Symmetric Brownian motor

In this paper, we present a model of a symmetric Brownian motor which changes the sign of its velocity when the temperature gradient is inverted. The velocity, external work, and efficiency are studied as a function of the temperatures of the baths and other relevant parameters. The motor shows a current reversal when another parameter (a phase shift) is varied. Analytical predictions and results from numerical simulations are performed and agree very well. Generic properties of this type of motor are discussed.

Brownian pump powered by a white-noise flasing ratchet

A Brownian pump of particles powered by a stochastic flashing ratchet mechanism is studied. The pumping device is embedded in a finite region and bounded by particle reservoirs. In the steady state, we exactly calculate the spatial density profile, the concentration ratio between both reservoirs and the particle flux. We propose a simulation framework for the consistent evaluation of such observable quantities.

Heat fluctuations in Brownian transducers

The heat fluctuation probability distribution function in Brownian transducers operating between two heat reservoirs is studied. We find, both analytically and numerically, that the recently proposed fluctuation theorem for heat exchange [C. Jarzynski and D. K. Wojcik, Phys. Rev. Lett. 92, 230602 (2004)] has to be applied carefully when the coupling mechanism between both baths is considered. We also conjecture how to extend such a relation when an external work is present.

Tight coupling in thermal Brownian motors

We study analytically a thermal Brownian motor model and calculate exactly the Onsager coefficients. We show how the reciprocity relation holds and that the determinant of the Onsager matrix vanishes. Such a condition implies that the device is built with tight coupling. This explains why Carnot¿s efficiency can be achieved in the limit of infinitely slow velocities. We also prove that the efficiency at maximum power has the maximum possible value, which corresponds to the Curzon-Alhborn bound. Finally, we discuss the model acting as a Brownian refrigerator.

La caverna de José Saramago: imatge platònica versus metafísica

La caverna de José Saramago té com a referència indubtable la imatge de la caverna del llibre VII de la República de Plató, i, tanmateix, Saramago no és un escriptor idealista o metafísic. Aquest article mostra com, tot aprofitant l¿aplicabilitat amb què Plató dota la seva imatge, Saramago defensa la necessitat de saber rebre els missatges de la terra, de la matèria, de no esdevenir presoners en las cavernes daurades de la societat occidental, i d¿esdevenir lliures en la natura, phýsis, i no pas lluny o més enllà, metá, d¿ella.

La caverna de José Saramago: imagen platónica versus metafísica

La caverna de José Saramago tiene como referencia indudable la imagen de la caverna del libro VII de la República de Platón, y, sin embargo, Saramago no és un escritor idealista o metafísico. Este artículo muestra cómo, aprovechando la aplicabilidad con que Platón dotó a su imagen, Saramago defiende la necesidad de saber recibir los mensajes de la tierra, de la materia, de no convertirnos en prisioneros en las cavernas doradas de la sociedad occidental, y de ser libres en la naturaleza, phýsis, y no lejos o más allá, metá, de ella.

The Cave by José Saramago: Platonic image versus metaphysics

The cave by José Saramago has as a certain reference the image of the cave of book VII of Plato's Republic and, however, Saramago is not an idealistic or metaphysical writer. This article, taking advantage of the applicability with which Plato endowed his image, defends the urge to be open to the messages sent by the earth, by matter, the urge not to become prisoners in the golden caves of the Western society and, finally, the urge to find our freedom in Nature, phýsis, and not far or beyond, metá, it.

La caverna de José Saramago: imagen platónica versus metafísica

La caverna de José Saramago tiene como referencia indudable la imagen de la caverna del libro VII de la República de Platón, y, sin embargo, Saramago no és un escritor idealista o metafísico. Este artículo muestra cómo, aprovechando la aplicabilidad con que Platón dotó a su imagen, Saramago defiende la necesidad de saber recibir los mensajes de la tierra, de la materia, de no convertirnos en prisioneros en las cavernas doradas de la sociedad occidental, y de ser libres en la naturaleza, phýsis, y no lejos o más allá, metá, de ella.

From Galilean-invariant to relativistic wave equations

Through an imaginary change of coordinates in the Galilei algebra in 4 space dimensions and making use of an original idea of Dirac and Lvy-Leblond, we are able to obtain the relativistic equations of Dirac and of Bargmann and Wigner starting with the (Galilean-invariant) Schrdinger equation.

