14,400 research outputs found
Wigner's Spins, Feynman's Partons, and Their Common Ground
The connection between spin and symmetry was established by Wigner in his
1939 paper on the Poincar\'e group. For a massive particle at rest, the little
group is O(3) from which the concept of spin emerges. The little group for a
massless particle is isomorphic to the two-dimensional Euclidean group with one
rotational and two translational degrees of freedom. The rotational degree
corresponds to the helicity, and the translational degrees to the gauge degree
of freedom. The question then is whether these two different symmetries can be
united. Another hard-pressing problem is Feynman's parton picture which is
valid only for hadrons moving with speed close to that of light. While the
hadron at rest is believed to be a bound state of quarks, the question arises
whether the parton picture is a Lorentz-boosted bound state of quarks. We study
these problems within Einstein's framework in which the energy-momentum
relations for slow particles and fast particles are two different
manifestations one covariant entity.Comment: LaTex 12 pages, 3 figs, based on the lectures delivered at the
Advanced Study Institute on Symmetries and Spin (Prague, Czech Republic, July
2001
CVD of solid oxides in porous substrates for ceramic membrane modification
The deposition of yttria-doped zirconia has been experimented systematically in various types of porous ceramic substrates by a modified chemical vapor deposition (CVD) process operating in an opposing reactant geometry using water vapor and corresponding metal chloride vapors as reactants. The effects of substrate pore dimension and structure, bulk-phase reactant concentration, reactant diffusivity in substrate pores and deposition temperature are experimentally studied and explained qualitatively by a theoretical modeling analysis. The experimental and theoretical results suggest a reaction mechanism which depends on water vapor and chloride vapor concentrations. Consequently, the diffusivity, bulk-phase reactant concentration, and substrate pore dimension are important in the CVD process. Effects of deposition temperature on the deposition results and narrow deposition zone compared to the substrate thickness also suggest a Langmuir-Hinshelwood reaction mechanism involved in the CVD process with a very fast CVD reaction rate. Gas permeation data indicate that whether deposition of solid in substrate pores could result in the pore-size reduction depends strongly on the initial pore-size distribution of the substrate
Wave Mechanics of Two Hard Core Quantum Particles in 1-D Box
The wave mechanics of two impenetrable hard core particles in 1-D box is
analyzed. Each particle in the box behaves like an independent entity
represented by a {\it macro-orbital} (a kind of pair waveform). While the
expectation value of their interaction, ,
satisfies (or , with being the size
of the box). The particles in their ground state define a close-packed
arrangement of their wave packets (with , phase position
separation and momentum ) and experience a
mutual repulsive force ({\it zero point repulsion}) which
also tries to expand the box. While the relative dynamics of two particles in
their excited states represents usual collisional motion, the same in their
ground state becomes collisionless. These results have great significance in
determining the correct microscopic understanding of widely different many body
systems.Comment: 12 pages, no figur
Experimental studies on pore size change of porous ceramic membranes after modification
Experimental results on pore size change of a microfiltration (MF) -alumina membrane and an ultrafiltration (UF) Îł-alumina membrane after modification by chemical vapor deposition (CVD) of solid oxides in the membrane pores are presented and explained using the results of a theoretical analysis. With an approx. 10-fold reduction in permeability, the average pore size of the MF membrane is found to increase after CVD modification, due to its relatively broader initial pore size distribution with a small amount of large pores and due to the particular CVD conditions (heterogeneous deposition mechanism) which give a pore narrowing rate independent of pore size. The effective pore size of the UF membrane appears to remain unchanged after modification with an approx. 50-fold reduction in permeability, as a result of the slit-shaped pores of the Îł-alumina film and the particular modification conditions. The experimental and theoretical results suggest that, in order to reduce effectively the average pore size of a membrane by a modification process, the membrane should have a rather uniform pore size distribution, or the modification process should be conducted under conditions which give a pore narrowing rate proportional to the pore size
Topological Excitation in Skyrme Theory
Based on the -mapping topological current theory and the decomposition
of gauge potential theory, we investigate knotted vortex lines and monopoles in
Skyrme theory and simply discuss the branch processes (splitting, merging and
intersection) during the evolution of the monopoles.Comment: 10 pages, 0 figure
Iron loss in permanent-magnet brushless AC machines under maximum torque per ampere and flux weakening control
The airgap flux density distribution, flux density loci in the stator core, and the associated iron loss in two topologies of brushless AC motor, having a surface-mounted magnet rotor and an interior-mounted magnet rotor, respectively, are investigated when operated under maximum torque per ampere control in the constant torque mode and maximum power control in the flux-weakening mode. It is shown that whilst the interior magnet topology is known to be eminently suitable for flux-weakening operation, due to its high demagnetization withstand capability, its iron loss can be significantly higher than for a surface-mounted magnet machine
Generation of electric fields and currents by neutral flows in weakly ionized plasmas through collisional dynamos
In weakly ionized plasmas neutral flows drag plasma across magnetic field lines generating intense electric fields and currents. An example occurs in the Earth's ionosphere near the geomagnetic equator. Similar processes take place in the Solar chromosphere and magnetohydrodynamic generators. This paper argues that not all convective neutral flows generate electric fields and currents and it introduces the corresponding universal criterion for their formation, âĂ(UĂB)â âB/ât, where U is the neutral flow velocity, B is the magnetic field, and t is time. This criterion does not depend on the conductivity tensor, ÏË. For many systems, the displacement current, âB/ât, is negligible making the criterion even simpler. This theory also shows that the neutral-dynamo driver that generates E-fields and currents plays the same role as the DC electric current plays for the generation of the magnetic field in the Biot-Savart law.This work was supported by NSF/DOE Grant No. PHY-1500439, NASA Grant Nos. NNX11A096G and NNX14AI13G, and NSF-AGS Postdoctoral Research Fellowship Award No. 1433536. (PHY-1500439 - NSF/DOE; NNX11A096G - NASA; NNX14AI13G - NASA; 1433536 - NSF-AGS
Recommended from our members
A Knowledge-based Decision Support System for RP&M Process Selection
Due to the large variety of RP&M material/machines and the. strengths/weaknesses
associated with different RP&M processes, the decision·to select a suitableRP&M system
becomes increasingly difficult. This paper presents a knowledge-based approach for the selection
ofsuitable RP&M material/machine to meet specific. requirements ofRP&Mapplications. The
system receives input data on the CAD modelandthe user's specifications,andgenerates outputs
that provide the most appropriate combination ofRP&Mmaterial/machine.·Optimal orientations,
together with estimated manufacturing time and cost, are considered and given in the final
outcome to help the user make the choice.Mechanical Engineerin
From StoCharts to MoDeST: a comparative reliability analysis of train radio communications
StoCharts have been proposed as a UML statechart extension for performance and dependability evaluation, and have been applied in the context of train radio reliability assessment to show the principal tractability of realistic cases with this approach. In this paper, we extend on this bare feasibility result in two important directions. First, we sketch the cornerstones of a mechanizable translation of StoCharts to MoDeST. The latter is a process algebra-based formalism supported by the Motor/Möbius tool tandem. Second, we exploit this translation for a detailed analysis of the train radio case study
- âŠ