12,346 research outputs found
Vacuum fluctuations of a scalar field near a reflecting boundary and their effects on the motion of a test particle
The contribution from quantum vacuum fluctuations of a real massless scalar
field to the motion of a test particle that interacts with the field in the
presence of a perfectly reflecting flat boundary is here investigated. There is
no quantum induced dispersions on the motion of the particle when it is alone
in the empty space. However, when a reflecting wall is introduced, dispersions
occur with magnitude dependent on how fast the system evolves between the two
scenarios. A possible way of implementing this process would be by means of an
idealized sudden switching, for which the transition occurs instantaneously.
Although the sudden process is a simple and mathematically convenient
idealization it brings some divergences to the results, particularly at a time
corresponding to a round trip of a light signal between the particle and the
wall. It is shown that the use of smooth switching functions, besides
regularizing such divergences, enables us to better understand the behavior of
the quantum dispersions induced on the motion of the particle. Furthermore, the
action of modifying the vacuum state of the system leads to a change in the
particle energy that depends on how fast the transition between these states is
implemented. Possible implications of these results to the similar case of an
electric charge near a perfectly conducting wall are discussed.Comment: 17 pages, 8 figure
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On the relation between Transversal and Longitudinal Scaling in Cities
Given that a group of cities follows a scaling law connecting urban population with socio-economic or infrastructural metrics (transversal scaling), should we expect that each city would follow the same behavior over time (longitudinal scaling)? This assumption has important policy implications, although rigorous empirical tests have been so far hindered by the lack of suitable data. Here, we advance the debate by looking into the temporal evolution of the scaling laws for 5507 municipalities in Brazil. We focus on the relationship between population size and two urban variables, GDP and water network length, analyzing the time evolution of the system of cities as well as their individual trajectory. We find that longitudinal (individual) scaling exponents are city-specific, but they are distributed around an average value that approaches to the transversal scaling exponent when the data are decomposed to eliminate external factors, and when we only consider cities with a sufficiently large growth rate. Such results give support to the idea that the longitudinal dynamics is a micro-scaling version of the transversal dynamics of the entire urban system. Finally, we propose a mathematical framework that connects the microscopic level to global behavior, and, in all analyzed cases, we find good agreement between theoretical prediction and empirical evidence
Avaliação de três acessos de Paspalum e dois cultivares (capim pojuca e Brachiaria decumbens) quanto à capacidade de rebrota.
No Brasil, o gênero Paspalum se destaca dentre as gramíneas com potencial forrageiro. A ampliação da variabilidade genética das forrageiras permite tornar os sistemas de produção animal menos vulneráveis às variações do ecossistema. O acúmulo de matéria seca em pastagens estabelecidas advém do aumento das áreas foliares que, conseqüentemente, incrementam a produção de assimilados, bem como de acréscimo do número de perfilhos e elevação dos entre nós [1]. O objetivo deste experimento foi avaliar a taxa de alongamento foliar de acessos do gênero Paspalum sp
Production, breeding and potential of cowpea crop in Brazil.
About 50 years ago cowpea was reported as a relatively minor tropical legume. However, in the last years, it has been emerging as one of the most important food legume of the 21st century (SINGH, 2010). Brazil is not out of this panorama. Brazilian agriculture is undergoing major technological changes and, in addition, globalization in agribusiness has caused impacts on the production chain of several crops, particularly those heavily dependent on the use of a large volume of agricultural inputs, mainly fertilizers and pesticides. Such crops have had a higher production cost each year. On the other hand, this situation has brought new opportunities. Business farmers have sought new alternatives for their production arrangements. In this context cowpea constitutes one of the best options.bitstream/item/85620/1/Doc-216-Production.pd
Irreversible magnetization under rotating fields and lock-in effect on ErBa_2Cu_3O_7 single crystal with columnar defects
We have measured the irreversible magnetization M_i of an ErBa_2Cu_3O_7
single crystal with columnar defects (CD), using a technique based on sample
rotation under a fixed magnetic field H. This method is valid for samples whose
magnetization vector remains perpendicular to the sample surface over a wide
angle range - which is the case for platelets and thin films - and presents
several advantages over measurements of M_L(H) loops at fixed angles. The
resulting M_i(\Theta) curves for several temperatures show a peak in the CD
direction at high fields. At lower fields, a very well defined plateau
indicative of the vortex lock-in to the CD develops. The H dependence of the
lock-in angle \phi_L follows the H^{-1} theoretical prediction, while the
temperature dependence is in agreement with entropic smearing effects
corresponding to short range vortex-defects interactions.Comment: 7 pages, 6 figures, to be published in Phys. Rev.
Avaliação da produtividade e teor de óleo em nove genótipos de mamoneira cultivados em baixa altitude.
A conjugate for the Bargmann representation
In the Bargmann representation of quantum mechanics, physical states are
mapped into entire functions of a complex variable z*, whereas the creation and
annihilation operators and play the role of
multiplication and differentiation with respect to z*, respectively. In this
paper we propose an alternative representation of quantum states, conjugate to
the Bargmann representation, where the roles of and
are reversed, much like the roles of the position and momentum operators in
their respective representations. We derive expressions for the inner product
that maintain the usual notion of distance between states in the Hilbert space.
Applications to simple systems and to the calculation of semiclassical
propagators are presented.Comment: 15 page
Citrate selective electrodes for the flow injection analysis of soft drinks, beers and pharmaceutical products
Aiming the establishment of simple and accurate readings of citric acid (CA) in complex samples, citrate (CIT) selective
electrodes with tubular configuration and polymeric membranes plus a quaternary ammonium ion exchanger were constructed.
Several selective membranes were prepared for this purpose, having distinct mediator solvents (with quite different polarities)
and, in some cases, p-tert-octylphenol (TOP) as additive. The latter was used regarding a possible increase in selectivity. The
general working characteristics of all prepared electrodes were evaluated in a low dispersion flow injection analysis (FIA)
manifold by injecting 500µl of citrate standard solutions into an ionic strength (IS) adjuster carrier (10−2 mol l−1) flowing at
3ml min−1. Good potentiometric response, with an average slope and a repeatability of 61.9mV per decade and ±0.8%, respectively,
resulted from selective membranes comprising additive and bis(2-ethylhexyl)sebacate (bEHS) as mediator solvent.
The same membranes conducted as well to the best selectivity characteristics, assessed by the separated solutions method
and for several chemical species, such as chloride, nitrate, ascorbate, glucose, fructose and sucrose. Pharmaceutical preparations,
soft drinks and beers were analyzed under conditions that enabled simultaneous pH and ionic strength adjustment
(pH = 3.2; ionic strength = 10−2 mol l−1), and the attained results agreed well with the used reference method (relative
error < 4%). The above experimental conditions promoted a significant increase in sensitivity of the potentiometric response,
with a supra-Nernstian slope of 80.2mV per decade, and allowed the analysis of about 90 samples per hour, with a relative
standard deviation <1.0%
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