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Prediction of good glass formers in the Al-Ni-La and Al-Ni-Gd systems using topological instability and electronegativity

GARGARELLA, P.; OLIVEIRA, M. F. de; KIMINAMI, C. S.; PAULY, S.; KUEHN, U.; BOLFARINI, C.; BOTTA, W. J.; ECKERT, J.
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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A new criterion has been recently proposed combining the topological instability (lambda criterion) and the average electronegativity difference (Delta e) among the elements of an alloy to predict and select new glass-forming compositions. In the present work, this criterion (lambda.Delta e) is applied to the Al-Ni-La and Al-Ni-Gd ternary systems and its predictability is validated using literature data for both systems and additionally, using own experimental data for the Al-La-Ni system. The compositions with a high lambda.Delta e value found in each ternary system exhibit a very good correlation with the glass-forming ability of different alloys as indicated by their supercooled liquid regions (Delta T(x)) and their critical casting thicknesses. In the case of the Al-La-Ni system, the alloy with the largest lambda.Delta e value, La(56)Al(26.5)Ni(17.5), exhibits the highest glass-forming ability verified for this system. Therefore, the combined lambda.Delta e criterion is a simple and efficient tool to select new glass-forming compositions in Al-Ni-RE systems. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3563099]; FAPESP; Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES); CNPq (Brazil); (DAAD) Deutscher Akademischer Austausch Dienst...

Modeling of the Barkhausen jump in low carbon steel

Benitez, Jose Alberto Pérez; Sanchez, Julio Antonio Capó; Padovese, Linilson Rodrigues
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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This work presents a model for the magnetic Barkhausen jump in low carbon content steels. The outcomes of the model evidence that the Barkhausen jump height depends on the coercive field of the pinning site and on the mean free path of the domain wall between pinning sites. These results are used to deduce the influence of the microstructural features and of the magnetizing parameters on the amplitude and duration of the Barkhausen jumps. In particular, a theoretical expression, establishing the dependence of the Barkbausen jump height on the carbon content and grain size, is obtained. The model also reveals the dependence of the Barkhausen jump on the applied frequency and amplitude. Theoretical and experimental results are presented and compared, being in good agreement. (C) 2008 American Institute of Physics.

Three-dimensional finite element thermal analysis of dental tissues irradiated with Er,Cr:YSGG laser

ANA, Patricia Aparecida; VELLOSO JR., Walter F.; ZEZELL, Denise Maria
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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In the present study, a finite element model of a half-sectioned molar tooth was developed in order to understand the thermal behavior of dental hard tissues (both enamel and dentin) under laser irradiation. The model was validated by comparing it with an in vitro experiment where a sound molar tooth was irradiated by an Er,Cr:YSGG pulsed laser. The numerical tooth model was conceived to simulate the in vitro experiment, reproducing the dimensions and physical conditions of the typical molar sound tooth, considering laser energy absorption and calculating the heat transfer through the dental tissues in three dimensions. The numerical assay considered the same three laser energy densities at the same wavelength (2.79 mu m) used in the experiment. A thermographic camera was used to perform the in vitro experiment, in which an Er, Cr: YSGG laser (2.79 mu m) was used to irradiate tooth samples and the infrared images obtained were stored and analyzed. The temperature increments in both the finite element model and the in vitro experiment were compared. The distribution of temperature inside the tooth versus time plotted for two critical points showed a relatively good agreement between the results of the experiment and model. The three dimensional model allows one to understand how the heat propagates through the dentin and enamel and to relate the amount of energy applied...

Zero-phonon emission and magnetic polaron parameters in EuTe

Henriques, Andre Bohomoletz; Galgano, Giovanni Decot; ABRAMOF, E.; DIAZ, B.; RAPPL, P. H. O.
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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A phonon structure in the photoluminescence of EuTe was discovered, with a well-defined zero-phonon emission line (ZPL). The ZPL redshifts linearly with the intensity of applied magnetic field, indicating spin relaxation of the photoexcited electron, and saturates at a lower magnetic field than the optical absorption bandgap, which is attributed to formation of magnetic polarons. From the difference in these saturation fields, the zero-field polaron binding energy and radius are estimated to be 43 meV and 3.2 (in units of the EuTe lattice parameter), respectively. (C) 2011 American Institute of Physics. [doi:10.1063/1.3634030]; CNPq; FAPESP

