Stability and instability in nineteenth-century fluid mechanics

The stability or instability of a few basic flows was conjectured, debated, and sometimes proved in the nineteenth century. Motivations varied from turbulence observed in real flows to permanence expected in hydrodynamic theories of matter. Contemporary mathematics often failed to provide rigorous answers, and personal intuitions sometimes gave wrong results. Yet some of the basic ideas and methods of the modern theory of hydrodynamic instability occurred to the elite of British and German mathematical physics, including Stokes, Kelvin, Helmholtz, and Rayleigh. This usually happened by reflecting on concrete specific problems, with a striking variety of investigative styles

Spin-to-Orbital Angular Momentum Conversion and Spin-Polarization Filtering in Electron Beams

We propose the design of a space-variant Wien filter for electron beams that induces a spin half-turn and converts the corresponding spin angular momentum variation into orbital angular momentum of the beam itself by exploiting a geometrical phase arising in the spin manipulation. When applied to a spatially coherent input spin-polarized electron beam, such a device can generate an electron vortex beam, carrying orbital angular momentum. When applied to an unpolarized input beam, the proposed device, in combination with a suitable diffraction element, can act as a very effective spin-polarization filter. The same approach can also be applied to neutron or atom beams.Comment: 9 pages, 5 figure

Equatorial Guinea: a link between Africa, Iberian Peninsula and Latin America

In the past, exchanges between Sub-Saharan Africa and Latin America were reduced to Atlantic slave trade. Sub-Saharan Africa supplied slaves for plantations in Latin America. The abolition of slavery, the presence of Afro-descendants, the independence of Latin American and African countries changed the relations between the two regions of the world. For historic and linguistic reasons, Equatorial Guinea is a link between Africa, Iberian Peninsula and Latin America. In 1984, the international Hispanic and African Congress of Culture took place in Bata. In 2013, Malabo hosted the third Africa and South America Summit. During the same period, Equatorial Guinea joined Portuguese speaking countries community. In 2011, the country adopted Portuguese as its third official language. In 2014, Equatorial Guinea became full member of Portuguese speaking countries organization. This article analyses origins and impact of the international Hispanic and African Congress of Culture of 1984. Then, it studies issues and perspectives of Africa and South America Summit of 2013. To that end, it relies on archive documents, interviews and political speeches

Poincaré and the Reaction Principle in Electrodynamics

When Henri Poincaré reviewed the then competing theories of electrodynamics in the 1890s, he required their compatibility with two principles of mechanical origin—the reaction principle and the relativity principle. Historians of relativity theory have usually focused on the relativity principle and neglected or misinterpreted Poincaré’s concern with the reaction principle. In particular, most of them have interpreted his crucial article of 1900 on “Lorentz’s theory and the principle of reaction” as an attempt to save this principle by assuming an electromagnetic momentum in addition to the momentum of matter. The purpose of this article is to dismiss this interpretation and show that Poincaré had the opposite intention as he spelled out the paradoxical consequences of Lorentz’s violation of the reaction principle. In doing so, he formally introduced a quantity that could later be interpreted as a genuine electromagnetic momentum, developed a fundamentally new understanding of Lorentz’s transformations in relation with the relativity principle and identified the paradoxes that Einstein would later solve by assuming the inertia of energy.Quand, dans les années 1890, Henri Poincaré passa en revue les diverses théories électrodynamiques alors en compétition, il exigea leur compatibilité avec deux principes hérités de la mécanique : le principe de réaction et le principe de relativité. Les historiens de la Relativité se sont généralement concentrés sur ce second principe et ils ont négligé ou mal compris son souci de respecter le principe de réaction. En particulier, la plupart d’entre eux ont interprété son article crucial de 1900 sur la théorie de Lorentz et le principe de réaction comme une tentative de sauver ce principe en admettant une quantité de mouvement électromagnétique en sus de la quantité de mouvement de la matière. Le but du présent article est de réfuter cette interprétation et de montrer que Poincaré avait l’intention opposée de mettre en évidence les conséquences paradoxales de la violation du principe de réaction dans la théorie de Lorentz. Ce faisant, il introduisit formellement une quantité que d’autres interprétèrent plus tard comme une véritable quantité de mouvement électromagnétique ; il développa une conception fondamentalement nouvelle des transformations de Lorentz en relation avec le principe de relativité ; et il identifia les paradoxes qu’Albert Einstein résolut quelques années plus tard en admettant l’inertie de l’énergie

Capillary-Gravity Waves on Depth-Dependent Currents: Consequences for the Wave Resistance

