arXiv subjects

E. N. Miranda

Publications and source records attributed to E. N. Miranda.

16 recordsLinked to original sources

arXiv2020

Size and temperature dependent magnetization of iron nanoclusters

The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and 8 nm, is investigated by means of spin dynamics (SD) simulations coupled to molecular dynamics (MD-SD), using a distance-dependent exchange interaction. Finite-size effects in the total magnetization as well as the influence of the free surface and the surface/core proportion of the nanoclusters are analyzed in detail for a wide temperature range, going beyond the cluster and bulk Curie temperatures. Comparison is made with experimental data and with theoretical models based on the mean-field Ising model adapted to small clusters, and taking into account the influence of low coordinated spins at free surfaces. Our results for the temperature dependence of the average magnetization per atom M(T), including the thermalization of the transnational lattice degrees of freedom, are in very good agreement with available experimental measurements on small Fe nanoclusters. In contrast, significant discrepancies with experiment are observed if the translational degrees of freedom are artificially frozen. The finite-size effects on M(T) are found to be particularly important near the cluster Curie temperature. Simulated magnetization above the Curie temperature scales with cluster size as predicted by models assuming short-range magnetic ordering (SRMO). Analytical approximations to the magnetization as a function of temperature and size are proposed.

cond-mat.mes-hall↗

arXiv2016

The Einstein nanocrystal

We study the simplest possible model of nanocrystal consisting in a simple cubic lattice with a small number of atoms (NA ~ 10-10^3), where each atom is linked to its nearest neighbor by a quantum harmonic potential. Some properties (entropy, temperature, specific heat) of the nanocrystal are calculated numerically but exactly within the framework of the microcanonical ensemble. We find that the presence of a surface in the nanocrystal modifies the thermostatistic properties to a greater extent than the small number of atoms in the system. The specific heat Cv behaves similarly to the Einstein solid, with an asymptotic value for high temperatures that differs from that of the Dulong-Petit law by a term of the order of NA^(-1/3) and that can be explained easily in terms of the surface. The entropy is non-additive, but this is due to the presence of the surface and we show that the additivity is recovered in the thermodynamic limit. Finally, we find that, when calculations follow the canonical ensemble, results differ little for small systems (NA = 27) and are inexistent for larger systems (NA = 1000).

physics.gen-ph↗

arXiv2015

Thermostatistics of small systems: Exact results in the microcanonical formalism

Several approximations are made to study the microcanonical formalism that are valid in the thermodynamics limit. Usually it is assumed that: 1)Stirling approximation can be used to evaluate the number of microstates; 2) the surface entropy can be replace by the volumen entropy; and 3)derivatives can be used even if the energy is not a continuous variable. It is also assumed that the results obtained from the microcanonical formalism agree with those from the canonical one. However, it is not clear if these assumptions are right for very small systems (10-100 particles). To answer this questions, two systems with exact solutions (the Einstein model of a solid and the two-level system)have been solve with and without these approximations.

physics.gen-ph↗

arXiv2012

What lies between a free adiabatic expansion and a quasi-static one?

An expression is found that relates the initial and final volumes and temperatures for any adiabatic process. It is given in terms of a parameter r that smoothly interpolates between a free adiabatic expansion (r = 0) and a quasi-static one (r = 1). The parameter has to be evaluated numerically, but an approximate expression is given.

physics.gen-ph↗

arXiv2012

Minimum and terminal velocities in projectile motion

The motion of a projectile with horizontal initial velocity V0, moving under the action of the gravitational field and a drag force is studied analytically. As it is well known, the projectile reaches a terminal velocity Vterm. There is a curious result concerning the minimum speed Vmin; it turns out that the minimum velocity is lower than the terminal one if V0 > Vterm and is lower than the initial one if V0 < Vterm. These results show that the velocity is not a monotonous function. If the initial speed is not horizontal, there is an angle range where the velocity shows the same behavior mentioned previously. Out of that range, the volocity is a monotonous function. These results come out from numerical simulations.

physics.gen-ph↗

arXiv2012

What fuel for a rocket?

