Showing papers on "Field (physics) published in 1987"

Two applications of axion electrodynamics.

Abstract: The equations of axion electrodynamics are studied. Variations in the axion field can give rise to peculiar distributions of charge and current. These effects provide a simple understanding of the fractional electric charge on dyons and of some recently discovered oddities in the electrodynamics of antiphase boundaries in PbTe. Some speculations regarding the possible occurrence of related phenomena in other solids are presented.

Electrostatics, Principles, Problems and Applications

Abstract: Historical background and elementary theory: Historical background Fundamental definitions Orders of magnitude Summary of electrostatic equations in uniform fields Gauss's law - the electric field due to an assembly of charges Electrification of solids and liquids: Contact and frictional charging Corona charging Charging by capture of small particles Induction charging Double-layer charging References Measurements and instrumentation: Introduction Potential dividers and resistance probes Electrometers and electrostatic voltmeters Field meters and non-contacting voltmeters Charge measurement - the Faraday cup Measurement of the charge and mobility of individual particles Field and potential probes The use of the Kerr and Pockels effect for measurement of electric fields Capacitance meters Measurement of resistance and resistivity Energy of electrostatic sparks Electrostatics in gas filtration: Introduction Electrostatic precipitation Electron beam desulphurisation and denitrisation Electrostatically enhanced cyclone separators Electrostatic scrubbers and granular bed filters Electrostatically enhanced fabric filters Appendix Miscellaneous applications: Electrostatic atomisation Electrostatic spray coating Electrostatic separation Electrokinetic phenomena in liquids Dielectrophoresis Applications of the corona discharge Electrodynamic containment and control of particles Applications of electromechanical forces Electrostatic copying Electrostatics and the textile industry Electrostatic crystals Electrostatic generators Hazards and problems: Fire and explosion hazards Electrostatic sparks Non-electrostatic sparks Electrostratic eliminators Antistatic agents Electrostatic problems in the electronics industry Adhesion Appendix Theory: Fundamental laws of electrostatics Capacitance and capacitors Polarisation Identification of field lines and equipotentials Electric field and potential by solution of Laplace's equation Earnshaw's theorem Corona discharge theory Appendix Index

Transverse relaxation of solvent protons induced by magnetized spheres: application to ferritin, erythrocytes, and magnetite

Abstract: Since 1/T2 of protons of tissue water is generally much greater than 1/T1 at typical imaging fields, small single-ion contrast agents--such as Gd(DTPA), which make comparable incremental contributions and therefore smaller fractional contributions to 1/T2 compared to 1/T1--are not as desirable for contrast-enhancement as agents that could enhance 1/T2 preferentially. In principle, such specialized agents will only be effective at higher fields because the field dependence (dispersion) of 1/T1 is such that it approaches zero at high fields whereas 1/T2 approaches a constant value. The residual 1/T2 is called the "secular" contribution and arises from fluctuations in time--as sensed by the protons of diffusing solvent or tissue water molecules--of the component of the magnetic field parallel to the static applied field. For solutions or suspensions of sufficiently large paramagnetic or ferromagnetic particles (greater than or equal to 250 A diameter), the paramagnetic contributions to the relaxation rates satisfy 1/T2 much greater than 1/T1 at typical imaging fields. We examine the theory of secular relaxation in some detail, particularly as it applies to systems relevant to magnetic resonance imaging, and then analyze the data for solutions, suspensions, or tissue containing ferritin, erythrocytes, agar-bound magnetite particles, and liver with low-density composite polymer-coated magnetite. In most cases we can explain the relaxation data, often quantitatively, in terms of the theory of relaxation of protons (water molecules) diffusing in the outer sphere environments of magnetized particles. The dipolar field produced by these particles has a strong spatial dependence, and its apparent fluctuations in time as seen by the diffusing protons produce spin transitions that contribute to both 1/T1 and /T2 comparably at low fields; for the larger particles, because of dispersion, the secular term dominates at fields of interest. On the basis of the agreement of theory with data for solutions of small paramagnetic complexes, large magnetite particles, and liver containing low-density polymer-coated magnetite agglomerates, it is argued that the theory is sufficiently reliable so that, e.g., for ferritin--for which 1/T2 is unexpectedly large--the source of its large relaxivity must reside in nonideal chemistry of the ferritin core. For blood, it appears that diffusion through intracellular gradients determines 1/T2.

