of the interaction force with a new charge was determined experimentally. Coulomb's experiments led to the establishment of a law strikingly reminiscent of the law of universal gravitation.
The force of interaction between two stationary point charges located in a vacuum is directly proportional to the product of the moduli of these charges, inversely proportional to the square of the distance between them and is directed along the straight line connecting these charges.
How do macroscopic bodies acquire an electrical charge? This will be discussed now.
Charge of a macroscopic body
Electrodynamics, created by Maxwell, considers electromagnetic interactions not of individual charged elementary particles, but of macroscopic bodies.
Macroscopic bodies, as a rule, are 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.
Body large sizes 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.
Electrification of bodies
In order to obtain an electrically charged macroscopic body or, as they say, electrify it, you need to separate part of the negative charge from the one associated with it
positive1.
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 be charged positively.
With the help of a simple experiment, it can be proven that during electrification by friction, both bodies acquire charges of opposite sign, but equal in magnitude.
1 Here and in what follows, for brevity, we will often talk about charges, the movement of charges, etc. In reality, we mean charged bodies (or particles), the movement of charged particles, etc., since a charge without a particle does not exist.
Rice. 1.2
Rice. 1.1
Let's take an electrometer (an electroscope in a metal case) with a metal sphere with a hole attached to its rod and two plates on long handles: one made of ebonite and the other made of plexiglass. When rubbing against each other, the plates become electrified. Let's bring one of the plates inside the sphere without touching its walls. If the plate is charged positively, then some of the electrons from the needle and rod of the electrometer will be attracted to the plate and collected on the inner surface of the sphere. At the same time, the arrow will be charged positively and will be pushed away from the rod (Fig. 1.1).
If you place another plate inside the sphere, having first removed the first one, then the electrons of the sphere and the rod will be repelled from the plate and will accumulate in excess on the arrow. This will cause the arrow to deflect at the same angle as in the first experiment. Having lowered both plates inside the sphere, we will not detect any deviation of the arrow (Fig. 1.2). This proves that the charges of the plates are equal in magnitude and opposite in sign. This conclusion follows directly from the law of conservation of charge.
How does electrification of bodies occur?
It is very simple to electrify bodies using friction. But explaining how this happens turned out to be a very difficult task. For many decades the following explanation has been given, and is still being given. 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 a 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.
However, recently this explanation of electrification by friction has become controversial. 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. Employees of the Institute of Physics and Technology in St. Petersburg offered another explanation.
For 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 in 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 mix, 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.
Thus, we see that such a seemingly simple phenomenon as electrification by friction contains a lot of mystery.
Electrification of bodies and its application in technology
Significant electrification occurs during friction of synthetic fabrics. When removing a nylon shirt in dry air, you can hear a characteristic crackling sound. Small sparks jump between the charged areas of the rubbing surfaces. A similar phenomenon has to be taken into account in production. Thus, threads of yarn in textile factories are electrified due to friction, attracted to the spindles and torn. Yarn attracts dust and becomes dirty. Therefore it is necessary to take various measures against electrification of threads.
When unwinding large rolls of paper in a printing plant, workers wear rubber gloves to protect themselves from electrical discharges that occur between the electrified paper and their hands.
Large electrical charges accumulate when tires rub against asphalt in dry weather. There is a danger of a spark jumping. Therefore, metal chains are attached to the back of cars - fuel tanks - and drag along the road. Sometimes even passenger cars are equipped with an elastic band made of conductive rubber.
Due to electrification by friction, a conventional electro-static machine operates.
The phenomenon of electrification of bodies in close contact is used in modern electrocopying 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 - negatively. 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 influence of light, the plate is discharged, and a positive charge remains only in areas corresponding to the dark areas of the image. The plate is then covered with a thin layer of beads. Due to the attraction of opposite charges, the resin powder is attracted to the positively charged areas of the plate. After this, the balls are shaken off and, pressing a sheet of paper tightly to the plate, an imprint is made on it. The print is fixed using heat.
A macroscopic body is electrically charged if it contains an excess amount 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 a lack of electrons.
? 1. The ebonite stick became negatively charged when electrified. Has the mass of the stick remained the same? 2. It is known that a glass rod rubbed against silk becomes positively charged. Determine experimentally the sign of the charge of a plastic handle rubbed on wool.