Vitorvani Soares

Relevance of cooling rates on formation from the melt of superconducting bismuth-based cuprate phases

Abstract This work presents a study on phase formation processes in the Bi–Sr–Ca–Cu–O system. A liquid phase is employed as nutrient in order to optimize the system’s components mixing and the cooling rate is the relevant experimental variable. Different cooling rates are employed to promote changes in component’s mobility inside the liquid, as well as to collect information regarding the metastable feature of the stoichiometrically ideal superconducting phases on this system. The solidification process is evaluated in real time by means of differential thermal analysis and thermogravimetry. Conventional X-ray crystallography is employed to characterize the samples at room temperature in powder form. Crystal structure refinement employs the Rietveld method. The results suggest that the amount of 2201 increases and of 2212 decreases as the cooling rate increases, and that formation of 2201 proceeds at high temperature.

A note on the conservation of mechanical energy and the Galilean principle of relativity

A reexamination of simple examples that we usually teach to our students in introductory courses is the starting point for a discussion about the principle of conservation of energy and Galilean invariance.

Adiabatic nucleation in the liquid–vapor phase transition

The fundamental difference between classical (isothermal) nucleation theory (CNT) and adiabatic nucleation theory (ANT) is discussed. CNT uses the concept of isothermal heterophase fluctuations, while ANT depends on common fluctuations of the thermodynamic variables. Applications to the nonequilibrium liquid to vapor transition are shown. However, we cannot yet calculate nucleation frequencies. At present, we can only indicate at what temperatures and pressures copious homogeneous nucleation is expected in the liquid to vapor phase transition. It is also explained why a similar general indication cannot be made for the inverse vapor to liquid transition. Simultaneously, the validity of Peng–Robinson’s equation of state [D.-Y. Peng and D. B. Robinson, Ind. Eng. Chem. Fundam. 15, 59 (1976)] is confirmed for highly supersaturated liquids.

Adiabatic nucleation in supersaturated liquids

Abstract Adiabatic nucleation theory (ANT) is shown to be in good agreement with experimental data of superheated liquids (boiling). Pure superheated liquids nucleate just a few K before reaching the spinodal, calculated by the Peng–Robinson equation of state. In spite of the correlation between the nucleation curve and the spinodal, nucleation is usually explained by the (isothermal) classical nucleation theory (CNT), which does not take into account the proximity of the spinodal. An alternative explanation is given by ANT. When the liquid is close to the spinodal, the spinodal is reached and overpassed by volume fluctuations and the nucleus appears as the result of spinodal phase separation.

Negative Pressures in Classical and Cosmological Fluids and Their Thermodynamic Stability

The purpose of this paper is to establish reasonable limits to negative pressures in classical and cosmological fl uids based on well-known present physical theories and discuss their thermodynamic stability. We begin the paper deriving the well-known equation of state of an ideal gas from thermodynamics and kinetic theory, using the equipartition of energy. Both non-relativistic and relativistic cases are discussed. From the results it is clear that for an ideal gas pressure can only be positive and proportional to the density (of mass or energy). In order to discuss a system where pressure can be negative we review the van der Waals fluid, which presents negative pressure only in liquid metastable states. Then we present the requirements that should be fulfilled by a Chaplygin type fluid, where pressure is negative and goes with a negative power of the energy density, in order to satisfy thermodynamical conditions of stability. Finally we consider the equation of state P = �Bρ ( B> 0) and show that it satisfies the conditions of stability but violates the third law of thermodynamics. Besides that, the temperature of this fluid increases during an adiabatic expansion. Subject Index: 070, 401

O paradoxo cinemático de Galileu

We describe a low-cost demonstration setup and present a short video that can be used by the teacher to discuss an apparent kinematical paradox described by Galileo in a letter to his friend Guidobaldo del Monte and also in Two New Sciences. The kinematical solution of this apparent paradox is also discussed.

Galileo's kinematical paradox and the expanding circle of simultaneity

Consider a vertical circle and the diameter BA and representative chords BC, BD, which start from point B at the top of the circle, and also the chord EA and the line segment GF (see figure 1). If we simultaneously release two or more beads from B in such a way that one of the beads falls along diameter BA and the others slide along chords BC and BD or EA, then the beads will hit the circumference of the circle at the same instant, even though the chords have different lengths [1, 2]. This apparent paradox can easily be explained if we relate the length of diameter BA, let us call it h, to the length of a chord, say EA, let us call it . Then, by making use of Thales’ theorem, which states that any triangle inscribed in a semicircle is a right triangle, we can write

Determinação experimental da constante de Boltzmann a partir da curva característica corrente-voltagem de um diodo

We present in this work a didactic experimental activity with a diode, set up with affordable instruments, and that can be easily adapted for an undergraduate or a high school class. The activity described establishes the current ID-voltage VD characteristics of a diode from measurements of the potential difference VD on it and the current ID through it, for three different temperatures T = 0 °C, 23 °C, and 100 °C. From the experimental results: (i) we build up an algebraic expression for the current ID as a function of the voltage VD, and the temperature T, and (ii) we determine the value of the Boltzmann’s constant kB, one of the physical fundamental constants.

Sensores digitais de temperatura com tecnologia one-wire: Um exemplo de aplicação didática na área de condução térmica

Neste trabalho apresentamos uma experiencia simples de conducao termica usando sensores de temperatura digitais com tecnologia one-wire(um-fio) associados a uma placa Arduino para controle e aquisicao de dados. Os sensores utilizados sao acessoveis e podem ser facilmente encontrados no mercado nacional. O modelo experimental reproduz uma experiencia classica de conducao termica usada comumente em livros didaticos de fisica para apresentar e discutir a lei de Fourier da conducao termica. Os resultados se mostraram suficientemente sensiveis para revelar uma distribuicao nao linear de temperaturas numa situacao em que condicoes de contorno pouco realistas, e comumente aplicadas a lei de Fourier nos livros didaticos, resulta num comportamento linear.

Geometric construction of the Mars’s orbit by the method of Kepler

In this work we present the construction method of the martian orbit around the sun as suggested by Kepler in 1605, from the observational measurements of the position of this planet were available at that time. We show that this construction is not a simple task, and that exercise is a good teaching tool to make clear the difficulty of choosing an ellipse with very low eccentricity, instead of a circle, as the curve that describes the orbits of the planets around the sun.