7,396 research outputs found
Understanding the tsunami with a simple model
In this paper, we use the approximation of shallow water waves (Margaritondo
G 2005 Eur. J. Phys. 26 401) to understand the behaviour of a tsunami in a
variable depth. We deduce the shallow water wave equation and the continuity
equation that must be satisfied when a wave encounters a discontinuity in the
sea depth. A short explanation about how the tsunami hit the west coast of
India is given based on the refraction phenomenon. Our procedure also includes
a simple numerical calculation suitable for undergraduate students in physics
and engineering
Discrimination of soil phases by dual energy x-ray tomography
Numerous soil ecological functions are influenced by soil structure through its impact on spatial and temporal distributions of soil particles, water, and air within the soil profile. The nondestructive technique of x‐ray computed tomography (CT) was used for studying soil structure. X‐ray attenuation determined for two energy levels (80 kV and 120 kV) was used to calculate distributions of water, air, and solids, as well as the voxel dry bulk density for two silt loam subsoils. The spatial resolution during scanning was 0.25 mm in the horizontal and 1 mm in the vertical direction. For different voxel sizes, the weighted mean of the derived volumetric water, air, and solid contents, and the dry bulk densities agreed with the sample's phase composition and dry bulk density obtained by weighing. The use of dual energy scanning to study the heterogeneity of soil structure and the spatial distribution of water, air, and solids is discussed
ELKO Spinor Fields: Lagrangians for Gravity derived from Supergravity
Dual-helicity eigenspinors of the charge conjugation operator (ELKO spinor
fields) belong -- together with Majorana spinor fields -- to a wider class of
spinor fields, the so-called flagpole spinor fields, corresponding to the
class-(5), according to Lounesto spinor field classification based on the
relations and values taken by their associated bilinear covariants. There
exists only six such disjoint classes: the first three corresponding to Dirac
spinor fields, and the other three respectively corresponding to flagpole,
flag-dipole and Weyl spinor fields. Using the mapping from ELKO spinor fields
to the three classes Dirac spinor fields, it is shown that the
Einstein-Hilbert, the Einstein-Palatini, and the Holst actions can be derived
from the Quadratic Spinor Lagrangian (QSL), as the prime Lagrangian for
supergravity. The Holst action is related to the Ashtekar's quantum gravity
formulation. To each one of these classes, there corresponds a unique kind of
action for a covariant gravity theory. Furthermore we consider the necessary
and sufficient conditions to map Dirac spinor fields (DSFs) to ELKO, in order
to naturally extend the Standard Model to spinor fields possessing mass
dimension one. As ELKO is a prime candidate to describe dark matter and can be
obtained from the DSFs, via a mapping explicitly constructed that does not
preserve spinor field classes, we prove that in particular the
Einstein-Hilbert, Einstein-Palatini, and Holst actions can be derived from the
QSL, as a fundamental Lagrangian for supergravity, via ELKO spinor fields. The
geometric meaning of the mass dimension-transmuting operator - leading ELKO
Lagrangian into the Dirac Lagrangian - is also pointed out, together with its
relationship to the instanton Hopf fibration.Comment: 11 pages, RevTeX, accepted for publication in
Int.J.Geom.Meth.Mod.Phys. (2009
Geometric scaling in the spectrum of an electron captured by a stationary finite dipole
We examine the energy spectrum of a charged particle in the presence of a
{\it non-rotating} finite electric dipole. For {\emph{any}} value of the dipole
moment above a certain critical value p_{\mathrm{c}}$ an infinite series of
bound states arises of which the energy eigenvalues obey an Efimov-like
geometric scaling law with an accumulation point at zero energy. These
properties are largely destroyed in a realistic situation when rotations are
included. Nevertheless, our analysis of the idealised case is of interest
because it may possibly be realised using quantum dots as artificial atoms.Comment: 5 figures; references added, outlook section reduce
Crater Morphometry and Scaling in Coarse, Rubble-Like Targets: Insights from Impact Experiments
Spacecraft images reveal that the asteroids Itokawa, Ryugu, and Bennu are covered with coarse, boulder-rich material [13]. Impactors that collide with these bodies encounter a target with extreme physical heterogeneity. Other bodies can also possess significant physical heterogeneity (e.g., megaregolith, layering, etc.). Such heterogeneities establish free surfaces and impedance contrasts that can affect shock propagation and attenuation. Therefore, such heterogeneities may also affect crater formation and excavation [4], melt generation [57] and crater scaling [4]. As described by [8,9], the extent to which target heterogeneity affects crater formation likely depends on how the length scale, d, of the heterogeneity (e.g., boulder size on a rubble-pile asteroid) compares to the width of the shock, w, generated by impact. Here we further test this hypothesis using impact experiments across a broad range of impact velocities and target grain sizes to systematically vary the ratio between the width of the shock and the diameter of target grains
A Transgenic Rat for Investigating the Anatomy and Function of Corticotrophin Releasing Factor Circuits.
