527 research outputs found

    Action at a distance as a full-value solution of Maxwell equations: basis and application of separated potential's method

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    The inadequacy of Li\'{e}nard-Wiechert potentials is demonstrated as one of the examples related to the inconsistency of the conventional classical electrodynamics. The insufficiency of the Faraday-Maxwell concept to describe the whole electromagnetic phenomena and the incompleteness of a set of solutions of Maxwell equations are discussed and mathematically proved. Reasons of the introduction of the so-called ``electrodynamics dualism concept" (simultaneous coexistence of instantaneous Newton long-range and Faraday-Maxwell short-range interactions) have been displayed. It is strictly shown that the new concept presents itself as the direct consequence of the complete set of Maxwell equations and makes it possible to consider classical electrodynamics as a self-consistent and complete theory, devoid of inward contradictions. In the framework of the new approach, all main concepts of classical electrodynamics are reconsidered. In particular, a limited class of motion is revealed when accelerated charges do not radiate electromagnetic field.Comment: ReVTeX file, 24pp. Small corrections which do not have influence results of the paper. Journal reference is adde

    Breakdown of superfluidity of an atom laser past an obstacle

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    The 1D flow of a continuous beam of Bose-Einstein condensed atoms in the presence of an obstacle is studied as a function of the beam velocity and of the type of perturbing potential (representing the interaction of the obstacle with the atoms of the beam). We identify the relevant regimes: stationary/time-dependent and superfluid/dissipative; the absence of drag is used as a criterion for superfluidity. There exists a critical velocity below which the flow is superfluid. For attractive obstacles, we show that this critical velocity can reach the value predicted by Landau's approach. For penetrable obstacles, it is shown that superfluidity is recovered at large beam velocity. Finally, enormous differences in drag occur when switching from repulsive to attractive potential.Comment: 15 pages, 6 figure

    Microwave-induced control of Free Electron Laser radiation

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    The dynamical response of a relativistic bunch of electrons injected in a planar magnetic undulator and interacting with a counterpropagating electromagnetic wave is studied. We demonstrate a resonance condition for which the free electron laser (FEL) dynamics is strongly influenced by the presence of the external field. It opens up the possibility of control of short wavelength FEL emission characteristics by changing the parameters of the microwave field without requiring change in the undulator's geometry or configuration. Numerical examples, assuming realistic parameter values analogous to those of the TTF-FEL, currently under development at DESY, are given for possible control of the amplitude or the polarization of the emitted radiation.Comment: 14 pages, 5 figures, accepted for publication in Phys. Rev.

    ‘As a woman…’; ‘As a Muslim…’: Subjects, positions and counter-terrorism powers in the United Kingdom

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    This article presents findings from original focus group research on the importance of identity claims within public understandings of counter-terrorism across the UK. Following a review of existing literature on the terrorism/counter-terrorism/identity nexus, the article introduces four prominent subject positions inhabited within public articulations of counter-terrorism powers: the ‘Muslim’, the ‘target’, the ‘woman’ and the ‘unaffected’. Positions such as these, we argue, both enable and inhibit particular normative, political and anecdotal claims about counter-terrorism frameworks and their impact upon the body politic. This, we suggest, is demonstrative of the co-constitutive role between counter-terrorism and identity claims. Thus, on the one hand, counter-terrorism initiatives work to position individuals socially, politically, and culturally: (re)producing various religious, ethnic and other identities. Yet, at the same time, specific subject positions are integral to the articulation of people’s attitudes toward developments in counter-terrorism. The article concludes by thinking through some of the implications of this, including for resistance toward securitising moves and for citizenship more generally

    Gauging the Effectiveness of Educational Technology Integration in Education: What the Best-Quality Meta-Analyses Tell Us

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    This chapter examines quantitative research in the literature of technology integration in education from the perspective of the meta-analyses of primary studies conducted from 1982 to 2015. The intent is to identify and review the best of these meta-analyses. Fifty-two meta-analyses were originally identified and evaluated for methodological quality using the Meta-Analysis Methodological Quality Review Guide (MMQRG), and the best 20 were selected and are included for review here. Some describe the effects of technology integration within specific content areas and some are more general. Technology integration in education is one of the most fluid areas of research, reflecting the incredible pace of the evolution of computer-based tools and applications. Just navigating through the vast primary empirical literature presents a real challenge to those interested in evaluating the educational effectiveness of technology. Systematic reviews in the field are numerous and quite diverse in their methodological quality, introducing potential bias in the interpretation of findings (Bernard RM, Borokhovski E, Schmid RF, Tamim RM. J Comput High Educ 26(3):183–209, 2014), thus bringing into question their applied value. This chapter identifies and reviews the best of these meta-analyses. In addition to overall statistical analyses of this collection, the findings of six of the most recent and best meta-analyses (after 2010) are summarized in more detail. The discussion focuses on the interpretation of the current findings, considers future alternatives to primary research in this area, and examines how meta-analysts might address them