Amplitude-squared squeezing of the electromagnetic field

Abstract: Operators are defined which correspond to the real and imaginary parts of the square of the field amplitude. These operators obey an uncertainty relation. Squeezing with respect to these operators is defined and minimum uncertainty states are discussed. A number of nonlinear optical processes are examined and shown to produce this effect. Interactions which couple one mode amplitude to the square of another convert this type of squeezing into normal squeezing. An example of such an interaction is given.

A stable finite element solution for two-dimensional magnetotelluric modelling

Abstract: Summary. We report herein on a finite element algorithm for 2-D magnetotelluric modelling which solves directly for secondary variations in the field parallel to strike, plus the subsequent vertical and transverse auxiliary fields, for both transverse electric and transverse magnetic modes. The governing Helmholtz equations for the secondary fields along strike are the same as those for total field algorithms with the addition of source terms involving the primary fields and the conductivity difference between the body and the host. Our approach has overcome a difficulty with numerical accuracy at low frequencies observed in total field solutions with 32-bit arithmetic far the transverse magnetic mode especially, but also for the transverse electric mode. Matrix ill-conditioning, which affects total field solutions, increases with the number of element rows with the square of the maximum element aspect ratio and with the inverse of frequency. In the secondary formulation, the field along strike and the auxiliary fields do not need to be extracted in the face of an approximately computed primary field which increasingly dominates the total field solution towards low frequencies. In addition to low-frequency stability, the absolute accuracy of our algorithm is verified by comparison with the TM and the TE mode analytic responses of a segmented overburden model.

Vibrational spectroscopy of molecules on surfaces

Abstract: The observation of the vibrational spectra of adsorbed species provides one of the most incisive methods for und erst an ding chemical and physical phenomena on surfaces. At the present time, many approaches may be applied to studies of molecular vibrations on surfaces. Some of these are used on high-area solids of technological importance (e.g., heterogeneous catalysts) while others are applied to single-crystal substrates to gain better understanding under conditions of controlled surface structure. This book has attempted to bring together in one place a discussion of the major methods used to measure vibrational spectra of surface species. The emphasis is on basic concepts and experimental methods rather than a current survey of the extensive literature in this field. Two introductory chapters describe the basic theoretical aspects of vibrational spectroscopy on surfaces, dealing with normal modes and excitation mechanisms in vibrational spectroscopy. The remaining seven chapters deal with various methods employed to observe surface vibra tions. These are arranged in an order that first treats the use of various methods on surfaces that are not of the single-crystal type. It is in this area that the field first got started in the late 1940s with pioneering work by Terenin and others in the Soviet Union, and by Eisehens and others in the United States in the 1950s. The last four chapters deal with relatively recent methods that permit vibrational studies to be made on single crystal substrates."

Analytical Dirac-Hartree-Fock-Slater screening function for atoms (Z=1-92).

Abstract: An analytical approximation, depending on five parameters, for the atomic screening function is proposed. The corresponding electrostatic potential takes a simple analytical form (superposition of three Yukawa potentials) well suited to most practical applications. Parameters in the screening function, determined by an analytical fitting procedure to Dirac-Hartree-Fock-Slater (DHFS) self-consistent data, are given for Z=1--92. The reliability of this analytical approach is demonstrated by showing that (a) Born cross sections for elastic scattering of fast charged particles by the present analytical field and by the DHFS field practically coincide and (b) one-electron binding energies computed from the independent-particle model with our analytical field (corrected for exchange and electrostatic self-interaction) agree closely with the DHFS energy eigenvalues.

Magnetostatic trapping fields for neutral atoms

Abstract: In view of the recent successful confinement of decelerated sodium atoms in a magnetostatic trap, it is of interest to evaluate possible trap-field configurations. Neutral atoms in a Zeeman sublevel whose energy increases with field can be confined by a field whose magnitude \ensuremath{\Vert}B\ensuremath{\Vert} increases with distance from the center. Because this same basic requirement applies also to traps for neutrons and for plasmas (in the guiding-center approximation), trap configurations developed previously for these purposes are of interest for neutral atoms. However, the desired properties differ considerably because of very different objectives and different behavior of very cold atoms as compared with hot plasmas. We characterize basic trap configurations using both the exact expressions for the field, and a multipole polynomial expansion that facilitates studies of symmetry properties and classical or quantum orbits. Polynomial terms for the field components are derived and coefficients obtained by comparison with Taylor-series expansions and by global fit. Contours of \ensuremath{\Vert}B\ensuremath{\Vert} for various trap configurations are also presented. Under certain restrictive conditions, \ensuremath{\Vert}B\ensuremath{\Vert}, and hence the effective potential, can be made isotropic to second order.

