2,059 research outputs found
The mechanical waves conceptual survey: An analysis of university students' performance, and recommendations for instruction
Indexación: Web of Science; Scopus.The Mechanical Waves Conceptual Survey (MWCS), presented in 2009, is the most important test to date that has been designed to evaluate university students' understanding of four main topics: propagation, superposition, reflection, and standing waves. In a literature review, we detected a significant need for a study that uses this test as an assessment tool and presents a complete analysis of students' difficulties on the test. This article addresses this need. We administered the MWCS at a private university in Mexico to 541 students. In this article, we present a complete description of these students' performance on the test, a description of their main difficulties, an elaboration of these main difficulties in terms of students' inappropriate conceptions, and recommendations for instruction based on the results obtained by the test. Our analyses may be used by instructors and researchers who intend to use the MWCS or create new instructional material.http://www.iserjournals.com/journals/eurasia/articles/10.12973/eurasia.2017.00651
S-Branes, Negative Tension Branes and Cosmology
A general class of solutions of string background equations is studied and
its physical interpretations are presented. These solutions correspond to
generalizations of the standard black p-brane solutions to surfaces with
curvature k=-1,0. The relation with the recently introduced S-branes is
provided. The mass, charge, entropy and Hawking temperature are computed,
illustrating the interpretation in terms of negative tension branes. Their
cosmological interpretation is discussed as well as their potential instability
under small perturbations.Comment: 10 pages. Talks given at: SUSY'02, ``the 10th International
Conference on Supersymmetry and Unification of Fundamental Interactions'',
DESY, Hamburg, Germany, 17-23 June 2002, and ``The 1st International
Conference on String Phenomenology'', Oxford, July 6 - 11, 2002. References
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Time Variable Faraday Rotation Measures of 3C-273 and 3C-279
Multifrequency polarimetry with the VLBA confirms the previously reported
time-varying Faraday rotation measure (RM) in the quasar 3C-279. Variability in
the RM and electric vector position angle (EVPA) of the jet component (C4) is
seen making it an unreliable absolute EVPA calibrator. 3C-273 is also shown to
vary its RM structure on 1.5 year time-scales. Variation in the RM properties
of quasars may result from a Faraday screen which changes on time-scales of a
few years, or from the motion of jet components which sample spatial variations
in the screen. A new component emerging from the core of 3C-279 appears to be
starting to sample such a spatial variation. Future monitoring of this
component and its RM properties is suggested as a diagnostic of the narrow line
region in 3C-279. We also present a new method of EVPA calibration using the
VLA Monitoring Program.Comment: Accepted to ApJ Letters. 12 pages, 5 figure
Addressing neuroticism in psychological treatment
Neuroticism has long been associated with psychopathology and there is increasing evidence that this trait represents a shared vulnerability responsible for the development and maintenance of a range of common mental disorders. Given that neuroticism may be more malleable than previously thought, targeting this trait in treatment, rather than its specific manifestations (e.g., anxiety, mood, and personality disorders), may represent a more efficient and cost-effective approach to psychological treatment. The goals of the current manuscript are to (a) review the role of neuroticism in the development of common mental disorders, (b) describe the evidence of its malleability, and (c) review interventions that have been explicitly developed to target this trait in treatment. Implications for shifting the focus of psychological treatment to underlying vulnerabilities, such as neuroticism, rather than on the manifest symptoms of mental health conditions, are also discussed.First author draf
Effect of Plasma Composition on the Interpretation of Faraday Rotation
Faraday rotation (FR) is widely used to infer the orientation and strength of
magnetic fields in astrophysical plasmas. Although the absence of
electron-positron pairs is a plausible assumption in many astrophysical
environments, the magnetospheres of pulsars and black holes and their
associated jets may involve a significant pair plasma fraction. This motivates
being mindful of the effect of positrons on FR. Here we derive and interpret
exact expressions of FR for a neutral plasma of arbitrary composition. We focus
on electron-ion-positron plasmas in which charge neutrality is maintained by an
arbitrary combination of ions and positrons. Because a pure electron-positron
plasma has zero FR, the greater the fraction of positrons the higher the field
strength required to account for the same FR. We first obtain general formulae
and then specifically consider parameters relevant to active galctic nuclei
(AGN) jets to illustrate the significant differences in field strengths that FR
