2,190 research outputs found
Entropy puzzle in small exploding systems
We use a simple hard-core gas model to study the dynamics of small exploding
systems. The system is initially prepared in a thermalized state in a spherical
container and then allowed to expand freely into the vacuum. We follow the
expansion dynamics by recording the coordinates and velocities of all particles
until their last collision points (freeze-out). We have found that the entropy
per particle calculated for the ensemble of freeze-out points is very close to
the initial value. This is in apparent contradiction with the Joule experiment
in which the entropy grows when the gas expands irreversibly into a larger
volume.Comment: 10 pages, 4 figures, accepted June 17 2003 for publication in Physics
Letters
3d laser scanning and modelling of the Dhow heritage for the Qatar National Museum
This is the final version of the article. Available from SPIE via the DOI in this record.Curating boats can be difficult. They are complex structures, often demanding to conserve whether in or out of the water; they are usually large, difficult to move on land, and demanding of gallery space. Communicating life on board to a visiting public in the terra firma context of a museum can be difficult. Boats in their native environment are inherently dynamic artifacts. In a museum they can be static and divorced from the maritime context that might inspire engagement. New technologies offer new approaches to these problems. 3D laser scanning and digital modeling offers museums a multifaceted means of recording, monitoring, studying and communicating watercraft in their care. In this paper we describe the application of 3D laser scanning and subsequent digital modeling. Laser scans were further developed using computer-generated imagery (CGI) modeling techniques to produce photorealistic 3D digital models for development into interactive, media-based museum displays. The scans were also used to generate 2D naval lines and orthographic drawings as a lasting curatorial record of the dhows held by the Nationa
An Investigation of Orientational Symmetry-Breaking Mechanisms in High Landau Levels
The principal axes of the recently discovered anisotropic phases of 2D
electron systems at high Landau level occupancy are consistently oriented
relative to the crystal axes of the host semiconductor. The nature of the
native rotational symmetry breaking field responsible for this preferential
orientation remains unknown. Here we report on experiments designed to
investigate the origin and magnitude of this symmetry breaking field. Our
results suggest that neither micron-scale surface roughness features nor the
precise symmetry of the quantum well potential confining the 2D system are
important factors. By combining tilted field transport measurements with
detailed self-consistent calculations we estimate that the native anisotropy
energy, whatever its origin, is typically ~ 1 mK per electron.Comment: Reference added, minor notational changes; final published versio
Impact of pressure anisotropy on tokamak equilibria and the toroidal magnetic precession
Using a generalized anisotropic tokamak equilibrium and an exact guiding centre drift formulation, the effect of parallel and perpendicular anisotropy on the toroidal precession drift is investigated. SigniïŹcant differences between parallel and perpendicular pressure anisotropy are observed. While the Shafranov shift is not sensitive to the ratio of the parallel and perpendicular pressures p â„ /pâ„ , the deepening of the magnetic well is found to be sensitive to p â„ /pâ„ . Here, the diamagnetic effect identiïŹed by Connor et al 1983 Nucl. Fusion 23 1702 is generalized and found to depend crucially on the deposition of the energetic ions on which the equilibrium depends, and leads to test particle precessional drifts that depend sensitively on pitch angl
The ACTN3 Gene and Differences between Playing Positions in Bone Mineral Content, Fat Mass and Lean Tissue Mass in the Arms, Legs and Trunk Of Rugby Union Football Players
Aim: The function of the present study was to identify differences
between individual playing positions in bone mineral content, fat
mass, and lean tissue mass, in the arms, trunk and legs of young adult
Rugby Union football players who carried the ACTN3 gene.
Subjects and methods: A cross-sectional case control study
was carried out using a candidate gene approach (n=55). Individuals
belonged to a homogeneous group of players relative to age, gender,
ability, and ethnicity. Players were allocated to their preferred playing
position. These were the front row (n=14), second and back rows
(n=16), scrum and outside-half (n=11), and centres, wings and fullbacks
(n=14). A 5 ml sample of saliva was obtained from each player
and specimens stored at 4oC until buccal cell DNA extraction was
carried out. Height was measured to the nearest 0.1 cm and body mass
to the closest 0.1 kg. Dual-energy X-ray absorptiometry was measured
using a Hologic QDR Discovery fan beam model. Statistical analyses
were undertaken using ANOVA, ANCOVA and MANOVA.
