23,098 research outputs found
Gambling in Great Britain:a response to Rogers
A recent issue of Practice: Social Work in Action featured a paper by Rogers that examined whether the issue of problem gambling was a suitable case for social work. Rogers’ overview was (in various places) out of date, highly selective, contradictory, presented unsupported claims and somewhat misleading. Rogers’ paper is to be commended for putting the issue of problem gambling on the social work agenda. However, social workers need up-to-date information and contextually situated information if they are to make informed decisions in helping problem gamblers
Analysis of a Three Phase Induction Motor Directly from Maxwell's Equations
The torque developed in a three phase AC squirrel cage motor is usually
expressed in terms of resistances and reactances of the stator, the rotor, and
the motor as a whole. We use Maxwell's equations to find the torque in terms of
geometrical parameters. This allows us to estimate the torque developed by a
motor without knowing the details of its circuitry
Reply to "Comment(s) on `Preacceleration without radiation: The non-existence of preradiation phenomenon," by J. D. Jackson [Am. J. Phys. 75, 844-845 (2007)] and V. Hnizdo [Am. J. Phys. 75, 845-846 (2007)
This paper replies the comments by J. D. Jackson [Am. J. Phys. 75, 844-845
(2007)] and V. Hnizdo [Am. J. Phys. 75, 845-846 (2007)].Comment: 9 pages. See also the related paper: "E. Eriksen and O. Gron, Does
preradiation exist? [Phys. Scr. 76, 60-63 (2007)].
Preacceleration without radiation: the non-existence of preradiation phenomenon
An unexpected prediction of classical electrodynamics is that a charge can
accelerate before a force is applied. We would expect that a preaccelerated
charge would radiate so that there would be spontaneous preradiation, an
acausal phenomenon. We reexamine the subtle relation between the Larmor formula
for the power radiated by a point charge and the Abraham-Lorentz equation and
find that for well-behaved external forces acting for finite times, the charge
does not radiate in time intervals where there is preacceleration. That is, for
these forces preradiation does not exist even though the charge is
preaccelerated. The radiative energy is emitted only in time intervals when the
external force acts on the charge.Comment: Equation (37) of the published paper in AJP has been correcte
The quantum Gaussian well
Different features of a potential in the form of a Gaussian well have been
discussed extensively. Although the details of the calculation are involved,
the general approach uses a variational method and WKB approximation,
techniques which should be familiar to advanced undergraduates. A numerical
solution of the Schr\"odinger equation through diagonalization has been
developed in a self-contained way, and physical applications of the potential
are mentioned.Comment: 11 pages, 4 figures, To be published in American Journal of Physic
On the parameters of the Kerr-NUT-(anti-)de Sitter space-time
Different forms of the metric for the Kerr-NUT-(anti-)de Sitter space-time
are being widely used in its extension to higher dimensions. The purpose of
this note is to relate the parameters that are being used to the physical
parameters (mass, rotation, NUT and cosmological constant) in the basic four
dimensional situation.Comment: 4 pages. To appear as a Note in Classical and Quantum Gravit
Carbon Nanotubes in Helically Modulated Potentials
We calculate effects of an applied helically symmetric potential on the low
energy electronic spectrum of a carbon nanotube in the continuum approximation.
The spectrum depends on the strength of this potential and on a dimensionless
geometrical parameter, P, which is the ratio of the circumference of the
nanotube to the pitch of the helix. We find that the minimum band gap of a
semiconducting nanotube is reduced by an arbitrarily weak helical potential,
and for a given field strength there is an optimal P which produces the biggest
change in the band gap. For metallic nanotubes the Fermi velocity is reduced by
this potential and for strong fields two small gaps appear at the Fermi surface
in addition to the gapless Dirac point. A simple model is developed to estimate
the magnitude of the field strength and its effect on DNA-CNT complexes in an
aqueous solution. We find that under typical experimental conditions the
predicted effects of a helical potential are likely to be small and we discuss
several methods for increasing the size of these effects.Comment: 12 pages, 10 figures. Accepted for publication in Physical Review B.
Image quality reduced to comply with arxiv size limitation
Quantum probe and design for a chemical compass with magnetic nanostructures
Magnetic fields as weak as Earth's may affect the outcome of certain
photochemical reactions that go through a radical pair intermediate. When the
reaction environment is anisotropic, this phenomenon can form the basis of a
chemical compass and has been proposed as a mechanism for animal
magnetoreception. Here, we demonstrate how to optimize the design of a chemical
compass with a much better directional sensitivity simply by a gradient field,
e.g. from a magnetic nanostructure. We propose an experimental test of these
predictions, and suggest design principles for a hybrid metallic-organic
chemical compass. In addition to the practical interest in designing a
biomimetic weak magnetic field sensor, our result shows that gradient fields
can server as powerful tools to probe spin correlations in radical pair
reactions.Comment: 8 pages, 6 figures, comments are welcom
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