Showing papers on "Electric potential published in 1992"

General relativistic electric potential drops above pulsar polar caps

Abstract: We study the general relativistic electrodynamics of an isolated, rotating, magnetic neutron star. We consider the region of a neutron star magnetosphere with steady, space charge limited flow along open magnetic field lines. The explicit solutions to the Maxwell equations are obtained. Being the simplest, this model enables one to carry out analytically a general relativistic treatment, and to demonstrate the influence of the effects of General Relativity on the creation of an electric field in the afore-mentioned region.

Membrane light modulating systems.

Abstract: A membrane light modulator (18) comprising a charge transfer plate (30), and having a multiplicity of conductors (35) extending from the rear surface (52) to the front surface (19) of the plate (30). The conductors (35) are supported in an insulating matrix (37). The front side (19) of the transfer plate (30) has a plurality of potential wells (32) defined by insulating walls (15), each potential well (32) constituting a pixel. A plurality of conductors (35) is provided for each pixel. A deformable reflecting membrane (34, 36) comprising a metal layer (36) spans the potential wells (32). An electric potential is provided on the metal layer (36), and a source of electrons (17) is provided for impacting the rear surface (52) of the charge transfer plate (30).

alpha decay calculations with a realistic potential

Abstract: We have previously used a cluster model, employing a square-well nuclear potential plus a surface-charge Coulomb potential, to satisfactorily describe {alpha}-decay half-lives for more than 400 nuclei. Here, we investigate a more realistic version of this cluster model, in which the square-well nuclear potential is replaced by a cosh'' potential geometry having nonzero diffuseness, and the Coulomb potential by one appropriate to a point charge {alpha} particle interacting with a uniformly charged spherical core. By varying the adjustable parameters of this more realistic model, we find several potentials which give comparable fits to the {alpha}-decay data, and select the one which best tallies with information from other areas of nuclear physics. In addition, we find that the {alpha}-particle preformation probability needed to describe favored transitions in odd-mass nuclei is only 60% of the equivalent quantity required for the ground state to ground state transitions of even-even nuclei.

On the mapping of electrostatic properties from the multipole description of the charge density.

Abstract: A method is presented to calculate the electrostatic potential, the electric field and the electric field gradient in a crystal from the atomic multipole expansion of the experimental charge density, as described by the Hansen-Coppens formalism [Hansen & Coppens (1978), Acta Cryst. A34, 909–921]. The electrostatic properties are expressed in terms of the positions and the charge-density parameters of the individual atoms. Contributions due to the procrystal charge density and the deformation charge density are compared. The method is illustrated by the calculation of the electrostatic potential maps of fully deuterated benzene and of iron(II) tetraphenylporphyrin.

Integral equation theory for charged liquids: Model 2-2 electrolytes and the bridge function

Abstract: The integral equation theory for a model 2–2 electrolyte is studied in detail. In this model electrolyte, the ions are assumed to be the same size, and interact via a continuous potential energy which behaves as the Coulomb potential at large distances and an inverse ninth power repulsion at short distances. The ions are embedded in a dielectric continuum of fixed dielectric constant, here taken to be 78.3 e0 in order to model water at 25 °C. The bridge function for this model is studied as a function of concentration (a) for six proposed closures, and (b) via ‘‘exact’’ inversion of data from computer simulations. A proposed closure derived from examination of the inverted bridge function yields predictions in good agreement with computer simulations. We emphasize the importance of choosing an ‘‘optimized’’ long‐range potential, as opposed to the traditional Coulomb choice. A simple functional form for the bridge function results from this optimized choice of long‐range potential.

Theoretical study of short- and long-range forces and atom transfer in scanning force microscopy.

Abstract: We investigate the interaction energy, the short-range force components, and the electron potential between two Al(001) slabs, which mimic a blunt tip close to an atomically corrugated sample in scanning force microscopy. The adhesive energy and perpendicular force calculated using the self-consistent-field pseudopotential method in the local-density approximation are site dependent, but can be accurately represented by a universal function in terms of scaled variables in the attractive range. The lateral force which determines friction variations on an atomic scale is not simply proportional to the perpendicular force and is typically one order of magnitude smaller. At larger separations the effect of the total long-range Van der Waals force and of its gradient are estimated to be small for a sharp conical support tip, but quite appreciable for a rounded support tip with a radius as small as 200 \AA{}. By calculating the interaction energy of an Al atom between two slabs, we also study the possibility of single-atom transfer between tip and sample, and show that the double well in the interaction energy collapses into a single minimum at a slab separation larger than two bulk interlayer spacings. The atom is preferentially located on the side of the deeper minimum, but can hop between the two wells at finite temperatures. Moreover, the position of the deeper minimum relative to the electrodes can vary as the tip is scanned against the sample. Finally we explore possible relations between the short-range perpendicular force and the tunneling conductance through the potential barrier between two semi-infinite jellium slabs as a function of their separation.

