155 research outputs found
Classical diamagnetism, magnetic interaction energies, and repulsive forces in magnetized plasmas
The Bohr-van Leeuwen theorem is often summarized as saying that there is no
classical magnetic susceptibility, in particular no diamagnetism. This is
seriously misleading. The theorem assumes position dependent interactions but
this is not required by classical physics. Since the work of Darwin in 1920 it
has been known that the magnetism due to classical charged point particles can
only be described by allowing velocity dependent interactions in the
Lagrangian. Legendre transformation to an approximate Hamiltonian can give an
estimate of the Darwin diamagnetism for a system of charged point particles.
Comparison with experiment, however, requires knowledge of the number of
classically behaving electrons in the sample. A new repulsive effective
many-body force, which should be relevant in plasmas, is predicted by the
Hamiltonian.Comment: added references, revise
The exact Darwin Lagrangian
Darwin (1920) noted that when radiation can be neglected it should be
possible to eliminate the radiation degrees-of-freedom from the action of
classical electrodynamics and keep the discrete particle degrees-of-freedom
only. Darwin derived his well known Lagrangian by series expansion in
keeping terms up to order . Since radiation is due to acceleration the
assumption of low speed should not be necessary. A Lagrangian is suggested that
neglects radiation without assuming low speed. It cures deficiencies of the
Darwin Lagrangian in the ultra-relativistic regime.Comment: 2.5 pages, no figure
Electromagnetic wave scattering by a superconductor
The interaction between radiation and superconductors is explored in this
paper. In particular, the calculation of a plane standing wave scattered by an
infinite cylindrical superconductor is performed by solving the Helmholtz
equation in cylindrical coordinates. Numerical results computed up to
of Bessel functions are presented for different wavelengths
showing the appearance of a diffraction pattern.Comment: 3 pages, 3 figure
Normal families and fixed points of iterates
Let F be a family of holomorphic functions and let K be a constant less than
4. Suppose that for all f in F the second iterate of f does not have fixed
points for which the modulus of the multiplier is greater than K. We show that
then F is normal. This is deduced from a result about the multipliers of
iterated polynomials.Comment: 5 page
Improving Standards of Care in Obstructed Labour: A Criteria-Based Audit at a Referral Hospital in a Low-Resource Setting in Tanzania
Objective In low-resource settings, obstructed labour is strongly associated with severe maternal morbidity and intrapartum asphyxia, and consequently maternal and perinatal deaths. This study evaluated the impact of a criteria-based audit of the diagnosis and management of obstructed labour in a low-resource setting.
Methods A baseline criteria-based audit was conducted from October 2013 to March 2014, followed by a workshop in which stakeholders gave feedback on interventions agreed upon to improve obstetric care. The implemented interventions included but were not limited to introducing standard guidelines for diagnosis and management of obstructed labour, agreeing on mandatory review by specialist for cases that are assigned caesarean section, re-training and supervision on use and interpretation of partograph and, strengthening team work between doctors, mid-wives and theatre staff. After implementing these interventions in March, a re-audit was performed from July 2015 to November, 2015, and the results were compared to those of the baseline audit.
Results Two hundred and sixty deliveries in the baseline survey and 250 deliveries in the follow-up survey were audited. Implementing the new criteria improved the diagnosis from 74% to 81% (p = 0.049) and also the management of obstructed labour from 4.2% at baseline audit to 9.2% at re-audit (p = 0.025). Improved detection of prolonged labour through heightened observation of regular contractions, protracted cervical dilatation, protracted descent of presenting part, arrested cervical dilation, and severe moulding contributed to improved standards of diagnosis (all p \u3c 0.04). Patient reviews by senior obstetricians increased from 34% to 43% (p = 0.045) and reduced time for caesarean section intervention from the median time of 120 to 90 minutes (p = 0.001) improved management (all p \u3c 0.05). Perinatal outcomes, neonatal distress and fresh stillbirths, were reduced from 16% to. 8.8% (p = 0.01).
Conclusion A criteria-based audit proved to be a feasible and useful tool in improving diagnosis and management of obstructed labour using available resources. Some of the observed changes in practice were of modest magnitude implying demand for further improvements, while sustaining those already put in place
The comfortable roller coaster -- on the shape of tracks with constant normal force
A particle that moves along a smooth track in a vertical plane is influenced
by two forces: gravity and normal force. The force experienced by roller
coaster riders is the normal force, so a natural question to ask is: what shape
of the track gives a normal force of constant magnitude? Here we solve this
problem. It turns out that the solution is related to the Kepler problem; the
trajectories in velocity space are conic sections.Comment: 10 pages, 4 figure
Static deformation of heavy spring due to gravity and centrifugal force
The static equilibrium deformation of a heavy spring due to its own weight is
calculated for two cases. First for a spring hanging in a constant
gravitational field, then for a spring which is at rest in a rotating system
where it is stretched by the centrifugal force. Two different models are
considered. First a discrete model assuming a finite number of point masses
connected by springs of negligible weight. Then the continuum limit of this
model. In the second case the differential equation for the deformation is
obtained by demanding that the potential energy is minimized. In this way a
simple application of the variational calculus is obtained.Comment: 11 pages, 2 figure
On the regular-geometric-figure solution to the N-body problem
The regular-geometric-figure solution to the -body problem is presented in
a very simple way. The Newtonian formalism is used without resorting to a more
involved rotating coordinate system. Those configurations occur for other kinds
of interactions beyond the gravitational ones for some special values of the
parameters of the forces. For the harmonic oscillator, in particular, it is
shown that the -body problem is reduced to one-body problems.Comment: To appear in Eur. J. Phys. (5 pages
The non-self-adjointness of the radial momentum operator in n dimensions
The non self-adjointness of the radial momentum operator has been noted
before by several authors, but the various proofs are incorrect. We give a
rigorous proof that the -dimensional radial momentum operator is not self-
adjoint and has no self-adjoint extensions. The main idea of the proof is to
show that this operator is unitarily equivalent to the momentum operator on
which is not self-adjoint and has no self-adjoint
extensions.Comment: Some text and a reference adde
Mechanics, cosmology and Mach's principle
It is pointed out that recent cosmological findings seem to support the view
that the mass/energy distribution of the universe defines the Newtonian
inertial frames as originally suggested by Mach. The background concepts of
inertial frame, Newton's second law, and fictitious forces are clarified. A
precise definition of Mach's principle is suggested. Then an approximation to
general relativity discovered by Einstein, Infeld, and Hoffmann is used and it
is found that this precise formulation of Mach's principle is realized provided
the mass/energy density of the universe has a specific value. This value turns
out to be twice the critical density. The implications of this approximate
result is put into context.Comment: 9 pages, 34 references, 0 figure
- …