Showing papers on "Electric potential published in 1997"

Influence of tissue resistivities on neuromagnetic fields and electric potentials studied with a finite element model of the head

Abstract: Modeling in magnetoencephalography (MEG) and electroencephalography (EEG) requires knowledge of the in vivo tissue resistivities of the head. The aim of this paper is to examine the influence of tissue resistivity changes on the neuromagnetic field and the electric scalp potential. A high-resolution finite element method (FEM) model (452162 elements, 2-mm resolution) of the human head with 13 different tissue types is employed for this purpose. Our main finding was that the magnetic fields are sensitive to changes in the tissue resistivity in the vicinity of the source. In comparison, the electric surface potentials are sensitive to changes in the tissue resistivity in the vicinity of the source and in the vicinity of the position of the electrodes. The magnitude (strength) of magnetic fields and electric surface potentials is strongly influenced by tissue resistivity changes, while the topography is not as strongly influenced. Therefore, an accurate modeling of magnetic field and electric potential strength requires accurate knowledge of tissue resistivities, while for source localization procedures this knowledge might not be a necessity.

Introduction to ground surface self-potential tomography†

Abstract: A new approach to self-potential (SP) data interpretation for the recognition of a buried causative SP source system is presented. The general model considered is characterized by the presence of primary electric sources or sinks, located within any complex resistivity structure with a flat air-earth boundary. First, using physical considerations of the nature of the electric potential generated by any arbitrary distribution of primary source charges and the related secondary induced charges over the buried resistivity discontinuity planes, a general formula is derived for the potential and the electric field component along any fixed direction on the ground surface. The total effect is written as a sum of elementary contributions, all of the same simple mathematical form. It is then demonstrated that the total electric power associated with the standing natural electric field component can be written in the space domain as a sum of cross-correlation integrals between the observed component of the total electric field and the component of the field due to each single constitutive elementary charge. By means of the cross-correlation bounding inequality, the concept of a scanning function is introduced as the key to the new interpretation procedure. In the space domain, the scanning function is the unit strength electric field component generated by an elementary positive charge. Next, the concept of charge occurrence probability is introduced as a suitable function for the tomographic imaging of the charge distribution geometry underground. This function is defined as the cross-correlation product of the total observed electric field component and the scanning function, divided by the square root of the product of the respective variances. Using this physical scheme, the tomographic procedure is described. It consists of scanning the section, through any SP survey profile, by the unit strength elementary charge, which is given a regular grid of space coordinates within the section, at each point of which the charge occurrence probability function is calculated. The complete set of calculated grid values can be used to draw contour lines in order to single out the zones of highest probability of concentrations of polarized, primary and secondary electric charges. An extension to the wavenumber domain and to three-dimensional tomography is also presented and discussed. A few simple synthetic examples are given to demonstrate the resolution power of the new SP inversion procedure.

Plasma processing method and apparatus

Abstract: A plasma processing method and apparatus are provided for processing the surface of a semiconductor device or the like through the effect of plasma A pulsed plasma discharge is performed by switching on and off the high frequency electric power for generating the plasma with a specified off period of the plasma generation, to control an inflow amount of positive and negative charges to sparse and dense portions of device patterns and suppress an electric potential on a gate oxide film Thereby, a highly accurate etching process with no charging damage can be carried out

Distribution of charge along the lightning channel : Relation to remote electric and magnetic fields and to return-stroke models

