« Physics - 10th grade"
First, let's consider the simplest case, when electrically charged bodies are at rest.
The branch of electrodynamics devoted to the study of the equilibrium conditions of electrically charged bodies is called electrostatics.
What is an electric charge?
What charges are there?
With words electricity, electric charge, electricity you have met many times and managed to get used to them. But try to answer the question: “What is an electric charge?” The concept itself charge- this is a basic, primary concept that cannot be reduced to modern level development of our knowledge to some simpler, elementary concepts.
Let us first try to find out what is meant by the statement: “This body or particle has an electric charge.”
All bodies are made of tiny particles, which are indivisible into simpler ones and are therefore called elementary.
Elementary particles have mass and due to this they are attracted to each other according to the law universal gravity. As the distance between particles increases, the gravitational force decreases in inverse proportion to the square of this distance. Most elementary particles, although not all, also have the ability to interact with each other with a force that also decreases in inverse proportion to the square of the distance, but this force is many times greater than the force of gravity.
So in the hydrogen atom, shown schematically in Figure 14.1, the electron is attracted to the nucleus (proton) with a force 10 39 times greater than the force of gravitational attraction.
If particles interact with each other with forces that decrease with increasing distance in the same way as the forces of universal gravity, but exceed the gravitational forces many times, 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.
The interaction of charged particles is called electromagnetic.
Electric charge determines the intensity of electromagnetic interactions, just as mass determines the intensity of gravitational interactions.
The electric charge of an elementary particle is not a special mechanism in 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 force interactions between them.
We, in essence, know nothing about charge if we do not know the laws of these interactions. Knowledge of the laws of interactions should be included in our ideas about charge. These laws are not simple, and it is impossible to outline them in a few words. Therefore, it is impossible to give a sufficiently satisfactory short definition concept electric charge.
Two signs of electric charges.
All bodies have mass and therefore attract each other. Charged bodies can both attract and repel each other. This the most important fact, familiar to you, means that in nature there are particles with electric charges of opposite signs; in the case of charges of the same sign, the particles repel, and in the case of different signs, they attract.
Charge of elementary particles - protons, which are part of all atomic nuclei, are called positive, and the charge electrons- negative. There are no internal differences between positive and negative charges. If the signs of the particle charges were reversed, then the nature of electromagnetic interactions would not change at all.
Elementary charge.
In addition to electrons and protons, there are several other types of charged elementary particles. But only electrons and protons can exist in a free state indefinitely. The rest of the charged particles live less than a millionth of a second. They are born during collisions of fast elementary particles and, having existed for an insignificantly short time, decay, turning into other particles. You will become familiar with these particles in 11th grade.
Particles that do not have an electrical charge include neutron. Its mass is only slightly greater than the mass of a proton. Neutrons, together with protons, are part of atomic nucleus. If an elementary particle has a charge, then its value is strictly defined.
Charged bodies Electromagnetic forces in nature play a huge role due to the fact that all bodies contain electrically charged particles. The constituent parts of atoms - nuclei and electrons - have an electrical charge.
Direct action electromagnetic forces between bodies is not detected, 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 bonded to each other electrical forces and form neutral systems.
A macroscopic body is electrically charged if it contains an excess amount of elementary particles with any one sign of charge. So, negative charge body is caused by an excess of electrons compared to the number of protons, and a positive one 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 or transfer a negative charge to a neutral body.
This can be done using friction. 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 charge positively.
Equality of charges during electrification
With the help of experiment, it can be proven that when electrified by friction, both bodies acquire charges that are opposite in sign, but identical in magnitude.
Let's take an electrometer, on the rod of which there is a metal sphere with a hole, and two plates on long handles: one made of hard rubber 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 positively charged, then some of the electrons from the needle 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 pushed away from the electrometer rod (Fig. 14.2, 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 deviate from the rod, and 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 at all (Fig. 14.2, b). This proves that the charges of the plates are equal in magnitude and opposite in sign.
