1,720,965 research outputs found
On vectorial internal variables and dielectric and magnetic relaxation phenomena
No full textIn a previous paper it has been shown by the author that a vectorial internal variable may give rise to dielectric relaxation phenomena and that if such a variable occurs the polarization P may be written in the form P = P(0) + P(1), where changes in P(0) are reversible processes and changes in P(1) are irreversible.
In this paper we introduce a somewhat more general assumption concerning the entropy. This generalization leads to the possibility that both changes in P(0) and in P(1) are irreversible phenomena. In this way a formalism is obtained with two relaxation times for dielectric relaxation. In particular we investigate the linearized form of the theory. It is seen that in the linear case the relation between the electric field E and the polarization P has the form of a linear relation among E, P, the first derivatives with respect to time of E and P, and the second derivative with respect to time of P. Debye's equation for dielectric relaxation in polar liquids and the equation derived by De Groot and Mazur are special cases of the equation which has been obtained in this paper. Analogous results can be derived for magnetic relaxation phenomena. Snoek's equation and the equation obtained by De Groot and Mazur are special cases of the equation for magnetic relaxation which is derived in this paper
On the thermodynamics of viscosity and plasticity
No full textThe thermodynamic theory for viscosity and plasticity phenomena, given by the author in some previous papers, is further developed. A rather general Theological equation is derived from which the rheological equations for Poynting-Thomson, Jeffreys, Maxwell, Kelvin, Hooke, Newton, Prandtl-Reuss, and Bingham media may be considered as degeneracies. The physical meaning of these degeneracies is discussed. In particular it is seen that a Bingham medium may be considered as a special case of a Jeffreys medium. Explicit expressions are given for the free energy, the internal energy, and the entropy of the substances just mentioned. Temperature phenomena are taken into account. Some higher order effects (Poynting effect, structural viscosity, and thixotropy) are discussed from the point of view of the developed theory. A generalization of the Von Mises yield criterion is proposed. With the help of this criterion the Bauschinger effect may be explained as a consequence of memory phenomena. For media without memory this new criterion and the Von Mises criterion coalesce. It will be seen that for plastic media with memory a rheological equation holds, which may be considered as a generalization of the Poynting-Thomson equation. Both distortional and volumetric phenomena are considered. It is assumed that the deformations and rotations are small
On dielectric and magnetic relaxation phenomena and vectorial internal degrees
No full textIt has been shown by the author in a previous paper that thermodynamic vectorial internal degrees of freedom which influence the polarization or the magnetization of a medium may give rise to dielectric or magnetic relaxation phenomena. Snoek's equation for magnetic relaxation phenomena was derived and it was shown that Debye's theory for dielectric after-effects in polar liquids is a special case of the developed theory. In this paper it is shown that if Z is some vectorial internal degree of freedom which influences the polarization a new internal degree of freedom bip(int) may be defined which is a function of biZ, which may replace biZ as vectorial internal degree of freedom and which is a part of the total specific polarization. Furthermore, p(int) may be introduced in such a way that the remaining part of the polarization, p(el) (defined by p(el)=p- pint), where p is the total polarization per unit of mass), has the property that it vanishes for all values of p(int) if the medium is in a state where the electric field E and the mechanical elastic stresses vanish and the temperature of the medium equals some reference temperature. If the equations of state are linearized the latter result implies for an isotropic medium E=¿a(0,0)(bdp) p(el), where ¿ is the mass density and a(0,0)(P) a constant. On the other hand p(int) satifies a relaxation equation. It is seen that the use of p(int) as an internal degree of freedom is of great advantage. This is connected with the fact that p(int) is a measurable quantity in contradistinction to an arbitrary hidden vectorial internal degree of freedom. Analogous results may be obtained for magnetic after-effects
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
On plasticity, elastic relaxation phenomena, and a stress-strain relation which characterizes Schofield-Scott-Blair media
No full textA generalization is given of the author's theory for plasticity phenomena. The theory includes the possibility of both stress and strain relaxation in the preplastic range. Methods of non-equilibrium thermodynamics are used. The plasticity phenomenon is explained by introducing a physical assumption concerning the phenomenological coefficients. A yield function is proposed which includes the Bauschinger effect and strain hardening. If the free energy f has the form f=f (1)+f (2), where f (1) is a function of the temperature and the elastic strains and f (2) is a function of the temperature and the inelastic strains, and if cross effects between the plastic flow and elastic relaxation phenomena may be neglected, the proposed yield function is such that the derivative with respect to time of the deviator of the plastic strain tensor is given by , where F is the yield function, taß is the mechanical stress tensor, and ¿ is a coefficient which vanishes in the preplastic range. If the equations of state may be linearized the proposed yield function reduces to a function which is analogous to a yield function proposed by Freudenthal. If the plastic flow phenomenon is not associated with changes in the microscopic structure of the medium the proposed yield function reduces to the Von Mises function. It follows from the theory that in a first approximation elastic relaxation phenomena in the preplastic range may be described by the equation for Poynting-Thomson media (standard linear solids). An equation which characterizes Schofield-Scott Blair media is also derived from the developed theory
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