The positive and negative charges in the dielectricelectric fieldUnder the action of force, only small displacement can be made within the range of atoms or molecules, which are called bound charges.
In an external electric field, the interior of a uniform medium is still electrically neutral, but there will be charges on the surface of the medium. Such charges cannot leave the dielectric to other charged bodies, nor can they move freely inside the dielectric.It is called bound charge or polarized charge.
Chinese name
Bound charge
Foreign name
Bound charge
Alias
Polarized charge
Field
electromagnetics
Nature
Only small displacement in the atomic or molecular range
Dielectric polarizationAfter that, additional charges can be generated inside and on the surface of the dielectric. Because this charge can not be led away by conduction like the free charge in the conductor, it can only make small displacement within the atomic or molecular range under the action of electric field force, so it is called bound charge or polarized charge.
Unlike the free charge in the conductor, which can be led away by conduction, the phenomenon of bound charge in the external electric field is calledDielectric polarization。
Polarization intensity relation
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(1) In a uniform dielectric, the bound charges cancel each other, and the bound charges only appear on the surface of the dielectric.When a homogeneous medium is polarized, the polarization charge surface density of a point on its surface is equal to that pointElectric polarization intensityThe component on the outer normal.All macroscopic effects produced by dielectrics are reflected by the binding charges that are not offset.The relationship between bound charge distribution and polarization intensity is derived below.
Generally defined
Is the normal direction outside the media.
Surface:
Inside the media:
because
, so
。
(2) In heterogeneous dielectrics, there is bound charge accumulation.In the electric field, the polarization flux passing through any closed surface is equal to the negative value of the total amount of polarized charges in the closed surface.
Surface:
Inside:
。
The negative value of polarization charge in any curved surface is equal to the flux of polarization intensity.
Density correlation
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Relationship between bulk density of bound charge and polarization intensity
Take an arbitrarily shaped volume V in the medium, the boundary surface of V is S, and then find the polarization charge quantity q 'in V.
If V tends to be physically infinitesimal, then the ratio
Is the bound charge volume density at that point.
Analysis: Dipoles cut into two sections by the boundary surface S of V contribute to the polarized charge q '.
Take a small bin dS on S
And this point
In the same direction and assuming
Normal to panel
The included angle is
。
DS is very small, and it is considered that the points above it
The dipole near dS has the same dipole moment
。
Make two panels dS parallel to dS on both sides of dSoneAnd dStwo, the vertical distance between them and dS is
Obviously, the dipole with the center inside the interlayer must be intercepted by dS, contributing to the polarization charge q ', while the dipole with the center outside the interlayer has no contribution to the polarization charge q'.
If the number of molecules per unit volume in the medium is n, the volume of interlayer is
；
The number of dipoles (molecular number) centered in the interlayer is
