15,542 research outputs found
Compaction and mobility in randomly agitated granular assemblies
We study the compaction and mobility properties of a dense granular material
under weak random vibration. By putting in direct contact millimetric glass
beads with piezoelectric transducers we manage to inject energy to the system
in a disordered manner with accelerations much smaller than gravity, resulting
in a slow compaction dynamics and no convection. We characterize the mobility
inside the medium by pulling through it an intruder grain at constant velocity.
We present an extensive study of the relation between drag force and velocity
for different vibration conditions and sizes of the intruder.Comment: 4 pages, 6 figures, to appear in the proceedings of Powders and
Grains 200
Rheology of a sonofluidized granular packing
We report experimental measurements on the rheology of a dry granular
material under a weak level of vibration generated by sound injection. First,
we measure the drag force exerted on a wire moving in the bulk. We show that
when the driving vibration energy is increased, the effective rheology changes
drastically: going from a non-linear dynamical friction behavior - weakly
increasing with the velocity- up to a linear force-velocity regime. We present
a simple heuristic model to account for the vanishing of the stress dynamical
threshold at a finite vibration intensity and the onset of a linear
force-velocity behavior. Second, we measure the drag force on spherical
intruders when the dragging velocity, the vibration energy, and the diameters
are varied. We evidence a so-called ''geometrical hardening'' effect for
smaller size intruders and a logarithmic hardening effect for the velocity
dependence. We show that this last effect is only weakly dependent on the
vibration intensity.Comment: Accepted to be published in EPJE. v3: Includes changes suggested by
referee
On the void explanation of the Cold Spot
The integrated Sachs-Wolfe (ISW) contribution induced on the cosmic microwave
background by the presence of a supervoid as the one detected by Szapudi et al.
(2015) is reviewed in this letter in order to check whether it could explain
the Cold Spot (CS) anomaly. Two different models, previously used for the same
purpose, are considered to describe the matter density profile of the void: a
top hat function and a compensated profile produced by a Gaussian potential.
The analysis shows that, even enabling ellipticity changes or different values
for the dark-energy equation of state parameter , the ISW contribution
due to the presence of the void does not reproduce the properties of the CS.
Finally, the probability of alignment between the void and the CS is also
questioned as an argument in favor of a physical connection between these two
phenomena
Spin dependent Momentum Distributions in Deformed Nuclei
We study the properties of the spin dependent one body density in momentum
space for odd--A polarized deformed nuclei within the mean field approximation.
We derive analytic expressions connecting intrinsic and laboratory momentum
distributions. The latter are related to observable transition densities in
{\bf p}--space that can be probed in one nucleon knock--out reactions from
polarized targets. It is shown that most of the information contained in the
intrinsic spin dependent momentum distribution is lost when the nucleus is not
polarized. Results are presented and discussed for two prolate nuclei,
Ne and Mg, and for one oblate nucleus, Ar. The effects of
deformation are highlighted by comparison to the case of odd--A nuclei in the
spherical model.Comment: Latex 2.09. 25 pages and 6 figures (available from
[email protected]), to appear in Ann. of Phy
Low-resolution spectroscopy and spectral energy distributions of selected sources towards sigma Orionis
Aims: We investigated in detail nine sources in the direction of the young
sigma Orionis cluster, which is considered a unique site for studying stellar
and substellar formation. The nine sources were selected because of some
peculiar properties, such as extremely red infrared colours or too strong
Halpha emission for their blue optical colours. Methods: We took high-quality,
low-resolution spectroscopy (R ~ 500) of the nine targets with ALFOSC at the
Nordic Optical Telescope. We also re-analyzed [24]-band photometry from
MIPS/Spitzer and compiled the best photometry available at the ViJHKs passbands
and the four IRAC/Spitzer channels for constructing accurate spectral energy
distributions covering from 0.55 to 24 mum. Results: The nine targets were
classified into: one Herbig Ae/Be star with a scatterer edge-on disc, two
G-type stars, one X-ray flaring, early-M, young star with chromospheric Halpha
emission, one very low-mass, accreting, young spectroscopic binary, two young
objects at the brown dwarf boundary with the characteristics of classical T
Tauri stars, and two emission-line galaxies, one undergoing star formation, and
another one whose spectral energy distribution is dominated by an active
galactic nucleus. Besides, we discover three infrared sources associated to
overdensities in a cold cloud in the cluster centre. Conclusions:
Low-resolution spectroscopy and spectral energy distributions are a vital tool
for measuring the physical properties and the evolution of young stars and
candidates in the sigma Orionis cluster.Comment: Accepted for publication in A&
Upper-division Student Understanding of Coulomb's Law: Difficulties with Continuous Charge Distributions
Utilizing the integral expression of Coulomb's Law to determine the electric
potential from a continuous charge distribution is a canonical exercise in
Electricity and Magnetism (E&M). In this study, we use both think-aloud
interviews and responses to traditional exam questions to investigate student
difficulties with this topic at the upper-division level. Leveraging a
theoretical framework for the use of mathematics in physics, we discuss how
students activate, construct, execute and reflect on the integral form of
Coulomb's Law when solving problems with continuous charge distributions. We
present evidence that junior-level E&M students have difficulty mapping
physical systems onto the mathematical expression for the Coulomb potential.
Common challenges include difficulty expressing the difference vector in
appropriate coordinates as well as determining expressions for the differential
charge element and limits of integration for a specific charge distribution. We
discuss possible implications of these findings for future research directions
and instructional strategies.Comment: 5 pages, 1 figure, 2 tables, accepted to 2012 PERC Proceeding
Superscaling and neutral current quasielastic neutrino-nucleus scattering
The superscaling approach is applied to studies of neutral current neutrino
reactions in the quasielastic regime. Using input from scaling analyses of
electron scattering data, predictions for high-energy neutrino and antineutrino
cross sections are given and compared with results obtained using the
relativistic Fermi gas model. The influence of strangeness content inside the
nucleons in the nucleus is also explored.Comment: 28 pages, 8 figures, accepted for publication in Phys.Rev.
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