Electromagnetic Energy, Momentum, and Angular Momentum in an Inhomogeneous Linear Dielectric

In a previous work, Optics Communications 284 (2011) 2460--2465, we considered a dielectric medium with an anti-reflection coating and a spatially uniform index of refraction illuminated at normal incidence by a quasimonochromatic field. Using the continuity equations for the electromagnetic energy density and the Gordon momentum density, we constructed a traceless, symmetric energy--momentum tensor for the closed system. In this work, we relax the condition of a uniform index of refraction and consider a dielectric medium with a spatially varying index of refraction that is independent of time, which essentially represents a mechanically rigid dielectric medium due to external constraints. Using continuity equations for energy density and for Gordon momentum density, we construct a symmetric energy--momentum matrix, whose four-divergence is equal to a generalized Helmholtz force density four-vector. Assuming that the energy-momentum matrix has tensor transformation properties under a symmetry group of space-time coordinate transformations, we derive the global conservation laws for the total energy, momentum, and angular momentum.Comment: added publication informatio

Energy-Momentum Tensor for the Electromagnetic Field in a Dielectric

The total momentum of a thermodynamically closed system is unique, as is the total energy. Nevertheless, there is continuing confusion concerning the correct form of the momentum and the energy-momentum tensor for an electromagnetic field interacting with a linear dielectric medium. Here we investigate the energy and momentum in a closed system composed of a propagating electromagnetic field and a negligibly reflecting dielectric. The Gordon momentum is easily identified as the total momentum by the fact that it is, by virtue of being invariant in time, conserved. We construct continuity equations for the energy and the Gordon momentum and use the continuity equations to construct an array that has the properties of a traceless, diagonally symmetric energy-momentum tensor. Then the century-old Abraham-Minkowski momentum controversy can be viewed as a consequence of attempting to construct an energy-momentum tensor from continuity equations that contain densities that correspond to nonconserved quantities.Comment: added publication informatio

Chlorate and disinfectant modify Salmonella enterica shedding in weaned pigs

The effects of chlorate administration, age at weaning (10 and 21 days), and topically applied disinfectant on Salmonella enterica shedding were evaluated in 80 weaned pigs naturally exposed to Salmonella positive dams. Salmonella spp were qualitatively detected in samples collected on post-weaning days (PWD) -2, 10 and 14 and the concentration estimated for samples collected PWD 0, 5 and for PWD 14 cecal content

The “Peer-Effect” in Counterterrorist Policies

AbstractExisting accounts posit that defensively oriented counterterrorist policies create negative externalities and result in regulatory competition that induces governments to increasingly tighten their policies. We argue that rather than causing an unconditional global “race to the top,” spatial dependence in counterterrorist policies is limited to within groups of countries exposed to a similar level of threat from international terrorism. Countries strongly differ in their propensity to become the target of an international terror attack. Governments can safely ignore counterterrorist policies enacted by countries outside their “peer group,” but they must pay attention to measures undertaken by their peers. We test several predictions derived from our theory in an empirical analysis of counterterrorist regulations in twenty Western developed-country democracies over the period 2001 to 2008.</jats:p

Survey of serum vitamin D status across stages of swine production and evaluation of supplemental bulk vitamin D premixes used in swine diets

The objectives of this investigation were to evaluate the serum 25-hydroxyvitamin D [25(OH)D] concentrations in pigs of different age groups, to determine if 25(OH)D concentrations varied with season, and to assess the quality of vitamin D supplements used in swine diets from multiple commercial suppliers. Serum samples (n = 1200) submitted to a diagnostic laboratory for routine surveillance were assayed for serum 25(OH)D concentrations. Vitamin D premix samples were obtained from suppliers and analyzed at two laboratories over a 9-month period. In all age categories, 25(OH)D concentrations in numerous serum samples were lower than reference values. In the nursery, finisher, and boar age categories, there was a difference between the months of January and June (P \u3c .05), with June samples containing higher quantities of circulating 25(OH)D. Serum samples from outdoor herds had higher 25(OH)D concentrations than samples from confined pigs (P \u3c .01). Among the supplement samples evaluated, no individual supplement had a concentration of 25(OH)D significantly lower than 500,000 IU per g. These results revealed that commercial swine may be deficient in serum vitamin D at varying times of the year, and feed-supplement concentrations may vary

β-Catenin signals regulate cell growth and the balance between progenitor cell expansion and differentiation in the nervous system

Abstractβ-Catenin is an essential component of the canonical Wnt signaling system that controls decisive steps in development. We employed here two conditional β-catenin mutant alleles to alter β-catenin signaling in the central nervous system of mice: one allele to ablate β-catenin and the second allele to express a constitutively active β-catenin. The tissue mass of the spinal cord and brain is reduced after ablation of β-catenin, and the neuronal precursor population is not maintained. In contrast, the spinal cord and brain of mice that express activated β-catenin is much enlarged in mass, and the neuronal precursor population is increased in size. β-Catenin signals are thus essential for the maintenance of proliferation of neuronal progenitors, controlling the size of the progenitor pool, and impinging on the decision of neuronal progenitors to proliferate or to differentiate

