535 research outputs found
Dependences of the Casimir-Polder interaction between an atom and a cavity wall on atomic and material properties
The Casimir-Polder and van der Waals interactions between an atom and a flat
cavity wall are investigated under the influence of real conditions including
the dynamic polarizability of the atom, actual conductivity of the wall
material and nonzero temperature of the wall. The cases of different atoms near
metal and dielectric walls are considered. It is shown that to obtain accurate
results for the atom-wall interaction at short separations, one should use the
complete tabulated optical data for the complex refractive index of the wall
material and the accurate dynamic polarizability of an atom. At relatively
large separations in the case of a metal wall, one may use the plasma model
dielectric function to describe the dielectric properties of wall material. The
obtained results are important for the theoretical interpretation of
experiments on quantum reflection and Bose-Einstein condensation.Comment: 5 pages, 1 figure, iopart.cls is used, to appear in J. Phys. A
(special issue: Proceedings of QFEXT05, Barcelona, Sept. 5-9, 2005
Compliance of the L5-S1 spinal unit: a comparative study between an unconstrained and a partially constrained system
A comparison between an unconstrained and a partially constrained system for in vitro biomechanical testing of the L5-S1 spinal unit was conducted. The objective was to compare the compliance and the coupling of the L5-S1 unit measured with an unconstrained and a partially constrained test for the three major physiological motions of the human spine. Very few studies have compared unconstrained and partially constrained testing systems using the same cadaveric functional spinal units (FSUs). Seven human L5-S1 units were therefore tested on both a pneumatic, unconstrained, and a servohydraulic, partially constrained system. Each FSU was tested along three motions: flexion-extension (FE), lateral bending (LB) and axial rotation (AR). The obtained kinematics on both systems is not equivalent, except for the FE case, where both motions are similar. The directions of coupled motions were similar for both tests, but their magnitudes were smaller in the partially constrained configuration. The use of a partially constrained system to characterize LB and AR of the lumbosacral FSU decreased significantly the measured stiffness of the segment. The unconstrained system is today's "gold standard” for the characterization of FSUs. The selected partially constrained method seems also to be an appropriate way to characterize FSUs for specific applications. Care should be taken using the latter method when the coupled motions are importan
Dependences of the van der Waals atom-wall interaction on atomic and material properties
The 1%-accurate calculations of the van der Waals interaction between an atom
and a cavity wall are performed in the separation region from 3 nm to 150 nm.
The cases of metastable He and Na atoms near the metal,
semiconductor or dielectric walls are considered. Different approximations to
the description of wall material and atomic dynamic polarizability are
carefully compared. The smooth transition to the Casimir-Polder interaction is
verified. It is shown that to obtain accurate results for the atom-wall van der
Waals interaction at shortest separations with an error less than 1% one should
use the complete optical tabulated data for the complex refraction index of the
wall material and the accurate dynamic polarizability of an atom. The obtained
results may be useful for the theoretical interpretation of recent experiments
on quantum reflection and Bose-Einstein condensation of ultracold atoms on or
near surfaces of different nature.Comment: 14 pages, 5 figures, 3 tables, accepted for publication in Phys. Rev.
El tiempo como contexto y pretexto educativo en la sociedad red
Les transformacions que se van produint en els temps socials són cada cop més determinades per la creixent desregulació i deshumanització del temps de treball. L’hegemonia del mercat es manifesta amb intensitat en la revisió i deconstrucció de les relacions espacialtemporals: la desregulació i la deslocalització són expressions genuïnes d’aquesta deriva. Aquestes tendències desafien l’educació Pel que fa a promoure processos que acreixen la presa de consciència sobre els diversos temps que conformen la vida quotidiana o sobre les conseqüències que els seus diferents usos provoquen en el desenvolupament personal i col·lectiu, tant en l’interior dels sistemes educatius com en altres contextos pedagògics i socials que procuren una formació més integral i integradora.The transformations on social times that have been occurring are more and more brought about by the increasing de-regularisation and dehumanization of working times. The dominance of the markets has a strong presence in the revision of space-time relationships: de-regularisation and off shoring are key concepts
