A parity breaking Ising chain Hamiltonian as a Brownian motor

We consider the translationally invariant but parity (left-right symmetry) breaking Ising chain Hamiltonian \begin{equation} {\cal H} = -U_2\sum_{k} s_{k}s_{k+1} - U_3\sum_{k} s_{k}s_{k+1}s_{k+3} \nonumber \end{equation} and let this system evolve by Kawasaki spin exchange dynamics. Monte Carlo simulations show that perturbations forcing this system off equilibrium make it act as a Brownian molecular motor which, in the lattice gas interpretation, transports particles along the chain. We determine the particle current under various different circumstances, in particular as a function of the ratio $U_3/U_2$ and of the conserved magnetization $M=\sum_k s_k$. The symmetry of the $U_3$ term in the Hamiltonian is discussedComment: 11 pages, 4 figure

The two-dimensional two-component plasma plus background on a sphere : Exact results

An exact solution is given for a two-dimensional model of a Coulomb gas, more general than the previously solved ones. The system is made of a uniformly charged background, positive particles, and negative particles, on the surface of a sphere. At the special value $\Gamma = 2$ of the reduced inverse temperature, the classical equilibrium statistical mechanics is worked out~: the correlations and the grand potential are calculated. The thermodynamic limit is taken, and as it is approached the grand potential exhibits a finite-size correction of the expected universal form.Comment: 23 pages, Plain Te

School Co-ordinators: Leaders of Learning in Professional Experience

It is widely accepted that professional experience or practicum is ‘a critically important part of teacher education courses and is consistently valued highly by student teachers’ (eg Ramsey, 2000; Teaching Australia, 2006; Parliament of Australia, 2007). In Australia and overseas there is a growing emphasis on teacher educators working in partnership with schools to construct professional experiences that maximise student teacher engagement and learning (Parliament of Australia, 2007). The literature on professional experience in pre-service teacher education provides varied and detailed accounts of the roles of the Pre-service Teacher, the Mentor Teacher and the University Mentor (see for example, Gaffey & Dobbins, 1996; Guyton & McIntyre, 1990; Zeichner, 1999). However the School based Professional Experience Co-ordinator, usually the principal or deputy principal, has received very little attention in the literature. The study on which this article is primarily based fills a gap in the existing literature on professional experience with its focus on the School based Co-ordinator role. It will be argued that Co-ordinators are essential in developing ‘new’ school-university partnerships which are necessary in ensuring high quality professional experiences

Granular Rough Sphere in a Low-Density Thermal Bath

We study the stationary state of a rough granular sphere immersed in a thermal bath composed of point particles. When the center of mass of the sphere is fixed the stationary angular velocity distribution is shown to be Gaussian with an effective temperature lower than that of the bath. For a freely moving rough sphere coupled to the thermostat via inelastic collisions we find a condition under which the joint distribution of the translational and rotational velocities is a product of Gaussian distributions with the same effective temperature. In this rather unexpected case we derive a formula for the stationary energy flow from the thermostat to the sphere in accordance with Fourier law

Charge renormalization and other exact coupling corrections in the dipolar effective interaction in an electrolyte near a dielectric wall

The aim of the paper is to study the renormalizations of the charge and of the screening length that appear in the large-distance behavior of the effective pairwise interaction between two charges in a dilute electrolyte solution, both along a dielectric wall and in the bulk. The electrolyte is described by the primitive model in the framework of classical statistical mechanics and the electrostatic response of the wall is characterized by its dielectric constant.Comment: 60 pages 9 figure

A modern Fizeau experiment for education and outreach purposes

On the occasion of the laser's 50th anniversary, we performed a modern Fizeau experiment, measuring the speed of light with a laser beam passing over the city centre of Marseille. For a round trip distance of almost five kilometers, the measurement has reached an uncertainty of about 10$^{-4}$, mainly due to atmospheric fluctuations. We present the experimental and pedagogical challenges of this brilliant outreach experiment.Comment: accepted by Eur J Phys in november 201

