Free charge is an equivalent concept, which usually refers to the charge that exists inside the substance and can make directional motion under the action of external electric field.
The macroscopic charge not caused by polarization, such as the macroscopic charge presented by the dielectric due to friction or contact with the charged body, and the macroscopic charge presented by the conductor due to the loss or acquisition of free electrons, are all free charges.
Chinese name
Free charge
Foreign name
free charge
Include
Free cation
Features
The internal binding of objects is relatively weak
Free charge includes free cation（positive charge）, free anion (negative charge) andfree electron。Such as free electrons in metals,electrolyte solutionPositive and negative ions in the gas, electrons and ions in the rarefied gas, etc.[1]
The characteristic of free charge is that the binding on them inside the object is relatively weak, and they can move freely inside the object;At the same time, free charge does not really exist, but is only a model used to describe the movement of free electrons.
The amount of free charge inside an object determines its conductivity;
The types of free charges inside objects can be different, either negative charges (such as electrons, chloride ions in electrolytic solutions, etc.) orpositive charge(e.g. hydrogen ion in solution).
When dealing with practical problems, different charged particles are often equivalent to a certain charge according to needs, for example, free electrons in wires are equivalent to an equal amount of free positive charges moving in the opposite direction.
charge
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An attribute of physical objects.Its simplest and most intuitive expression is the attraction to small and light objects (such as feathers and hair crumbs). If an object has the property of attracting small and light objects, it is said that it is charged or charged.The term electricity comes from the Greek word "elektron", which means amber.As early as 600 BC, there were records about friction electrification. In 1600, the British physicist W·GiberIt is found that not only amber can attract light and small objects after friction, but also many other substances, such as diamond, sapphiresulfur, hard resin and alum also have the property of attracting light and small objects after friction.He noticed that these materials did not have the north-south nature of magnets after friction.Charge has the following three basic properties: ① there are two kinds of charges in nature - positive charge and negative charge; ②Charge conservation; ③Quantization of electric charge.A large number of experiments have shown that objects or elementary particles can be positively or negatively charged, but their electric quantities are equal to the amount charged by the electron or its integral multiple.The same kind of charges repel each other and the different kinds of charges attract each other.When the barycenter of the equal amount of different sign charge distribution coincides, the external electric effects of the same amount of different sign charge will cancel each other and become neutral, which is considered as uncharged.There is an electric field around the charge and a magnetic field around the moving charge;The electric charge in the electric field is affected by the electric force, and the moving charge in the magnetic field is affected by the magnetic force.
polarization
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① Under the action of an external electric field, electric charges appear on the surface or inside of the dielectric.PressdielectricThe structure of can be divided into two categories.The first type is non-polar molecules without dipole moment. Under the action of external electric field, the positive and negative charges (such as electrons and atomic nuclei) that make up the molecules move relatively to form electric dipoles, which have a certain dipole moment. Their direction follows the direction of the external electric field. Therefore, positive and negative bound charges appear on the two surfaces of the dielectric respectively.The stronger the external electric field, the greater the dipole moment of the molecule, and the more bound charges, that is, the higher the degree of polarization, which is induced polarization.Usually, the vector sum of the dipole moments of each molecule in the unit volume is used as a measure of the degree of polarization.The second type is originally polar molecules with a certain dipole moment. Due to the thermal movement of molecules, the direction of the dipole moment is confused.Under the influence of the external electric field, the dipole moment has a tendency to rotate along the external electric field, but under the influence of the molecular thermal movement, not all the dipole moments of molecules are aligned along the direction of the electric field;The stronger the external electric field, the more orderly the molecular arrangement, which is directional polarization.At this time, positive and negative bound charges also appear on the two surfaces of the dielectric.This polarization process is more significant in liquid dielectrics than in solid dielectrics, because molecules are easier to rotate in liquid dielectrics. ②Also called "polarization".When the current passes through the galvanic cell or electrolytic cell, the double electric layer balance on the electrode electrolyte interface is destroyed, which makes the potential value of the electrode deviate from the balanced electrode potential (the difference between the two is the overvoltage).The degree of polarization is related to the current intensity passing through the primary cell or electrolytic cell. When no current passes through,OvervoltageIs zero, and the electrode potential value is equal to the balance electrode potential value.There are two main types, namelyElectrochemical polarizationAndConcentration polarization。