G. Ares de Parga

On Some Nonendoreversible Engine Models with Nonlinear Heat Transfer Laws

In this work, we analyze a nonendoreversible thermal engine model with a nonlinear heat transfer law between the heat reservoirs and the working fluid under two optimization criteria: the maximum power regime and the so-called ecological criterion. We find that this nonendoreversible model has a similar behaviour to that shown by De Vos (Am. J. Phys. 53, 570 (1985)) for endoreversible models with two thermal conductances with only one superior conductance and with only one inferior conductance, respectively. The model is compared with two sets of real power plants, the first one containing power plants of old design (before 1960s) and the second one being formed by modern nuclear power plants. Our results suggest that the first group was designed under conditions which, are reminiscent of a maximum power regime and the second one under an ecological-like criterion. We also study some general properties of nonendoreversible thermal engine models.

A variational approach to ecological-type optimization criteria for finite-time thermal engine models

In this paper we apply variational calculus procedures for the optimization of a Curzon-Ahlborn thermal cycle under the so-called modified ecological criterion. Our result for the optimum efficiency is the same that Velasco et al (2000 J. Phys. D 33 355) previously obtained by means of the method of the saving functions. Besides, we show that both the saving functions and the modified ecological criteria are equivalent.

A proposal for relativistic transformations in thermodynamics

Since the beginning of the past century many proposals for relativistic transformations in thermodynamics have been suggested. A general consensus about this matter has not been reached. In this work, we propose a scheme of thermodynamic relativistic transformations inspired by the Planck–Einstein theory, but changing the relativistic transformation of energy. This change permits the form invariance of thermodynamics. Also by means of finite-time thermodynamics we demonstrate the relativistic invariance of thermal efficiency.

A variational optimization of a finite-time thermal cycle with a nonlinear heat transfer law

In this paper we use a variational approach to study an endoreversible Curzon–Ahlborn–Novikov (CAN) heat engine under both maximum power and maximum ecological function conditions. By means of this procedure we analyze the performance of a CANheat engine with a nonlinear heat transfer law (the Dulong–Petit law) to describe the heat exchanges between the working substance and its thermal reservoirs. Our results are consistent with previous ones obtained by means of other procedures. In addition, we obtain expressions for the temperatures of the isothermal branches of the working fluid under maximum power conditions. Finally, we present an expression for a kind of nonendoreversible Carnot efficiency.

A Variational Ecological-Type Optimization of Some Thermal-Engine Models

In a recent work [23] we have analyzed a nonendoreversible thermal engine model under two maximization criteria: the maximum power regime and the so-called ecological criterion. In the present work, we extend the study of the same class of nonendoreversible models, but by using the so-named generalized ecological criterion [15]. This criterion is based on a family of ecological-type functions depending on a parameter associated to the particular heat transfer law used in the engine model. By means of this criterion we find a simplification of some of the results obtained in [23] by using both conventional and variational calculus. Besides, we propose a simple procedure to calculate the parameter involved in the generalized ecological function.

The Thomas program and the canonical proper-time theory

In a seminal paper, Thomas [1] suggested that a many-particle relativistic dynamics can be constructed provided that we give up the assumption of invariant world-lines. In the first part of this paper, we show that, using only retarded potentials, a completely consistent action-at-a-distance classical electrodynamics can be constructed, which provides the correct dissipative force back reaction in the equations of motion. In particular, we can account for radiation reaction without the Lorentz-Dirac equation, which requires self-energy divergence, advanced potentials and mass renormalization. What is equally interesting is that the Bakamjian and Thomas [2] program is a special case and, the calculations could have been completed by them, had they considered the problem. The general theory provides a new invariance group which is related to the Lorentz group by a scale transformation. This theory also provides a natural and unique definition of simultaneity for all observers, completing Thomas program. As an unexpected side benefit, the theory is noninvariant under time reversal and requires no assumptions about the structure of the charged particles.

An analytical approximation of a pendulum trajectory

An analytical approximation of a pendulum trajectory is developed for large initial angles. Instead of using a perturbation method, a succession of just two polynomials is used in order to get simple integrals. By obtaining the approximated period, the result is compared with the Kidd–Frogg and Hite formulas for the period which are very close to the exact solution for the considered angle.

Hammond versus Ford radiation reaction force with the attractive Coulomb field

The classical central field is analyzed within the Hammond theory of radiation reaction force. For the attractive Coulomb field, the trajectories deduced from Ford and Hammond equations are numerically obtained. Ford and Hammond equations are rewritten by using a recent correction to the non-relativistic equations for charged point particles which include a radiation reaction force term. Also, for the attractive Coulomb case, the trajectories are numerically obtained for both corrected equations. A comparison between all these trajectories is made. It is proved that Hammond equation satisfies the constraint proposed by Dirac of getting an equation of motion which should make the electron in the hydrogen atom spiralling inwards and ultimately falling into the nucleus. A further analysis of the applicability of such a theory is described for experiments particularly in Plasma Physics and some comments are made for the generalization of Hammond equation to General Relativity.

A semiclassical approach to the matte black-body

In this paper, a semiclassical approach is used to describe a kind of black-body which we will call a matte black-body. Although the frequency energy density of a black-body is deduced using a semiclassical method which includes the electromagnetic reaction force and the quantization of the energy, a phenomenological damping force, as in the explanation of the anomalous dispersion of some fluids, is considered in order to obtain the corresponding frequency energy density of the matte black-body. The concept of emissivity is incorporated into the new body in order to explain the experimental data of the radiation measured in the Earths atmosphere. The purpose of this article consists of showing students the applicability of semiclassical approaches in obtaining physical results.