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Electromotive Force (electromotive + force)
Selected AbstractsChemInform Abstract: Thermodynamic Studies on Chromium Carbides by the Electromotive Force (emf) MethodCHEMINFORM, Issue 34 2001Heiko Kleykamp Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Electrical penetration graphs of the nymphal stage of Bemisia argentifoliiENTOMOLOGIA EXPERIMENTALIS ET APPLICATA, Issue 2 2003Y.X. Jiang Abstract Electrical penetration graph (EPG, DC system) waveforms were recorded from first, second, and third instar Bemisia argentifolii nymphs. Waveforms recorded were similar among the three instars. Four waveforms were recorded and were named C, J, L, and H. Waveform J is new, whereas waveforms C, L, and H of B. argentifolii nymphs were similar to those published previously from greenhouse whitefly nymphs. As in the previous study on greenhouse whitefly nymphs, there was variation in each of waveforms C, L, and H. Waveform C was recorded at an extracellular voltage level, and represents a pathway phase where the stylets penetrate the plant tissue in an intercellular pathway. At the end of waveform C, the voltage dropped to an intracellular level, indicating penetration of a living cell, and the stylet tips then remained in that cell for the rest of the EPG recording, which was sometimes as long as 16 h. Three waveforms (J, L, and H) were recorded during this intracellular phase, beginning with J, a brief (average = 31 s), low amplitude, irregular waveform. J appeared only at the beginning of the intracellular phase, and was followed by either L (five out of eight times) or H (three out of eight times). Waveforms L and H then alternated with one another for the remainder of the intracellular phase. The most conspicuous difference between L and H was the frequency of their voltage fluctuations; L had a lower frequency and H a higher frequency. Usually the shape of waveform L was dominated by voltage peaks in a positive direction, while waveform H was characterized by strong voltage peaks in a negative direction; although some variants of both L and H had distinct voltage peaks in both directions. The electrical origin of both the positive and negative voltage peaks was electromotive force (emf) fluctuation rather than resistance fluctuation. During waveform H, copious amounts of honeydew were produced, indicating that the penetrated cell was a sieve element. We conclude, therefore, that H represents phloem sap ingestion; and because J and L are produced in the same cell as H, then phloem phase is represented by waveforms J, L, and H. The biological correlations for J and L are not yet known. [source] Non-ideal evolution of non-axisymmetric, force-free magnetic fields in a magnetarMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2008A. Mastrano ABSTRACT Recent numerical magnetohydrodynamic calculations by Braithwaite and collaborators support the ,fossil field' hypothesis regarding the origin of magnetic fields in compact stars and suggest that the resistive evolution of the fossil field can explain the reorganization and decay of magnetar magnetic fields. Here, these findings are modelled analytically by allowing the stellar magnetic field to relax through a quasi-static sequence of non-axisymmetric, force-free states, by analogy with spheromak relaxation experiments, starting from a random field. Under the hypothesis that the force-free modes approach energy equipartition in the absence of resistivity, the output of the numerical calculations is semiquantitatively recovered: the field settles down to a linked poloidal,toroidal configuration, which inflates and becomes more toroidal as time passes. A qualitatively similar (but not identical) end state is reached if the magnetic field evolves by exchanging helicity between small and large scales according to an ,-dynamo-like, mean-field mechanism, arising from the fluctuating electromotive force produced by the initial random field. The impossibility of matching a force-free internal field to a potential exterior field is discussed in the magnetar context. [source] Analysis of the effect of a mean velocity field on a mean field dynamoMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007Alejandra Kandus ABSTRACT We study semi-analytically and in a consistent manner the generation of a mean velocity field by helical magnetohydrodynamical (MHD) turbulence, and the effect that this field can have on a mean field dynamo. Assuming a prescribed, maximally helical small-scale velocity field, we show that large-scale flows can be generated in MHD turbulent flows via small-scale Lorentz force. These flows back-react on the mean electromotive force of a mean field dynamo through new terms, leaving the original , and , terms explicitly unmodified. Cross-helicity plays the key role in interconnecting all the effects. In the minimal , closure that we chose to work with, the effects are stronger for large relaxation times. [source] A self-consistent treatment of the electromotive force in magnetohydrodynamics for large diffusivitiesASTRONOMISCHE NACHRICHTEN, Issue 7 2010A. Courvoisier Abstract The coupled equations that describe the effect of large-scale magnetic and velocity fields on forced high-diffusivity magnetohydrodynamic flows are investigated through an extension of mean field electrodynamics. Our results generalise those of Rädler & Brandenburg (2010), who consider a similar situation but assume that the effect of the Lorentz force on the momentum equation can be neglected. New mean coupling terms are shown to appear, which can lead to large-scale growth of magnetic and velocity fields even when the usual a-effects are absent (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Mean electromotive force proportional to mean flow in MILD turbulenceASTRONOMISCHE NACHRICHTEN, Issue 1 2010K.-H. Rädler Abstract In mean-field magnetohydrodynamics the mean electromotive force due to velocity and magnetic-field fluctuations plays a crucial role. In general it consists of two parts, one independent of and another one proportional to the mean magnetic field. The first part may be nonzero only in the presence of mhd turbulence, maintained, e.g., by small-scale dynamo action. It corresponds to a battery, which lets a mean magnetic field grow from zero to a finite value. The second part, which covers, e.g., the , effect, is important for large-scale dynamos. Only a few examples of the aforementioned first part of the mean electromotive force have been discussed so far. It is shown that a mean electromotive force proportional to the mean fluid velocity, but independent of the mean magnetic field, may occur in an originally homogeneous isotropic mhd turbulence if there are nonzero correlations of velocity and electric current fluctuations or, what is equivalent, of vorticity and magnetic field fluctuations. This goes beyond the Yoshizawa effect, which consists in the occurrence of mean electromotive forces proportional to the mean vorticity or to the angular velocity defining the Coriolis force in a rotating frame and depends on the cross-helicity defined by the velocity and magnetic field fluctuations. Contributions to the mean electromotive force due to inhomogeneity of the turbulence are also considered. Possible consequences of the above findings for the generation of magnetic fields in cosmic bodies are discussed (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Do mean-field dynamos in nonrotating turbulent shear-flows exist?ASTRONOMISCHE NACHRICHTEN, Issue 4 2006G. Rüdiger Abstract A plane-shear flow in a fluid with forced turbulence is considered. If the fluid is electrically-conducting then a mean electromotive force (EMF) results even without basic rotation and the magnetic diffusivity becomes a highly anisotropic tensor. It is checked whether in this case self-excitation of a large-scale magnetic field is possible (so-called W, × J, -dynamo) and the answer is NO. The calculations reveal the cross-stream components of the EMF perpendicular to the mean current having the wrong signs, at least for small magnetic Prandtl numbers. After our results numerical simulations with magnetic Prandtl number of about unity have only a restricted meaning as the Prandtl number dependence of the diffusivity tensor is rather strong. If, on the other hand, the turbulence field is strati.ed in the vertical direction then a dynamo-active , -effect is produced. The critical magnetic Reynolds number for such a self-excitation in a simple shear flow is slightly above 10 like for the other , but much more complicated , flow patterns used in existing dynamo experiments with liquid sodium or gallium. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||