157 research outputs found
Tale of two curricula: The performance of 2000 students in introductory electromagnetism
The performance of over 2000 students in introductory calculus-based electromagnetism (E&M) courses at four large research universities was measured using the Brief Electricity and Magnetism Assessment (BEMA). Two different curricula were used at these universities: a traditional E&M curriculum and the Matter & Interactions (M&I) curriculum. At each university, postinstruction BEMA test averages were significantly higher for the M&I curriculum than for the traditional curriculum. The differences in post-test averages cannot be explained by differences in variables such as preinstruction BEMA scores, grade point average, or SAT Reasoning Test (SAT) scores. BEMA performance on categories of items organized by subtopic was also compared at one of the universities; M&I averages were significantly higher in each topic. The results suggest that the M&I curriculum is more effective than the traditional curriculum at teaching E&M concepts to students, possibly because the learning progression in M&I reorganizes and augments the traditional sequence of topics, for example, by increasing early emphasis on the vector field concept and by emphasizing the effects of fields on matter at the microscopic level
Comparing large lecture mechanics curricula using the Force Concept Inventory: A five thousand student study
The performance of over 5000 students in introductory calculus-based
mechanics courses at the Georgia Institute of Technology was assessed using the
Force Concept Inventory (FCI). Results from two different curricula were
compared: a traditional mechanics curriculum and the Matter & Interactions
(M&I) curriculum. Post-instruction FCI averages were significantly higher for
the traditional curriculum than for the M&I curriculum; the differences between
curricula persist after accounting for factors such as pre-instruction FCI
scores, grade point averages, and SAT scores. FCI performance on categories of
items organized by concepts was also compared; traditional averages were
significantly higher in each concept. We examined differences in student
preparation between the curricula and found that the relative fraction of
homework and lecture topics devoted to FCI force and motion concepts correlated
with the observed performance differences. Limitations of concept inventories
as instruments for evaluating curricular reforms are discussed.Comment: 21 pages, 4 figures, submitted to Am. J. Phys. arXiv admin note:
substantial text overlap with arXiv:1112.559
Nonergodicity transitions in colloidal suspensions with attractive interactions
The colloidal gel and glass transitions are investigated using the idealized
mode coupling theory (MCT) for model systems characterized by short-range
attractive interactions. Results are presented for the adhesive hard sphere and
hard core attractive Yukawa systems. According to MCT, the former system shows
a critical glass transition concentration that increases significantly with
introduction of a weak attraction. For the latter attractive Yukawa system, MCT
predicts low temperature nonergodic states that extend to the critical and
subcritical region. Several features of the MCT nonergodicity transition in
this system agree qualitatively with experimental observations on the colloidal
gel transition, suggesting that the gel transition is caused by a low
temperature extension of the glass transition. The range of the attraction is
shown to govern the way the glass transition line traverses the phase diagram
relative to the critical point, analogous to findings for the fluid-solid
freezing transition.Comment: 11 pages, 7 figures; to be published in Phys. Rev. E (1 May 1999
Mapping photodissociation and shocks in the vicinity of Sgr A*
We have obtained maps of the molecular emission within the central five
arcminutes (12 pc) of the Galactic center (GC) in selected molecular tracers:
SiO(2-1), HNCO(5_{0,5}-4_{0,4}), and the J=1-->0 transition of H^{13}CO+,
HN^{13}C, and C^{18}O at an angular resolution of 30" (1.2 pc). The mapped
region includes the circumnuclear disk (CND) and the two surrounding giant
molecular clouds (GMCs) of the Sgr A complex, known as the 20 and 50 km s^{-1}
molecular clouds.Additionally, we simultaneously observed the J=2-1 and 3-2
transitions of SiO toward selected positions to estimate the physical
conditions of the molecular gas. The SiO(2-1) and H^{13}CO+(1-0) emission
covers the same velocity range and presents a similar distribution. In
contrast, HNCO(5-4) emission appears in a narrow velocity range mostly
concentrated in the 20 and 50 km s^{-1} GMCs. The HNCO column densities and
fractional abundances present the highest contrast, with difference factors of
60 and 28, respectively. Their highest values are found toward the cores
