This is a kind ofdielectricPolarization phenomenon.First, when a piece of uniform dielectric composed of non-polar molecules is placed in an external electric field, the positive and negative charges in the molecule will be subject to the electric field force in the opposite direction, so the center of the positive and negative charges will have a small relative displacement, thus formingElectric dipole, whichElectric dipole momentWill be arranged along the direction of the external electric field.At this time, positive and negative polarization charges will also appear on the front and rear surfaces of the dielectric along the direction of the external electric field.This is an elastic, instantaneous and energy free polarization mode.
Polar molecule: the positive charge center of the molecule coincides with the negative charge center.
Equivalent electric dipole moment of non-polar molecule:
The electrodeless molecular dielectrics are also neutral as a whole.[1]
classification
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Displacement polarization can be divided into electronic displacement polarization andIon displacement polarization。
Electron displacement polarization
Electron displacement polarizationUnder the action of external electric fielddielectricThe periphery of an atom or ionElectronic cloudInduction due to elastic displacement relative to atomic nucleusdipole momentPhenomenon.Under the action of the alternating electric field, it can be regarded as a spring oscillator, and the elastic restoring force is - kx.
Establish Newton equation:
；
Electric dipole moment:
；
Natural frequency of spring vibrator:
yes
, get
When
yes
。
Ion displacement polarization
The polarization caused by the ion shifting from the equilibrium position under the action of electric field.
Under the action of alternating electric field, the movement of ions in the electric field is assumed to be a spring oscillator.
The induced electric dipole moment is
As above, the natural frequency of the elastic vibrator is obtained:
Ion displacement polarizability:
When
yes
。
polarization of dielectric
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Dielectric polarization mechanism
The orientation of a large number of particles constituting a macro object is in a state of chaos due to thermal motion. No matter whether the particles themselves have electric moments, the contribution of particles to the macro electrode is always zero due to the average result of thermal motion. Only under the effect of an external electric field,Only particles can contribute an electric moment along the electric field direction that can be accumulated to give the macroscopic polarization intensity. In the case that the effect of the macroscopic external electric field is much smaller than the interaction inside the structural particles (composite particles), the local electric field acting on the particles polarizes the particles and produces electric dipole moments.
α is called microscopic polarizability.
The contribution of a particle to the susceptibility can have different reasons:
1. The displacement of the negative charge center caused by the distortion of the electron cloud produces an induced electric moment, calledElectron displacement polarization, its polarizability
2. The relative displacement of the positive and negative ion center and the induced electric moment occur, which is called ion displacement polarization, and its polarizability
3. The inherent electric dipole moment turns to symmetrical orientation polarization along the direction of external electric field, and its polarizability
4. The actual dielectric may have interlayer or a large number of crystal defects due to non-uniformity
The first two polarizations are displacement polarization, and the last two are relaxation polarization.
nature:
（1）Electron displacement polarizationThe electrons with relative displacement caused by the distortion of the electron cloud under the action of the electric field of atoms or ions are mainly valence electrons, because these electrons are in the outermost and secondary outer layers of the orbit, farthest from the nucleus and least bound by the nucleus;
(2) The response time of electron displacement polarization to external field is very short, about
Also called optical frequency polarization;
(3) Under the action of electric field, any dielectric has electronic displacement polarization, and the induced dipole moment generated by the electronic displacement polarization of atoms, molecules and ions.
Displacement polarization of non-polar molecules
After the external electric field is added, the positive and negative charge centers of the dielectric molecule no longer coincide under the action of the electric field force, forming an electric dipole. Their equivalent electric dipole moments P are all along the direction of the electric field.
Two and external electric field strength of dielectric
In the vertical surface layer, there will be positive charges and negative charges respectively.These charges cannot leave the medium, nor can they move in the dielectric itself. We call them polarized charges.This phenomenon of polarization charge appearing in the dielectric under the action of external electric field is called dielectric polarization.
Because the polarization of non-polar molecules lies in the relative displacement of positive and negative charge centers, it is often called displacement polarization.
Orientation polarization of polar molecules
When there is no external electric field, the electric dipole moment orientation of polar molecules is different, and the entire medium is not charged.
The intrinsic electric moment of polar molecules in the external electric field is subject to a torque, and the direction of the electric moment turns to be consistent with the direction of the external electric field.
Polarization of polar molecules is the direction in which the equivalent electric dipole turns to the external electric field, so it is called orientation polarization.
Generally speaking, displacement polarization will also occur when molecules are oriented. However, for polar dielectrics, the effect of orientation polarization is much stronger than that of displacement polarization under the action of electrostatic field, so the polarization mechanism of polar molecules is orientation polarization.
