48,438 research outputs found
Maximum entropy principle for stationary states underpinned by stochastic thermodynamics
The selection of an equilibrium state by maximising the entropy of a system,
subject to certain constraints, is often powerfully motivated as an exercise in
logical inference, a procedure where conclusions are reached on the basis of
incomplete information. But such a framework can be more compelling if it is
underpinned by dynamical arguments, and we show how this can be provided by
stochastic thermodynamics, where an explicit link is made between the
production of entropy and the stochastic dynamics of a system coupled to an
environment. The separation of entropy production into three components allows
us to select a stationary state by maximising the change, averaged over all
realisations of the motion, in the principal relaxational or nonadiabatic
component, equivalent to requiring that this contribution to the entropy
production should become time independent for all realisations. We show that
this recovers the usual equilibrium probability density function (pdf) for a
conservative system in an isothermal environment, as well as the stationary
nonequilibrium pdf for a particle confined to a potential under nonisothermal
conditions, and a particle subject to a constant nonconservative force under
isothermal conditions. The two remaining components of entropy production
account for a recently discussed thermodynamic anomaly between over- and
underdamped treatments of the dynamics in the nonisothermal stationary state
Charge control of nickel-cadmium batteries by coulometer and third electrode method
Combined coulometer/third electrode control circuit for a nickel-cadmium battery included at least one cell of the third electrode type is illustrated. The coulometer/third electrode sensing circuit controls the series regulator as necessary to maintain the sensing voltage at the preset sensing level
Control of equilibrium pressure-temperature conditions in cryogenic storage
Metered vent controls the pressure within a liquid hydrogen tank. Vent size is chosen to permit a gas flow which corresponds to the boil-off rate necessary to maintain the desired bulk temperature of the cryogen
Bounds on negative energy densities in flat spacetime
We generalise results of Ford and Roman which place lower bounds -- known as
quantum inequalities -- on the renormalised energy density of a quantum field
averaged against a choice of sampling function. Ford and Roman derived their
results for a specific non-compactly supported sampling function; here we use a
different argument to obtain quantum inequalities for a class of smooth, even
and non-negative sampling functions which are either compactly supported or
decay rapidly at infinity. Our results hold in -dimensional Minkowski space
() for the free real scalar field of mass . We discuss various
features of our bounds in 2 and 4 dimensions. In particular, for massless field
theory in 2-dimensional Minkowski space, we show that our quantum inequality is
weaker than Flanagan's optimal bound by a factor of 3/2.Comment: REVTeX, 13 pages and 2 figures. Minor typos corrected, one reference
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