of the GMCs, whereas the lowest ones are measured at the CND. SiO abundances do
not follow this trend, with high values found toward the CND, as well as the
GMCs. By comparing our abundances with those of prototypical Galactic sources
we conclude that HNCO, similar to SiO, is ejected from grain mantles into
gas-phase by nondissociative C-shocks. This results in the high abundances
measured toward the CND and the GMCs. However, the strong UV radiation from the
Central cluster utterly photodissociates HNCO as we get closer to the center,
whereas SiO seems to be more resistant against UV-photons or it is produced
more efficiently by the strong shocks in the CND. Finally, we discuss the
possible connections between the molecular gas at the CND and the GMCs using
the HNCO/SiO, SiO/CS, and HNCO/CS intensity ratios as probes of distance to the
Central cluster.Comment: 26 pages plus 2 appendixes with additional figures. 17 figures in
total. Accepted for publication in A&
Tagging the world : descrying consciousness in cognitive processes
Although having conscious experiences is a fundamental feature of our everyday life,
our understanding of what consciousness is is very limited. According to one of the
main conclusions of contemporary philosophy of mind, the qualitative aspect of
consciousness seems to resist functionalisation, i.e. it cannot be adequately defined
solely in terms of functional or causal roles, which leads to an epistemic gap between
phenomenal and scientific knowledge. Phenomenal qualities, then, seem to be, in
principle, unexplainable in scientific terms. As a reaction to this pessimistic
conclusion it is a major trend in contemporary science of consciousness to turn away
from subjective experiences and re-define the subject of investigations in
neurological and behavioural terms. This move, however, creates a gap between
scientific theories of consciousness, and the original phenomenon, which we are so
intimately connected with.
The thesis focuses on this gap. It is argued that it is possible to explain features of
consciousness in scientific terms. The thesis argues for this claim from two
directions. On the one hand, a specific identity theory is formulated connecting
phenomenal qualities to certain intermediate level perceptual representations which
are unstructured for central processes of the embedding cognitive system. This
identity theory is hypothesised on the basis of certain similarities recognised between
the phenomenal and the cognitive-representational domains, and then utilised in
order to uncover further similarities between these two domains. The identity theory
and the further similarities uncovered are then deployed in formulating explanations
of the philosophically most important characteristics of the phenomenal domain—i.e.
why phenomenal qualities resist functionalisation, and why the epistemic gap occurs.
On the other hand, the thesis investigates and criticises existing models of reductive
explanation. On the basis of a detailed analysis of how successful scientific
explanations proceed a novel account of reductive explanation is proposed, which utilises so-called prior identities. Prior identities are prerequisites rather than
outcomes of reductive explanations. They themselves are unexplained but are
nevertheless necessary for mapping the features to be explained onto the features the
explanation relies on. Prior identities are hypothesised in order to foster the
formulation of explanatory claims accounting for target level phenomena in terms of
base level processes—and they are justified if they help projecting base level
explanations to new territories of the target level.
The thesis concludes that the identity theory proposed is a prior identity, and the
explanations of features of the phenomenal domain formulated with the aid of this
identity are reductive explanations proper. In this sense, the thesis introduces the
problem of phenomenal consciousness into scientific discourse, and therefore offers
a bridge between the philosophy and the science of consciousness: it offers an
approach to conscious experience which, on the one hand, tries to account for the
philosophically most important features of consciousness, whereas, on the other
hand, does it in a way which smoothly fits into the everyday practice of scientific
research
Common variation at 12q24.13 (OAS3) influences chronic lymphocytic leukemia risk
Common variation at 12q24.13 (OAS3) influences chronic lymphocytic leukemia ris
Star Formation and Dynamics in the Galactic Centre
The centre of our Galaxy is one of the most studied and yet enigmatic places
in the Universe. At a distance of about 8 kpc from our Sun, the Galactic centre
(GC) is the ideal environment to study the extreme processes that take place in
the vicinity of a supermassive black hole (SMBH). Despite the hostile