The reason for polarization comes from the kinetics of electrode process, because the electrode process is a multiphase oxidation reduction reaction with a double electric layer and a surface electric field on the electrode surface. When any of these steps becomes a control step, the electrode potential can significantly deviate from the balanced electrode potential.[2]
polarization of dielectric
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Under the action of an external electric field, the dielectric shows the phenomenon of electricity.In general,dielectricMacroscopically, it does not show electricity.Under the action of external electric field, the local movement of bound charge causes it to show electricity on the macro scale, and the macro charge appears at the uneven places on the surface and inside of the dielectric. This phenomenon is calledpolarization。This macroscopic charge due to polarization is called polarized charge (also called bound charge).The polarized charge in the volume element △ I divided by △ I is the polarized charge volume density of the point.Similarly, the polarized charge surface density exists on the polarized dielectric surface.For the sake of clarity, the macroscopic charge not caused by polarization is called free charge.For example, the macroscopic charge of a dielectric due to friction or contact with a charged body and the macroscopic charge of a conductor due to the loss or acquisition of free electrons belong to free charge.According to the microscopic mechanism of polarization, it can be divided into displacement polarization of non-polar molecules and orientation polarization of polar molecules.There are two cases of displacement polarization, one is Htwo、NtwoAnd other gas molecules, because the electronic mass is much smaller than the nuclear mass.Under the action of the electric field force, the electron center of gravity moves a section of displacement along the opposite direction of the field strength.Each molecule forms a small electric dipole, and the electric dipole moment of the electric dipole p ∝ E is arranged along the direction of the external electric field.The polarization of such non-polar molecules is often calledElectron displacement polarization。The other is that the positive and negative ions of the dielectric consisting of positive and negative ions move for a period of displacement along the positive and negative directions of the field strength in the external electric field, forming an electric dipole. The electric dipole moment of the electric dipole, p ∝ E, is called ion polarization.The displacement polarization of a homogeneous medium results in the appearance of polarized surface charges on the surface.In polar molecular dielectrics, each molecule has an electric moment p, but in the absence of an external field, it is not electrically sensible due to thermal motion.Under the action of external electric field, each molecular electric moment p is subjected to a torque, which makes the molecular electric moment turn to the E direction. However, due to thermal movement, this orientation is not complete, that is, all molecular dipoles are not neatly arranged along the direction of external electric field.
Of course, the stronger the E, the more orderly the orientation.This polarization mechanism is called orientation polarization.The polarized surface charge also appears on the surface as a result of the directional polarization of homogeneous media.Displacement polarization at anyDielectric polarizationThe orientation polarization only exists for polar molecules.However, in the dielectric with polar molecules, the orientation polarization is about an order of magnitude larger than the displacement polarization, so the orientation polarization is the main one.In a very high frequency electric field, due to the large inertia of molecules, the orientation polarization cannot keep up with the change of the external electric field, while the inertia of electrons is small, so at this time, no matter what kind of dielectric, only the electron displacement polarization mechanism plays a role.
Bound charge
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The positive and negative charges that exist inside the substance and can only make small relative displacement under the action of the ordinary external electric field.For example, when a dielectric is polarized in an external electric field, two kinds of polarized charges with equal quantity and different signs on its surface or inside are bound charges.[3]
A charged particle in a dielectric molecule that cannot undergo macroscopic displacement due to the constraint of the internal force of the molecule.Under the action of external electric field, these confined charged particles can do microscopic movement. Polarized charge is the macroscopic effect caused by the small displacement of bound charge in the dielectric.The polarized charge is also called the bound charge.
