2,239 research outputs found
High field magnetotransport in composite conductors: the effective medium approximation revisited
The self consistent effective medium approximation (SEMA) is used to study
three-dimensional random conducting composites under the influence of a strong
magnetic field {\bf B}, in the case where all constituents exhibit isotropic
response. Asymptotic analysis is used to obtain almost closed form results for
the strong field magnetoresistance and Hall resistance in various types of two-
and three-constituent isotropic mixtures for the entire range of compositions.
Numerical solutions of the SEMA equations are also obtained, in some cases, and
compared with those results. In two-constituent
free-electron-metal/perfect-insulator mixtures, the magnetoresistance is
asymptotically proportional to at {\em all concentrations above the
percolation threshold}. In three-constituent metal/insulator/superconductor
mixtures a line of critical points is found, where the strong field
magnetoresistance switches abruptly from saturating to non-saturating
dependence on , at a certain value of the
insulator-to-superconductor concentration ratio. This transition appears to be
related to the phenomenon of anisotropic percolation.Comment: 16 pages, 3 figure
Critical point for the strong field magnetoresistance of a normal conductor/perfect insulator/perfect conductor composite with a random columnar microstructure
A recently developed self-consistent effective medium approximation, for
composites with a columnar microstructure, is applied to such a
three-constituent mixture of isotropic normal conductor, perfect insulator, and
perfect conductor, where a strong magnetic field {\bf B} is present in the
plane perpendicular to the columnar axis. When the insulating and perfectly
conducting constituents do not percolate in that plane, the
microstructure-induced in-plane magnetoresistance is found to saturate for
large {\bf B}, if the volume fraction of the perfect conductor is greater
than that of the perfect insulator . By contrast, if , that
magnetoresistance keeps increasing as without ever saturating. This
abrupt change in the macroscopic response, which occurs when , is a
critical point, with the associated critical exponents and scaling behavior
that are characteristic of such points. The physical reasons for the singular
behavior of the macroscopic response are discussed. A new type of percolation
process is apparently involved in this phenomenon.Comment: 4 pages, 1 figur
Eddy currents in a conducting sphere
This report analyzes the eddy current induced in a solid conducting sphere by a sinusoidal current in a circular loop. Analytical expressions for the eddy currents are derived as a power series in the vectorial displacement of the center of the sphere from the axis of the loop. These are used for first order calculations of the power dissipated in the sphere and the force and torque exerted on the sphere by the electromagnetic field of the loop
Grackles
Numbering in the tens of millions of birds, grackle populations in North America can cause a variety of conflicts with people. Grackles eat agricultural crops and livestock feed, damage property, spread pathogens, and collide with aircraft. Their large roosts can be a nuisance in urban and suburban areas. A combination of dispersal techniques, exclusion, and lethal removal may help to reduce grackle damage.
Grackles adapt easily to human-dominated environments, and exploit human food and other features of human landscapes. Thus, an integrated damage management approach to grackle damage focuses on reducing and eliminating the damage, rather than simply controlling grackle populations.
Three species of grackles are present in North America: the common grackle, the boat-tailed grackle, and the great-tailed grackle. A fourth species, the greater Antillean grackle (Q. niger) is present in Puerto Rico. All are part of the Family Icteridae that includes blackbirds, orioles, cowbirds, meadowlarks, and bobolinks. The population status of all three grackle species is considered common to overabundant
Microscopic colitis : epidemiology, death and associated disorders
Microscopic colitis (MC) is the most recently recognized inflammatory condition of the large
intestine. MC is an umbrella term for two disease entities, namely lymphocytic colitis (LC)
and collagenous colitis (CC). These subtypes are distinguished by their histopathological
presentation and cannot be separated based on clinical observation or symptoms. The most
prominent clinical presentation is watery, non-bloody diarrhea. However, a proportion of
patients also suffer from abdominal pain, weight loss, fecal incontinence and reduced quality
of life. MC primarily affects the elderly and shows a clear female preponderance with some
2/3 of patients being women. Large scale, epidemiological research of the disease has
historically been hampered by insufficient awareness. Therefore, using data from Swedish
health care registers, this thesis aims to elucidate temporal patterns of MC as well as the
association with mortality, cancer and celiac disease (CD).
In study I, we examined the validity of having a MC diagnosis recorded in Swedish regional
pathology registers. Through manual review of medical charts (n=211), carried out by two
independent reviewers, we computed a positive predictive value for MC of 95%. Thus, we
concluded that Swedish pathology registers are a reliable source for identifying patients with
MC.
In study II, we identified every patient with a first-time diagnosis of MC recorded in Swedish
pathology registers from 1995-2015. Using this cohort, we examined temporal trends, age
distribution and sex differences in MC. As expected, a majority of patients (72%) were
female and mean age at diagnosis was 60.2 years. Incidence rates increased appreciably from
1995 to 2012, after which rates have stabilized. The mean age-standardized incidence rate
from 2006 to 2015 was 10.5 cases/100,000 person-years, with a female to male incidence rate
ratio of 2.4, adjusted for age and calendar period. When analyzing age-specific incidence,
incidence rates increased up to 75-79 years after which they declined. Furthermore, we
estimated that during a life-time 1 in 115 women and 1 in 286 men are expected to be
diagnosed with MC.
In study III we examined mortality in patients with MC. This was done using a matched
cohort study design where each exposed individual (MC) (n=14,333) was matched according
to age, sex, county of residence and year of biopsy to five reference individuals from the
general population. During the study period (1990 to 2017) patients with MC had a higher
probability of death. However, after adjustment for comorbidities the association vanished.
Thus, we concluded that the increased risk of death is attributable to the burden of
concomitant disease.
Study IV aimed to investigate the association between MC and cancer. Again, this was done
using the matched cohort design described above. In total, we identified 11,758 patients
diagnosed with MC between 1990 and 2016 with MC that were matched to 50,828 reference
individuals. After adjustments for the matching variables and comorbidities (CD and
diabetes), we estimated an adjusted hazard ratio (aHR) of 1.08 (95%CI=1.02-1.16) for overall
cancer. In secondary analyses, we found a decreased probability of colorectal cancer (aHR,
0.52 (95%CI_0.40-0.66)). The same pattern was observed for gastrointestinal cancers overall
(aHR, 0.72(95%CI=0.60-0.85)).
In study V we examined the association between CD and MC. Using the same matched
cohort study design described above, we identified 45,267 patients with CD and 224,568
reference individuals between 1990 and 2016. 456 patients with CD were diagnosed with MC
during the study period compared to 198 reference individuals that developed MC during the
same period. These figures correspond to an aHR of 11.5 (95%CI=9.3-13.7). However, as the
proportional hazards assumption was violated, the main result should not be interpreted as the
probability for a CD patient to develop MC at any instant during the study period compared
to the reference population, but rather a mean aHR based on all lengths of follow up.
However, as the increased risk remained even after >10 years of follow up, our results
indicate that the concomitance of these diagnoses should be considered if symptoms persist or
reoccur despite a gluten free diet
- …