New duality relation for the Discrete Gaussian SOS model on a torus

We construct a new duality for two-dimensional Discrete Gaussian models. It is based on a known one-dimensional duality and on a mapping, implied by the Chinese remainder theorem, between the sites of an $N\times M$ torus and those of a ring of $NM$ sites. The duality holds for an arbitrary translation invariant interaction potential $v(\mathbf{r})$ between the height variables on the torus. It leads to pairs $(v,\widetilde{v})$ of mutually dual potentials and to a temperature inversion according to $\widetilde{\beta}=\pi^2/\beta$. When $v(\mathbf{r})$ is isotropic, duality renders an anisotropic $\widetilde{v}$. This is the case, in particular, for the potential that is dual to an isotropic nearest-neighbor potential. In the thermodynamic limit this dual potential is shown to decay with distance according to an inverse square law with a quadrupolar angular dependence. There is a single pair of self-dual potentials $v^\star=\widetilde{v^\star}$. At the self-dual temperature $\beta^\star=\widetilde{\beta^\star}=\pi$ the height-height correlation can be calculated explicitly; it is anisotropic and diverges logarithmically with distance.Comment: 26 pages, 2 figure

The Ideal Conductor Limit

This paper compares two methods of statistical mechanics used to study a classical Coulomb system S near an ideal conductor C. The first method consists in neglecting the thermal fluctuations in the conductor C and constrains the electric potential to be constant on it. In the second method the conductor C is considered as a conducting Coulomb system the charge correlation length of which goes to zero. It has been noticed in the past, in particular cases, that the two methods yield the same results for the particle densities and correlations in S. It is shown that this is true in general for the quantities which depend only on the degrees of freedom of S, but that some other quantities, especially the electric potential correlations and the stress tensor, are different in the two approaches. In spite of this the two methods give the same electric forces exerted on S.Comment: 19 pages, plain TeX. Submited to J. Phys. A: Math. Ge

Alimentation de sevrage et interventions nutritionnelles au Congo

Au Congo les états de dénutrition apparaissent tôt et leur prévalence est particulièrement élevée pendant la période du sevrage. En vue d'aider à l'élaboration et à la mise en oeuvre d'interventions nutritionnelles (éducation, amélioration de l'alimentation de sevrage) des données relatives à l'alimentation des enfants de moins de 24 mois, en milieu rural et en milieu urbain, ont été collectées. La pratique de l'allaitement marternel est encore généralisée et de longue durée, en dépit d'une tendance au raccourcissement en milieu urbain. En revanche les pratiques alimentaires, notamment de sevrage, et la qualité des bouillies contribuent fortement au mauvais état nutritionnel et aux déficits de croissance des enfants. En milieu rural les bouillies sont données trop tôt puis arrêtées également trop tôt pour être remplacées sans véritable transition par les aliments du plat familial. En milieu urbain l'utilisation des bouillies de sevrage est plus rationnelle et les enfants sont amenés plus progressivement au sevrage total. Leur alimentation est également moins monotone. Les bouillies préparées localement sont toutes de qualité médiocre, notamment au plan énergétique. Des solutions peuvent être proposées pour les améliorer. Elles devront tenir compte des aspects culturels des pratiques alimentaires. Elles devront également ne pas se traduire pour les mères par un surcroît de travail. Les différences positives constatées en milieu urbain laissent penser que des changements efficaces sont possibles à moyen terme. (Résumé d'auteur

Casimir force between two ideal-conductor walls revisited

The high-temperature aspects of the Casimir force between two neutral conducting walls are studied. The mathematical model of "inert" ideal-conductor walls, considered in the original formulations of the Casimir effect, is based on the universal properties of the electromagnetic radiation in the vacuum between the conductors, with zero boundary conditions for the tangential components of the electric field on the walls. This formulation seems to be in agreement with experiments on metallic conductors at room temperature. At high temperatures or large distances, at least, fluctuations of the electric field are present in the bulk and at the surface of a particle system forming the walls, even in the high-density limit: "living" ideal conductors. This makes the enforcement of the inert boundary conditions inadequate. Within a hierarchy of length scales, the high-temperature Casimir force is shown to be entirely determined by the thermal fluctuations in the conducting walls, modelled microscopically by classical Coulomb fluids in the Debye-H\"{u}ckel regime. The semi-classical regime, in the framework of quantum electrodynamics, is studied in the companion letter by P.R.Buenzli and Ph.A.Martin, cond-mat/0506363, Europhys.Lett.72, 42 (2005).Comment: 7 pages.One reference updated. Domain of validity of eq.(11) correcte