2,478 research outputs found
New concept in brazing metallic honeycomb panels
Aluminum oxide coating provides surface which will not be wetted by brazing alloy and which stops metallic diffusion welding of tooling materials to part being produced. This method eliminates loss of tooling materials and parts from braze wetting and allows fall-apart disassembly of tooling after brazing
Dynamics of a liquid dielectric attracted by a cylindrical capacitor
The dynamics of a liquid dielectric attracted by a vertical cylindrical
capacitor is studied. Contrary to what might be expected from the standard
calculation of the force exerted by the capacitor, the motion of the dielectric
is different depending on whether the charge or the voltage of the capacitor is
held constant. The problem turns out to be an unconventional example of
dynamics of a system with variable mass, whose velocity can, in certain
circumstances, suffer abrupt changes. Under the hypothesis that the voltage
remains constant the motion is described in qualitative and quantitative
details, and a very brief qualitative discussion is made of the constant charge
case.Comment: To appear in European Journal of Physic
Force on a neutral atom near conducting microstructures
We derive the non-retarded energy shift of a neutral atom for two different
geometries. For an atom close to a cylindrical wire we find an integral
representation for the energy shift, give asymptotic expressions, and
interpolate numerically. For an atom close to a semi-infinite halfplane we
determine the exact Green's function of the Laplace equation and use it derive
the exact energy shift for an arbitrary position of the atom. These results can
be used to estimate the energy shift of an atom close to etched microstructures
that protrude from substrates.Comment: 7 pages, 5 figure
Can a charged ring levitate a neutral, polarizable object? Can Earnshaw's Theorem be extended to such objects?
Stable electrostatic levitation and trapping of a neutral, polarizable object
by a charged ring is shown to be theoretically impossible. Earnshaw's Theorem
precludes the existence of such a stable, neutral particle trap.Comment: 11 pages, 1 figur
On the exact electric and magnetic fields of an electric dipole
We derive from Jefimenko's equations a multipole expansion in order to obtain
the exact expressions for the electric and magnetic fields of an electric
dipole with an arbitrary time dependence. A few comments are also made about
the usual expositions found in most common undergraduate and graduate textbooks
as well as in the literature on this topic
Theory of Tunneling for Rough Junctions
A formally exact expression for the tunneling current, for its separation
into specular and diffuse components, and for its directionality, is given for
a thick tunnel junction with rough interfaces in terms of the properties of
appropriately defined scattering amplitudes. An approximate evaluation yields
the relative magnitudes of the specular and diffuse components, and the angular
dependence of the diffuse component, in terms of certain statistical properties
of the junction interfaces.Comment: 4 page
Unified description of ballistic and diffusive carrier transport in semiconductor structures
A unified theoretical description of ballistic and diffusive carrier
transport in parallel-plane semiconductor structures is developed within the
semiclassical model. The approach is based on the introduction of a
thermo-ballistic current consisting of carriers which move ballistically in the
electric field provided by the band edge potential, and are thermalized at
certain randomly distributed equilibration points by coupling to the background
of impurity atoms and carriers in equilibrium. The sum of the thermo-ballistic
and background currents is conserved, and is identified with the physical
current. The current-voltage characteristic for nondegenerate systems and the
zero-bias conductance for degenerate systems are expressed in terms of a
reduced resistance. For arbitrary mean free path and arbitrary shape of the
band edge potential profile, this quantity is determined from the solution of
an integral equation, which also provides the quasi-Fermi level and the
thermo-ballistic current. To illustrate the formalism, a number of simple
examples are considered explicitly. The present work is compared with previous
attempts towards a unified description of ballistic and diffusive transport.Comment: 23 pages, 10 figures, REVTEX
The impact of long-term soccer-specific training on the physical development of elite junior soccer players
The goal of any soccer talent development programme is to guide players towards professional characteristics. In order to maximise this process it is essential to gain an insight into their individual characteristics. Within this specific population there are many factors from maturation, hormonal, anatomical and musculoskeletal changes that accompany paediatric development and consequently have a direct impact upon their development. The aim of this thesis was to determine the relative influence of changes in maturation and long-term systematic training on the physical development of elite junior soccer players.