3-Hydroxybutyrate coinfused with noradrenaline decreases resulting plasma levels of noradrenaline in Wistar rats

Pentobarbital-anaesthetized male Wistar rats were infused with 6microgkg-1min-1 of noradrenaline. The infusion was supplemented with 8.5 mgkg-1min-1 of D-3-hydroxybutyrate (3-OHB) for 15 min in order to determine its effect on the adrenergic response of the rat. Plasma levels of noradrenaline rose to a plateau of approximately 50 nmoll-1 with infusion. In the group infused with noradrenaline alone, noradrenaline levels were maintained for 1h. Supplementation with 3-OHB induced a decrease in plasma noradrenaline level that was inversely correlated with 3-OHB level. Aortic and interscapular brown adipose tissue temperatures increased with noradrenaline infusion, but the rise was arrested by 3-OHB; replacing 3-OHB with glucose had no effect. Infusion of saline, glucose or 3-OHB in the absence of noradrenaline did not induce a rise in temperature in either tissue. Blood 3-OHB concentration increased to 1.2 mmoll-1 during 3-OHB infusion, decreasing rapidly at the end of infusion. Blood glucose levels increased with noradrenaline infusion; the presence of high 3-OHB levels decreased glucose concentration. The effects observed were transient and dependent on 3-OHB concentration; these effects may help explain most of the other effects of noradrenaline described here. The role of 3-OHB as a regulator of adrenergic responses seems to be part of a complex fail-safe mechanism which prevents wasting.

Rat insulin turnover in vivo

Zucker lean and obese rats were injected under pentobarbital anesthesia with 125I-labeled insulin; at timed intervals from 30 to 120 sec, blood samples were extracted and used for the estimation of insulin levels by RIA. A group of rats from each series was maintained under a constant infusion of noradrenaline. For each insulin determination, a duplicate blood sample containing the same amount of insulin as that used in the RIA, but without the radioactive label, was used as a blank for insulin measurement. The radioactivity in these tubes was then used for the measurement of insulin label per ml blood. From plasma label decay curves and insulin concentrations, the insulin pool size, half-life, and rate of degradation were calculated. Obese rats had higher insulin levels (2.43 nM) and showed less effect of noradrenaline than their lean counterparts, in which insulin distribution volume shrank with noradrenaline treatment. The half-life of plasma insulin was similar in all groups (range, 226-314 sec). Pool size and overall degradation rates were higher in obese (198 femtokatals) than in lean rats (28 femtokatals). It is postulated that obese rats synthesize and cleave much more insulin than lean controls despite their higher circulating levels of insulin.

Estudiar el ocio a través de ejercicios teatrales. Una aproximación al tiempo libre

El tema del ocio, estudiado desde la perspectiva de la psicosociología del tiempo libre (Munné, 1980), establece la necesidad de estimular el comportamiento autocondicionado y las habilidades para superar los heterocondicionamientos deeste tiempo social (Casas y Codina, 1998). Esto sugiere la búsqueda de herramientas metodológicas que permitan el desarrollo de la persona a través del autoconocimiento, proceso éste en el que la noción de sí mismo (self) adquiere unpapel preponderante.Si, dentro del ocio, se considera el aspecto contrafuncional de lo creativo (Munné, ibídem), como un elemento que permite transitar del tiempo liberador al tiempo liberado (Munné y Codina, 1996), podrían utilizarse los ejercicios teatrales paraindagar los procesos que conducen al tiempo libre. En psicología social, el uso de estas herramientas ha estado considerado, fundamentalmente, dentro de las perspectivas del psicodrama iniciado por Moreno, ciertos enfoques de lasdenominadas teorías del rol, y más recientemente como una expresión del denominado ocio serio (Stebbins, 1992).En esta comunicación se analizan las ventajas y desventajas de cada una de estas tres perspectivas, y se sugiere cómo podrían aplicarse los ejercicios teatrales en los estudios de ocio; a esto último se añade la consideración de nuevas epistemologías como las teorías de la complejidad en lo que respecta al tiempo libre y al sí mismo (Munné, 1997; Codina, 1998, 1999a, 1999b), derivándose de todo esto una posible alternativa en la intervención psicosocial desde los estudios de ocio.

El Prat de Llobregat de fa un segle segons una topografia mèdica: 1904

Aquest llibre va lligat a l'estudi que un dels seus autors, el doctor Vallribera i Puig, havia fet prèviament sobre els 145 treballs que restaven inèdits i que havien estat presentats al concurs de Topografies Mèdiques a la Reial Acadèmia de Medicina de Catalunya, on restaven conservats. Aquí es recupera un d'aquest treballs, datat el 1904 i aquí atribuït a Segimon Salgot i Baucells. Aquest treball centra el seu estudi a les topografies mèdiques a la comarca del Baix Llobregat.

José Ramón Mélida. Arqueología Española. Edición de Margarita Díaz Andreu

Dentro de la serie de Urgoiti Editores sobre los grandes nombres de la ciencia histórica en España, se publica la tercera obra dedicada a los arqueólogos y prehistoriadores de la primera mitad del siglo XX. Tras los estudios de Jordi Cortadella sobre Pere Bosch Gimpera (1981-1974) y Fernando Wulff sobre Adolf Schulten (1870-1960), y a la espera del trabajo de Gloria Mora sobre Hugo Obermaier (1877-1946), ve la luz el estudio que Margarita Díaz Andreu ha dedicado a una de las figuras más nombradas, pero a la par más desconocidas, de la arqueología española: José Ramón Mélida y Alinari (1856-1933).