Dipoles at rest

Abstract: In a world populated by magnetic monopoles (as well as ordinary electric charges), there are two kinds of electric dipoles: those due to separated electric charges, and those due to current loops of magnetic charge. Similarly, there are two kinds of magnetic dipoles: those due to separated magnetic monopoles, and those due to electric current loops. This paper derives the potentials and fields of each of the four dipole species, and calculates the force, torque, energy, momentum, and angular momentum of each type, when placed (at rest) in a static external field (which may itself be produced by electric charges and currents, magnetic charges and currents, or all of these). Some implications and applications of the various results are discussed.

Computation of electric fields and potential on polluted insulators using a boundary element method

Abstract: The authors present a calculation method for electric fields and potential applicable in the case of a polluted insulator. This method, based on boundary integral equations, is suitable for 3-D geometries and polluted layers. Results obtained by this method are compared with analytical solutions (potential, currents) and with measured values (potential, leakage current). >

Solvation free energies and solvent force constants

Abstract: A theoretical formulation for the solvent force constant k{sub q}, which gauges electrical potential fluctuations for an ion in solution and whose charge dependence is a measure of nonlinear aspects of solvation, is presented in terms of the solute charge (q) variation of the solvation free energy. This formulation allows the calculation of k{sub q} via integral equation theories. This is illustrated by a series of calculations for ionic solutes in model dipolar-quadrupolar solvents via the reference hypernetted chain (RHNC) integral equation approach. It is found that the q variation of k{sub q} can be comprehended in terms of the cooperative (or competing) contributions of the solvent dipole and quadrupole to the acceleration of the solvation free energy. By contrast, traditional notions of dielectric saturation prove to be of much less direct relevance, due in part to the importance of competing electrostriction effects. The formalism is also applied to available simulation and integral equation solvation free energy studies of aqueous ionic solvation to infer to behavior of k{sub q}. The extensions for the formalism to more complex solutes (e.g., ion pairs), to higher order fluctuations (e.g., electric field), and to the solvent frequency and effective mass are briefly indicated. 51more » refs., 11 figs., 1 tab.« less

A semi-empirical many-body interatomic potential for modelling dynamical processes in gallium arsenide

Abstract: An empirical many-body potential for GaAs is developed. General agreement is obtained between the experimental and predicted values of bulk GaAs properties and the energetics of small As, Ga and GaxAsy clusters. The potential is suitable for use in molecular dynamics computer codes for modelling atomic collisions in GaAs where low energy collisions are important, such as the recrystallisation process that takes place at the end of an atomic collision cascade. For small interatomic separation (hard collisions) the potential can be splined to a screened Coulomb potential in the usual way.

Ericson fluctuations in the chaotic ionization of the hydrogen atom in crossed magnetic and electric fields.

Abstract: We report exact quantum calculations for the hydrogen atom in crossed magnetic and electric fields. Employing the complex-coordinate-rotation method we are able to extend the calculations of eigenstates far into the continuum region. Calculated photoionization cross sections are found to exhibit strong Ericson fluctuations, a characteristic feature of chaotic scattering. This interpretation is supported by classical trajectory calculations which reveal a fractal dependence of the classical ionization time on the initial conditions.

Algebraic treatment of a general noncentral potential

Abstract: It is shown that the problem of a particle moving in a noncentral potential generalizing the Coulomb potential and the Hartmann ring‐shaped potential accepts SO(2,1)⊗SO(2,1) as a dynamical group, within the framework of the Kustaanheimo–Stiefel transformation. The Green’s function relative to this compound potential is calculated through the algebraic approach so(2,1) and with the help of the Baker–Campbell–Hausdorff formulas. The energy spectrum and the normalized wave functions of the bound states can then be deduced. Eventually, the Coulomb potential, the Hartmann ring‐shaped potential and also, due to its close link with the latter, the compound Coulomb plus Aharonov–Bohm potential may all be considered as particular cases.

