1,433 research outputs found
Entropy production for velocity-dependent macroscopic forces: the problem of dissipation without fluctuations
In macroscopic systems, velocity-dependent phenomenological forces are
used to model friction, feedback devices or self-propulsion. Such forces
usually include a dissipative component which conceals the fast energy
exchanges with a thermostat at the environment temperature , ruled by a
microscopic Hamiltonian . The mapping - even if effective
for many purposes - may lead to applications of stochastic thermodynamics where
an fluctuating entropy production (FEP) is derived. An
enlightening example is offered by recent macroscopic experiments where
dissipation is dominated by solid-on-solid friction, typically modelled through
a deterministic Coulomb force . Through an adaptation of the microscopic
Prandtl-Tomlinson model for friction, we show how the FEP is dominated by the
heat released to the -thermostat, ignored by the macroscopic Coulomb model.
This problem, which haunts several studies in the literature, cannot be cured
by weighing the time-reversed trajectories with a different auxiliary dynamics:
it is only solved by a more accurate stochastic modelling of the thermostat
underlying dissipation.Comment: 6 pages, 3 figure
Nickel hydrogen bipolar battery electrode design
The preferred approach of the NASA development effort in nickel hydrogen battery design utilizes a bipolar plate stacking arrangement to obtain the required voltage-capacity configuration. In a bipolar stack, component designs must take into account not only the typical design considerations such as voltage, capacity and gas management, but also conductivity to the bipolar (i.e., intercell) plate. The nickel and hydrogen electrode development specifically relevant to bipolar cell operation is discussed. Nickel oxide electrodes, having variable type grids and in thicknesses up to .085 inch are being fabricated and characterized to provide a data base. A selection will be made based upon a system level tradeoff. Negative (hydrpogen) electrodes are being screened to select a high performance electrode which can function as a bipolar electrode. Present nickel hydrogen negative electrodes are not capable of conducting current through their cross-section. An electrode was tested which exhibits low charge and discharge polarization voltages and at the same time is conductive. Test data is presented
Bipolar Nickel-hydrogen Batteries for Aerospace Applications
A bipolar nickel-hydrogen battery which effectively addresses all key requirements for a spacecraft power system, including long-term reliability and low mass, is discussed. The design of this battery is discussed in the context of system requirements and nickel-hydrogen battery technology in general. To achieve the ultimate goal of an aerospace application of a bipolar Ni-H2 battery several objectives must be met in the design and development of the system. These objectives include: maximization of reliability and life; high specific energy and energy density; reasonable cost of manufacture, test, and integration; and ease in scaling for growth in power requirements. These basic objectives translate into a number of specific design requirements, which are discussed
What is the temperature of a granular medium?
In this paper we discuss whether thermodynamical concepts and in particular
the notion of temperature could be relevant for the dynamics of granular
systems. We briefly review how a temperature-like quantity can be defined and
measured in granular media in very different regimes, namely the glassy-like,
the liquid-like and the granular gas. The common denominator will be given by
the Fluctuation-Dissipation Theorem, whose validity is explored by means of
both numerical and experimental techniques. It turns out that, although a
definition of a temperature is possible in all cases, its interpretation is far
from being obvious. We discuss the possible perspectives both from the
theoretical and, more importantly, from the experimental point of view
On anomalous diffusion and the out of equilibrium response function in one-dimensional models
We study how the Einstein relation between spontaneous fluctuations and the
response to an external perturbation holds in the absence of currents, for the
comb model and the elastic single-file, which are examples of systems with
subdiffusive transport properties. The relevance of non-equilibrium conditions
is investigated: when a stationary current (in the form of a drift or an energy
flux) is present, the Einstein relation breaks down, as is known to happen in
systems with standard diffusion. In the case of the comb model, a general
relation, which has appeared in the recent literature, between the response
function and an unperturbed suitable correlation function, allows us to explain
the observed results. This suggests that a relevant ingredient in breaking the
Einstein formula, for stationary regimes, is not the anomalous diffusion but
the presence of currents driving the system out of equilibrium.Comment: 10 pages, 4 figure
Shear viscosity to electric conductivity ratio of the QGP
The transport coefficients of strongly interacting matter are currently subject of intense theoretical and phenomenological studies due to their relevance for the characterization of the quark-gluon plasma produced in ultra-relativistic heavy-ion collisions (uRHIC). We predict that (η/s)/(σel/T), independently on the running coupling αs(T), should increase up to about ~ 20 for T → Tc, while it goes down to a nearly flat behavior around ≃ 4 for T ≥ 4 Tc. Therefore we find a stronger T-dependence of σel/T with respect to η/s that in a quasiparticle approach is constrained by lQCD thermodynamics. A conformal theory, instead, predicts a similar T dependence of η/s and σel/T
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