Active waveguide effects from porous anodic alumina: An optical sensor proposition

TRIVINHO-STRIXINO, F.; GUERREIRO, H. A.; GOMES, C. S.; PEREIRA, E. C.; GUIMARAES, Francisco Eduardo Gontijo
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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We present in this paper an active waveguide effect observed in porous anodic alumina (PA), which can be applied in optical sensors. The spectral position, shape, and polarization effect of the narrow waveguide modes is described. An analytical test with a commercial pesticide was performed. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3447375]; CNPq; FAPESP

Substrate/semiconductor interface effects on the emission efficiency of luminescent polymers

THEREZIO, Eralci M.; PIOVESAN, Erick; ANNI, Marco; SILVA, R. A.; OLIVEIRA JUNIOR, Osvaldo Novais de; MARLETTA, Alexandre
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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The importance of interface effects for organic devices has long been recognized, but getting detailed knowledge of the extent of such effects remains a major challenge because of the difficulty in distinguishing from bulk effects. This paper addresses the interface effects on the emission efficiency of poly(p-phenylene vinylene) (PPV), by producing layer-by-layer (LBL) films of PPV alternated with dodecylbenzenesulfonate. Films with thickness varying from similar to 15 to 225 nm had the structural defects controlled empirically by converting the films at two temperatures, 110 and 230 degrees C, while the optical properties were characterized by using optical absorption, photoluminescence (PL), and photoluminescence excitation spectra. Blueshifts in the absorption and PL spectra for LBL films with less than 25 bilayers (<40-50 nm) pointed to a larger number of PPV segments with low conjugation degree, regardless of the conversion temperature. For these thin films, the mean free-path for diffusion of photoexcited carriers decreased, and energy transfer may have been hampered owing to the low mobility of the excited carriers. The emission efficiency was then found to depend on the concentration of structural defects, i.e., on the conversion temperature. For thick films with more than 25 bilayers...

Quantum yield excitation spectrum (UV-visible) of CdSe/ZnS core-shell quantum dots by thermal lens spectrometry

CRUZ, Renato A.; PILLA, Viviane; CATUNDA, Tomaz
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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A recently developed thermal lens spectrometry configuration has been used to study CdSe/ZnS core-shell quantum dots (QDs) suspended in toluene and tetrahydrofuran (THF) solvents. The special features of this configuration make it very attractive to measure fluorescence quantum yield (eta) excitation spectrum since it simplifies the measurement procedure and consequently improve the accuracy. Furthermore, the precision reached is much higher than in conventional photoluminescence (PL) technique. Two methods, called reference sample and multiwavelength have been applied to determine eta, varying excitation wavelength in the UV-visible region (between 335-543 nm). The eta and PL spectra are practically independent of the excitation wavelength. For CdSe/ZnS QDs suspended in toluene we have obtained eta=76 +/- 2%. In addition, the aging effect on eta and PL has been studied over a 200 h period for QDs suspended in THF. (C) 2010 American Institute of Physics. [doi:10.1063/1.3343517]; FAPESP; Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES); CNPq

Effective coordination concept applied for phase change (GeTe)(m)(Sb(2)Te(3))(n) compounds

SILVA, Juarez L. F. Da
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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In this work, we employed the effective coordination concept to study the local environments of the Ge, Sb, and Te atoms in the Ge(m)Sb(2n)Te(m+3n) compounds. From our calculations and analysis, we found an average effective coordination number (ECN) reduction of 1.59, 1.42, and 1.37, for the Ge, Sb, Te atoms in the phase transition from crystalline, ECN=5.55 (Ge), 5.73 (Sb), 4.37 (Te), to the amorphous phase, ECN=3.96 (Ge), 4.31 (Sb), 3.09 (Te), for the Ge(2)Sb(2)Te(5) composition. Similar changes are observed for other compositions. Thus, our results indicate that the coordination changes from the crystalline to amorphous phase are not large as previously assumed in the literature, i.e., from sixfold to fourfold for Ge, which can contribute to obtain a better understanding of the crystalline to amorphous phase transition. (C) 2011 American Institute of Physics. [doi:10.1063/1.3533422]; Sao Paulo Science Foundation (FAPESP