measurements from radio frequency measurements. Complementarily, using galaxy
cluster core plasmas as examples, we discuss how plasma composition can be
constrained if independent measurements of the field strength and number
density are available and combined with FR.Comment: Submitted to MNRA
Modulated Reheating and Large Non-Gaussianity in String Cosmology
A generic feature of the known string inflationary models is that the same
physics that makes the inflaton lighter than the Hubble scale during inflation
often also makes other scalars this light. These scalars can acquire
isocurvature fluctuations during inflation, and given that their VEVs determine
the mass spectrum and the coupling constants of the effective low-energy field
theory, these fluctuations give rise to couplings and masses that are modulated
from one Hubble patch to another. These seem just what is required to obtain
primordial adiabatic fluctuations through conversion into density perturbations
through the `modulation mechanism', wherein reheating takes place with
different efficiency in different regions of our Universe. Fluctuations
generated in this way can generically produce non-gaussianity larger than
obtained in single-field slow-roll inflation; potentially observable in the
near future. We provide here the first explicit example of the modulation
mechanism at work in string cosmology, within the framework of LARGE Volume
Type-IIB string flux compactifications. The inflationary dynamics involves two
light Kaehler moduli: a fibre divisor plays the role of the inflaton whose
decay rate to visible sector degrees of freedom is modulated by the primordial
fluctuations of a blow-up mode (which is made light by the use of
poly-instanton corrections). We find the challenges of embedding the mechanism
into a concrete UV completion constrains the properties of the non-gaussianity
that is found, since for generic values of the underlying parameters, the model
predicts a local bi-spectrum with fNL of order `a few'. However, a moderate
tuning of the parameters gives also rise to explicit examples with fNL O(20)
potentially observable by the Planck satellite.Comment: 42 pages, 2 figure
Faraday rotation and polarization gradients in the jet of 3C~120: Interaction with the external medium and a helical magnetic field?
We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA
observations of the radio galaxy 3C 120 revealing a systematic presence of
gradients in Faraday rotation and degree of polarization across and along the
jet. The degree of polarization increases with distance from the core and
toward the jet edges, and has an asymmetric profile in which the northern side
of the jet is more highly polarized. The Faraday rotation measure is also
stratified across the jet width, with larger values for the southern side. We
find a localized region of high Faraday rotation measure superposed on this
structure between approximately 3 and 4 mas from the core, with a peak of about
6000 rad/m^2. Interaction of the jet with the external medium or a cloud would
explain the confined region of enhanced Faraday rotation, as well as the
stratification in degree of polarization and the flaring of superluminal knots
when crossing this region. The data are also consistent with a helical field in
a two-fluid jet model, consisting of an inner, emitting jet and a sheath
containing nonrelativistic electrons. However, this helical magnetic field
model cannot by itself explain the localized region of enhanced Faraday
rotation. The polarization electric vectors, predominantly perpendicular to the
jet axis once corrected for Faraday rotation, require a dominant component
parallel to the jet axis (in the frame of the emitting plasma) for the magnetic
field in the emitting region.Comment: Accepted for publication in ApJ Letters. 4 pages (including 5
figures
Swift development of protective effector functions in naive CD8(+) T cells against malaria liver stages.
We generated T cell receptor transgenic mice specific for the liver stages of the rodent malaria parasite Plasmodium yoelii and studied the early events in the development of in vivo effector functions in antigen-specific CD8(+) T cells. Differently to activated/memory cells, naive CD8(+) T cells are not capable of exerting antiparasitic activity unless previously primed by parasite immunization. While naive cells need to differentiate before achieving effector status, the time required for this process is very short. Indeed, interferon (IFN)-gamma and perforin mRNA are detectable 24 h after immunization and IFN-gamma secretion and cytotoxic activity are detected ex vivo 24 and 48 h after immunization, respectively. In contrast, the proliferation of CD8(+) T cells begins after 24 h and an increase in the total number of antigen-specific cells is detected only after 48 h. Remarkably, a strong CD8(+) T cell-mediated inhibition of parasite development is observed in mice challenged with viable parasites only 24 h after immunization with attenuated parasites. These results indicate that differentiation of naive CD8(+) T cells does not begin only after extensive cell division, rather this process precedes or occurs simultaneously with proliferation
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