1,433 research outputs found
On the uniqueness of the surface sources of evoked potentials
The uniqueness of a surface density of sources localized inside a spatial
region and producing a given electric potential distribution in its
boundary is revisited. The situation in which is filled with various
metallic subregions, each one having a definite constant value for the electric
conductivity is considered. It is argued that the knowledge of the potential in
all fully determines the surface density of sources over a wide class of
surfaces supporting them. The class can be defined as a union of an arbitrary
but finite number of open or closed surfaces. The only restriction upon them is
that no one of the closed surfaces contains inside it another (nesting) of the
closed or open surfaces.Comment: 16 pages, 5 figure
Electronic states and optical properties of PbSe nanorods and nanowires
A theory of the electronic structure and excitonic absorption spectra of PbS
and PbSe nanowires and nanorods in the framework of a four-band effective mass
model is presented. Calculations conducted for PbSe show that dielectric
contrast dramatically strengthens the exciton binding in narrow nanowires and
nanorods. However, the self-interaction energies of the electron and hole
nearly cancel the Coulomb binding, and as a result the optical absorption
spectra are practically unaffected by the strong dielectric contrast between
PbSe and the surrounding medium. Measurements of the size-dependent absorption
spectra of colloidal PbSe nanorods are also presented. Using room-temperature
energy-band parameters extracted from the optical spectra of spherical PbSe
nanocrystals, the theory provides good quantitative agreement with the measured
spectra.Comment: 35 pages, 12 figure
Stuffed Black Holes
Initial data corresponding to spacetimes containing black holes are
considered in the time symmetric case. The solutions are obtained by matching
across the apparent horizon different, conformally flat, spatial metrics. The
exterior metric is the vacuum solution obtained by the well known conformal
imaging method. The interior metric for every black hole is regular everywhere
and corresponds to a positive energy density. The resulting matched solutions
cover then the whole initial (Cauchy) hypersurface, without any singularity,
and can be useful for numerical applications. The simpler cases of one black
hole (Schwarzschild data) or two identical black holes (Misner data) are
explicitly solved. A procedure for extending this construction to the multiple
black hole case is also given, and it is shown to work for all time symmetric
vacuum solutions obtained by the conformal imaging method. The numerical
evolution of one such 'stuffed' black hole is compared with that of a pure
vacuum or 'plain' black hole in the spherically symmetric case.Comment: 12 pages, Latex, 4 postscript figures, corrected some typos, new
section about physical interpretatio
VetPestX: Finally! An Online, Searchable, Pesticide Label Database Just for Pests of Animals
Almost all online pesticide databases contain crop-specific product labels; very few include products labeled for animal use. A single online location for veterinary pesticide labels was needed. Led by Alec Gerry of the University of California at Riverside (UCR), veterinary entomologists from California, Washington, New Mexico, and Oklahoma contributed information on animal pesticide products registered in five western states (CA, WA, OR, ID, NM) and OK to a new, online, searchable, veterinary pesticide labels database named VetPestX, developed and maintained on UC Riverside\u27s website. Animal producers and owners requesting pesticide information to manage pests are now routinely directed to VetPestX
Diffraction of light by a planar aperture in a metallic screen
We present a complete derivation of the formula of Smythe [Phys.Rev.72, 1066
(1947)] giving the electromagnetic field diffracted by an aperture created in a
perfectly conducting plane surface. The reasoning, valid for any excitating
field and any hole shape, makes use only of the free scalar Green function for
the Helmoltz equation without any reference to a Green dyadic formalism. We
compare our proof with the one previously given by Jackson and connect our
reasoning to the general Huygens Fresnel theorem.Comment: J. Math. Phys. 47, 072901 (2006
Equation of the field lines of an axisymmetric multipole with a source surface
Optical spectropolarimeters can be used to produce maps of the surface magnetic fields of stars and hence to determine how stellar magnetic fields vary with stellar mass, rotation rate, and evolutionary stage. In particular, we now can map the surface magnetic fields of forming solar-like stars, which are still contracting under gravity and are surrounded by a disk of gas and dust. Their large scale magnetic fields are almost dipolar on some stars, and there is evidence for many higher order multipole field components on other stars. The availability of new data has renewed interest in incorporating multipolar magnetic fields into models of stellar magnetospheres. I describe the basic properties of axial multipoles of arbitrary degree ℓ and derive the equation of the field lines in spherical coordinates. The spherical magnetic field components that describe the global stellar field topology are obtained analytically assuming that currents can be neglected in the region exterior to the star, and interior to some fixed spherical equipotential surface. The field components follow from the solution of Laplace’s equation for the magnetostatic potential
Electrostatics of ions inside the nanopores and trans-membrane channels
A model of a finite cylindrical ion channel through a phospholipid membrane
of width separating two electrolyte reservoirs is studied. Analytical
solution of the Poisson equation is obtained for an arbitrary distribution of
ions inside the trans-membrane pore. The solution is asymptotically exact in
the limit of large ionic strength of electrolyte on the two sides of membrane.
