108 research outputs found
Irreversible Performance of a Quantum Harmonic Heat Engine
The unavoidable irreversible losses of power in a heat engine are found to be
of quantum origin. Following thermodynamic tradition a model quantum heat
engine operating by the Otto cycle is analyzed. The working medium of the model
is composed of an ensemble of harmonic oscillators. A link is established
between the quantum observables and thermodynamical variables based on the
concept of canonical invariance. These quantum variables are sufficient to
determine the state of the system and with it all thermodynamical variables.
Conditions for optimal work, power and entropy production show that maximum
power is a compromise between the quasistatic limit of adiabatic following on
the compression and expansion branches and a sudden limit of very short time
allocation to these branches. At high temperatures and quasistatic operating
conditions the efficiency at maximum power coincides with the endoreversible
result. The optimal compression ratio varies from the square root of the
temperature ratio in the quasistatic limit where their reversibility is
dominated by heat conductance to the temperature ratio to the power of 1/4 in
the sudden limit when the irreversibility is dominated by friction. When the
engine deviates from adiabatic conditions the performance is subject to
friction. The origin of this friction can be traced to the noncommutability of
the kinetic and potential energy of the working medium.Comment: 25 pages, 7 figures. Revision added explicit heat-transfer expression
and extended the discussion on the quantum origin of frictio
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
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
The different paths to entropy
In order to undestand how the complex concept of entropy emerged,we propose a
trip towards the past reviewing the works of Clausius, Boltzmann, Gibbs and
Planck. In particular, since the Gibbs's work is not very well known, we
present a detailed analysis, recalling the three definitions of the entropy
that Gibbs gives. May be one of the most important aspect of the entropy is to
see it as a thermodynamic potential like the other thermodynamic potentials as
proposed by Callen. We close with some remarks on entropy and irreversibility.Comment: 32 page
Erythropoietin levels are not independently associated with malaria-attributable severe disease in Mozambican children.
BACKGROUND: Severe malaria is difficult to differentiate from other forms of malaria or other infections with similar symptoms. Any parameter associated to malaria-attributable severe disease could help to improve severe malaria diagnosis. METHODOLOGY: This study assessed the relation between erythropoietin (EPO) and malaria-attributable severe disease in an area of Mozambique with moderate malaria transmission. 211 children <5 years, recruited at Manhiça District Hospital or in the surrounding villages, were included in one of the following groups: severe malaria (SM, n = 44), hospital malaria without severity (HM, n = 49), uncomplicated malaria (UM, n = 47), invasive bacterial infection without malaria parasites (IBI, n = 39) and healthy community controls (C, n = 32). Malaria was diagnosed by microscopy and IBI by blood/cerebrospinal fluid culture. PRINCIPAL FINDINGS: Mean EPO concentration in the control group was 20.95 U/l (SD = 2.96 U/l). Values in this group were lower when compared to each of the clinical groups (p = 0.026 C versus UM, p<0.001 C vs HM, p<0.001 C vs SM and p<0.001 C vs IBI). In the 3 malaria groups, values increased with severity [mean = 40.82 U/l (SD = 4.07 U/l), 125.91 U/l (SD = 4.99U/l) and 320.87 U/l (SD = 5.91U/l) for UM, HM and SM, respectively, p<0.001]. The IBI group [mean = 101.75 U/l (SD = 4.12 U/l)] presented lower values than the SM one (p = 0.002). In spite of the differences, values overlapped between study groups and EPO levels were only associated to hemoglobin. Hemoglobin means of the clinical groups were 93.98 g/dl (SD = 14.77 g/dl) for UM, 75.96 g/dl (SD = 16.48 g/dl) for HM, 64.34 g/dl (SD = 22.99 g/dl) for SM and 75.67 g/dl (SD = 16.58 g/dl) for IBI. CONCLUSIONS: Although EPO levels increase according to malaria severity and are higher in severe malaria than in bacteremia, the utility of EPO to distinguish malaria-attributable severe disease is limited due to the overlap of values between the study groups and the main role of hemoglobin in the expression of EPO
Projecting the Medium-Term: Outcomes and Errors for GDP Growth
The focus of this paper is the evaluation of a very popular method for potential output estimation and medium-term forecasting - the production function approach - in terms of predictive performance. For this purpose, a forecast evaluation for the three to five years ahead predictions of GDP growth for the individual G7 countries is conducted. To carry out the forecast performance check a particular testing framework is derived that allows the computation of robust test statistics given the specific nature of the generated out-of sample forecasts. In addition, medium-term GDP projections from national and international institutions are examined and it is assessed whether these projections convey a reliable view about future economic developments and whether there is scope for improving their predictive content
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