Surface effects in the magnetic properties of crystalline 3 nm ferrite nanoparticles chemically synthesized

LIMA JR., E.; BIASI, E. De; MANSILLA, M. Vasquez; SALETA, M. E.; EFFENBERG, F.; ROSSI, L. M.; COHEN, R.; RECHENBERG, H. R.; ZYSLER, R. D.
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
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We have systematically studied the magnetic properties of ferrite nanoparticles with 3, 7, and 11 nm of diameter with very narrow grain size distributions. Samples were prepared by the thermal decomposition of Fe (acac)(3) in the presence of surfactants giving nanoparticles covered by oleic acid. High resolution transmission electron microscopy (HRTEM) images and XRD diffraction patterns confirms that all samples are composed by crystalline nanoparticles with the spinel structure expected for the iron ferrite. ac and dc magnetization measurements, as well in-field Mossbauer spectroscopy, indicate that the magnetic properties of nanoparticles with 11 and 7 nm are close to those expected for a monodomain, presenting large M(S) (close to the magnetite bulk). Despite the crystalline structure observed in HRTEM images, the nanoparticles with 3 nm are composed by a magnetically ordered region (core) and a surface region that presents a different magnetic order and it contains about 66% of Fe atoms. The high saturation and irreversibility fields in the M(H) loops of the particles with 3 nm together with the misalignment at 120 kOe in the in-field Mossbauer spectrum of surface component indicate a high surface anisotropy for the surface atoms...

Optimization of Heat Treatment Profiles Applied to Nanometric-Scale Nb(3)Sn Wires With Cu-Sn Artificial Pinning Centers

RODRIGUES JR., D.; SILVA, L. B. S. Da; RODRIGUES, C. A.; OLIVEIRA JR., N. F.; BORMIO-NUNES, C.
Fonte: IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC Publicador: IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Tipo: Artigo de Revista Científica
ENG
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Nb(3)Sn is one of the most used superconducting materials for applications in high magnetic fields. The improvement of the critical current densities (J(c)) is important, and must be analyzed together with the optimization of the flux pinning acting in the material. For Nb(3)Sn, it is known that the grain boundaries are the most effective pinning centers. However, the introduction of artificial pinning centers (APCs) with different superconducting properties has been proved to be beneficial for J(c). As these APCs are normally in the nanometric-scale, the conventional heat treatment profiles used for Nb(3)Sn wires cannot be directly applied, leading to excessive grain growth and/or increase of the APCs cross sections. In this work, the heat treatment profiles for Nb(3)Sn superconductor wires with Cu(Sn) artificial pinning centers in nanometric-scale were analyzed in an attempt to improve J(c) . It is described a methodology to optimize the heat treatment profiles in respect to diffusion, reaction and formation of the superconducting phases. Microstructural, transport and magnetic characterization were performed in an attempt to find the pinning mechanisms acting in the samples. It was concluded that the maximum current densities were found when normal phases (due to the introduction of the APCs) are acting as main pinning centers in the global behavior of the Nb(3)Sn superconducting wire.; FAPESP; CNPq; CAPES...

A geometrical approach of 3-D FEA for educational purposes applied to electrostatic fields

Rossi, Luiz Natal; Silva, Viviane Cristine; Martinho, Lucas Blattner; Cardoso, Jose Roberto
Fonte: IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC Publicador: IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Tipo: Artigo de Revista Científica
ENG
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A geometrical approach of the finite-element analysis applied to electrostatic fields is presented. This approach is particularly well adapted to teaching Finite Elements in Electrical Engineering courses at undergraduate level. The procedure leads to the same system of algebraic equations as that derived by classical approaches, such as variational principle or weighted residuals for nodal elements with plane symmetry. It is shown that the extension of the original procedure to three dimensions is straightforward, provided the domain be meshed in first-order tetrahedral elements. The element matrices are derived by applying Maxwell`s equations in integral form to suitably chosen surfaces in the finite-element mesh.