in this drift. This trends challenge education to promote new dynamics that would increasing the awareness of the different times that conform daily life, or the consequences that the different uses of them have on personal and collective development. Both in the inside of educational systems as in further pedagogic and social contexts that search for a more global and Integrative education.Las transformaciones que se vienen produciendo en los tiempos sociales están cada vez más determinadas por la creciente desregulación y deshumanización del tiempo de trabajo. La hegemonía del mercado se manifiesta con intensidad en la revisión y deconstrucción de las relaciones espacio-temporales: la desregulación y la deslocalización son expresiones genuinas de esta deriva. Estas tendencias desafían a la educación en cuanto a promover procesos que acrecienten la toma de conciencia sobre los diversos tiempos que conforman la vida cotidiana o sobre las consecuencias que sus diferentes usos provocan en el desarrollo personal y colectivo, tanto en el interior de los sistemas educativos como en otros contextos pedagógicos y sociales que procuran una formación más integral e integradora
Lateral projection as a possible explanation of the nontrivial boundary dependence of the Casimir force
We find the lateral projection of the Casimir force for a configuration of a
sphere above a corrugated plate. This force tends to change the sphere position
in the direction of a nearest corrugation maximum. The probability distribution
describing different positions of a sphere above a corrugated plate is
suggested which is fitted well with experimental data demonstrating the
nontrivial boundary dependence of the Casimir force.Comment: 5 pages, 1 figur
Thermal quantum field theory and the Casimir interaction between dielectrics
The Casimir and van der Waals interaction between two dissimilar thick
dielectric plates is reconsidered on the basis of thermal quantum field theory
in Matsubara formulation. We briefly review two main derivations of the
Lifshitz formula in the framework of thermal quantum field theory without use
of the fluctuation-dissipation theorem. A set of special conditions is
formulated under which these derivations remain valid in the presence of
dissipation. The low-temperature behavior of the Casimir and van der Waals
interactions between dissimilar dielectrics is found analytically from the
Lifshitz theory for both an idealized model of dilute dielectrics and for real
dielectrics with finite static dielectric permittivities. The free energy,
pressure and entropy of the Casimir and van der Waals interactions at low
temperatures demonstrate the same universal dependence on the temperature as
was previously discovered for ideal metals. The entropy vanishes when
temperature goes to zero proving the validity of the Nernst heat theorem. This
solves the long-standing problem on the consistency of the Lifshitz theory with
thermodynamics in the case of dielectric plates. The obtained asymptotic
expressions are compared with numerical computations for both dissimilar and
similar real dielectrics and found to be in excellent agreement. The role of
the zero-frequency term in Matsubara sum is investigated in the case of
dielectric plates. It is shown that the inclusion of conductivity in the model
of dielectric response leads to the violation of the Nernst heat theorem. The
applications of this result to the topical problems of noncontact atomic
friction and the Casimir interaction between real metals are discussed.Comment: 39 pages, 4 figures, to appear in Phys. Rev.
A theory of Plasma Membrane Calcium Pump stimulation and activity
The ATP-driven Plasma Membrane Calcium pump or Ca(2+)-ATPase (PMCA) is
characterized by a high affinity to calcium and a low transport rate compared
to other transmembrane calcium transport proteins. It plays a crucial role for
calcium extrusion from cells. Calmodulin is an intracellular calcium buffering
protein which is capable in its Ca(2+) liganded form of stimulating the PMCA by
increasing both the affinity to calcium and the maximum calcium transport rate.
We introduce a new model of this stimulation process and derive analytical
expressions for experimental observables in order to determine the model
parameters on the basis of specific experiments. We furthermore develop a model
for the pumping activity. The pumping description resolves the seeming
contradiction of the Ca(2+):ATP stoichiometry of 1:1 during a translocation
step and the observation that the pump binds two calcium ions at the
intracellular site. The combination of the calcium pumping and the stimulation
model correctly describes PMCA function. We find that the processes of
calmodulin-calcium complex attachment to the pump and of stimulation have to be
separated. Other PMCA properties are discussed in the framework of the model.
The presented model can serve as a tool for calcium dynamics simulations and
provides the possibility to characterize different pump isoforms by different
type-specific parameter sets.Comment: 24 pages, 6 figure
Exact Casimir-Polder potential between a particle and an ideal metal cylindrical shell and the proximity force approximation
We derive the exact Casimir-Polder potential for a polarizable microparticle
inside an ideal metal cylindrical shell using the Green function method. The
exact Casimir-Polder potential for a particle outside a shell, obtained
recently by using the Hamiltonian approach, is rederived and confirmed. The
exact quantum field theoretical result is compared with that obtained using the
proximity force approximation and a very good agreement is demonstrated at
separations below 0.1, where is the radius of the cylinder. The
developed methods are applicable in the theory of topological defects.Comment: 8 pages, 4 figures, Accepted for publication in Eur. Phys. J.
Ampliação da jornada escolar: configurações próprias para diferentes contextos - Brasil e Europa
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