Results: The study sample comprised 22% RR, 60% RX, and 18%
XX genotypes of the ACTN3 gene respectively. Players in the second
and back rows were significantly taller than other positions. Body
mass differences, were significantly greater in forwards than backs.
There were non-significant differences between positions in adjusted
bone mineral content or adjusted lean tissue mass. Adjusted fat mass
reflected differences between left and right arms, but not left and right
legs.
Conclusion: At a developmental level of performance, an
understanding and practical application of the structural, physiological
and body composition characteristics of individual players, will
facilitate personal and team accomplishment, efficiency of training
and conditioning, and nurture the potential of young adult players
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Applications of Solid Freeform Fabrication at the Naval Research Laboratory
Solid Freeform Fabrication (SFF) and related techniques are used at the Naval Research
Laboratory (NRL) for a variety of materials related investigations. Research and applications
conducted over the past few years are described including: Helisys Laminated Object
Manufacturing System (LOMS) fabrication of: ceramic piezoelectric actuators, tooling for
multifunctional materials, and anatomical prototypes for surgical visualization; fabrication of
mesoscale electronic and sensor components using a laser forward transfer direct write
technique; and visualization of complex, 3-D microstructures using a Stratasys Fused-Deposition
Modeler. The paper closes with a brief overview of future SFF related work at the NRL.Support for this work from DARPA, Office of Naval Research, and the Naval Research
Laboratory Core Research Program is gratefully acknowledged.Mechanical Engineerin
The Onset of Anisotropic Transport of Two-Dimensional Electrons in High Landau Levels: An Isotropic-to-Nematic Liquid Crystal Phase Transition?
The recently discovered anisotropy of the longitudinal resistance of
two-dimensional electrons near half filling of high Landau levels is found to
persist to much higher temperatures T when a large in-plane magnetic field B||
is applied. Under these conditions we find that the longitudinal resistivity
scales quasi-linearly with B||/T. These observations support the notion that
the onset of anisotropy at B||=0 does not reflect the spontaneous development
of charge density modulations but may instead signal an isotropic-to-nematic
liquid crystal phase transition.Comment: 5 pages, 4 figure
Gazeau-Klauder type coherent states for hypergeometric type operators
The hypergeometric type operators are shape invariant, and a factorization
into a product of first order differential operators can be explicitly
described in the general case. Some additional shape invariant operators
depending on several parameters are defined in a natural way by starting from
this general factorization. The mathematical properties of the eigenfunctions
and eigenvalues of the operators thus obtained depend on the values of the
involved parameters. We study the parameter dependence of orthogonality, square
integrability and of the monotony of eigenvalue sequence. The obtained results
allow us to define certain systems of Gazeau-Klauder coherent states and to
describe some of their properties. Our systematic study recovers a number of
well-known results in a natural unified way and also leads to new findings.Comment: An error occurring in Theorem 12 and Theorem 13 has been correcte
Transport coefficients from the 2PI effective action
We show that the lowest nontrivial truncation of the two-particle irreducible
(2PI) effective action correctly determines transport coefficients in a weak
coupling or 1/N expansion at leading (logarithmic) order in several
relativistic field theories. In particular, we consider a single real scalar
field with cubic and quartic interactions in the loop expansion, the O(N) model
in the 2PI-1/N expansion, and QED with a single and many fermion fields.
Therefore, these truncations will provide a correct description, to leading
(logarithmic) order, of the long time behavior of these systems, i.e. the
approach to equilibrium. This supports the promising results obtained for the
dynamics of quantum fields out of equilibrium using 2PI effective action
techniques.Comment: 5 pages, explanation in introduction expanded, summary added; to
appear in PR
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