Corticotrophin-releasing factor (CRF) is a 41 amino acid neuropeptide that coordinates adaptive responses to stress. CRF projections from neurons in the central nucleus of the amygdala (CeA) to the brainstem are of particular interest for their role in motivated behavior. To directly examine the anatomy and function of CRF neurons, we generated a BAC transgenic Crh-Cre rat in which bacterial Cre recombinase is expressed from the Crh promoter. Using Cre-dependent reporters, we found that Cre expressing neurons in these rats are immunoreactive for CRF and are clustered in the lateral CeA (CeL) and the oval nucleus of the BNST. We detected major projections from CeA CRF neurons to parabrachial nuclei and the locus coeruleus, dorsal and ventral BNST, and more minor projections to lateral portions of the substantia nigra, ventral tegmental area, and lateral hypothalamus. Optogenetic stimulation of CeA CRF neurons evoked GABA-ergic responses in 11% of non-CRF neurons in the medial CeA (CeM) and 44% of non-CRF neurons in the CeL. Chemogenetic stimulation of CeA CRF neurons induced Fos in a similar proportion of non-CRF CeM neurons but a smaller proportion of non-CRF CeL neurons. The CRF1 receptor antagonist R121919 reduced this Fos induction by two-thirds in these regions. These results indicate that CeL CRF neurons provide both local inhibitory GABA and excitatory CRF signals to other CeA neurons, and demonstrate the value of the Crh-Cre rat as a tool for studying circuit function and physiology of CRF neurons
Modulation of Localized States in Electroconvection
We report on the effects of temporal modulation of the driving force on a
particular class of localized states, known as worms, that have been observed
in electroconvection in nematic liquid crystals. The worms consist of the
superposition of traveling waves and have been observed to have unique, small
widths, but to vary in length. The transition from the pure conduction state to
worms occurs via a backward bifurcation. A possible explanation of the
formation of the worms has been given in terms of coupled amplitude equations.
Because the worms consist of the superposition of traveling waves, temporal
modulation of the control parameter is a useful probe of the dynamics of the
system. We observe that temporal modulation increases the average length of the
worms and stabilizes worms below the transition point in the absence of
modulation.Comment: 4 pages, 4 figure
Possible detection of two giant extrasolar planets orbiting the eclipsing polar UZ Fornacis
We present new high-speed, multi-observatory, multi-instrument photometry of
the eclipsing polar UZ For in order to measure precise mid-eclipse times with
the aim of detecting any orbital period variations. When combined with
published eclipse times and archival data spanning ~27 years, we detect
departures from a linear and quadratic trend of ~60 s. The departures are
strongly suggestive of two cyclic variations of 16(3) and 5.25(25) years. The
two favoured mechanisms to drive the periodicities are either two giant
extrasolar planets as companions to the binary (with minimum masses of
6.3(1.5)M(Jupiter) and 7.7(1.2)M(Jupiter)) or a magnetic cycle mechanism (e.g.
Applegate's mechanism) of the secondary star. Applegate's mechanism would
require the entire radiant energy output of the secondary and would therefore
seem to be the least likely of the two, barring any further refinements in the
effect of magnetic fieilds (e.g. those of Lanza et al.). The two planet model
can provide realistic solutions but it does not quite capture all of the
eclipse times measurements. A highly eccentric orbit for the outer planet would
fit the data nicely, but we find that such a solution would be unstable. It is
also possible that the periodicities are driven by some combination of both
mechanisms. Further observations of this system are encouraged.Comment: 10 pages, 4 figures, 2 table
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