Analytical and Numerical Studies of the Beta-Effect in Tropical Cyclone Motion. Part I: Zero Mean Flow

Abstract: The β-effect on tropical cyclone motion is studied using an analytical as well as a numerical model in a nondivergent barotropic framework. The analytical model and the linear version of the numerical model give essentially the same result: the linear β-effect causes a westward stretching of the model vortex but no significant movement of the vortex center. An east-west asymmetry in the meridional wind field is also created. It is the inclusion of the nonlinear term that produces the northwestward movement of the vortex previously found by other investigators (e.g., Kitade, 1981). This northwestward movement increases with both the maximum wind speed and the radius of maximum wind in a constant-shape vortex. A wind maximum is also found to the northeast of the vortex, which appears to be consistent with the observational findings of Shea and Gray. This asymmetry plays an important role in the vortex motion.

A model for the separation of large DNA molecules by crossed field gel electrophoresis

Abstract: The idea that large DNA molecules adopt a stretched conformation as they pass through gels suggests a simple mechanism for the separation of DNA by crossed field electrophoresis: at each change in field direction a DNA molecule takes off in the new direction of the field by a movement which is led by what was formerly its back end. The effect of this ratcheting motion is to subtract from the DNA molecule's forward movement, at each step, an amount which is proportional to its length. We find that this model explains most of the features of the separation, and we describe experiments, using a novel electrophoresis apparatus, which support the model. The apparatus turns the gel between two preset orientations in a uniform electric field at preset time intervals. This separation method has the practical advantage over some others that the DNA molecules follow straight tracks. A further advantage is that the parameters which determine the separation are readily predicted from the simple theory describing their motion.

Hyperfine fields of 3d and 4d impurities in nickel.

Abstract: We present detailed calculations of the electronic structure and the hyperfine fields of 3d and 4d impurities in nickel. The calculations are based on the local-density approximation of density-functional theory and the Korringa-Kohn-Rostoker Green's-function method for impurity calculations. We self-consistently calculate the local moments and hyperfine fields of the impurities and their nearest neighbors. We derive new formulas for the proper relativistic generalizations of the contact, orbital, and dipolar contributions to the hyperfine field and explicitly calculate relativistic corrections to the contact interaction which are important for 4d impurities. The hyperfine fields can be split up into local and transferred contributions which are directly related to the local moments and to the moments of the neighboring atoms. The calculated hyperfine fields are in reasonable agreement with the experimental data.

Evaluating the cost and performance of field-scale granular activated carbon systems.

Abstract: typical ambient allyl chloride concentrations are roughly 1000 times lower (18) than propylene concentrations. It is clear a t this point, however, that a variety of reactive atmospheric hydrocarbons (toluene, propylene, acetaldehyde, allyl chloride) can be converted into products that are potentially much more mutagenic than the reactants themselves. Much more work is necessary, however, to fully establish the potential human health effects brought about by exposure to atmospheric photooxidation products.

Time‐dependent self‐consistent field (TDSCF) approximation for a reaction coordinate coupled to a harmonic bath: Single and multiple configuration treatments

Abstract: This paper explores the usefulness of the time‐dependent self‐consistent field (TDSCF) approximation for treating the dynamics of a reaction coordinate coupled to a bath of harmonic degrees of freedom. The reaction coordinate is a one‐dimensional double well potential, typical of a hydrogen atom isomerization process. The standard (i.e., one configuration) TDSCF approximation is found to provide a very poor description of the effect of coupling to the bath on the isomerization rate. A multiconfiguration (MC‐TDSCF) treatment is thus developed and found to provide a much improved description.

Mapping the boundaries of distance education: Problems in defining the field

Abstract: (1987). Mapping the boundaries of distance education: Problems in defining the field. American Journal of Distance Education: Vol. 1, No. 1, pp. 7-13.

