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Search Results: 1 - 10 of 61 matches for " Heckler "
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Sorgo e girassol no outono-inverno, em sistema plantio direto, no Mato Grosso do Sul, Brasil
Heckler, Jo?o Carlos;
Ciência Rural , 2002, DOI: 10.1590/S0103-84782002000300024
Abstract: the grain production systems at the brazilian western region, mainly in mato grosso do sul state, have some weak points, among them the lack of alternative crops for the fall-winter season. with the purpose of studying the behavior of sorghum and sunflower genotypes, under the no-till cropping system and at the fall-winter season, two experiments were carried out in the year 2000, at embrapa western agriculture, dourados, mato grosso do sul state, brazil. the randomized blocks experimental design was used. the sunflower results showed significant differences among the genotypes grain yields. in average, the yield was 2.176kg ha-1. the highest yield was obtained with the m 734 hybrid, which yielded 3.028kg ha-1. the sorghum average grain yield was 7.861 kg ha-1, noting that the br 304, m 51, and ag 2005e genotypes yielded 9.865, 9.771, and 9.055kg ha-1, respectively.
Sorgo e girassol no outono-inverno, em sistema plantio direto, no Mato Grosso do Sul, Brasil
Heckler Jo?o Carlos
Ciência Rural , 2002,
Abstract: A falta de culturas alternativas para o cultivo de outono-inverno é importante problema para os sistemas de produ o de gr os da Regi o Oeste do Brasil, em particular no Estado do Mato Grosso do Sul. Com o propósito de estudar o comportamento de genótipos de girassol (Helianthus annuus L.) e sorgo (Sorghum bicolor L. Moench) em Sistema Plantio Direto, no outono-inverno do ano 2000, foram conduzidos dois experimentos na Embrapa Agropecuária Oeste, em Dourados, MS, utilizando-se o delineamento de blocos ao acaso. Os resultados das avalia es do girassol mostraram diferen as significativas entre os tratamentos quanto ao rendimento de gr os que, em média, produziram 2.176kg ha-1. O maior rendimento foi do híbrido M 734 (3.028kg ha-1). O rendimento médio de gr os alcan ado pelos genótipos de sorgo foi de 7.861kg ha-1, com destaque para: BR 304, M 51 e AG 2005E, com 9.865, 9.771 e 9.055kg ha-1, respectivamente.
On the formation of a Hawking-radiation photosphere around microscopic black holes
Andrew F. Heckler
Physics , 1996, DOI: 10.1103/PhysRevD.55.480
Abstract: We show that once a black hole surpasses some critical temperature $T_{crit}$, the emitted Hawking radiation interacts with itself and forms a nearly thermal photosphere. Using QED, we show that the dominant interactions are bremsstrahlung and electron-photon pair production, and we estimate $T_{crit} \sim m_{e}/\alpha^{5/2}$, which when calculated more precisely is found to be $T_{crit} \approx $45 GeV. The formation of the photosphere is purely a particle physics effect, and not a general relativistic effect, since the the photosphere forms roughly $\alpha^{-4}$ Schwarzschild radii away from the black hole. The temperature $T$ of the photosphere decreases with distance from the black hole, and the outer surface is determined by the constraint $T\sim m_{e}$ (for the QED case), since this is the point at which electrons and positrons annihilate, and the remaining photons free stream to infinity. Observational consequences are discussed, and it is found that, although the QED photosphere will not affect the Page-Hawking limits on primordial black holes, which is most important for 100MeV black holes, the inclusion of QCD interactions may significantly effect this limit, since for QCD we estimate $T_{crit}\sim \Lambda_{QCD}$. The photosphere greatly reduces possibility of observing individual black holes with temperatures greater than $T_{crit}$, since the high energy particles emitted from the black hole are processed through the photosphere to a lower energy, where the gamma ray background is much higher. The temperature of the plasma in the photosphere can be extremely high, and this offers interesting possibilities for processes such as symmetry restoration.
The Effects of Electro-weak Phase Transition Dynamics on Baryogenesis and Primordial Nucleosynthesis
Andrew F. Heckler
Physics , 1994, DOI: 10.1103/PhysRevD.51.405
Abstract: The evolution of the electro-weak phase transition, including reheating due to the release of latent heat in shock waves, is calculated for various values of as yet unknown parameters of electro-weak theory such as latent heat and bubble wall surface tension. We show that baryon production, which occurs in the vicinity of the bubble walls of the phase transition, can be a sensitive function of bubble wall velocity, and this velocity dependence is important to include in the calculation of the baryon density of the universe. There is a sensitive velocity dependence for all mechanisms of baryon production, depending on the magnitude of velocity of the bubble wall, and we examine in particular an inverse velocity dependence on baryon production, which is predicted by the charge transport mechanism of baryon production. For this mechanism we find both an enhancement of baryon production and the generation of inhomogeneities during the electro-weak phase transition. We calculate the magnitude of the baryon enhancement, which can be as large as a few orders of magnitude, depending on the parameters of the theory, and we calculate the size and amplitude of the inhomogeneities generated. We determine that the inhomogeneities generated in a thermally nucleated electro-weak phase transition are to small to survive diffusive processes and effect the nucleosynthesis epoch. We also examine the possibility that a phase transition nucleated by other means, such as by the presence of cosmic strings, may produce inhomogeneities that could effect
Calculation of the emergent spectrum and observation of primordial black holes
Andrew F. Heckler
Physics , 1997, DOI: 10.1103/PhysRevLett.78.3430
Abstract: We calculate the emergent spectrum of microscopic black holes, which emit copious amounts of thermal ``Hawking'' radiation, taking into account the proposition that (contrary to previous models) emitted quarks and gluons do not directly fragment into hadrons, but rather interact and form a photosphere and decrease in energy before fragmenting. The resulting spectrum emits copious amount of photons at energies around 100MeV. We find that the limit on the average universal density of black holes is not significantly affected by the photosphere. However we also find that gamma ray satellites such as EGRET and GLAST are well suited to look for nearby black holes out to a distance on the order of 0.3 parsecs, and conclude that if black holes are clustered locally as much as luminous matter, they may be directly detectable.