On the energy conversion efficiency in magnetic hyperthermia applications: A new perspective to analyze the departure from the linear regime

Landi, G. T.; Bakuzis, A. F.
Fonte: AMER INST PHYSICS; MELVILLE Publicador: AMER INST PHYSICS; MELVILLE
Tipo: Artigo de Revista Científica
ENG
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The success of magnetic hyperthermia cancer treatments rely strongly on the magnetic properties of the nanoparticles and their intricate dependence on the externally applied field. This is particularly more so as the response departs from the low field linear regime. In this paper we introduce a new parameter, referred to as the efficiency in converting electromagnetic energy into thermal energy, which is shown to be remarkably useful in the analysis of the system response, especially when the power loss is investigated as a function of the applied field amplitude. Using numerical simulations of dynamic hysteresis, through the stochastic Landau-Lifshitz model, we map in detail the efficiency as a function of all relevant parameters of the system and compare the results with simple-yet powerful-predictions based on heuristic arguments about the relaxation time. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4705392]; Brazilian funding agency CNPq; Brazilian funding agency CAPES; Brazilian funding agency FINEP; Brazilian funding agency FAPEG; Brazilian funding agency FAPESP; Brazilian funding agency FUNAPE

Charge generation, charge transport, and residual charge in the electrospinning of polymers: A review of issues and complications

Collins, George; Federici, John; Imura, Yuki; Catalani, Luiz Henrique
Fonte: AMER INST PHYSICS; MELVILLE Publicador: AMER INST PHYSICS; MELVILLE
Tipo: Artigo de Revista Científica
ENG
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46.203457%
Electrospinning has become a widely implemented technique for the generation of nonwoven mats that are useful in tissue engineering and filter applications. The overriding factor that has contributed to the popularity of this method is the ease with which fibers with submicron diameters can be produced. Fibers on that size scale are comparable to protein filaments that are observed in the extracellular matrix. The apparatus and procedures for conducting electrospinning experiments are ostensibly simple. While it is rarely reported in the literature on this topic, any experience with this method of fiber spinning reveals substantial ambiguities in how the process can be controlled to generate reproducible results. The simplicity of the procedure belies the complexity of the physical processes that determine the electrospinning process dynamics. In this article, three process domains and the physical domain of charge interaction are identified as important in electrospinning: (a) creation of charge carriers, (b) charge transport, (c) residual charge. The initial event that enables electrospinning is the generation of region of excess charge in the fluid that is to be electrospun. The electrostatic forces that develop on this region of charged fluid in the presence of a high potential result in the ejection of a fluid jet that solidifies into the resulting fiber. The transport of charge from the charge solution to the grounded collection device produces some of the current which is observed. That transport can occur by the fluid jet and through the atmosphere surrounding the electrospinning apparatus. Charges that are created in the fluid that are not dissipated remain in the solidified fiber as residual charges. The physics of each of these domains in the electrospinning process is summarized in terms of the current understanding...

Electroluminescence and electric current response spectroscopy applied to the characterization of polymer light-emitting electrochemical cells

Gozzi, Giovani; Faria, Roberto Mendonça; Santos, Lucas Fugikawa
Fonte: AMER INST PHYSICS; MELVILLE Publicador: AMER INST PHYSICS; MELVILLE
Tipo: Artigo de Revista Científica
ENG
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56.457754%
Frequency-dependent electroluminescence and electric current response spectroscopy were applied to polymeric light-emitting electrochemical cells in order to obtain information about the operation mechanism regimes of such devices. Three clearly distinct frequency regimes could be identified: a dielectric regime at high frequencies; an ionic transport regime, characterized by ionic drift and electronic diffusion; and an electrolytic regime, characterized by electronic injection from the electrodes and electrochemical doping of the conjugated polymer. From the analysis of the results, it was possible to evaluate parameters like the diffusion speed of electronic charge carriers in the active layer and the voltage drop necessary for operation. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752438]; FAPESP; FAPESP; CNPq; CNPq; National Institute of Organic Electronics (INEO); National Institute of Organic Electronics (INEO); CAPES; CAPES

In vitro cytotoxicity of Selol-loaded magnetic nanocapsules against neoplastic cell lines under AC magnetic field activation