We study theoretically the capillary-gravity waves created at the water-air interface by a small two-dimensional perturbation when a depth-dependent current is initially present in the fluid. Assuming linear wave theory, we derive a general expression of the wave resistance experienced by the perturbation as a function of the current profile in the case of an inviscid fluid. We then analyze and discuss in details the behavior of the wave resistance in the particular case of a linear current, a valid approximation for some wind generated currents.Comment: Submitted to EP

Dissertation critique et apologétique sur la langue basque

Digitalización. Vitoria-Gasteiz : Archivos y Bibliotecas, Mayo 199

Extended Einstein-Cartan theory a la Diakonov: the field equations

Diakonov formulated a model of a primordial Dirac spinor field interacting gravitationally within the geometric framework of the Poincar\'e gauge theory (PGT). Thus, the gravitational field variables are the orthonormal coframe (tetrad) and the Lorentz connection. A simple gravitational gauge Lagrangian is the Einstein-Cartan choice proportional to the curvature scalar plus a cosmological term. In Diakonov's model the coframe is eliminated by expressing it in terms of the primordial spinor. We derive the corresponding field equations for the first time. We extend the Diakonov model by additionally eliminating the Lorentz connection, but keeping local Lorentz covariance intact. Then, if we drop the Einstein-Cartan term in the Lagrangian, a nonlinear Heisenberg type spinor equation is recovered in the lowest approximation.Comment: 13 pages, no figure

CONFIGURACIÓN DEL CUERPO MÉDICO ESCOLAR EN URUGUAY (1903-1915)

El propósito del presente trabajo es examinar y analizar, a partir de fuentes institucionales del período seleccionado, las diversas configuraciones de sentido sobre la arquitectura escolar y los sujetos de la educación en el contexto de configuración del Cuerpo Médico Escolar en Uruguay. El recorte temporal obedece a las fuentes halladas que, a su vez, buscan abarcar los dos períodos presidenciales de José Batlle y Ordóñez del siglo xx (1903-1907 y 1911-1915). Con ello pretendemos no solo detenernos en la creación del Cuerpo Médico Escolar, sino aproximarnos a los argumentos de carácter higienista de los años previos y posteriores a su creación, en el marco de las nuevas condiciones de la vida urbana del Uruguay modernizador. Para la realización del análisis se recurre a las siguientes fuentes: Anales de Instrucción Primaria; Memoria correspondiente a los años 1911 a 1914 del Dr. Abel J. Pérez; Reglamento del Cuerpo Médico Escolar Nacional (1910); Reglas de higiene para los escolares realizado por el Cuerpo Médico Escolar (1913); Algunas ideas sobre nuevas orientaciones de la enseñanza de Abel J. Pérez, y Organización Médico-Escolar en el Uruguay de R. Schiaffino. Palabras clave: higienismo, Cuerpo Médico Escolar, arquitectura escolar, sujetos de la educación SCHOLAR MEDICAL CORPS CONFIGURATION IN URUGUAY(1903-1915)  Meanings of school architecture and education subjects Abstract The purpose of this article is to examine and analyze, considering the institutional resources of the selected period, the diferent sense configurations on school architecture and education subjects within the context of the Scholar Medical Corps in Uruguay. The temporary cutbacks respond to the resources found, which also intend to include both presidencies of José Batlle y Ordóñez in the twentieth century (1903-1907 and 1911-1915). We pretend not only to focus on the Scholar Medical Corps’s creation, but also to approach the hygienist arguments before and after its creation; within the context of the new conditions of urban life of the modernizing Uruguay. With the intention of performing such analysis the following resources are used: Annual document of Primary Instruction; Memory correspondant to 1911 to 1914 of Dr. Abel J. Pérez; National Scholar Medical Corps (1910); Hygiene rules for schoolchildren performed by the Scholar Medical Corps (1913); Some ideas about new guidelines on teaching of Abel J. Pérez, and School‑Medical Organization in Uruguay of R. Schiaffino. Keywords: hygienism, Scholar Medical Corps, school architecture, education subject

Carnot cycle for an oscillator

Carnot established in 1824 that the efficiency of cyclic engines operating between a hot bath at absolute temperature $T_{hot}$ and a bath at a lower temperature $T_{cold}$ cannot exceed $1-T_{cold}/T_{hot}$. We show that linear oscillators alternately in contact with hot and cold baths obey this principle in the quantum as well as in the classical regime. The expression of the work performed is derived from a simple prescription. Reversible and non-reversible cycles are illustrated. The paper begins with historical considerations and is essentially self-contained.Comment: 19 pages, 3 figures, sumitted to European Journal of Physics Changed content: Fluctuations are considere