However, even for physiological concentrations of electrolyte, the
electrostatic barrier sizes found using the theory are in excellent agreement
with the numerical solution of the Poisson equation. The analytical solution is
used to calculate the electrostatic potential energy profiles for pores
containing charged protein residues. Availability of a semi-exact interionic
potential should greatly facilitate the study of ionic transport through
nanopores and ion channels
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"Engaging with birth stories in pregnancy: a hermeneutic phenomenological study of women's experiences across two generations"
BACKGROUND: The birth story has been widely understood as a crucial source of knowledge about childbirth. What has not been reported is the effect that birth stories may have on primigravid women's understandings of birth. Findings are presented from a qualitative study exploring how two generations of women came to understand birth in the milieu of other's stories. The prior assumption was that birth stories must surely have a positive or negative influence on listeners, steering them towards either medical or midwifery-led models of care.
METHODS: A Heideggerian hermeneutic phenomenological approach was used. Twenty UK participants were purposively selected and interviewed. Findings from the initial sample of 10 women who were pregnant in 2012 indicated that virtual media was a primary source of birth stories. This led to recruitment of a second sample of 10 women who gave birth in the 1970s-1980s, to determine whether they were more able to translate information into knowledge via stories told through personal contact and not through virtual technologies
RESULTS: Findings revealed the experience of 'being-in-the-world' of birth and of stories in that world. From a Heideggerian perspective, the birth story was constructed through 'idle talk' (the taken for granted assumptions of things, which come into being through language). Both oral stories and those told through technology were described as the 'modern birth story'. The first theme 'Stories are difficult like that', examines the birth story as problematic and considers how stories shape meaning. The second 'It's a generational thing', considers how women from two generations came to understand what their experience might be. The third 'Birth in the twilight of certainty,' examines women's experience of Being in a system of birth as constructed, portrayed and sustained in the stories being shared.
CONCLUSIONS: The women pregnant in 2012 framed their expectations in the language of choice, whilst the women who birthed in the 1970s-1980s framed their experience in the language of safety. For both, however, the world of birth was the same; saturated with, and only legitimised by the birth of a healthy baby. Rather than creating meaningful understanding, the 'idle talk' of birth made both cohorts fearful of leaving the relative comfort of the 'system', and of claiming an alternative birth
Diffraction by a small aperture in conical geometry: Application to metal coated tips used in near-field scanning optical microscopy
Light diffraction through a subwavelength aperture located at the apex of a
metallic screen with conical geometry is investigated theoretically. A method
based on a multipole field expansion is developed to solve Maxwell's equations
analytically using boundary conditions adapted both for the conical geometry
and for the finite conductivity of a real metal. The topological properties of
the diffracted field are discussed in detail and compared to those of the field
diffracted through a small aperture in a flat screen, i. e. the Bethe problem.
The model is applied to coated, conically tapered optical fiber tips that are
used in Near-Field Scanning Optical Microscopy. It is demonstrated that such
tips behave over a large portion of space like a simple combination of two
effective dipoles located in the apex plane (an electric dipole and a magnetic
dipole parallel to the incident fields at the apex) whose exact expressions are
determined. However, the large "backward" emission in the P plane - a salient
experimental fact that remained unexplained so far - is recovered in our
analysis which goes beyond the two-dipole approximation.Comment: 21 pages, 6 figures, published in PRE in 200
Passive magnetic shielding in static gradient fields
The effect of passive magnetic shielding on dc magnetic field gradients imposed by both external and internal sources is studied for two idealized shield models: concentric spherical and infinitely-long cylindrical shells of linear material. It is found that higher-order multipoles of an externally applied magnetic field are always shielded progressively better for either geometry by a factor related to the order of the multipole. In regard to the design of internal coil systems, we determine reaction factors for the general multipole field and provide examples of how one can take advantage of the coupling of the coils to the innermost shell to optimize the uniformity of the field. Furthermore, we provide formulae relevant to active magnetic compensation systems which attempt to stabilize the interior fields by sensing and cancelling the exterior fields close to the outermost shell. Overall this work provides a comprehensive framework that is useful for the analysis and optimization of dc magnetic shields, serving as a theoretical and conceptual design guide as well as a starting point and benchmark for finite-element analysis."We gratefully acknowledge the support of the Natural Sciences and Engineering Research
Council of Canada."https://aip.scitation.org/doi/10.1063/1.487371
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