Talk the talk, walk the walk: Defining Critical Race Theory in research

Over the last decade there has been a noticeable growth in published works citing Critical Race Theory (CRT). This has led to a growth in interest in the UK of practical research projects utilising CRT as their framework. It is clear that research on 'race' is an emerging topic of study. What is less visible is a debate on how CRT is positioned in relation to methodic practice, substantive theory and epistemological underpinnings. The efficacy of categories of data gathering tools, both traditional and non-traditional is a discussion point here to explore the complexities underpinning decisions to advocate a CRT framework. Notwithstanding intersectional issues, a CRT methodology is recognisable by how philosophical, political and ethical questions are established and maintained in relation to racialised problematics. This paper examines these tensions in establishing CRT methodologies and explores some of the essential criteria for researchers to consider in utilising a CRT framework. © 2012 Copyright Taylor and Francis Group, LLC

X-ray Observations of the Seyfert galaxy LB 1727 (1H 0419-577)

We discuss the properties of the Seyfert 1.5 galaxy LB 1727, also known as 1H 0419-577, from X-ray observations obtained by ASCA and ROSAT along with optical observations from earlier epochs. ASCA shows only modest (< 20%) variations in X-ray flux within or between the observations. In contrast, a daily monitoring campaign over 1996 Jun - Sept by the ROSAT HRI instrument reveals the soft X-ray (0.1-2 keV) flux to have increased by a factor ~3. The 2 - 10 keV continuum can be parameterized as a power-law with a photon index Gamma ~ 1.45-1.68 across ~0.7 - 11 keV in the rest-frame. We also report the first detection of iron Kalpha line emission in this source. Simultaneous ASCA and ROSAT data show the X-ray spectrum to steepen sharply at a rest-energy \~0.75 keV, the spectrum below this energy can be parameterized as a power-law of slope Gamma ~3.6. We show that LB 1727 is one of the few Seyferts for which we can rule out the possibility that the presence of a warm absorber is solely responsible for the spectral steepening in the soft X-ray regime. Consideration of the overall spectral-energy-distribution for this source indicates the presence of a pronounced XUV-bump visible in optical, ultraviolet and soft X-ray data.Comment: 27 pages, 7 figures. LaTeX with encapsulated postscript. To appear in the Astrophysical Journa

The strain-rate sensitivity of high-strength high-toughness steels.

The present study examines the strain-rate sensitivity of four high strength, high-toughness alloys at strain rates ranging from 0.0002 s-1 to 200 s-1: Aermet 100, a modified 4340, modified HP9-4-20, and a recently developed Eglin AFB steel alloy, ES-1c. A refined dynamic servohydraulic method was used to perform tensile tests over this entire range. Each of these alloys exhibit only modest strain-rate sensitivity. Specifically, the strain-rate sensitivity exponent m, is found to be in the range of 0.004-0.007 depending on the alloy. This corresponds to a {approx}10% increase in the yield strength over the 7-orders of magnitude change in strain-rate. Interestingly, while three of the alloys showed a concominant {approx}3-10% drop in their ductility with increasing strain-rate, the ES1-c alloy actually exhibited a 25% increase in ductility with increasing strain-rate. Fractography suggests the possibility that at higher strain-rates ES-1c evolves towards a more ductile dimple fracture mode associated with microvoid coalescence

A simple model for AGN feedback in nearby early-type galaxies

Recent work indicates that star-forming early-type galaxies (ETGs) residing in the blue cloud migrate rapidly to the red sequence within around a Gyr, passing through several phases of increasingly strong AGN activity in the process (Schawinski et al. 2007, MNRAS, 382, 1415; S07 hereafter). We show that natural depletion of the gas reservoir through star formation (i.e. in the absence of any feedback from the AGN) induces a blue-to-red reddening rate that is several factors lower than that observed in S07. This is because the gas depletion rate due to star formation alone is too slow, implying that another process needs to be invoked to remove gas from the system and accelerate the reddening rate. We develop a simple phenomenological model, in which a fraction of the AGN's luminosity couples to the gas reservoir over a certain 'feedback timescale' and removes part of the gas mass from the galaxy, while the remaining gas continues to contribute to star formation. We use the model to investigate scenarios which yield migration times consistent with the results of S07. We find that acceptable models have feedback timescales <0.2 Gyrs. The mass fraction in young stars in the remnants is <5% and the residual gas fractions are less than 0.6%, in good agreement with the recent literature. At least half of the initial gas reservoir is removed as the galaxies evolve from the blue cloud to the red sequence. If we restrict ourselves to feedback timescales similar to the typical duty cycles of local AGN (a few hundred Myrs) then a few tenths of a percent of the luminosity of an early-type Seyfert (10^11 LSun) must couple to the gas reservoir in order to produce migration times that are consistent with the observations.Comment: MNRAS in press (minor revisions to version 1