Abstract: We derive exact expressions for remote electric and magnetic fields as a function of the time- and height-varying charge density on the lightning channel for both leader and return-stroke processes. Further, we determine the charge density distributions for six return-stroke models. The charge density during the return-stroke process is expressed as the sum of two components, one component being associated with the return-stroke charge transferred through a given channel section and the other component with the charge deposited by the return stroke on this channel section. After the return-stroke process has been completed, the total charge density on the channel is equal to the deposited charge density component. The charge density distribution along the channel corresponding to the original transmission line (TL) model has only a transferred charge density component so that the charge density is everywhere zero after the wave has traversed the channel. For the Bruce-Golde (BG) model there is no transferred, only a deposited, charge density component. The total charge density distribution for the version of the modified transmission line model that is characterized by an exponential current decay with height (MTLE) is unrealistically skewed toward the bottom of the channel, as evidenced by field calculations using this distribution that yield (1) a large electric field ramp at ranges of the order of some tens of meters not observed in the measured electric fields from triggered-lightning return strokes and (2) a ratio of leader-to-return-stroke electric field at far distances that is about 3 times larger than typically observed. The BG model, the traveling current source (TCS) model, the version of the modified transmission line model that is characterized by a linear current decay with height (MTLL), and the Diendorfer-Uman (DU) model appear to be consistent with the available experimental data on very close electric fields from triggered-lightning return strokes and predict a distant leader-to-return-stroke electric field ratio not far from unity, in keeping with the observations. In the TCS and DU models the distribution of total charge density along the channel during the return-stroke process is influenced by the inherent assumption that the current reflection coefficient at ground is equal to zero, the latter condition being invalid for the case of a lightning strike to a well-grounded object where an appreciable reflection is expected from ground.

Inactivation kinetics of salmonella dublin by pulsed electric field

Abstract: Microbial inactivation kinetic models are needed to predict treatment dosage in food pasteurization processes. In this study, we determined inactivation kinetic models of Salmonella dublin in skim milk with a co-field flow high voltage pulsed electric field (PEF) treatment system. Electric field strength of 15-40 kV/cm, treatment time of 12-127 μs, medium temperatures of 10-50C were tested. A new inactivation kinetic model that combines the effect of treatment time to electric field strength or medium temperature was developed.

Central collisions of charged dust particles in a plasma

Abstract: Elastic dust particle particle interaction in an rf plasma provides a charge measurement method independent of the knowledge of plasma parameters. Assuming a screened Coulomb potential surrounding each particle with a constant charge at fixed plasma conditions, the charge and the screening length can be calculated from the trajectories of two colliding particles. With this method, we determine the charge and screening length of dust particles in the sheath of an rf discharge. The method can also be used to determine plasma parameters taking the dust particles as local probes. {copyright} {ital 1997} {ital The American Physical Society}

Electric potential changes associated with slip failure of granite: Preseismic and coseismic signals

Abstract: Electric potential changes were measured for stick-slip events in granite samples with a three-block direct shear arrangement at 8 MPa normal stress. Two electrodes were mounted on the left- and right-hand blocks, and the electric potential difference between each electrode and the ground was measured with a high input impedance recording system of frequency range from DC to 100 Hz. As well as coseismic electric signals of about 1.5 V which appeared the moment of the dynamic slip event, preseismic signals were detected just before the slip event. The coseismic signal rises stepwise with opposite polarities at the two electrodes and exponentially decays with a time constant of e/s, where e is the permittivity and s is the conductivity of the rock sample. We conducted a simple test of rapid stress drop without slipping and observed almost the same electric signal as the coseismic signal. This suggests that the electric signal is generated by the piezoelectric effect. We proposed a generation model based on the piezoelectric effect and the resultant relaxation process and obtained a theoretical frequency response, which is in agreement with experimental data. The preseismic signal appears about 2–3 s before the dynamic event with an amplitude of about 50 mV. The local strains along two sliding surfaces were also measured to monitor the growth of the rupture nucleation zone. When the growth of the rupture nucleation zone occurred on the left sliding surface, a clear preseismic signal was detected at the electrode mounted on the left granite block. When the growth occurred on the right-hand surface, a signal was detected at the electrode on the right block. This shows that the preseismic electric signal is caused by stress change in the rupture nucleation zone. These preseismic and coseismic signals were also detected with an antenna, which was placed away from the sample surface.