Membrane Curvature Elasticity in Weakly Charged Lamellar Phases

Abstract: We study the effect of electrostatic interactions on the membrane bending energies of weakly charged, swollen, lamellar phases of surfactant solutions. We treat the surface charge density of the lamellae as a constant and consider only situations where it is low enough so that the distance 2d between lamellae is the smallest relevant length scale in the problem. In the presence of salt (the short-distance DebyeHuckel regime) we show that the electrostatic contribution to the bending energy of a membrane is in general small, in disagreement with a previous result, is proportional to d3, and is independent of ionic strength. Identical results are obtained for membranes undulating sinusoidally in phase and for concentric cylindrical membranes. The bending constant is also calculated for membranes held at constant electric potential and is compared to the constant charge density case. In the absence of salt, continuity arguments predict an electrostatic contribution to the bending energy that scales as d3. Furthermore, a direct calculation for concentric cylindrical membranes gives exactly the same scaling behavior (including the numerical prefactor) as in the presence of salt.

A new two-dimensional model for the potential distribution of short gate-length MESFET's and its applications

Abstract: A new analytical technique for calculating the 2-D potential distribution of a MESFET device operated in the subthreshold region is proposed, in which the 2-D Poisson's equation is solved by the Green's function technique. The potential and electric-field distributions of a non-self-aligned MESFET device are calculated exactly from different types of Green's function in different boundary regions, and the sidewall potential at the interface between these regions can be determined by the continuation of the electric field at the sidewall boundary. The remarkable feature of the proposed method is that the implanted doping profile in the active channel can be treated. Furthermore, a simplified technique is developed to derive a set of quasi-analytical models for the sidewall potential at both sides of the gate edge, the threshold voltage of short gate-length devices, and the drain-induced barrier lowering. Moreover, the developed quasi-analytical models are compared with the results of 2-D numerical analysis and good agreements are obtained. >

Large-Scale Electric and Magnetic Fields Generated by the Oceans

Abstract: The magnetostatic equations are used to derive a consistent set of equations capable of describing the global-scale, low-frequency electric and magnetic fields induced by the motion of the ocean through the geomagnetic field. The equations are solved numerically with realistic 2° × 2° topography in a global domain with ocean flow simulated by a detailed ocean circulation model. Estimates of the annual mean and the first annual harmonic of the electric potential, the vertical component of the oceanic magnetic field, and the vertically integrated electric current density stream function are obtained. With the idea of using electric and magnetic fields to deduce large-scale oceanic flow, emphasis is placed on the geographical location of interesting features. The fields are not found to be basin-wide but rather are found to be localized and strongest in shallow regions. The magnetic fields generated by ocean currents are of the order of 1 nT, and while these can be measured by magnetometers, they would be difficult to detect owing to contamination from other sources of magnetic variation. In finding the electric field, electric currents cannot be neglected where ocean currents cut across isobaths. However, in regions where the ocean flow is aligned with the isobaths, measurement of electric fields is sufficient to find the ocean flow.

Adiabatic pair potential of highly charged plates in an electrolyte

Abstract: The effective interaction between highly charged plates immersed in an electrolyte solution is investigated using mean field theory. The Helmholtz free energy is formulated as a functional of the mean electric potential which satisfies the Poisson-Boltzmann equation. Exact solutions of the Poisson-Boltzmann equation under Dirichlet and Neumann boundary conditions enable us to express the free energy analytically in terms of Legendre's elliptic integrals of the first and second kinds. The adiabatic potentials of the charged plates, which are determined as the parts of the free energy that depend on the interplate distance, turn out to have long-range weak attractive parts as well as medium-range strong repulsive parts irrespective of the type of boundary conditions. While the repulsion originates mainly in the osmotic pressure of the excess ions trapped between the plates by the large surface charges, the attraction arises from an electric pull from the intermediate cloud of excess counterions between the ...