    Electrophysiological correlates of reinforcement learning in young people with Tourette syndrome with and without co-occurring ADHD symptoms

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    Altered reinforcement learning is implicated in the causes of Tourette syndrome (TS) and attention-deficit/hyperactivity disorder (ADHD). TS and ADHD frequently co-occur but how this affects reinforcement learning has not been investigated. We examined the ability of young people with TS (n = 18), TS+ADHD (N = 17), ADHD (n = 13) and typically developing controls (n = 20) to learn and reverse stimulus-response (S-R) associations based on positive and negative reinforcement feedback. We used a 2 (TS-yes, TS-no) x 2 (ADHD-yes, ADHD-no) factorial design to assess the effects of TS, ADHD, and their interaction on behavioural (accuracy, RT) and event-related potential (stimulus-locked P3, feedback-locked P2, feedback-related negativity, FRN) indices of learning and reversing the S-R associations. TS was associated with intact learning and reversal performance and largely typical ERP amplitudes. ADHD was associated with lower accuracy during S-R learning and impaired reversal learning (significantly reduced accuracy and a trend for smaller P3 amplitude). The results indicate that co-occurring ADHD symptoms impair reversal learning in TS+ADHD. The implications of these findings for behavioural tic therapies are discussed

    How important is the context of an adolescent's first alcoholic drink? Evidence that parental provision may reduce later heavy episodic drinking

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    Objective: This study examined the extent to which a retrospective measure of parental provision of the first alcoholic beverage was related to current heavy episodic drinking and current responsible drinking practices. Sample: 608 14- to 17-year-olds from the 2007 Australian National Drug Strategy Household Survey. Measures: Source of first alcoholic beverage (friends/parents/others), source of current alcohol, age of onset of alcohol use, current responsible drinking practices, and proportion of current friends who drink. Results: Binary logistic and multiple regression procedures revealed that parental provision of an adolescent's first alcoholic beverage predicted lower current heavy episodic drinking, and responsible drinking mediated this association. Discussion: The results suggested that for adolescents who become alcohol users, parental provision of the first drink may reduce subsequent alcohol-related risks compared to introduction to alcohol by friends and other sources. Alcohol-related risks remain significant for adolescents who consume alcohol, independent of who is the provider. Copyright (C) 2012 S. Karger AG, Base

    Virus infection and grazing exert counteracting influences on survivorship of native bunchgrass seedlings competing with invasive exotics

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    1.  Invasive annual grasses introduced by European settlers have largely displaced native grassland vegetation in California and now form dense stands that constrain the establishment of native perennial bunchgrass seedlings. Bunchgrass seedlings face additional pressures from both livestock grazing and barley and cereal yellow dwarf viruses (B/CYDVs), which infect both young and established grasses throughout the state. 2.  Previous work suggested that B/CYDVs could mediate apparent competition between invasive exotic grasses and native bunchgrasses in California. 3.  To investigate the potential significance of virus-mediated mortality for early survivorship of bunchgrass seedlings, we compared the separate and combined effects of virus infection, competition and simulated grazing in a field experiment. We infected two species of young bunchgrasses that show different sensitivity to B/CYDV infection, subjected them to competition with three different densities of exotic annuals crossed with two clipping treatments, and monitored their growth and first-year survivorship. 4.  Although virus infection alone did not reduce first-year survivorship, it halved the survivorship of bunchgrasses competing with exotics. Within an environment in which competition strongly reduces seedling survivorship (as in natural grasslands), virus infection therefore has the power to cause additional seedling mortality and alter patterns of establishment. 5.  Surprisingly, clipping did not reduce bunchgrass survivorship further, but rather doubled it and disproportionately increased survivorship of infected bunchgrasses. 6.  Together with previous work, these findings show that B/CYDVs can be potentially powerful elements influencing species interactions in natural grasslands. 7.  More generally, our findings demonstrate the potential significance of multitrophic interactions in virus ecology. Although sometimes treated collectively as plant ‘predators’, viruses and herbivores may exert influences that are distinctly different, even counteracting
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