Convergence, electrostatic potential, and density measurements in a spherically convergent ion focus

Abstract: Unique measurements of the basic plasma-flow characteristics in a low pressure (⩽53 mPa H2) spherically convergent ion focus are obtained using high-voltage (⩽5 kV) emissive and double probes. The radial plasma potential distribution agrees with a collisionless, recirculating, space-charge-limited current model. Flow convergence increases with voltage and neutral pressure and decreases with cathode grid wire spacing and current. Core radii within 4–5 times the ideal geometric limit are measured, and the observed core sizes are consistent with predictions from a multipass orbit model which includes asymmetries in the accelerating potential well. A virtual anode is observed in the converged core region, and no evidence for multiple potential well structures in the core is found. Measurements of the core ion density (nic∼1015 m−3) are consistent with simple flow convergence models.

Photoconductivity nonlinearity at high excitation power in quantum well infrared photodetectors

Abstract: Nonlinear photoconductivity effects at high excitation power in quantum well infrared photodetectors (QWIPs) are studied both experimentally and theoretically. The photoconductivity nonlinearity is mainly caused by a redistribution of the electric potential at high power, which leads to a decrease of electric field in the bulk of the QWIP. As a result of the decreased field, the photoexcited electron escape probability and drift velocity decrease resulting in a decrease of responsivity.

An effective modelling method to analyze the electric field around transmission lines and substations using a generalized finite line charge

Abstract: For the rather complicated and time-consuming three-dimensional electric field calculation in the vicinity of transmission lines and substations, this paper proposes an effective numerical calculation method based on the charge simulation method (CSM). In order to better represent nonuniform charge distribution on an electrode, it is subdivided into small segments with linear charge density. Each segment with linear charge density can be easily represented by a generalized finite line type of charge whose expressions for potential and electric field were analytically derived and which was named "finite slant line charge" in this paper. As for the arrangement of small segments of a subdivided electrode, it has been found that the unequally spaced arrangement method is superior to the equally spaced one. In order to arrange segments fast and effectively, effective formulas were derived from multiple regression analysis of many simulations. The proposed method is applied to the electric field calculation around the transmission lines with significant change in direction and substation busbars.

Viscosity equation for concentrated suspensions of charged colloidal particles

Abstract: Concentrated suspensions of charged stabilized colloidal particles exhibit very large viscosity at low shear rate, a strong shear-thinning behavior at intermediate shear rate, and a constant second Newtonian viscosity at high shear rate. This type of non-Newtonian behavior is affected by many factors such as the particle volume fraction φ, the particle diameter, the surface electric potential ψ0, salt concentration, etc. The generalized equation for the viscosity η of this system is proposed by applying Eyring’s transition state theory. The surface electric potential ψ0 and the thickness of the electric double layer κ−1 are determined by applying the theory to experimental data. Systematic experiments of η of the model colloidal dispersion systems are carried out as the function of φ and shear rate and the results are satisfactorily reproduced by the present theory. The effects of hydrodynamic diameter dh and ψ0 of the colloidal particle on η are also quantitatively explained.

A stationary approach to inverse scattering for schrodinger operators with first order perturbation

Abstract: In this paper, we study the inverse scattering of Schrodinger operators with short-range (resp. long-range) electric and magnetic potentials. We develop a stationary approach to determine the high energy asymptotics of the scattering operator (resp. modified scattering operator). As a corollary, we show that the electric potential and the magnetic field are uniquely determined by the first two terms of this asymptotic expansion.

Active Transport in Biological Membranes and Stochastic Resonances

Abstract: The role of intrinsic fluctuations of the membrane electric potential (barrier height) in the active transport of ions through cell membranes is examined. Experimental data [D.-S. Liu et al., J. Biol. Chem. 265, 7260 (1990)] on active transport of ${\mathrm{Na}}^{+}$ in human erythrocytes under the influence of ac electric fields can be interpreted as the evidence of stochastic resonance between the external field and the fluctuations of the membrane potential. The calculations show that in the considered system one can expect also the appearance of aperiodic stochastic resonance.