The aim of the first study (Chapter 4) was to assess the reliability of a range of anthropometric and performance measures in aged matched academy and non-academy soccer players (U14-18). The results demonstrated that all anthropometric (%CV values of 0.1 – 1.3%) and performance measures (%CV values of 1.8-6.2%) were highly reproducible in both the academy and non-academy soccer players. These assessments would be subsequently used to determine the impact of long-term training on changes in physical development of junior soccer players.
The aim of study two (Chapter 5) was to determine the validity of a non-invasive approach (maturity offset) for predicting end height stature in academy soccer players (U14-18). Overall, agreement between estimates of end height stature in elite youth soccer players’ using skeletal x-ray and the maturity-offset method were poor with a SEM and 95 % LOA of 4 cm and +11cm being observed respectively. These findings indicate that care must be taken when predicating end height stature in academy soccer players when using maturity offset method.
The aim of the third study (Chapter 6) was to examine the typical weekly training load experienced by academy and non-academy soccer players (U12-U16) during the in-season competitive period. Physiological loading associated with training sessions and match-play were monitored using heart rate (HR) and ratings of perceived exertion (RPE). Training and match loads were calculated by multiplying global session RPE and duration (RPE-TL). Weekly training load in the academy players (U12, 737±51; U14, 646±106; U16, 750±81) was higher than the non-academy players (U12, 157±28; U14, 161±19; U16, 193±26) across the three age groups. Similarly, match load and % time spent >90%HRmax was higher in the academy players compared to the non-academy players. The present findings indicate that the overall load and intensity of training is greater in academy players compared to aged match non-academy players.
The aim of the fourth study (Chapter 7) was to determine the relative influence of changes in maturation and long-term systematic training on changes in physical performance in age matched academy and non-academy junior soccer player. The three-year change in the physical performance of twenty-seven academy and eighteen non-academy soccer players (U12-U16) were monitored. When corrected for differences in both baseline performance and change in maturity status (maturity offset), greater changes in countermovement jump (7.3 + 2.6 cm, 5.4 + 2.5 cm), 10 m (-0.15 + 0.05 s, -0.10 + 0.04 s) and 20 m sprint (-0.30 + 0.16 s, -0.15 + 0.13 s), agility (-0.19 + 0.01 s, -0.08 + 0.08 s), repeated sprint (-0.60 + 0.26 s, -0.41 + 2.1 s) and intermittent endurance capacity (1128 + 406 m, 315 + 370 m) were observed in the Academy players compared with non-academy players (p0.7). These findings indicate that long-term player development programs accelerate the rate physical development of academy soccer players relative to age and maturity matched non-academy players.
In summary, the present thesis highlights that academy soccer players experience greater rates of improvement in physical performance indicators compared to non-academy players, independently from the initial performance level of the child and change in maturation over the same time period. These difference are likely to reflect the increased volume and intensity of soccer-specific training experienced by the young soccer players as part of the academy’s approach to long-term athlete development. Future research is warranted in order to determine training loads in elite youth soccer players at different stages of biological maturity which serve to enhance performance whilst minimising the risk of injury
Optical precursors in transparent media
We theoretically study the linear propagation of a stepwise pulse through a
dilute dispersive medium when the frequency of the optical carrier coincides
with the center of a natural or electromagnetically induced transparency window
of the medium (slow-light systems). We obtain fully analytical expressions of
the entirety of the step response and show that, for parameters representative
of real experiments, Sommerfeld-Brillouin precursors, main field and second
precursors "postcursors" can be distinctly observed, all with amplitudes
comparable to that of the incident step. This behavior strongly contrasts with
that of the systems generally considered up to now
Brillouin-Wigner perturbation theory in open electromagnetic systems
A Brillouin-Wigner perturbation theory is developed for open electromagnetic
systems which are characterised by discrete resonant states with complex
eigenenergies. Since these states are exponentially growing at large distances,
a modified normalisation is introduced that allows a simple spectral
representation of the Green's function. The perturbed modes are found by
solving a linear eigenvalue problem in matrix form. The method is illustrated
on exactly solvable one- and three-dimensional examples being, respectively, a
dielectric slab and a microsphere.Comment: 6 pages, 2 figure
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