Electrification of bodies and its manifestations. Significant electrification occurs during friction of synthetic fabrics. When you take off a shirt made of synthetic material in dry air, you can hear a characteristic crackling sound. Small sparks jump between the charged areas of the rubbing surfaces.
In printing houses, paper is electrified during printing and the sheets stick together. To prevent this from happening, special devices are used to drain the charge. However, the electrification of bodies in close contact is sometimes used, for example, in various electrocopying installations, etc.
Law of conservation of electric charge.
Experience with the electrification of plates proves that during electrification by friction, a redistribution of existing charges occurs between bodies that were previously neutral. 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, law of conservation of electric charge. This law is valid for a system into which charged particles do not enter from the outside and from which they do not leave, i.e. for isolated system.
In an isolated system algebraic sum the charges of all bodies are conserved.
q 1 + q 2 + q 3 + ... + q n = const. (14.1)
where q 1, q 2, etc. are the charges of individual charged bodies.
The law of conservation of charge has 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; Charged particles also disappear only in pairs, turning into neutral ones. And in all these cases, the algebraic sum of the charges remains the same.
The validity of the law of conservation of charge is confirmed by observations of a huge number of transformations of elementary particles. This law expresses one of the most fundamental properties of electric charge. The reason for the charge conservation is still unknown.
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It is impossible to give a brief definition of charge that is satisfactory in all respects. We are accustomed to finding explanations that we understand very complex formations and processes like the atom, liquid crystals, 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 given body or particle have an electrical 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– this is physical scalar quantity, characterizing 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 is often used material point, which makes it possible to significantly simplify the solution of many problems, when studying the interaction of charges, the idea of a point charge turns out to be 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 we talk about the interaction of two point charges, then they thereby assume that the distance between the two charged bodies under consideration is significantly greater than their linear dimensions.
Electric charge of an elementary particle
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. At identical signs particles repel each other, but if they are different, they attract each other.
There is currently no explanation for the reasons for the existence of two types of electric charges. In any case, none fundamental differences between positive and negative charges is not detected. 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. None modern theory cannot 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 amazing was that quarks should have a fractional electric charge: 1/3 and 2/3 elementary charge. To build protons and neutrons, two types of quarks are enough. And their maximum number, apparently, does not exceed six.
Unit of measurement of electric charge
Can you briefly and succinctly answer the question: “What is an electric charge?” This may seem simple at first glance, but in reality it turns out to be much more complicated.
Do we know what electric charge is?
The fact is that at the current level of knowledge we cannot yet decompose the concept of “charge” into simpler components. This is a fundamental, so to speak, primary concept.
We know that this is specific property elementary particles, the mechanism of interaction of charges is known, we can measure the charge and use its properties.
However, all this is a consequence of data obtained experimentally. The nature of this phenomenon is still not clear to us. Therefore, we cannot unambiguously determine what an electric charge is.
To do this, it is necessary to unpack a whole range of concepts. Explain the mechanism of interaction of charges and describe their properties. Therefore, it is easier to understand what the statement means: “ this particle has (carries) an electric charge.”
The presence of an electric charge on a particle
However, later it was possible to establish that the number of elementary particles is much larger, and that the proton, electron and neutron are not indivisible and fundamental building materials of the Universe. They themselves can decompose into components and turn into other types of particles.
Therefore, the name "elementary particle" currently includes a fairly large class of particles smaller in size than atoms and atomic nuclei. In this case, particles can have a variety of properties and qualities.
However, such a property as electric charge comes in only two types, which are conventionally called positive and negative. The presence of a charge on a particle is its ability to repel or be attracted to another particle, which also carries a charge. The direction of interaction depends on the type of charges.
Like charges repel, unlike charges attract. Moreover, the force of interaction between charges is very large in comparison with the gravitational forces inherent in all bodies in the Universe without exception.
In the hydrogen nucleus, for example, an electron carrying a negative charge is attracted to a nucleus consisting of a proton and carrying positive charge, with a force 1039 times greater than the force with which the same electron is attracted by a proton due to gravitational interaction.