The polarization electric field and its effects in an anisotropic rotating magnetospheric plasma

Abstract: Spatial variations of density and temperature along a magnetic field line are evaluated for a plasma undergoing adiabatic motion in a rotating magnetosphere. The effects of centrifugal and gravitational forces are accounted for, as is anisotropy in the pitch angle distribution functions of individual species. A polarization electric field is invoked to eliminate the net electric charge density resulting from the aforementioned mass dependent forces and different anisotropies. The position of maximum density in a two-component, electron-ion plasma is determined both in the absence and in the presence of the polarization effect and compared. A scale height, generalized to include anisotropies, is derived for the density fall-off. The polarization electric field is also included in the parallel guiding center equation; equilibrium points are determined and compared in both individual and average senses with the position of density maximum. Finally a transverse (to magnetic field lines) electric component is deduced as a consequence of dissimilar charge neutralization on adjacent field lines. The E x B velocity resultant from such a 'fringing' electric field is calculated and compared with the magnitude of other drifts.

Effect of divertor bias on plasma flow in the DIII-D scrape-off layer

Abstract: A toroidal ring bias electrode in contact with the single-null divertor scrape-off layer in the DIII-D tokamak achieved electric field control of plasma flow in the layer. Bias strongly altered the static pressure of neutralized gas in the new divertor chamber (designed for future pumping) during Ohmic, L-mode and ELMing H-mode plasma operation. The pressure increased or decreased, depending on the signs of the applied electric potential and toroidal magnetic field, in agreement with the E*B drift direction. The variation of the core plasma density was opposite to the variation of the pump chamber gas pressure and was proportionally much smaller. A large bias induced pressure rise appeared even when the scrape-off layer was not directed at the pump entrance aperture-a surprising effect that is interpreted qualitatively in terms of radial potential discontinuities. Bias also affected divertor heat fluxes, and preliminary observations are discussed qualitatively

Scattering of an induced surface electromagnetic field by fatigue cracks in ferromagnetic metals

Abstract: In crack detection and sizing by the alternating current field measurement technique, U-shaped wires or coils excited by a high-frequency AC current source can be used to induce the surface field in the workpiece. The authors present a modeling technique for the interaction of a fatigue crack in a ferromagnetic metal with the surface field resulting from an inducer with two U-shaped wires. This work is an extension of a previous modeling technique to have developed for infinitely long (one-dimensional) cracks. In the present technique, the boundary of the fatigue crack is approximated by a circular arc, leading to a formulation for an efficient computation of the field-flaw interaction. Various numerical and experimental results supporting the modeling and illustrating the behavior of the magnetic field and electric potential at the metal surface around circular-arc cracks are presented. >

Ion and potential distributions in charged and non-charged primitive spherical pores in equilibrium with primitive electrolyte solution calculated by grand canonical ensemble Monte Carlo simulation. Comparison with generalized Debye–Hückel and Donnan theory

Abstract: Grand canonical ensemble Monte Carlo simulations (GCEMC) have been performed for dilute to moderately concentrated restricted primitive model electrolytes (1 : 1) in equilibrium with spherical micropores with radii from 1.5 to 10 times the ionic diameter. The pores are primitive : hard walls, with the same relative permittivity as the pore fluid and a smeared out fixed charge on the walls. The fixed charge is set to zero, one or five elementary charges. The constraining chemical potentials in the bulk solution are found by canonical Monte Carlo simulations by HNC calculations.The following topics are emphasized : 1, The need to move ions independently without regard to electroneutrality. 2, The deviation from electroneutrality in isolated small pores. 3, Electroneutrality may be artificially induced by the application of an overall ‘Donnan potential’. 4, Electroneutrality is a collective phenomenon of a ensemble of many pores. 5, Average mean activity coefficients in the pores depend slightly on the total applied electric potential for the dilute solutions corresponding to electrosorption. The dependence is even more pronounced for the average single ion activity coefficients. 6, Except in dilute solutions, the mean ionic singlet distribution functions G±(r) are unaffected by the presence of the wall charge. 7, G± near the wall is a compromise between the hard sphere ‘piling up’ at high concentrations (an ionic diameter effect) and the tendency of the ions at lower concentrations to avoid the zone, where symmetric ionic clouds cannot be formed (a Debye length effect). The latter effect points to the need for a generalisation of the simple Onsager–Samaras theory of surface adsorption and surface tension due to image charges. 8, The electric potential may be effectively found and smoothed by a Poisson equation integration of the GCEMC data for G+ to G–. 9, At low concentrations, a simple analytic generalisation of the Debye–Huckel theory explains well the observed potential distribution. 10, At higher concentrations, the potential distribution may still be well fitted by one or two eigenfunctions of the Laplace operator. 11, There is evidence of a quasi-crystalline structure induced by the wall charge in the middle of large pores at higher ionic concentrations. 12, The present simulation method may be used also for simulation of bulk properties of electrolytes without introduction of periodic boundary conditions, since bulk densities are obtained in a great volume fraction in a pore with radius only ca. 10 times the ionic diameter.We discuss some implications of the present model calculations and future generalisations for the theory of real desalination membranes with an alveolar structure of the skin layer and for the theory of ion-exchange membranes.