Falqueiro, A. M.; Siqueira-Moura, M. P.; Jardim, D. R.; Primo, F. L.; Morais, P. C.; Mosiniewicz-Szablewska, E.; Suchocki, P.; Tedesco, Antonio Claudio
Fonte: AMER INST PHYSICS; MELVILLE Publicador: AMER INST PHYSICS; MELVILLE
Tipo: Artigo de Revista Científica
ENG
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The goals of this study are to evaluate in vitro compatibility of magnetic nanomaterials and their therapeutic potential against cancer cells. Highly stable ionic magnetic fluid sample (maghemite, gamma-Fe2O3) and Selol were incorporated into polymeric nanocapsules by nanoprecipitation method. The cytotoxic effect of Selol-loaded magnetic nanocapsules was assessed on murine melanoma (B16-F10) and oral squamous cell carcinoma (OSCC) cell lines following AC magnetic field application. The influence of different nanocapsules on cell viability was investigated by colorimetric MTT 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay. In the absence of AC magnetic field Selol-loaded magnetic nanocapsules, containing 100 mu g/mL Selol plus 5 x 10(12) particle/mL, showed antitumoral activity of about 50% on B16-F10 melanoma cells while OSCC carcinoma cells demonstrated drug resistance at all concentrations of Selol and magnetic fluid (range of 100-500 mu g/mL Selol and 5 x 10(12) -2.5 x 10(13) particle/mL). On the other hand, under AC applied fields (1 MHz and 40 Oe amplitude) B16-F10 cell viability was reduced down to 40.5% (+/- 3.33) at the highest concentration of nanoencapsulated Selol. The major effect, however, was observed on OSCC cells since the cell viability drops down to about 33.3% (+/- 0.38) under application of AC magnetic field. These findings clearly indicate that the Selol-loaded magnetic nanocapsules present different toxic effects on neoplastic cell lines. Further...

Tópícos de Astrofísica e Cosmologia: uma aplicação de Física Moderna e Contemporânea no Ensino Médio; Topics for astrophysics and cosmology: a application of modern and contemporary physics in high school

Aguiar, Ricardo Rechi
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 05/11/2010 PT
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O mundo é moldado pela tecnologia: computadores, celulares, internet e vários outros equipamentos são responsáveis por mudanças nos hábitos de milhões de pessoas. Além disso, o conhecimento científico atual nos apresenta um Universo gigantesco e rico em figuras quase mitológicas, como aglomerados estelares e galácticos, expansão do espaço-tempo e um surgimento universal cataclísmico. E, no núcleo de todas estas inovações e informações, se encontra o conhecimento que chamamos de Física Moderna e Contemporânea. Seria de se esperar que o ensino de Física na escola média estivesse recheado de assuntos desta natureza, auxiliando os educandos a explorar e interpretar o mundo em que vivem e entender a ciência envolvida por trás destes equipamentos e descobertas. Porém, tais conteúdos ainda são a minoria dos tópicos abordados nesta disciplina. Este trabalho se insere neste contexto, procurando colaborar com a incorporação de conteúdos de Física Moderna e Contemporânea no ensino médio, através da apresentação do processo de construção de uma proposta curricular de Física diferenciada, aplicada a partir do ano de 2006 em uma escola particular paulistana. Nele explora-se o desenvolvimento e a aplicação da proposta que introduziu alguns tópicos de Astrofísica e Cosmologia no segundo semestre do curso da 1ª série do ensino médio da escola no ano de 2008. Observou-se...

Band structure calculations of InP wurtzite/zinc-blende quantum wells

Faria Junior, P. E.; Sipahi, Guilherme Matos
Fonte: AMER INST PHYSICS; MELVILLE Publicador: AMER INST PHYSICS; MELVILLE
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
46.40194%
Semiconductor nanowhiskers (NWs) made of III-V compounds exhibit great potential for technological applications. Controlling the growth conditions, such as temperature and diameter, it is possible to alternate between zinc-blende (ZB) and wurtzite (WZ) crystalline phases, giving origin to the so called polytypism. This effect has great influence in the electronic and optical properties of the system, generating new forms of confinement to the carriers. A theoretical model capable to accurately describe electronic and optical properties in these polytypical nanostructures can be used to study and develop new kinds of nanodevices. In this study, we present the development of a wurtzite/zinc-blende polytypical model to calculate the electronic band structure of nanowhiskers based on group theory concepts and the k.p method. Although the interest is in polytypical superlattices, the proposed model was applied to a single quantum well of InP to study the physics of the wurtzite/zinc-blende polytypism. By the analysis of our results, some trends can be predicted: spatial carriers' separation, predominance of perpendicular polarization (xy plane) in the luminescence spectra, and interband transition blueshifts with strain. Also, a possible range of values for the wurtzite InP spontaneous polarization is suggested. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767511]; Brazilian funding agency CAPES; Brazilian funding agency CAPES; Brazilian funding agency CNPq; Brazilian funding agency CNPq; Brazilian funding agency FAPESP; Brazilian funding agency FAPESP