Elementary concepts from general physics and thermodynamics have been used to analyze rocket propulsion. Making some reasonable assumptions, an expression for the exit velocity of the gases is found. From that expression one can conclude what are the desired properties for a rocket fuel.

physics.gen-ph↗

arXiv2012

Entropy production by simple electrical circuits

The entropy production by simple electrical circuits (R, RC, RL) is analyzed. It comes out that the entropy production is minimal, in agreement with a well known theorem due to Prigogine. In this way, it is wrong a recent result by Zupanovic, Juretic and Botric (Physica Review E 70, 056198) who claimed that the entropy production in simple electrical circuits is a maximum

physics.gen-ph↗

arXiv2012

Adiabatic reversible compression: a molecular view

The adiabatic compression (or expansion) of an ideal gas has been analysed. Using the kinetic theory of gases the usual relation between temperature and volume is obtained, while textbooks follow a thermodynamic approach. In this way we show once again the agreement between a macroscopic view (thermodynamics) and a microscopic one (kinetic theory).

physics.gen-ph↗

arXiv2012

An apparent paradox in hydrostatic

It is shown that for a liquid in any connected vessels system, it is not possible to fulfill simultaneously Pascal's principle, mass conservation, and energy conservation. The viscosity has to necessarily be taken into account to understand the system change.

physics.gen-ph↗

arXiv2012

On the maximum efficiency of realistic heat engines

In 1975, Courzon and Ahlborn studied a Carnot engine with thermal losses and got an expression for its efficiency that described better the performance of actual heat machines than the traditional result due to Carnot. In their original derivation, time appears explicitly and this is disappointing in the framework of classical thermodynamics. In this note a derivation is given without any explicit reference to time.

physics.gen-ph↗

arXiv2012

How to get mechanical work from a capacitor and two batteries

The work done by a parallel plate capacitor is evaluated when the plate separation is changed. Two cases are considered: 1) the capacitor has a constant charge; 2) the capacitor is at constant voltage. The net work is calculated when the device follows a closed cycle in the charge-voltage space. For certain conditions a net mechanical work can be obtained from the cycling capacitor. The analysis is simple enough to be explained in a general physics course.

physics.gen-ph↗

arXiv2012

Una paradoja hidrostatica

It is shown that for a liquid in a connected vessel system, it is not possible to fulfill simultaneously Pascal's principle, mass conservation and energy conservation when the system goes from an equilibrium state to another one.

physics.gen-ph↗

arXiv2012

Levitation in an "almost" electrostatic field

It is well known that a charged particle cannot be in stable equilibrium in a purely electrostatic field. The situation is different in a magnetostatic field; consequently, magnetic levitation is possible while electrostatic levitation is not. In this paper, motivated by an analogy with a mechanical system, we show that the addition of a small oscillating electrical field to an otherwise electrostatic configuration leads to the stabilisation of unstable equilibrium points. Therefore, levitation becomes possible in an "almost electrostatic" field.

physics.gen-ph↗

arXiv2012

Microscopic description of a nonequilibrium system

The empirical laws of chemical kinetics are studied from a microscopic point of view. An analysis based on elementary probability theory and combinatorics is enough to explain the kinetics law observed in experiments. Thus, an out of equilibrium system may be examined with tools available to a student who begins the study of statistical physics.

physics.gen-ph↗

arXiv2012

Entropy generation in a chemical reaction

Entropy generation in a chemical reaction is analyzed without using the general formalism of non-equilibrium thermodynamics at a level adequate for advanced undergraduates. In a first approach to the problem, the phenomenological kinetic equation of an elementary first order reaction is used to show that entropy production is always positive. A second approach assumes that the reaction is near equilibrium to prove that the entropy generated is always greater than zero, without any reference to the kinetics of the reaction. Finally, it is shown that entropy generation is related to fluctuations in the number of particles at equilibrium, i.e. it is associated to a microscopic process.

physics.gen-ph↗

arXiv2012

How to transform, with a capacitor, thermal energy into usable work

The temperature dependence of the dielectric permittivity is taken into account to study the energy change in a capacitor that follows a cycle between a cold and a hot thermal reservoirs. There is a net energy gain in the process that, in principle, can be transformed into usable work. The article is simple enough as to be used with keen undergraduates that have taken a university general physics or thermodynamics course. Further experimental work and a possible technological application are suggested.

physics.gen-ph↗