Analysis on the applicability of direct current electrical potential method to the detection of damage by multiple small internal cracks

Abstract: The direct current electric potential around a number of penny-shaped cracks was analyzed by Crack-Flow Modification Method (CFMM) in order to investigate the applicability of the Direct Current Electrical Potential Method (DC-EPM) to the detection of damage due to multiple small internal cracks. The potential difference was calculated for the cracking process in a smooth round bar specimen of a Type 304 stainless steel under a creep-fatigue condition. It was clarified by the analysis that, in the case of the present creep-fatigue test, damage could be detected at about a half of the lifetime assuming that a change of 1 percent in the potential difference was measurable by a voltmeter.

Voltage of the vacuum arc with a ring anode in an axial magnetic field

Abstract: The plasma jet focusing and voltage distribution in the interelectrode gap of a vacuum arc with a ring anode and subjected to an axial magnetic field were studied theoretically. A two-dimensional model was developed based on the free plasma jet expansion into vacuum, and the steady-state solution of the fully ionized plasma in the hydrodynamic approximation was analyzed. It was found that the imposition of an axial magnetic field reduces the radial expansion of the plasma jet. The characteristic jet angle decreases from about 40/spl deg/ in the zero magnetic field case and approaches a value of about 20/spl deg/ with a 0.02 T magnetic field. The arc voltage consisting of the cathode drop, the plasma voltage drop, and anode sheath drop increased, with the imposition of a magnetic field, and decreased with the anode length. The model was compared to experimental measurements of the vacuum arc voltage behavior in an axial magnetic field, and good agreement was found.

Sedimentation Velocity and Potential in a Dilute Suspension of Charged Composite Spheres

Abstract: The sedimentation of a charged composite particle composed of a solid core and a surrounding porous shell in an electrolyte solution is analytically studied In the solvent-permeable and ion-penetrable porous surface layer of the particle, idealized hydrodynamic frictional segments with fixed charges are assumed to distribute at a uniform density The equations which govern the ionic concentration distributions, the electric potential profile, and the fluid flow field inside and outside the surface layer of a charged composite particle migrating in an unbounded solution are linearized assuming that the system is only slightly distorted from equilibrium Using a perturbation method, these linearized equations are solved for a composite sphere with the charge densities of the rigid core surface and of the surface layer as the small perturbation parameters An analytical expression for the settling velocity of the composite sphere in closed form is obtained from a balance among its gravitational, electrostatic, and hydrodynamic forces The result demonstrates that the presence of the fixed charges in the composite sphere slows down its settling velocity relative to that of an uncharged one A closed-form formula for the sedimentation potential in a dilute suspension of identical charged composite spheres is also derived by using the requirement of zero net electric current The Onsager reciprocal relation is found to be satisfied between sedimentation and electrophoresis It is shown that spherically-symmetric “neutral” composite particles (bearing no net charge) can undergo electrophoresis, induce sedimentation potential, and experience a smaller settling velocity relative to corresponding uncharged particles The direction of the electrophoretic velocity or the induced potential gradient is determined by the fixed charges in the porous surface layers of the particles In the limiting cases, the analytical solutions describing the sedimentation velocity and sedimentation potential (or electrophoretic mobility) for charged composite spheres reduce to those for charged solid spheres and for charged porous spheres

V-shaped dc potential structure caused by current-driven electrostatic ion-cyclotron instability

Abstract: It is first demonstrated that a V-shaped dc potential structure is created by a current-driven electrostatic ion-cyclotron instability by means of an open two-dimensional particle simulation model. A positive dc potential difference along magnetic field lines is generated by anomalous resistivity caused by the ion-cyclotron instability. In the direction across the magnetic field lines, the dc potential rises from the high-current region to the low-current region. {copyright} {ital 1997} {ital The American Physical Society}

Fundamental solutions for plane problem of piezoelectric materials

Abstract: Based on the basic equations of two-dimensional, transversely isotropic, piezoelectric elasticity, a group of general solutions for body force problem is obtained. And by utilizing this group of general solutions and employing the body potential theory and the integral method, the closed-form solutions of displacements and electric potential for an infinite piezoelectric plane loaded by point forces and point charge are acquired. Therefore, the fundamental solutions, which are very important and useful in the boundary element method (BEM), are presented.

