Showing papers by "University of the Aegean published in 1989"

Philosophy of science: an empirical study of teachers’ views

Abstract: An empirical survey of science teachers’ views about scientific knowledge from a philosophical‐epistemological perspective was carried out by means of a questionnaire specially developed for the purpose. The themes investigated were views concerning the nature of scientific method, the criteria of demarcation of science from non‐science, the nature of change in scientific knowledge and the status of scientific knowledge. Individuals could be identified, from particular patterns of responses, as being in agreement or not with a number of broad philosophical positions. These results point to a version of contextualist thought (broadly Kuhnian) as being the most favoured system. This would represent a shift from findings of earlier research, which has tended to identify empirical inductivism as the view mostly held by science teachers. However, the extent of any such shift is shown to depend on the aspect of science under consideration. In addition, the analysis looks at correlations between teachers’ views ...

Hydrodynamic characteristics of the solid‐liquid fluidized bed developing during the backwash of filter media

Abstract: Fluid‐particle interactions in a fluidized bed are discussed and a model is pre‐sented for the prediction of the expansion of a nonspherical particle bed, by application of the equations governing the respective behaviour of spherical particle beds and after introduction of an appropriate spherical diameter. Experimental data provided by the backwashing literature, have been used to validate the model and to compare it with prediction equations presented by other authors. Recent backwashing literature is reviewed, while special emphasis is put on the concept of optimum backwashing conditions, where a specific correction to published views is proposed.

Location of Transfer Centres on Segments of a Communication Network with Proportional Traffic

Abstract: The problem of placing bridges to alleviate congestion in a heavily loaded local-area computer network is briefly described. It is suggested that the generalization to that of placing transfer centres in other situations in which demand is arranged linearly may also be profitable. In order to obtain a better understanding of the problem, this paper concentrates on models in which demand is proportional (that is, the traffic intensity from one user to another is proportional to the product of their activity rates as measured by the traffic each produces). The results are rather surprising, it being demonstrated that the extent to which congestion can be relieved is severely limited.

Protonic conductivity : a new application of soliton theory

Abstract: We propose a two sublattice model with a doubly periodic on-site potential to describe the proton transport in hydrogen-bonded quasi-one-dimensional networks. The discrete system is reduced to a continuum double Sine-Gordon equation for the protonic part plus a simple differential equation for the heavy ion part. Its two-component kink solitons correspond to the ionic and B j e m defects. The correct response of these solitonic defects to an externally applied dc electric field makes this system an excellent model for qualitative and quantitative description of the protonic conductivity in hydrogen-bonded networks. 1 INTRODUCTION Proton conductivity along quasi-one-dimensional hydrogen-bonded networks in some molecular systems can exceed the conductivity in the orthogonal direction by a factor of 1000. Ice-like structures of hydrogen-bonded systems show a proton mobility only an order of magnitude less than that in metals. For that reason, such a crystal is often considered as a \"protonic semiconductor\". Experimental evidence 11-41 show that high proton mobility in ice-like systems is due to their transfer along the hydrogen bonds. In spite of the profound importance that the hydrogen bond plays in Condensed Matter Physics and Biology, very little is known about the detailed mechanisms for proton transport along hydrogen-bonded networks. The hydrogen-bonded networks which we consider here are quasi one-dimensional clusters of molecular aggregates interacting with their first neighbors through hydrogen bonds. Considering the simplest case and focussing our attention on the main degrees of freedom, we obtain the following diatomic structure of hydrogen-bonded dimers : where the full line segments indicate a covalent or an, ionic bond, the dotted line segments indicate a hydrogen bond, and X denotes a negative ion. The atoms that are usually involved with hydrogen bonds representing the negative proton-accepting side of the heavy molecule or aggregate are 0, N, and F. In such a chain because of the symmetric one-dimensional environment of the hydrogen ion, protons can easily jump Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989301 C3-4 JOURNAL DE PHYSIQUE from their left equilibrium position to the energetically equivalent position at the right near the next X ion. This physical situation is usually modeled by the association with each proton of a double-well substrate potential representing the two degenerate proton equilibrium positions per unit cell [S]. Ice is a very common example of a hydrogen-bonded system. In the more stable Ih hexagonal ice structure 111, one can distinguish hydrogen-bonded networks either following a given crystallographic direction (for example, a zig-zag hydrogen-bonded linkage) or just following a Bemal-Fowler filarnental path [61. Before proceeding to better-defined but more complex quasi-one dimensional biological or polymeric hydrogen-bonded macromolecules, let us first study the zig-zag diatomic structure in Ih ice because of the extensive experimental data that are available for ice [I-31. One well-known property of ice, as well as of other hydrogen-bonded sytems, is the spontaneous formation of ionic and Bjenum-type defects. In the zig-zag hydrogen-bonded network (Fig.0, one can realize a configuration where a number of successive protons jumping in their second equilibrium position produce one hydroxyl ion (OH-) with negative effective charge and one hydronium ion (H30+) with a positive effective charge [Fig.l(a)l. In an 'infinite' chain we can generate a large number of negative and positive ionic defects without changing the number of protons in the system. On the other hand, following Bjerrum's theory 171, we can rotate a number of successive water dipoles in such a way that an 0-0 bond with two protons and positive effective charge and one without proton and with negative effective charge is generated [Fig.l(b)l. The number of positive (D) and negative (L) Bjerrum defects can increase or decrease by injection or removal of protons, respectively.