1,995 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
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
Test of Understanding Graphs in Calculus: Test of Students’ Interpretation of Calculus Graphs
Indexación: Scopus.Studies show that students, within the context of mathematics and science, have difficulties understanding the concepts of the derivative as the slope and the concept of the antiderivative as the area under the curve. In this article, we present the Test of Understanding Graphs in Calculus (TUG-C), an assessment tool that will help to evaluate students' understanding of these two concepts by a graphical representation. Data from 144 students of introductory courses of physics and mathematics at a university was collected and analyzed. To evaluate the reliability and discriminatory power of this test, we used statistical techniques for individual items and the test as a whole, and proved that the test's results are satisfactory within the standard requirements. We present the design process in this paper and the test in the appendix. We discuss the findings of our research, students' understanding of the relations between these two concepts, using this new multiple-choice test. Finally, we outline specific recommendations. The analysis and recommendations can be used by mathematics or science education researchers, and by teachers that teach these concepts. © Authors.http://www.ejmste.com/Test-of-Understanding-Graphs-in-Calculus-Test-of-Students-Interpretation-of-Calculus,78085,0,2.htm
Rotation-Measures across Parsec-scale Jets of FRI radio galaxies
We present the results of a parsec-scale polarization study of three FRI
radio galaxies - 3C66B, 3C78 and 3C264 - obtained with the Very Long Baseline
Array at 5, 8 and 15 GHz. Parsec-scale polarization has been detected in a
large number of beamed radio-loud active galactic nuclei, but in only a handful
of the relatively unbeamed radio galaxies. We report here the detection of
parsec-scale polarization at one or more frequencies in all three FRI galaxies
studied. We detect Faraday rotation measures of the order of a few hundred
rad/m^2 in the nuclear jet regions of 3C78 and 3C264. In 3C66B polarization was
detected at 8 GHz only. A transverse rotation measure gradient is observed
across the jet of 3C78. The inner-jet magnetic field, corrected for Faraday
rotation, is found to be aligned along the jet in both 3C78 and 3C264, although
the field becomes orthogonal further from the core in 3C78. The RM values in
3C78 and 3C264 are similar to those previously observed in nearby radio
galaxies. The transverse RM gradient in 3C78, the increase in the degree of
polarization at the jet edge, the large rotation in the polarization angles due
to Faraday rotation and the low depolarization between frequencies, suggests
that a layer surrounding the jet with a sufficient number of thermal electrons
and threaded by a toroidal or helical magnetic field is a good candidate for
the Faraday rotating medium. This suggestion is tentatively supported by Hubble
Space Telescope optical polarimetry but needs to be examined in a greater
number of sources.Comment: Accepted for publication in The Astrophysical Journal, March 2009 -
20 v694 issu
Quantification By Gas Chromatography of the Content of Amino Acids Present in Sausages Fortified with Quinoa Vegetable Protein
The objective of this research was to determine the quality of the protein present in sausages fortified with quinoa as a substitute for animal protein, through the identification and quantification of amino acids, using gas chromatography and precolumn derivatization. The amino acid composition found in the analyzed products was predominantly composed of: Threonine (THR) with a concentration of 1046.32µmol / L, aminobutyric acid (ABA) with a concentration of 9685.68 µmol / L and glutamic acid (GLU) with a concentration of 1178.71 µmol / L. These values were found in the treatment with the highest percentage of quinoa flour, establishing a directly proportional relationship between the concentrations of these amino acids and the percentage of quinoa. Gas chromatography was an adequate technique for determining the amino acid profile due to its speed and sensitivity.
Keywords: amino acids, sausages, quinoa, derivatization, gas chromatography.
RESUMEN
La presente investigación tiene por objetivo determinar la calidad de la proteína presente en embutidos fortificados con quinua como sustituyente de la proteína animal, a través de la identificación y cuantificación de aminoácidos mediante la aplicación de cromatografía de gases y la derivatización precolumna. La composición de aminoácidos encontrada en los productos analizados destaca la presencia mayoritaria de: Treonina (THR) con una concentración de 1046,32 µmol/L, ácido aminobutírico (ABA) con una concentración de 9685,68 µmol/L y ácido glutámico (GLU) con una concentración de 1178,71 µmol/L, todos estos valores se presentaron en el tratamiento con mayor porcentaje de harina de quinua estableciéndose una relación directamente proporcional entre las concentraciones de estos aminoácidos y el porcentaje de adición de quinua en los tratamientos estudiados. Se puede concluir que la cromatografía de gases empleada resultó una técnica adecuada para la determinación del perfil aminoacídico por la rapidez y sensibilidad presentada sobre las muestras estudiadas.
Palabras claves: aminoácidos, embutidos, quinua, derivatización, cromatografía de gases.
 
Three dimensional magnetic field structure of six parsec-scale active galactic nuclei jets
The parsec-scale Faraday rotation measure (RM) distribution of six "blazars"
is investigated using multi-frequency (4.6--43 GHz) polarization observations
taken on 2006 July 2 with the VLBA. Analysis of the RM provides the direction
of the line-of-sight (LoS) magnetic field component, as well as the intrinsic
2-D polarization distribution on the plane of the sky. Our results show that
the magnitude of the core RM increases systematically with frequency, and is
well described by a power-law, where |RM_{core}| \propto \nu^a. Our measured
values of vary from 0.9 to 3.8, providing information on the assumed
power-law fall-off in the electron density with distance from the central
engine for each source. RM gradients were detected across the jets of three
sources, supporting the presence of helical magnetic fields in a sheath or
boundary layer surrounding their jets. We find a bi-modal distribution of the
intrinsic jet polarization orientation; either aligned or orthogonal to the jet
direction. A helical magnetic field geometry can neatly explain both the
bi-model distribution of the jet polarization orientation and the ordered
polarization structure detected on these scales. In half the sources, we find
that the core RM changes sign with distance from the central engine. We provide
an explanation for this by considering a boundary layer of Faraday rotating
material threaded by a helical magnetic field, where bends in the relativistic
jet or accelerating/decelerating flows give rise to changes in the dominant LoS
components of the magnetic field, which in turn gives rise to different signs
of the RM. (abridged)Comment: 29 pages, 26 figures, accepted for publication in MNRAS, v2 -> proof
corrections: references update
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