environment, several tens of early-type stars populate the central parsec of
our Galaxy. A fraction of them lie in a thin ring with mild eccentricity and
inner radius ~0.04 pc, while the S-stars, i.e. the ~30 stars closest to the
SMBH (<0.04 pc), have randomly oriented and highly eccentric orbits. The
formation of such early-type stars has been a puzzle for a long time: molecular
clouds should be tidally disrupted by the SMBH before they can fragment into
stars. We review the main scenarios proposed to explain the formation and the
dynamical evolution of the early-type stars in the GC. In particular, we
discuss the most popular in situ scenarios (accretion disc fragmentation and
molecular cloud disruption) and migration scenarios (star cluster inspiral and
Hills mechanism). We focus on the most pressing challenges that must be faced
to shed light on the process of star formation in the vicinity of a SMBH.Comment: 68 pages, 35 figures; invited review chapter, to be published in
expanded form in Haardt, F., Gorini, V., Moschella, U. and Treves, A.,
'Astrophysical Black Holes'. Lecture Notes in Physics. Springer 201
Control of Cerebellar Long-Term Potentiation by P-Rex-Family Guanine-Nucleotide Exchange Factors and Phosphoinositide 3-Kinase
Long-term potentiation (LTP) at the parallel fibre-Purkinje cell synapse in the cerebellum is a recently described and poorly characterized form of synaptic plasticity. The induction mechanism for LTP at this synapse is considered reciprocal to "classical" LTP at hippocampal CA1 pyramidal neurons: kinases promote increased trafficking of AMPA receptors into the postsynaptic density in the hippocampus, whereas phosphatases decrease internalization of AMPA receptors in the cerebellum. In the hippocampus, LTP occurs in overlapping phases, with the transition from early to late phases requiring the consolidation of initial induction processes by structural re-arrangements at the synapse. Many signalling pathways have been implicated in this process, including PI3 kinases and Rho GTPases.We hypothesized that analogous phases are present in cerebellar LTP, and took as the starting point for investigation our recent discovery that P-Rex--a Rac guanine nucleotide exchange factor which is activated by PtdIns(3,4,5)P(3)--is highly expressed in mouse cerebellar Purkinje neurons and plays a role in motor coordination. We found that LTP evoked at parallel fibre synapses by 1 Hz stimulation or by NO donors was not sustained beyond 30 min when P-Rex was eliminated or Rac inhibited, suggesting that cerebellar LTP exhibits a late phase analogous to hippocampal LTP. In contrast, inhibition of PI3 kinase activity eliminated LTP at the induction stage.Our data suggest that a PI3K/P-Rex/Rac pathway is required for late phase LTP in the mouse cerebellum, and that other PI3K targets, which remain to be discovered, control LTP induction
Surviving the hole I: Spatially resolved chemistry around Sgr A*
The interstellar region within the few central parsecs around the
super-massive black hole, Sgr A* at the very Galactic center is composed by a
number of overlapping molecular structures which are subject to one of the most
hostile physical environments in the Galaxy. We present high resolution
(4"x3"~0.16x0.11 pc) interferometric observations of CN, 13CN, H2CO, SiO,
c-C3H2 and HC3N emission at 1.3 mm towards the central ~4 pc of the Galactic
center region. Strong differences are observed in the distribution of the
different molecules. The UV resistant species CN, the only species tracing all
previously identified circumnuclear disk (CND) structures, is mostly
concentrated in optically thick clumps in the rotating filaments around Sgr A*.
H2CO emission traces a shell-like structure that we interpret as the expansion
of Sgr A East against the 50 km/s and 20 km/s giant molecular clouds (GMCs). We
derive isotopic ratios 12C/13C~15-45 across most of the CND region. The densest
molecular material, traced by SiO and HC3N, is located in the southern CND. The
observed c-C3H2/HC3N ratio observed in the region is more than an order of
magnitude lower than in Galactic PDRs. Toward the central region only CN was
detected in absorption. Apart from the known narrow line-of-sight absorptions,
a 90 km/s wide optically thick spectral feature is observed. We find evidences
of an even wider (>100 km/s) absorption feature. Around 70-75% of the gas mass,
concentrated in just the 27% densest molecular clumps, is associated with
rotating structures and show evidences of association with each of the arcs of
ionized gas in the mini-spiral structure. Chemical differentiation has been
proven to be a powerful tool to disentangle the many overlapping molecular
components in this crowded and heavily obscured region.Comment: 12 pages, 22 figures. Accepted for publication in Astronomy and
Astrophysic
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