Particles may or may not carry a charge, depending on the type of particle. However, it is impossible to “remove” the charge from the particle, just as the existence of a charge outside the particle is impossible.
In addition to the proton and neutron, some other types of elementary particles carry a charge, but only these two particles can exist indefinitely.
From approximately 1000 seconds (for a free neutron) to a negligible fraction of a second (from 10 −24 to 10 −22 s for resonances).
The structure and behavior of elementary particles is studied by particle physics.
All elementary particles are subject to the principle of identity (all elementary particles of the same type in the Universe are completely identical in all their properties) and the principle of particle-wave dualism (each elementary particle corresponds to a de Broglie wave).
All elementary particles have the property of interconvertibility, which is a consequence of their interactions: strong, electromagnetic, weak, gravitational. Particle interactions cause transformations of particles and their collections into other particles and their collections, if such transformations are not prohibited by the laws of conservation of energy, momentum, angular momentum, electric charge, baryon charge, etc.
Main characteristics of elementary particles: lifetime, mass, spin, electric charge, magnetic moment, baryon charge, lepton charge, strangeness, isotopic spin, parity, charge parity, G-parity, CP-parity.
Classification
By lifetime
- Stable elementary particles are particles that have infinite big time life in a free state (proton, electron, neutrino, photon and their antiparticles).
- Unstable elementary particles are particles that decay into other particles in a free state in a finite time (all other particles).
By weight
All elementary particles are divided into two classes:
- Massless particles are particles with zero mass (photon, gluon).
- Particles with non-zero mass (all other particles).
By largest back
All elementary particles are divided into two classes:
By type of interaction
Elementary particles are divided into the following groups:
Compound particles
- Hadrons are particles that participate in all types of fundamental interactions. They consist of quarks and are divided, in turn, into:
- mesons are hadrons with integer spin, that is, they are bosons;
- baryons are hadrons with half-integer spin, that is, fermions. These, in particular, include the particles that make up the nucleus of an atom - proton and neutron.
Fundamental (structureless) particles
- Leptons are fermions that have the form of point particles (that is, not consisting of anything) up to scales of the order of 10 −18 m. They do not participate in strong interactions. Participation in electromagnetic interactions observed experimentally only for charged leptons (electrons, muons, tau leptons) and not observed for neutrinos. There are 6 known types of leptons.
- Quarks are fractionally charged particles that are part of hadrons. They were not observed in the free state (a confinement mechanism has been proposed to explain the absence of such observations). Like leptons, they are divided into 6 types and are considered structureless, however, unlike leptons, they participate in strong interactions.
- Gauge bosons are particles through the exchange of which interactions are carried out:
- photon is a particle that carries electromagnetic interaction;
- eight gluons - particles that carry the strong force;
- three intermediate vector bosons W + , W− and Z 0, which tolerate weak interaction;
- graviton - hypothetical particle, transferring gravitational interaction. The existence of gravitons, although not yet experimentally proven due to the weakness of gravitational interaction, is considered quite probable; however, the graviton is not included in the Standard Model of elementary particles.
Video on the topic
Sizes of elementary particles
Despite the wide variety of elementary particles, their sizes fit into two groups. The sizes of hadrons (both baryons and mesons) are about 10 −15 m, which is close to the average distance between the quarks included in them. The sizes of fundamental, structureless particles - gauge bosons, quarks and leptons - within the experimental error are consistent with their point nature ( upper limit diameter is about 10−18 m) ( see explanation). If in further experiments the final sizes of these particles are not discovered, then this may indicate that the sizes of gauge bosons, quarks and leptons are close to the fundamental length (which very likely may turn out to be the Planck length equal to 1.6 10 −35 m) .
