In electromagnetics, charge density is a measure that describes the density of charge distribution. The charge density can be classified into linear charge density, surface charge density, and bulk charge density.
Assuming that the charge is distributed on a curve or a straight rod, the linear charge density is the amount of charge per unit length, and the unit is coulomb/meter (coulomb/meter). Assuming that the charge is distributed on a plane or the surface of an object, the surface charge density is the amount of charge per unit area, and the unit is Coulomb/m^2. Assuming that the charge is distributed in a certain area or inside of an object in a three-dimensional space, the volume charge density is the amount of charge per unit volume, and the unit is Coulomb/m^3.
Definition
From the perspective of the macroscopic effect, the charge on the charged body can be considered as a continuous distribution. The density of charge distribution can be measured by charge density. The charge of the volume distribution is measured by the volume density of the charge, and the charge of the area distribution and the line distribution are measured by the charge surface density and the charge linear density respectively. A measure of the density of charge distribution. When the charge is distributed inside the object, the amount of electricity per unit volume is called the volume charge density; when it is distributed on the surface of the object, the amount of electricity per unit area is called the surface charge density; when it is distributed on the line, the amount of electricity per unit length is called the line charge density. When a solid is charged, the charge is distributed on the surface, and the surface charge density is the highest at the tip of the solid. The electric charge of the flowing liquid is mixed in the liquid. The charging status of the powder varies randomly with the dispersion, suspension, and deposition of the powder. Gas charging is the charging of powder particles (such as moisture, impurities) suspended in the gas.
Since there are two kinds of charges in nature, positive and negative, the charge density may be negative. The charge density may also depend on location. Take special care not to confuse charge density with charge carrier density.
The charge density is related to the volume of charge carriers. For example, due to the relatively small radius of lithium cations, its bulk charge density is greater than that of sodium cations.
Classical charge density
Assuming that a charge carrier with a volume of V has a uniform charge density regardless of position, then the total charge Q is
Suppose that there are N discrete point charges in a certain area, like electrons. Then, the charge density can be expressed by the Dirac delta function as
Among them, is the inspection position, and is the amount of charge of the i-th point where the position is.
Quantum charge density
In quantum mechanics, there is a positively charged nucleus in the center of a hydrogen-like atom, which surrounds an electron orbital around the nucleus, and its charge density can be expressed as a wave function
Among them, q is the amount of charge of electrons.
Note that is the probability of finding an electron. After normalization, the probability of finding an electron in all space is
For example, the wave function of a hydrogen atom is
Among them, is the radial function, is the spherical harmonic function, n is the principal quantum number, l is the angular quantum number, and m is the magnetic quantum number.
Relativistic
Relativistic charge density,
From the perspective of relativity, the length of the wire is related to the movement speed of the observer, so the charge density is a relativistic concept. Anthony French (Anthony French) showed in his work that the moving charge density produces magnetic field force, which attracts or repels other current-carrying wires. . Using the Minkowski diagram, Frankie explained why a neutral current-carrying wire appears to carry a net charge density to an observer in a moving reference frame. Through space-time coordinates, the field of studying electromagnetic phenomena is called relativistic electromagnetism.
Charge areal density
The amount of electricity per unit area of the sample is in μC/m2. LFY-403 Triboelectric Charge Tester (Faraday Cylinder Method) evaluates the electrostatic characteristics of fabrics charged in the form of friction under laboratory conditions.
Devices and tools
(1) Measuring range of electrostatic charge, 0μC-2μC,
(2) Inner cylinder, diameter 400mm～600mm, height 750mm～900mm,
(3) Outer cylinder, diameter 500mm～70mm, height 850mm～1000mm,
(4) Teflon insulation bracket
(5) Atmospheric conditions for the test, temperature: (20 ± 5) ℃, relative humidity: 35% ± 5%.
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