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Chips with Everything
Published in Sharon Ann Holgate, Understanding Solid State Physics, 2021
Some types of dielectric materials have an electric dipole moment when they are below a certain critical temperature even when they are not in an electric field. These materials are known as ferroelectrics. If the temperature is raised above this critical temperature, which is known as the Curie temperature, TC, the material loses its electric dipole moment unless it happens to be in an external electric field. In other words, below the Curie temperature the material is ferroelectric, whereas above the Curie temperature, it is an ordinary dielectric. The Curie temperature for several different ferroelectric materials is shown in Table 7.1.
Liquids: comparison with experiment
Published in Michael de Podesta, Understanding the Properties of Matter, 2020
Polar molecules have a permanent (intrinsic) electric dipole moment pP due to the distribution of electric charge within each molecule. An applied electric field exerts a torque on the molecules, which tends to align them with the applied field. In addition, the electronic charge distribution within the molecule can itself be changed by the applied field, resulting in an induced electric dipole moment. The magnitude of the induced dipole moment is related to α, the polarisability of the molecule.
Dielectric Properties
Published in Daniel D. Pollock, PHYSICAL PROPERTIES of MATERIALS for ENGINEERS 2ND EDITION, 2020
If a dielectric material is placed between the plates of a capacitor, its electrical effect is to increase the capacitance of the assembly. This is a result of the polarization of the molecules of the intervening material. Substances composed of polar molecules (those with a permanent electric dipole moment) have random molecular orientations in the absence of an external electric field. The field between the charged plates aligns, or polarizes, the molecular dipoles of the intervening material. This tendency of molecular alignment to become parallel to the field is called orientation polarization. If the intervening, insulating substance is a liquid or a gas, as is sometimes the case, the polar molecules can move much more freely and become aligned more readily than in most solids. The molecules of the insulating material become predominantly arrayed such that the negative poles at one of its surfaces are adjacent to the positive poles on the surface of the positive plate. The dipolar molecules in the interior of the dielectric material array themselves in a −, +, −, + order until, at the opposite surface of the material, the positive molecular poles are adjacent to the negative plate. A layer of negative charge thus forms on the surface of the dielectric adjacent to the surface of the positive plate and one of positive charge is created adjacent to the surface of the negative plate. The result is the effective cancellation of some of the charges on each of the plates and the consequent reduction of the electric field strength. This increases the ability of the plates to hold more charges; their capacity is increased. In cases in which the molecules are not polar (those with symmetric charge distributions), polarization may be induced by an applied field to produce results similar to those just noted. This occur in solids in which the molecules or ions are not free to rotate.
Study of diffusion characteristics of asphalt–aggregate interface with molecular dynamics simulation
Published in International Journal of Pavement Engineering, 2021
Man Huang, Hongliang Zhang, Yang Gao, Li Wang
Further in-depth analysis from the MD perspective indicates that due to the non-uniform and asymmetrical distribution of molecular charges, the positive and negative charge centres of the molecules do not coincide. As a result, the molecules acquire polarity which can be characterised by the dipole moment. Noteworthy, the dipole moment is of utmost importance in both physics and chemistry fields, and it is often used to determine the spatial configuration of molecules. The overall dipole moment of a molecule is approximately equal to the vector sum of the dipole moments of individual bonds and groups, which eventually relates the microstructure with macroscopic properties of the molecules. Owing to the continuous movement of molecules, orientation of electric dipole moment varies constantly; therefore, the magnitude of the electric dipole moment is more meaningful. The electric dipole moment is defined as the product of the electric quantity q of any particle in the electric dipole and the separation d between the two particles. The equation for calculating the molecular dipole moment is represented as follows:where the unit of μ is debye. μi denotes the dipole moment of atomic nucleus i; qi denotes the charge of atom A; ri denotes the radius vector corresponding to charge qi and n denotes the number of atoms in the molecule.
Optimisation of the dipole-Coulomb approximation for high-l Rydberg states of polar molecules
Published in Molecular Physics, 2020
Anastasia S. Chervinskaya, Dmitrii L. Dorofeev, Sergei V. Elfimov, Boris A. Zon
It is well known that quantum defects of high-l RSs are quite small as well as the difference between their wavefuctions and hydrogenic ones. However, it must be emphasised that this small difference is essential for analysis of the influence of the molecular core structure on the motion of the Rydberg electron in these states. It is of special importance for polar molecules due to the electric dipole moment of their cores. A simple way to use QDT for high-l RSs in polar molecules was proposed in [35,36], where the motion of the electron in a potential involving the point Coulomb and point dipole terms was considered. In the frame of this approach, the variables can be separated, and the solution of the one-electron Schrödinger equation can be written as follows: where the angular part is the dipole-spherical function [35], is the usual spherical harmonic, is the dipole moment, and . The axis z is directed along , and the molecule is supposed to be nonrotating (Born-Oppenheimer approximation). For . The function (2) is referred to as the DCA-function. The properties of functions (3) and the radial function in (2) are considered in detail in [57].
Guiding neutral polar molecules by electromagnetic vortex field
Published in Journal of Modern Optics, 2020
The dynamics of such a molecule results from the interaction between the electric dipole moment and the electric field of the electromagnetic wave. The following equations of motion constitute the complete set for translational and rotational degrees of freedom of a molecule of mass m: where denotes its position and the angular momentum. The latter can be written as where we have introduced the ‘projected’ angular velocity in place of the true angular velocity connected with the rotation of the molecule. This leads to some simplification of the equations of motion: In fact, when guiding a polar particle along a beam of radiation, we are not interested in the value of , but mainly in the position . Therefore, is a quantity equally good in our considerations as . Apparently might be (at least in principle) eliminated from the above equations, resulting in the motion independent of the value of κ. This is not true, since κ enters through the initial conditions for and . Nonetheless from (6) one can draw a conclusion that for any ‘oblateness’ the same trajectory can be obtained by means of the appropriate modification on the initial conditions, although the rotational states will be different. Equations containing are much more intricate because of the nontrivial time dependence of the moment of inertia components connected with the instantaneous orientation of the electric dipole moment.