Critical analysis of electric field modeling: formamide

Abstract: A detailed analysis of ab initio derived electric field and multipole modeling is presented using formamide as an example. Penetration effects, a problem encountered with most currently used modeling methods, are avoided using the Overlap Multipolar Expansion (OME) technique computed up to the hexadecapole level. This method is shown to be grid insensitive. The OME fields and subsequent atomic charge modeling are found to be more sensitive to correlation effects than to basis sets changes. Three different types of error analysis are applied to test the accuracy of regenerated fields from various multipole descriptions. It is shown that atom centered charge models, those most commonly used in the literature, have significant errors. Considerable improvement can be made by using various combinations of charge, dipole, and quadrupole atom- or bond-centered models. For molecular dynamic programs it is argued that the best compromise between accuracy and efficiency is to use both atom and fictive atom charge models calculated from MP2 level density matrices using at least DZP level basis sets. © 1992 by John Wiley & Sons, Inc.

Large Electric Fields in Acoustic Waves and the Stimulation of Lightning Discharges

Abstract: Fluid acceleration can play a role similar to that of gravity in the establishment of charge separation generated electric fields in weakly ionized plasma in which solid particles are the dominant carriers of both negative and positive charge. It is pointed out that the properties of fluid motions, the ionization structures, and grain size and charge distributions necessary to produce electric fields strong enough to induce discharge in the absence of gravity can be inferred from calculations of the electric fields in linear acoustic waves

Method of and apparatus for testing circuit boards and the like with an inhomogeneous electric field

Abstract: Selected test points of conductors on the substrate of a printed circuit board are tested by positioning the circuit board adjacent an array of electrodes or by utilizing a set of conductors in or on the substrate as an array of electrodes. An electric potential is set up by applying an inhomogeneous electric field to the array of electrodes or to the conductors to be tested, and signals denoting the characteristics of the electric potential are generated. Such signals are evaluated by a suitable circuit, for example, by comparing them with reference signals which are indicative of passable or non-defective conductors, i.e., of conductors which are devoid of open and/or short circuits, or by comparing the generated signals with each other. Test probes are utilized to apply an electric field to the conductors to be tested and/or to transmit signals to the evaluating circuit.

Behaviour of conductive particles in corona-dominated electric fields

Abstract: The authors present a theoretical analysis and an experimental study of the behavior of spherical and cylindrical conductive particles in contact with a metallic electrode, and affected by a corona-dominated electric field in atmospheric air. An accurate mathematical model of the conductive particle movement is introduced, taking into account the variation of the electric charge of the small object in a mono-ionized electric field, and the action of air convection produced by unipolar injection. Several experiments have been performed, using point-plane and wire-cylinder electrodes to quantify the contributions of the electric field, space charge density, and shape, size and mass of the particles. The results are compared to those for electric fields without space charge. >

A new equation for the electrical potential of liquid and PVC membranes containing both neutral carriers and ion-exchangers

Abstract: A new equation for the electrical potential of liquid and film membranes, containing both neutral and charged carriers, is presented The equation holds at any degree of association of electrolytes in the membrane, but it gives only an implicit description of the membrane potential Some results of corresponding digital simulations are discussed and compared with the experimental data

Ion Collection from Laser-Induced Plasma Using Positively Biased Wire Electrode

Abstract: In atomic vapor laser isotope separation (AVLIS), an ion collection method which has a short collection time and low applied potential is required. We demonstrated that by using a positively biased wire electrode, the ions are collected from the laser-induced plasma in a shorter time at the same applied potential or at lower electric potential in the same collection times compared with the conventional parallel electrode method. The ion collection times could be estimated using simple one-dimensional models for both the wire electrode method and the parallel electrode method.