1,262 research outputs found
Verschraenkung versus Stosszahlansatz: Disappearance of the Thermodynamic Arrow in a High-Correlation Environment
The crucial role of ambient correlations in determining thermodynamic
behavior is established. A class of entangled states of two macroscopic systems
is constructed such that each component is in a state of thermal equilibrium at
a given temperature, and when the two are allowed to interact heat can flow
from the colder to the hotter system. A dilute gas model exhibiting this
behavior is presented. This reversal of the thermodynamic arrow is a
consequence of the entanglement between the two systems, a condition that is
opposite to molecular chaos and shown to be unlikely in a low-entropy
environment. By contrast, the second law is established by proving Clausius'
inequality in a low-entropy environment. These general results strongly support
the expectation, first expressed by Boltzmann and subsequently elaborated by
others, that the second law is an emergent phenomenon that requires a
low-entropy cosmological environment, one that can effectively function as an
ideal information sink.Comment: 4 pages, REVTeX
Distortion and regulation characterization of a Mapham inverter
Output voltage Total Harmonic Distortion (THD) of a 20kHz, 6kVA Mapham resonant inverter is characterized as a function of its switching-to-resonant frequency ratio, f sub s/f sub r, using the EASY5 engineering analysis system. EASY5 circuit simulation results are compared with hardware test results to verify the accuracy of the simulations. The effects of load on the THD versus f sub s/f sub r ratio is investigated for resistive, leading, and lagging power factor load impedances. The effect of the series output capacitor on the Mapham inverter output voltage distortion and inherent load regulation is characterized under loads of various power factors and magnitudes. An optimum series capacitor value which improves the inherent load regulation to better than 3 percent is identified. The optimum series capacitor value is different than the value predicted from a modeled frequency domain analysis. An explanation is proposed which takes into account the conduction overlap in the inductor pairs during steady-state inverter operation, which decreases the effective inductance of a Mapham inverter. A fault protection and current limit method is discussed which allows the Mapham inverter to operate into a short circuit, even when the inverter resonant circuit becomes overdamped
Class of dilute granular Couette flows with uniform heat flux
In a recent paper [F. Vega Reyes et al., Phys. Rev. Lett. 104, 028001 (2010)]
we presented a preliminary description of a special class of steady Couette
flows in dilute granular gases. In all flows of this class the viscous heating
is exactly balanced by inelastic cooling. This yields a uniform heat flux and a
linear relationship between the local temperature and flow velocity. The class
(referred to as the LTu class) includes the Fourier flow of ordinary gases and
the simple shear flow of granular gases as special cases. In the present paper
we provide further support for this class of Couette flows by following four
different routes, two of them being theoretical (Grad's moment method of the
Boltzmann equation and exact solution of a kinetic model) and the other two
being computational (molecular dynamics and Monte Carlo simulations of the
Boltzmann equation). Comparison between theory and simulations shows a very
good agreement for the non-Newtonian rheological properties, even for quite
strong inelasticity, and a good agreement for the heat flux coefficients in the
case of Grad's method, the agreement being only qualitative in the case of the
kinetic model.Comment: 15 pages, 10 figures; v2: change of title plus some other minor
change
A real Lorentz-FitzGerald contraction
Many condensed matter systems are such that their collective excitations at
low energies can be described by fields satisfying equations of motion formally
indistinguishable from those of relativistic field theory. The finite speed of
propagation of the disturbances in the effective fields (in the simplest
models, the speed of sound) plays here the role of the speed of light in
fundamental physics. However, these apparently relativistic fields are immersed
in an external Newtonian world (the condensed matter system itself and the
laboratory can be considered Newtonian, since all the velocities involved are
much smaller than the velocity of light) which provides a privileged coordinate
system and therefore seems to destroy the possibility of having a perfectly
defined relativistic emergent world. In this essay we ask ourselves the
following question: In a homogeneous condensed matter medium, is there a way
for internal observers, dealing exclusively with the low-energy collective
phenomena, to detect their state of uniform motion with respect to the medium?
By proposing a thought experiment based on the construction of a
Michelson-Morley interferometer made of quasi-particles, we show that a real
Lorentz-FitzGerald contraction takes place, so that internal observers are
unable to find out anything about their `absolute ' state of motion. Therefore,
we also show that an effective but perfectly defined relativistic world can
emerge in a fishbowl world situated inside a Newtonian (laboratory) system.
