1,721,048 research outputs found
CLOSED-FORM EXPRESSION FOR THE POTENTIAL WITHIN A FACE-CENTRED CUBIC IONIC CRYSTAL
No full textIn this work we deal with the electrostatic potential generated by a periodical system of
charges in the space. Specifically, we consider a three-dimensional mesh having cubic cells,
whose nodes are occupied by point charges +Q or- Q; so that each positive (negative) charge
is surrounded by six negative (positive) charges at distances d: This system clearly models a
face centred cubic (FCC) ionic crystal that we further assume to extend to infinity in every
direction. Considering ions as point particles subject only to coulombic interactions, the
potential here concerned can be expressed by means of an alternating series that exhibits a very
slowconvergence, giving rise to computational difficulties.
In the following we illustrate an alternative formula for the potential, consisting in a closedform
expression involving the elliptic Jacobi Theta functions. Such formula provides either an
immediate interpretation from an analytical point of view, and a fast converging procedure for
numerical purposes.
Finally, we illustrate an application of our methodology concerning the calculus of the
Madelung constant in the case of an ideal FCC ionic crystal. Numerical accuracy and
computational costs are comparable with those of the most rapidly converging formulas
previously known
Mean activity coefficient of electrolyte solutions
No full textIn this paper, we deal with the mean activity coefficient, of electrolyte solutions. The case mean activity cofficient <= 1 is
investigated. As generally recognized, the most accepted models (specific ion interaction/Pitzer theory)
have the disadvantage of the dependence on semiempirical parameters. These are not directly accessible
from experimental measurements, but can only be estimated by means of best-fitting numerical techniques
from experimental data. In the general context of research devoted to the achievement of some reduction of
complexity, we propose a model of electrolyte solution that allows us to calculate the mean activity coefficient without using fitting
parameters where the (upper) concentration exists at which the electrolyte solution exhibits a mean activity coefficient = 1 (molality
scale). In the remaining cases, we show that a unique parameter is required, that is, the concentration that
should ideally give a mean activity coefficient =1 for the electrolyte. Compared to other models that do not require adjustable
parameters, the present one is generally applicable over a wider range of concentrations; moreover, it does
not impose any restriction on the ion-size variations. Our model follows a pseudolattice approach, starting
from the primitive idea of a disordered lattice of solute ions within a continuous solvent at extremely dilute
solutions and coming to a disordered lattice of local arrangements of both solute ions and solvent dipoles at
higher concentrations. Compared to other theories based on lattice models, this work stresses the role of
statistical deviations from any time-averaged (lattice) configuration. All formulas in this paper are applied
for 1:1, 2:2, 1:2, and 2:1 aqueous electrolytes at 25 °C
Effects of time-variant extremely-low-frequency (ELF) electromagnetic fields (EMF) on cholinesterase activity in Dictyostelium discoideum
No full textRecently, we detected propionylcholinesterase (PrChE) activity in single-cell amoebae of Dictyostelium discoideum using cytochemical, electrophoretic, and spectrophotometric methods. The involvement of this enzyme activity in cell-cell and cell-environment interactions was suggested. In this work, we found that exposure of single-cell amoebae to an extremely low-frequency electromagnetic fields (ELF-EMF) of 300 μT, 50 Hz, from 1 h up to 48 h at 21 ± 1 °C affected PrChE activity
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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