It should be noted, however, that the size of an elementary particle is a rather complex concept that is not always consistent with classical concepts. Firstly, the uncertainty principle does not allow one to strictly localize a physical particle. A wave packet, which represents a particle as a superposition of precisely localized quantum states, always has finite dimensions and a certain spatial structure, and the dimensions of the packet can be quite macroscopic - for example, an electron in an experiment with interference on two slits “feels” both slits of the interferometer, separated by a macroscopic distance . Secondly, physical particle changes the structure of the vacuum around itself, creating a “coat” of short-term virtual particles - fermion-antifermion pairs (see Polarization of the vacuum) and bosons that carry interactions. The spatial dimensions of this region depend on the gauge charges possessed by the particle and on the masses of the intermediate bosons (the radius of the shell of massive virtual bosons is close to their Compton wavelength, which, in turn, is inversely proportional to their mass). So, the electron radius from the point of view of neutrinos (between them it is only possible weak interaction) is approximately equal to the Compton wavelength of W bosons, ~3×10 −18 m, and the dimensions of the region strong interaction hadrons are determined by the Compton wavelength of the lightest hadron, the pi meson (~10 −15 m), which acts here as an interaction carrier.
Story
Initially, the term “elementary particle” meant something absolutely elementary, the first brick of matter. However, when hundreds of hadrons with similar properties were discovered in the 1950s and 1960s, it became clear that hadrons at least have internal degrees of freedom, that is, they are not elementary in the strict sense of the word. This suspicion was later confirmed when it turned out that hadrons consist of quarks.
Thus, physicists have moved a little deeper into the structure of matter: leptons and quarks are now considered the most elementary, point-like parts of matter. For them (together with gauge bosons) the term “ fundamental particles".
In string theory, which has been actively developed since about the mid-1980s, it is assumed that elementary particles and their interactions are consequences various types vibrations of especially small “strings”.
Standard model
The Standard Model of elementary particles includes 12 flavors of fermions, their corresponding antiparticles, as well as gauge bosons (photons, gluons, W- And Z-bosons), which carry interactions between particles, and the Higgs boson, discovered in 2012, which is responsible for the presence inert mass at particles. However, the Standard Model is largely viewed as a temporary theory rather than a truly fundamental one, since it does not include gravity and contains several dozen free parameters (particle masses, etc.), the values of which do not follow directly from the theory. Perhaps there are elementary particles that are not described Standard model- for example, such as the graviton (a particle that hypothetically carries gravitational forces) or supersymmetric partners of ordinary particles. In total, the model describes 61 particles.
Fermions
The 12 flavors of fermions are divided into 3 families (generations) of 4 particles each. Six of them are quarks. The other six are leptons, three of which are neutrinos, and the remaining three carry a unit negative charge: the electron, muon, and tau lepton.
| First generation | Second generation | Third generation |
|---|---|---|
| Electron: e− | Muon: μ − | Tau lepton: τ − |
| Electron neutrino: ν e | Muon neutrino: ν μ | Tau neutrino: ν τ (\displaystyle \nu _(\tau )) |
| u-quark (“up”): u | c-quark (“charmed”): c | t-quark (“true”): t |
| d-quark (“down”): d | s-quark (“strange”): s | b-quark (“lovely”): b |
Antiparticles
There are also 12 fermionic antiparticles corresponding to the above twelve particles.
| First generation | Second generation | Third generation |
|---|---|---|
| positron: e+ | Positive muon: μ + | Positive tau lepton: τ + |
| Electron antineutrino: ν ¯ e (\displaystyle (\bar (\nu ))_(e)) | Muon antineutrino: ν ¯ μ (\displaystyle (\bar (\nu ))_(\mu )) | Tau antineutrino: ν ¯ τ (\displaystyle (\bar (\nu ))_(\tau )) |
| u-antique: u ¯ (\displaystyle (\bar (u))) | c-antique: c ¯ (\displaystyle (\bar (c))) | t-antique: t ¯ (\displaystyle (\bar (t))) |
| d-antique: d ¯ (\displaystyle (\bar (d))) | s-antique: s ¯ (\displaystyle (\bar (s))) | b-antique: b ¯ (\displaystyle (\bar (b))) |
Quarks
Quarks and antiquarks have never been discovered in a free state - this is explained by the phenomenon