Investigations into the mechanisms of electrohydrodynamic spraying of liquids

Abstract: The mechanism of stable jet formation was investigated by observing the motion of the liquid in a jet close to the capillary tip from which it emerges under the influence of an electric field. Tracer particles of lycopodium were inserted in the liquid and streak photographs were taken of a stable jet formed under the application of 10.5 kV and a flow rate of 30 ml/h. These photographs show an axisymmetric circulation of the liquid in the conical base of the jet, invalidating previous theories that have assumed a uniform velocity profile in the liquid cone. This axisymmetric motion of the liquid in the jet was explained in terms of interfacial electrical shear stresses. Due to the semi-insulating nature of the liquid, there will exist a potential difference between the base of the capillary and the tip of the cone. This potential drop ensures that the interface is subjected to a tangential electric field in the direction of flow and hence an electric shear stress on the surface of the cone. Both tangential and normal fields on the cone were calculated from a knowledge of the jet profile and current. The tangential field on the surface of the cone was calculated by considering it to be a section of a sphere and dividing it into five sections. Each section was assumed to form an equipotential surface normal to the direction of flow and to have a constant current flowing through it. The normal field was calculated numerically using a computer program that estimates the potential distribution within a region subject to given boundary conditions using the finite element method. The results of calculation showed that the shear force acting on the cone was about 5 to 10 times smaller than the normal force. However, the normal force was found to be fairly constant and independent of applied voltage, within the range studied. In contrast, the shear force showed a tendency to decrease with an increase in applied voltage. It was also observed that as the voltage was increased, the length of the jet decreased. This could be explained in terms of the reduction of tangential shear force with an increase in applied voltage, thus reducing jet stability.

A mathematical theory of gravitational collapse

Abstract: We study the asymptotic behaviour, as the retarded timeu tends to infinity, of the solutions of Einstein's equations in the spherically symmetric case with a massless scalar field as the material model. We prove that when the final Bondi massM1 is different from zero, asu → ∞, a black hole forms of massM1 surrounded by vacuum. We find the rate of decay of the metric functions and the behaviour of the scalar field on the horizon.

Atoms in strong magnetic fields

Abstract: We review recent results of investigations of hydrogen-like systems at magnetic field strengths where the Lorentz forces are comparable to, or larger than, the Coulomb binding forces. This situation is realized for low-lying states at field strengths typical of magnetic white dwarfs and neutron stars, while for Rydberg states already laboratory field strengths are sufficient. We discuss the wavelength spectrum of the hydrogen atom in magnetic fields of arbitrary strength, and describe in which way the spectroscopy of "stationary lines", which appear in this spectrum, has made possible the detection of the largest magnetic field strength ever found in a white dwarf star to date. For Rydberg states in strong laboratory fields we perform a quantitative comparison between experimental and theoretical spectra, and demonstrate that symptoms of "quantum stochasticity" are recovered in the spectra of magnetized Rydberg atoms. In particular we point out that the breakdown of quasi-separability in the quantal problem is closely related to the disappearance of regular orbits in the classical problem. We conclude that magnetized Rydberg atoms lend themselves as ideal objects in which to study, theoretically and experimentally, manifestations of quantum stochasticity.

The Anisotropic Upper Critical Field of Single Crystal YBa2Cu3Ox

Abstract: We report a study of anisotropic upper critical field of single crystal samples of orthorhombic YBa2Cu3Ox. The critical field shows anisotropy characteristic of a quasi-two-dimensional superconductor, and is highest when the field is oriented perpendicular to the c-axis. For this field direction, the zero resistance state persists up to 86 K in a magnetic field of 90 kOe, which is lower by only 5 K than the zero resistance temperature in the absence of magnetic field. The anisotropy Hc2⊥/Hc2// is about 2 at 90 K, 5 at 86.5 K and tends to increase with decreasing temperature.

