1,544 research outputs found
Statistical mechanics of two-dimensional foams: Physical foundations of the model
In a recent series of papers [1--3], a statistical model that accounts for
correlations between topological and geometrical properties of a
two-dimensional shuffled foam has been proposed and compared with experimental
and numerical data. Here, the various assumptions on which the model is based
are exposed and justified: the equiprobability hypothesis of the foam
configurations is argued. The range of correlations between bubbles is
discussed, and the mean field approximation that is used in the model is
detailed. The two self-consistency equations associated with this mean field
description can be interpreted as the conservation laws of number of sides and
bubble curvature, respectively. Finally, the use of a '' Grand-Canonical ''
description, in which the foam constitutes a reservoir of sides and curvature,
is justified
Architecture of optimal transport networks
We analyze the structure of networks minimizing the global resistance to flow
(or dissipated energy) with respect to two different constraints: fixed total
channel volume and fixed total channel surface area. First, we determine the
shape of channels in such optimal networks and show that they must be straight
with uniform cross-sectional areas. Then, we establish a relation between the
cross-sectional areas of adjoining channels at each junction. Indeed, this
relation is a generalization of Murray's law, originally established in the
context of local optimization. Moreover, we establish a relation between angles
and cross-sectional areas of adjoining channels at each junction, which can be
represented as a vectorial force balance equation, where the force weight
depends on the channel cross-sectional area. A scaling law between the minimal
resistance value and the total volume or surface area value is also derived
from the analysis. Furthermore, we show that no more than three or four
channels meet in one junction of optimal bi-dimensional networks, depending on
the flow profile (e.g.: Poiseuille-like or plug-like) and the considered
constraint (fixed volume or surface area). In particular, we show that sources
are directly connected to wells, without intermediate junctions, for minimal
resistance networks preserving the total channel volume in case of plug flow
regime. Finally, all these results are illustrated with a simple example, and
compared with the structure of natural networks
Optimizing the bulk modulus of cellular networks
We present an alternative derivation of upper-bounds for the bulk modulus of
both two-dimensional and three-dimensional cellular materials. For
two-dimensional materials, we recover exactly the expression of the
Hashin-Shtrikman (HS) upper-bound in the low-density limit, while for
three-dimensional materials we even improve the HS bound. Furthermore, we
establish necessary and sufficient conditions on the cellular structure for
maximizing the bulk modulus, for a given solid volume fraction. These
conditions are found to be exactly those under which the electrical (or
thermal) conductivity of the material reaches its maximal value as well. These
results provide a set of straightforward criteria allowing to address the
design of optimized cellular materials, and shed light on recent studies of
structures with both maximal bulk modulus and maximal conductivity. Finally, we
discuss the compatibility of the criteria presented here with the geometrical
constraints caused by minimization of surface energy in a real foam
Frame tension governs the thermal fluctuations of a fluid membrane: new evidence
Two different tensions can be defined for a fluid membrane: the internal
tension, , conjugated to the real membrane area in the Hamiltonian, and
the frame tension, , conjugated to the projected (or frame) area.
