210 research outputs found
Efficiency at maximum power output of an irreversible Carnot-like cycle with internally dissipative friction
We investigate the efficiency at maximum power of an irreversible Carnot
engine performing finite-time cycles between two reservoirs at temperatures
and , taking into account of internally dissipative
friction in two "adiabatic" processes. In the frictionless case, the
efficiencies at maximum power output are retrieved to be situated between
and , with being
the Carnot efficiency. The strong limits of the dissipations in the hot and
cold isothermal processes lead to the result that the efficiency at maximum
power output approaches the values of and
, respectively. When dissipations of two isothermal
and two adiabatic processes are symmetric, respectively, the efficiency at
maximum power output is founded to be bounded between 0 and the Curzon-Ahlborn
(CA) efficiency , and the the CA efficiency is achieved in
the absence of internally dissipative friction
Intrinsic Periodicity of Time and Non-maximal Entropy of Universe
The universe is certainly not yet in total thermodynamical equilibrium,so
clearly some law telling about special initial conditions is needed. A universe
or a system imposed to behave periodically gets thereby required ``initial
conditions". Those initial conditions will \underline{not} look like having
already suffered the heat death, i.e. obtained the maximal entropy, like a
random state. The intrinsic periodicity explains successfully why entropy is
not maximal, but fails phenomenologically by leading to a
\underline{constant}entropy.Comment: 8 page
Quantum mechanical Carnot engine
A cyclic thermodynamic heat engine runs most efficiently if it is reversible.
Carnot constructed such a reversible heat engine by combining adiabatic and
isothermal processes for a system containing an ideal gas. Here, we present an
example of a cyclic engine based on a single quantum-mechanical particle
confined to a potential well. The efficiency of this engine is shown to equal
the Carnot efficiency because quantum dynamics is reversible. The quantum heat
engine has a cycle consisting of adiabatic and isothermal quantum processes
that are close analogues of the corresponding classical processes.Comment: 10 page
Efficiency of a thermodynamic motor at maximum power
Several recent theories address the efficiency of a macroscopic thermodynamic
motor at maximum power and question the so-called "Curzon-Ahlborn (CA)
efficiency." Considering the entropy exchanges and productions in an n-sources
motor, we study the maximization of its power and show that the controversies
are partly due to some imprecision in the maximization variables. When power is
maximized with respect to the system temperatures, these temperatures are
proportional to the square root of the corresponding source temperatures, which
leads to the CA formula for a bi-thermal motor. On the other hand, when power
is maximized with respect to the transitions durations, the Carnot efficiency
of a bi-thermal motor admits the CA efficiency as a lower bound, which is
attained if the duration of the adiabatic transitions can be neglected.
Additionally, we compute the energetic efficiency, or "sustainable efficiency,"
which can be defined for n sources, and we show that it has no other universal
upper bound than 1, but that in certain situations, favorable for power
production, it does not exceed 1/2
Shannon Meets Carnot: Generalized Second Thermodynamic Law
The classical thermodynamic laws fail to capture the behavior of systems with
energy Hamiltonian which is an explicit function of the temperature. Such
Hamiltonian arises, for example, in modeling information processing systems,
like communication channels, as thermal systems. Here we generalize the second
thermodynamic law to encompass systems with temperature-dependent energy
levels, , where denotes averaging over
the Boltzmann distribution and reveal a new definition to the basic notion of
temperature. This generalization enables to express, for instance, the mutual
information of the Gaussian channel as a consequence of the fundamental laws of
nature - the laws of thermodynamics
Carnot cycle for an oscillator
Carnot established in 1824 that the efficiency of cyclic engines operating
between a hot bath at absolute temperature and a bath at a lower
temperature cannot exceed . We show that linear
oscillators alternately in contact with hot and cold baths obey this principle
in the quantum as well as in the classical regime. The expression of the work
performed is derived from a simple prescription. Reversible and non-reversible
cycles are illustrated. The paper begins with historical considerations and is
essentially self-contained.Comment: 19 pages, 3 figures, sumitted to European Journal of Physics Changed
content: Fluctuations are considere
Law Behind Second Law of Thermodynamics --Unification with Cosmology--
In an abstract setting of a general classical mechanical system as a model
for the universe we set up a general formalism for a law behind the second law
of thermodynamics, i.e. really for "initial conditions". We propose a
unification with the other laws by requiring similar symmetry and locality
properties.Comment: 17 page
Dynamical typicality of embedded quantum systems
We consider the dynamics of an arbitrary quantum system coupled to a large
arbitrary and fully quantum mechanical environment through a random
interaction. We establish analytically and check numerically the typicality of
this dynamics, in other words the fact that the reduced density matrix of the
system has a self-averaging property. This phenomenon, which lies in a
generalized central limit theorem, justifies rigorously averaging procedures
over certain classes of random interactions and can explain the absence of
sensitivity to microscopic details of irreversible processes such as
thermalisation. It provides more generally a new ergodic principle for embedded
quantum systems.Comment: 9 pages. Accepted for publication in Phys. Rev. A. This article
supersedes the part on "dynamical typicality" in arXiv:1510.0435
Brownian Carnot engine
The Carnot cycle imposes a fundamental upper limit to the efficiency of a
macroscopic motor operating between two thermal baths. However, this bound
needs to be reinterpreted at microscopic scales, where molecular bio-motors and
some artificial micro-engines operate. As described by stochastic
thermodynamics, energy transfers in microscopic systems are random and thermal
fluctuations induce transient decreases of entropy, allowing for possible
violations of the Carnot limit. Despite its potential relevance for the
development of a thermodynamics of small systems, an experimental study of
microscopic Carnot engines is still lacking. Here we report on an experimental
realization of a Carnot engine with a single optically trapped Brownian
particle as working substance. We present an exhaustive study of the energetics
of the engine and analyze the fluctuations of the finite-time efficiency,
showing that the Carnot bound can be surpassed for a small number of
non-equilibrium cycles. As its macroscopic counterpart, the energetics of our
Carnot device exhibits basic properties that one would expect to observe in any
microscopic energy transducer operating with baths at different temperatures.
Our results characterize the sources of irreversibility in the engine and the
statistical properties of the efficiency -an insight that could inspire novel
strategies in the design of efficient nano-motors.Comment: 7 pages, 7 figure
The falling chain of Hopkins, Tait, Steele and Cayley
A uniform, flexible and frictionless chain falling link by link from a heap
by the edge of a table falls with an acceleration if the motion is
nonconservative, but if the motion is conservative, being the
acceleration due to gravity. Unable to construct such a falling chain, we use
instead higher-dimensional versions of it. A home camcorder is used to measure
the fall of a three-dimensional version called an -slider. After
frictional effects are corrected for, its vertical falling acceleration is
found to be . This result agrees with the theoretical
value of for an ideal energy-conserving -slider.Comment: 17 pages, 5 figure
- …