Open educational resources and teaching in the 21st century: questions concerning authority

As a source of materials for education the Web is, to a large extent, shifting ground. Open Educational Resources (OER) provided by Higher Education Institutions constitute, at least in principle, a reliable category of Web-based resources given their association with traditional forms of expert authority. Nevertheless, OER embody different aspects of academic thinking and practice, competing, in an unlevelled field, with other sources that may provide a much more immediate appeal in that they afford quick and easy consumption of information delivered in a piecemeal, often uncritical, fashion. This paper draws upon a piece of research in the area of ‘online informal learning’ to illustrate issues arising from the availability of open content and, in particular, OER. This research suggests a number of aspects related to the impact of open content on assumed boundaries between teacher/learner, formal/informal learning, training/education, content/presentation and, crucially, in how the blurring of these boundaries may have an impact on the location of ‘value’ within views of education in which marketing and business discourses predominate. The paper argues that, despite the need for critical debate on issues regarding validation, current arguments focusing on ‘expertise’ risk diluting its significance in subtle yet fundamental ways

Explicit Actions for Electromagnetism with Two Gauge Fields with Only one Electric and one Magnetic Physical Fields

We extend the work of Mello et al. based in Cabbibo and Ferrari concerning the description of electromagnetism with two gauge fields from a variational principle, i.e. an action. We provide a systematic independent derivation of the allowed actions which have only one magnetic and one electric physical fields and are invariant under the discrete symmetries $P$ and $T$. We conclude that neither the Lagrangian, nor the Hamiltonian, are invariant under the electromagnetic duality rotations. This agrees with the weak-strong coupling mixing characteristic of the duality due to the Dirac quantization condition providing a natural way to differentiate dual theories related by the duality rotations (the energy is not invariant). Also the standard electromagnetic duality rotations considered in this work violate both $P$ and $T$ by inducing Hopf terms (theta terms) for each sector and a mixed Maxwell term. The canonical structure of the theory is briefly addressed and the 'magnetic' gauge sector is interpreted as a ghost sector.Comment: v2: 12 pages; References added, discussion concerning degrees of freedom corrected; v3: is now used the standard normalization of 1/4 in the actions; the possibility of theta being a pseudo-scalar implied a title changing; eq (23) added; signs corrected in equations (39,45-47); references adde

Nontopological self-dual Maxwell-Higgs vortices

We study the existence of self-dual nontopological vortices in generalized Maxwell-Higgs models recently introduced in Ref. \cite{gv}. Our investigation is explicitly illustrated by choosing a sixth-order self-interaction potential, which is the simplest one allowing the existence of nontopological structures. We specify some Maxwell-Higgs models yielding BPS nontopological vortices having energy proportional to the magnetic flux, $\Phi_{B}$, and whose profiles are numerically achieved. Particularly, we investigate the way the new solutions approach the boundary values, from which we verify their nontopological behavior. Finally, we depict the profiles numerically found, highlighting the main features they present.Comment: 6 pages, 4 figure

O desafio da colheita de frutas e hortaliças.

bitstream/item/88140/1/2220001.pd

Magnetized Accretion-Ejection Structures: 2.5D MHD simulations of continuous Ideal Jet launching from resistive accretion disks

We present numerical magnetohydrodynamic (MHD) simulations of a magnetized accretion disk launching trans-Alfvenic jets. These simulations, performed in a 2.5 dimensional time-dependent polytropic resistive MHD framework, model a resistive accretion disk threaded by an initial vertical magnetic field. The resistivity is only important inside the disk, and is prescribed as eta = alpha_m V_AH exp(-2Z^2/H^2), where V_A stands for Alfven speed, H is the disk scale height and the coefficient alpha_m is smaller than unity. By performing the simulations over several tens of dynamical disk timescales, we show that the launching of a collimated outflow occurs self-consistently and the ejection of matter is continuous and quasi-stationary. These are the first ever simulations of resistive accretion disks launching non-transient ideal MHD jets. Roughly 15% of accreted mass is persistently ejected. This outflow is safely characterized as a jet since the flow becomes super-fastmagnetosonic, well-collimated and reaches a quasi-stationary state. We present a complete illustration and explanation of the `accretion-ejection' mechanism that leads to jet formation from a magnetized accretion disk. In particular, the magnetic torque inside the disk brakes the matter azimuthally and allows for accretion, while it is responsible for an effective magneto-centrifugal acceleration in the jet. As such, the magnetic field channels the disk angular momentum and powers the jet acceleration and collimation. The jet originates from the inner disk region where equipartition between thermal and magnetic forces is achieved. A hollow, super-fastmagnetosonic shell of dense material is the natural outcome of the inwards advection of a primordial field.Comment: ApJ (in press), 32 pages, Higher quality version available at http://www-laog.obs.ujf-grenoble.fr/~fcass