According to the standard statistical description of a membrane, the
fluctuation spectrum is governed by . However, using rotational
invariance arguments, several studies argued that fluctuation spectrum must be
governed by the frame tension instead. These studies disagree on the
origin of the result obtained with the standard description yet: either a
miscounting of configurations, quantified with the integration measure, or the
use of a quadratic approximation of the Helfrich Hamiltonian. Analyzing the
simplest case of a one-dimensional membrane, for which arc length offers a
natural parametrization, we give a new proof that the fluctuations are driven
by , and show that the origin of the issue with the standard description
is a miscounting of membrane configurations. The origin itself of this
miscounting depends on the thermodynamic ensemble in which calculations are
made
Concevoir des contenus de formations des enseignants à partir de l'analyse du système d'activité en classe
Cette communication présente une réflexion relative à la conception de formations issues de l'analyse de l'activité en classe. Les résultats d'une étude portant sur une séquence de lecture orale et collective au Cycle 2 du primaire montrent que l'action de l'enseignante et des élèves s'articulent localement et que ces articulations font émerger une organisation collective que nous dénommons configuration d'activité. Cette configuration est organisée au sens où un ordre émerge de ces interactions locales, et organisante dans la mesure où les actions individuelles sont contraintes par elle en retour. La discussion porte sur la possibilité de concevoir des contenus de formation d'enseignants sur l'analyse de l'activité réelle en classe, et notamment sur le (ou les) niveau(x) d'organisation de l'activité pertinent(s) pour la conception de ces contenus
Les préoccupations et la tonalité émotionnelle des enseignants débutants lors de leurs premières expériences en classe
International audienceThis study describes and interprets the beginning teacher's experience in Physical Education during her first teaching year. The method to describe and interpret her experience is based on a primacy allowed to her emotions during her experience, to her preoccupations in front of their students, and to the way she copes with her preoccupations into action. The results indicate how teacher's emotional tonality and preoccupations format her action during ordinary teaching situations. They show the usefulness of revealing the subjective dimension of beginning teacher's professional experience in teacher education programs.Cette recherche décrit et interprète l'expérience d'une enseignante débutante en Éducation Physique lors de sa première année d'enseignement. La méthode pour décrire et interpréter son expérience accorde le primat à ses émotions durant son expérience, à ses préoccupations face aux élèves, et à la manière de concrétiser ses préoccupations dans l'action. Les résultats montrent combien la tonalité émotionnelle et les préoccupations de l'enseignante influencent son action lors de situations d'enseignement ordinaires. Ils montrent la nécessité de dévoiler la dimension subjective de l'expérience professionnelle des enseignants débutants en formation initiale
La formation d’enseignants entre utilitarisme et cycles de Don : illustration en éducation physique
La formation des enseignants est gĂ©nĂ©ralement conçue en termes de transmission/construction utilitaire de connaissances et de compĂ©tences, et les formateurs valorisent le plus souvent l’adhĂ©sion, au moins formelle, Ă une logique utilitariste. Cependant, certains faits significatifs ne sont pas explicables dans cette logique. Ces faits sont analysĂ©s Ă partir de la thĂ©orie du Don dans des cas d’aide spontanĂ©e et des dispositifs de conseil chez des Ă©tudiants, des enseignants stagiaires et des formateurs. L’analyse montre que la nature des liens de confiance ou de dĂ©fiance qui se tissent entre formateurs et formĂ©s pourrait avoir une influence dans la formation des enseignants. Ces liens permettent la rĂ©alisation de cycles de don ou de vengeance qui autorisent ou non le transfert de connaissances chez les enseignants nouveaux venus.In general, teacher training is envisaged in terms of transmission/construction of knowledge and competencies, and trainers most often place value on a utilitarian logic, as least in a formal sense. However, certain significant events are not explained using this logic. These situations are analyzed based on the theory of the “Gift” in cases of spontaneous help and advice between students, teacher mentors, and teacher trainers. The analysis shows that nature of relationships, being either those of confidence or defiance, that develop between trainers and their students can impact on teacher training. These links show the presence of cycles of gift and of vengeance that facilitate or prevent the transfer of knowledge to newly arrived teachers.La formaciĂłn docente se concibe en general en tĂ©rminos de transmisiĂłn/construcciĂłn utilitaria de conocimientos y de competencias, y los formadores habitualmente valoran la adhesiĂłn, por lo menos formal, a una lĂłgica utilitarista. Sin embargo, algunos hechos significativos no se pueden explicar dentro de esta lĂłgica. Estos hechos son analizados a partir de la teorĂa del Don en unos casos de ayuda espontánea y de dispositivos de apoyo con estudiantes, docentes en prácticas y formadores. El análisis muestra que la naturaleza de los lazos de confianza o de desconfianza que se generan entre formadores y formados podrĂa influir en la formaciĂłn docente. Estos lazos permiten la realizaciĂłn de ciclos de don o de venganza que autorizan o no la transferencia de conocimientos en los docentes reciĂ©n llegados
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