O ensino de conceitos do eletromagnetismo, óptica, ondas e física moderna e contemporânea através de situações na medicina

Parisoto, Mara Fernanda
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Dissertação Formato: application/pdf
POR
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O ensino de Física, geralmente, está desvinculado não apenas de outras disciplinas, mas também, de qualquer contexto que lhe possa dar significado. No entanto, segue diretrizes estaduais e nacionais que apontam para necessidade de interdisciplinaridade e contextualização. Assim, enfatiza-se a importância da utilização de várias estratégias para atingir alunos que aprendem de diversas maneiras: cinestésica, visual e auditiva. Sob essa perspectiva, o presente trabalho tem por objetivo principal, a partir do desenvolvimento de vários recursos didáticos, promover a aprendizagem significativa nos alunos. Para tanto, utilizaram-se várias ferramentas, tais como texto de apoio, atividades experimentais, novas tecnologias, mapas conceituais, diagramas V de Gowin e situações-problemas. Tais ferramentas foram utilizadas em cursos para professores, futuros professores de Física do Ensino Médio e, também, para pessoas que utilizam radiações ionizantes para tecidos biológicos em seu trabalho, usando, para tal, conceitos de Eletromagnetismo, Óptica e Física Moderna e Contemporânea na área de Física aplicada à Medicina. Através de análise qualitativa e quantitativa, verificamos se tais recursos tiveram algum impacto positivo...

Statistical physics applied to stone-age civilization

Sumour, M. A.; Radwan, M. A.; Shabat, M. M.; El-Astal, Ali H.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 13/10/2011
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About 45,000 years ago, symbolic and technological complexity of human artefacts increased drastically. Computer simulations of Powell, Shennan and Thomas (2009) explained it through an increase of the population density, facilitating the spread of information about useful innovations. We simplify this demographic model and make it more similar to standard physics models. For this purpose, we assume that bands (extended families) of stone-age humans were distributed randomly on a square lattice such that each lattice site is randomly occupied with probability p and empty with probability 1-p. Information spreads randomly from an occupied site to one of its occupied neighbours. If we wait long enough, information spreads from one side of the lattice to the opposite site if and only if p is larger than the percolation threshold; this process was called "ant in the labyrinth" by deGennes 1976. We modify it by giving the diffusing information a finite lifetime, which shifts the threshold upwards.; Comment: 6 pages including three figures

Heavy quark physics on the lattice with improved nonrelativistic actions

Meinel, Stefan
Fonte: University of Cambridge; Department of Applied Mathematics and Theoretical Physics; St John's College Publicador: University of Cambridge; Department of Applied Mathematics and Theoretical Physics; St John's College
Tipo: Thesis; doctoral; PhD
EN
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Hadrons containing heavy quarks, in particular b quarks, play an important role in high energy physics. Measurements of their electroweak interactions are used to test the Standard Model and search for new physics. For the comparison of experimental results with theoretical predictions, nonperturbative calculations of hadronic matrix elements within the theory of quantum chromodymanics are required. Such calculations can be performed from first principles by formulating QCD on a Euclidean spacetime grid and computing the path integral numerically. Including b quarks in lattice QCD calculations requires special techniques as the lattice spacing in present computations usually can not be chosen fine enough to resolve their Compton wavelength. In this work, improved nonrelativistic lattice actions for heavy quarks are used to perform calculations of the bottom hadron mass spectrum and of form factors for heavy-to-light decays. In heavy-to-light decays, additional complications arise at high recoil, when the momentum of the light meson reaches a magnitude comparable to the cutoff imposed by the lattice. Discretisation errors at high recoil can be reduced by working in a frame of reference where the heavy and light mesons move in opposite directions. Using a formalism referred to as moving nonrelativistic QCD (mNRQCD)...