Simulation of Water in a Small Pore: Effect of Electric Field and Density

Abstract: The behavior of water in a pore of nanometer dimensions is studied by Monte Carlo simulation over a series of densities, and with five different electric charge configurations, providing external fields from zero to extremely high values, exceeding 3 × 109 V m-1. The pore contained a tapered section that had an opening of radius 0.25 nm into a secondary tapered section below. The pore wall was a medium of dielectric constant 4 (comparable to the value in a protein), and the electric charges were placed in the wall. A reservoir, with which the remainder of the volume could exchange water molecules, was kept at constant density. Quantities that were obtained included the energy of the system, the orientation of the water molecules in the tapered section of the pore (and the remainder of the volume, but that proved to show little orientation), the density in response to the density of the reservoir section, molecular distributions, and electric potential and field. We observed that a high field, as expected,...

Electric field-gradient contributions to the chemical shifts of liquid water

Abstract: Contributions to the gas-to-liquid chemical shifts of water arising from the electric field gradient of the surrounding molecules have been calculated as a function of the temperature. Since the theoretical model is based on perturbation theory, this part of the chemical shift may be calculated from quadrupole shielding polarizabilities and statistical mechanical ensemble averages of external electric field-gradients. The contributions from the electric field gradients are found to be substantial and are calculated to −9.7 ppm for the oxygen shift and 0.7 ppm for the proton shift at room temperature.

Modelling earth current precursors in earthquake prediction

Abstract: This paper deals with the theory of earth current precursors of earthquake. A dilatancy-diffusion-polarization model is proposed to explain the anomalies of the electric potential, which are observed on the ground surface prior to some earthquakes. The electric polarization is believed to be the electrokinetic effect due to the invasion of fluids into new pores, which are opened inside a stressed-dilated rock body. The time and space variation of the distribution of the electric potential in a layered earth as well as in a faulted half-space is studied in detail. It results that the surface response depends on the underground conductivity distribution and on the relative disposition of the measuring dipole with respect to the buried bipole source. A field procedure based on the use of an areal layout of the recording sites is proposed, in order to obtain the most complete information on the time and space evolution of the precursory phenomena in any given seismic region.

Virtual Anode in Ion Beam Emissions in Space: Numerical Simulations

Abstract: The plasma interactions associated with the emission of an unneutralized ion beam in space are discussed. Three-dimensional particle simulations, which follow the beam ions as well as the ambient electrons and ions in the self-consistent electric e eld, are performed for various emission conditions. Classical theories and laboratory experiments for one-dimensional beam e ow in diodes have shown that a potential hump will form when the beam current density exceeds a critical value. When the potential hump becomes so high that the beam kinetic energy is near zero there, it becomes a virtual electrode. We show that similar characteristics are also exhibited in high-density ion beams emitted from spacecraft to space plasma, although three-dimensional effects and the ambient plasma make the interaction more complex. Once a virtual anode has formed, it will partly block the beam transmission and discharge the spacecraft potential.

Use of transport coefficients to calculate properties of electrode sheaths of electric arcs

Abstract: Particle conservation equations for electrons and positive ions, together with Poisson"s equation to account for space-charge effects on the electric field, have been solved for the electrode sheath regions of electric arcs. For thermionic cathodes and the anode, we find that the ambipolar diffusion approximation is generally valid. At the surface of the anode we find that there is generally a small retarding electric field. For non-thermionic cathodes and no ionisation due to the electric field in the sheath, we calculate unrealistically high sheath voltages and even then, find that the electric fields at the cathode surface are insufficient for field emission. It is suggested that photoionisation in the region close to the cathode may be a principal source of electrons for non-thermionic cathodes.