Finite-Q cavity electrodynamics: dynamical and statistical aspects

Abstract: Solutions of the basic equations of a simplified model of the cavity quantum electrodynamics are presented under the condition that the single-photon Rabi frequency is much larger than the cavity decay rate. Such solutions are used to calculate the quantum-statistical properties of the field and atomic observables under a variety of initial conditions involving the states of the field and atom. Effects of increasing cavity damping and of the addition of thermal photons on collapse and revival phenomena are discussed. Phase-sensitive aspects of the cavity field are also treated. Quantum effects are shown to be most easily isolated by sending an atom initially prepared in a dressed state for such a state does not evolve further under the influence of a classical field. The appearance of squeezing in the cavity field is demonstrated. The squeezing is most prominent for a coherently prepared atom passing through an empty cavity. The quantum features of the complex dipole moment and their detectability are also discussed in detail.

Analysis of electromagnetic scattering from dielectric cylinders using a multifilament current model

Abstract: A moment solution is presented for the problem of transverse magnetic (TM) scattering from homogeneous dielectric cylinders. The moment solution uses fictitious filamentary currents to simulate both the field scattered by the cylinder and the field inside the cylinder and in turn point-matches the continuity conditions for the tangential components of the electric and magnetic fields across the cylinder surface. The procedure is simple to execute and is general in that cylinders of arbitrary shape and complex permittivity can be handled effectively. Metallic cylinders are treated as reduced cases of the general procedure. Results are given and compared with available analytic solutions, which demonstrate the very good performance of the procedure.

Angular discrimination of light transmission through polymer-dispersed liquid-crystal films

Abstract: The refractive index of the polymer np is adjusted relative to the ordinary refractive index of dispersed droplets of nematic liquid crystal no to regulate the angle of maximum transmission through a film in the presence of an applied field. Maximum transmission occurs at normal incidence when np≤no with the breadth of the angle of view being largest at np=no. When np>no, maximum transmission is peaked at an angle away from normal incidence, the value of the angle depending upon the ratio np/no. Angular discriminating filters are therefore possible with these films. The optical response of these materials as a function of applied field is also shown to be dependent on the ratio np/no as is the contrast exhibited by information displays. Methods for measuring the scattering cross section of a nematic droplet are also described.

Diffusion in a periodic Lorentz gas

Abstract: We use a constant “driving force”F d together with a Gaussian thermostatting “constraint force”F d to simulate a nonequilibrium steady-state current (particle velocity) in a periodic, two-dimensional, classical Lorentz gas. The ratio of the average particle velocity to the driving force (field strength) is the Lorentz-gas conductivity. A regular “Galton-board” lattice of fixed particles is arranged in a dense triangular-lattice structure. The moving scatterer particle travels through the lattice at constant kinetic energy, making elastic hard-disk collisions with the fixed particles. At low field strengths the nonequilibrium conductivity is statistically indistinguishable from the equilibrium Green-Kubo estimate of Machta and Zwanzig. The low-field conductivity varies smoothly, but in a complicated way, with field strength. For moderate fields the conductivity generally decreases nearly linearly with field, but is nearly discontinuous at certain values where interesting stable cycles of collisions occur. As the field is increased, the phase-space probability density drops in apparent fractal dimensionality from 3 to 1. We compare the nonlinear conductivity with similar zero-density results from the two-particle Boltzmann equation. We also tabulate the variation of the kinetic pressure as a function of the field strength,

The magnetic field of Uranus

Abstract: Aspherical harmonic model of the planetary magnetic field of Uranus is obtained from the Voyager 2 encounter observations using generalized inverse techniques which allow partial solutions to complex (underdetermined) problems. The Goddard Space Flight Center 'Q3' model is characterized by a large dipole tilt (58.6 deg) relative to the rotation axis, a dipole moment of 0.228 G R(Uranus radii cubed) and an unusually large quadrupole moment. Characteristics of this complex model magnetic field are illustrated using contour maps of the field on the planet's surface and discussed in the context of possible dynamo generation in the relatively poorly conducting 'ice' mantle.

Local and nonlocal surface nonlinearities for surface optical second-harmonic generation.

Abstract: We investigate the physical origins of the surface nonlinear susceptibility responsible for surface optical second-harmonic generation. Experiments performed on simple covalent systems were designed to distinguish between the nonlocal electric-quadrupole-type nonlinearity induced by field discontinuity and the local intrinsic, electric-dipole-type nonlinearity of a surface or interface. We find that both mechanisms could operate: The latter usually dominates when the surface layer has a strong structural asymmetry with the fundamental or the second-harmonic frequency at resonance with a dipole-allowed transition.