9,765 research outputs found
Kinetics of first-order phase transitions from microcanonical thermostatistics
More than a century has passed since van't Hoff and Arrhenius formulated
their celebrated rate theories, but there are still elusive aspects in the
temperature-dependent phase transition kinetics of molecular systems. Here I
present a theory based on microcanonical thermostatistics that establishes a
simple and direct temperature dependence for all rate constants, including the
forward and the reverse rate constants, the equilibrium constant, and the
nucleation rate. By considering a generic model that mimic the microcanonical
temperature of molecular systems in a region close to a first-order phase
transition, I obtain shape-free relations between kinetics and thermodynamics
physical quantities which are validated through stochastic simulations.
Additionally, the rate theory is applied to results obtained from protein
folding and ice nucleation experiments, demonstrating that the expressions
derived here can be used to describe the experimental data of a wide range of
molecular systems.Comment: 22 pages, 5 figure
A continuous population of variable stars up to about 1.5 mag above the horizontal branch?
Increasing samples of pulsating variable stars populating the classical
instability strip from the horizontal branch to a few magnitudes brighter are
being found in several Local Group galaxies, irrespective of the galaxy
morphological type. We will review the observational scenario focusing in
particular on the Anomalous Cepheids and related objects.Comment: 5 pages, 2 figures, invited review in JENAM 2003, "Minisymposium:
Asteroseismology and Stellar Evolution", Communications in Asteroseismology,
in pres
Spacetime geometries and light trapping in travelling refractive index perturbations
In the framework of transformation optics, we show that the propagation of a
locally superluminal refractive index perturbation (RIP) in a Kerr medium can
be described, in the eikonal approximation, by means of a stationary metric,
which we prove to be of Gordon type. Under suitable hypotheses on the RIP, we
obtain a stationary but not static metric, which is characterized by an
ergosphere and by a peculiar behaviour of the geodesics, which are studied
numerically, also accounting for material dispersion. Finally, the equation to
be satisfied by an event horizon is also displayed and briefly discussed.Comment: 14 pages, 7 figure
Space geometry of rotating platforms: an operational approach
We study the space geometry of a rotating disk both from a theoretical and operational approach, in particular we give a precise definition of the space of the disk, which is not clearly defined in the literature. To this end we define an extended 3-space, which we call relative space: it is recognized as the only space having an actual physical meaning from an operational point of view, and it is identified as the 'physical space of the rotating platform'. Then, the geometry of the space of the disk turns out to be non Euclidean, according to the early Einstein's intuition; in particular the Born metric is recovered, in a clear and self consistent context. Furthermore, the relativistic kinematics reveals to be self consistent, and able to solve the Ehrenfest's paradox without any need of dynamical considerations or ad hoc assumptions
Charged Particles and the Electro-Magnetic Field in Non-Inertial Frames of Minkowski Spacetime: I. Admissible 3+1 Splittings of Minkowski Spacetime and the Non-Inertial Rest Frames
By using the 3+1 point of view and parametrized Minkowski theories we develop
the theory of {\it non-inertial} frames in Minkowski space-time. The transition
from a non-inertial frame to another one is a gauge transformation connecting
the respective notions of instantaneous 3-space (clock synchronization
convention) and of the 3-coordinates inside them. As a particular case we get
the extension of the inertial rest-frame instant form of dynamics to the
non-inertial rest-frame one. We show that every isolated system can be
described as an external decoupled non-covariant canonical center of mass
(described by frozen Jacobi data) carrying a pole-dipole structure: the
invariant mass and an effective spin. Moreover we identify the constraints
eliminating the internal 3-center of mass inside the instantaneous 3-spaces. In
the case of the isolated system of positive-energy scalar particles with
Grassmann-valued electric charges plus the electro-magnetic field we obtain
both Maxwell equations and their Hamiltonian description in non-inertial
frames. Then by means of a non-covariant decomposition we define the
non-inertial radiation gauge and we find the form of the non-covariant Coulomb
potential. We identify the coordinate-dependent relativistic inertial
potentials and we show that they have the correct Newtonian limit. In the
second paper we will study properties of Maxwell equations in non-inertial
frames like the wrap-up effect and the Faraday rotation in astrophysics. Also
the 3+1 description without coordinate-singularities of the rotating disk and
the Sagnac effect will be given, with added comments on pulsar magnetosphere
and on a relativistic extension of the Earth-fixed coordinate system.Comment: This paper and the second one are an adaptation of arXiv 0812.3057
for publication on Int.J.Geom. Methods in Modern Phys. 77
On the origin of the negative energy-related contribution to the elastic modulus of rubber-like gels
We consider a coarse-grained polymer model in order to investigate the origin
of a recently discovered negative energy-related contribution to the elastic
modulus of rubber-like gels. From this model, we are able to compute an
exact expression for the free energy of the system, which allows us to evaluate
a stress-strain relationship that displays a non-trivial dependence on the
temperature . We validate our approach through comparisons between the
theoretical results and the experimental data obtained for tetra-PEG hydrogels,
which indicate that, although simple, the present model works well to describe
the experiments. Importantly, our approach unveiled aspects of the experimental
analysis which turned out to be different from the conventional entropic and
energetic analysis broadly used in the literature. Also, in contrast to the
linear dependence predicted by the traditional, {\it i.e.}, purely entropic,
models, our results suggest that the general expression of the elastic modulus
should be of the form , with being a
temperature-dependent correction factor that could be related to the
interaction between the chains in the network and the solvent. Accordingly, the
correction factor allows the expression found for the elastic modulus to
describe both rubber and rubber-like gels.Comment: 14 pages, 5 figure
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