This leads us to reflect on the various levels of description in physics, in
particular regarding the quest towards a theory of quantum gravity.Comment: 6 pages, no figures. Minor changes reflect published versio
Quasi-classical Molecular Dynamics Simulations of the Electron Gas: Dynamic properties
Results of quasi-classical molecular dynamics simulations of the quantum
electron gas are reported. Quantum effects corresponding to the Pauli and the
Heisenberg principle are modeled by an effective momentum-dependent
Hamiltonian. The velocity autocorrelation functions and the dynamic structure
factors have been computed. A comparison with theoretical predictions was
performed.Comment: 8 figure
‘Question Moments’: A Rolling Programme of Question Opportunities in Classroom Science
This article has been made available through the Brunel Open Access Publishing Fund.This naturalistic study integrates specific 'question moments' into lesson plans to
increase pupils' classroom interactions. A range of teaching tools has explored
students' ideas through opportunities to ask and write questions. Their oral and written
outcomes provide data on individual and group misunderstandings. Changes to the
schedule of lessons were introduced to discuss these questions and solve disparities.
Flexible lesson planning over fourteen lessons across a four-week period of highschool
chemistry accommodated students' contributions and increased student
participation, promoted inquiring and individualised teaching, with each teaching
strategy feeding forward into the next
The Hartree limit of Born's ensemble for the ground state of a bosonic atom or ion
The non-relativistic bosonic ground state is studied for quantum N-body
systems with Coulomb interactions, modeling atoms or ions made of N "bosonic
point electrons" bound to an atomic point nucleus of Z "electron" charges,
treated in Born--Oppenheimer approximation. It is shown that the (negative)
ground state energy E(Z,N) yields the monotonically growing function (E(l N,N)
over N cubed). By adapting an argument of Hogreve, it is shown that its limit
as N to infinity for l > l* is governed by Hartree theory, with the rescaled
bosonic ground state wave function factoring into an infinite product of
identical one-body wave functions determined by the Hartree equation. The proof
resembles the construction of the thermodynamic mean-field limit of the
classical ensembles with thermodynamically unstable interactions, except that
here the ensemble is Born's, with the absolute square of the ground state wave
function as ensemble probability density function, with the Fisher information
functional in the variational principle for Born's ensemble playing the role of
the negative of the Gibbs entropy functional in the free-energy variational
principle for the classical petit-canonical configurational ensemble.Comment: Corrected version. Accepted for publication in Journal of
Mathematical Physic
Bose-Einstein Condensation of Helium and Hydrogen inside Bundles of Carbon Nanotubes
Helium atoms or hydrogen molecules are believed to be strongly bound within
the interstitial channels (between three carbon nanotubes) within a bundle of
many nanotubes. The effects on adsorption of a nonuniform distribution of tubes
are evaluated. The energy of a single particle state is the sum of a discrete
transverse energy Et (that depends on the radii of neighboring tubes) and a
quasicontinuous energy Ez of relatively free motion parallel to the axis of the
tubes. At low temperature, the particles occupy the lowest energy states, the
focus of this study. The transverse energy attains a global minimum value
(Et=Emin) for radii near Rmin=9.95 Ang. for H2 and 8.48 Ang.for He-4. The
density of states N(E) near the lowest energy is found to vary linearly above
this threshold value, i.e. N(E) is proportional to (E-Emin). As a result, there
occurs a Bose-Einstein condensation of the molecules into the channel with the
lowest transverse energy. The transition is characterized approximately as that
of a four dimensional gas, neglecting the interactions between the adsorbed
particles. The phenomenon is observable, in principle, from a singular heat
capacity. The existence of this transition depends on the sample having a
relatively broad distribution of radii values that include some near Rmin.Comment: 21 pages, 9 figure
Criticality in strongly correlated fluids
In this brief review I will discuss criticality in strongly correlated
fluids. Unlike simple fluids, molecules of which interact through short ranged
isotropic potential, particles of strongly correlated fluids usually interact
through long ranged forces of Coulomb or dipolar form. While for simple fluids
mechanism of phase separation into liquid and gas was elucidated by van der
Waals more than a century ago, the universality class of strongly correlated
fluids, or in some cases even existence of liquid-gas phase separation remains
uncertain.Comment: Proceedings of Scaling Concepts and Complex Systems, Merida, Mexic
The Boltzmann Entropy for Dense Fluids Not in Local Equilibrium
We investigate, via computer simulations, the time evolution of the
(Boltzmann) entropy of a dense fluid not in local equilibrium. The
macrovariables describing the system are the (empirical) particle density
f=\{f(\un{x},\un{v})\} and the total energy . We find that is
monotone increasing in time even when its kinetic part is decreasing. We argue
that for isolated Hamiltonian systems monotonicity of
should hold generally for ``typical'' (the overwhelming majority of) initial
microstates (phase-points) belonging to the initial macrostate ,
satisfying . This is a direct consequence of Liouville's theorem
when evolves autonomously.Comment: 8 pages, 5 figures. Submitted to PR
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