Systematic study of student understanding of the relationships between the directions of force, velocity, and acceleration in one dimension
Rebecca Rosenblatt,Andrew F. Heckler
Physical Review Special Topics. Physics Education Research , 2011,
Abstract: We developed an instrument to systematically investigate student conceptual understanding of the relationships between the directions of net force, velocity, and acceleration in one dimension and report on data collected on the final version of the instrument from over 650 students. Unlike previous work, we simultaneously studied all six possible conditional relations between force, velocity, and acceleration in order to obtain a coherent picture of student understanding of the relations between all three concepts. We present a variety of evidence demonstrating the validity and reliability of the instrument. An analysis of student responses from three different course levels revealed three main findings. First, a significant fraction of students chose “partially correct” responses, and from pre- to post-test, many students moved from “misconception” to partially correct responses, or from partially correct to fully correct responses. Second, there were asymmetries in responding to conditional relations. For example, students answered questions of the form “Given the velocity, what can be inferred about the net force?” differently than converse questions “Given the net force, what can be inferred about the velocity?” Third, there was evidence of hierarchies in student responses, suggesting, for example, that understanding the relation between velocity and acceleration is necessary for understanding the relation between velocity and force, but the converse is not true. Finally, we briefly discuss how these findings might be applied to instruction.
Nonperturbative effects on nucleation
Marcelo Gleiser,Andrew F. Heckler
Physics , 1995, DOI: 10.1103/PhysRevLett.76.180
Abstract: A nonperturbative correction to the thermal nucleation rate of critical bubbles in a first order phase transition is estimated. The correction originates from large-amplitude fluctuations which may be present before the transition occurs. Using a simple model of a scalar field in a double-well potential, we present a method to obtain a corrected potential which incorporates the free-energy density available from large-amplitude fluctuations, which is not included in the usual perturbative calculation. For weaker phase transitions, the nucleation rate can be much larger than the rate calculated via perturbation theory. As an application of our method, we show how nonperturbative corrections can both qualitatively and quantitatively explain anomalously high nucleation rates observed in 2-d numerical simulations.
Peaks and decays of student knowledge in an introductory E&M course
Eleanor C. Sayre,Andrew F. Heckler
Physical Review Special Topics. Physics Education Research , 2009,
Abstract: A common format for assessment of learning is pretesting and post-testing. In this study, we collect student test data several times per week throughout a course, allowing for the measurement of the changes in student knowledge with a time resolution on the order of a few days. To avoid the possibility of test-retest effects, separate and quasirandom subpopulations of students are tested on a variety of tasks. We report on data taken in a calculus-based introductory E&M class populated primarily by engineering majors. Unsurprisingly for a traditional introductory course, there is little change in many conceptual questions. However, the data suggest that some student performance peaks and decays rapidly during a quarter, a pattern consistent with memory research yet unmeasurable by pretesting and post-testing. In addition, it appears that some course topics can interfere with prior knowledge, decreasing performance on questions related to earlier topics in the course.
Interference between electric and magnetic concepts in introductory physics
Thomas M. Scaife,Andrew F. Heckler
Physical Review Special Topics. Physics Education Research , 2011,
Abstract: We investigate student confusion of concepts of electric and magnetic force. At various times during a traditional university-level course, we administered a series of simple questions about the direction of force on a charged particle moving through either an electric or a magnetic field. We find that after electric force instruction but before magnetic force instruction most students answer electric force questions correctly, and we replicate well-known results that many students incorrectly answer that magnetic forces are in the same direction as the magnetic field. After magnetic force instruction, most students answer magnetic force questions correctly, but surprisingly many students incorrectly answer that electric forces are perpendicular to electric fields, as would happen if a student confused electric forces with magnetic forces. As a further indication of interference between electric and magnetic concepts, we also find that students’ responses depend on whether electric or magnetic force questions are posed first, and this effect depends on whether electric or magnetic force was most recently taught.
Searching for stellar mass black holes in the solar neighborhood
Andrew F. Heckler,Edward W. Kolb
Physics , 1996, DOI: 10.1086/310362
Abstract: We propose a strategy for searching for isolated stellar mass black holes in the solar neighborhood with the Sloan Digital Sky Survey. Due to spherical accretion of the inter-stellar medium and the ambient magnetic field, an isolated black hole is expected to emit a blended, thermal synchrotron spectrum with a roughly flat peak from the optical down to the far infra-red. We find that the Sloan Survey will be able to detect isolated black holes, in the considered mass range of 1--100$M_{\odot}$, out to a few hundred parsecs, depending on the local conditions of the ISM. We also find that the black holes are photmetrically distinguishable from field stars and they have a photometry similar to QSOs. They can be further singled out from QSO searches because they have a featureless spectrum with no emission lines. The Sloan Survey will likely find hundreds of objects that meet these criteria, and to further reduce the number of candidates, we suggest other selection criteria such as infra-red searches and proper motion measurements. Estimates indicate that dozens of black holes may exist out to a few hundred parsecs. If no black hole candidates are found in this survey, important limits can be placed on the local density of black holes and the halo fraction in black holes, especially for masses greater than about $20 M_{\odot}$.
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