28,383 research outputs found
Superconducting pipes and levitating magnets
Motivated by a beautiful demonstration of the Faraday's and Lenz's law in
which a small neodymium magnet falls slowly through a conducting
non-ferromagnetic tube, we consider the dynamics of a magnet falling through a
superconducting pipe. Unlike the case of normal conducting pipes, in which the
magnet quickly reaches the terminal velocity, inside a superconducting tube the
magnet falls freely. On the other hand, to enter the pipe the magnet must
overcome a large electromagnetic energy barrier. For sufficiently strong
magnets, the barrier is so large that the magnet will not be able to penetrate
it and will be suspended over the front edge. We calculate the work that must
done to force the magnet to enter a superconducting tube. The calculations show
that superconducting pipes are very efficient at screening magnetic fields. For
example, the magnetic field of a dipole at the center of a short pipe of radius
and length decays, in the axial direction, with a
characteristic length . The efficient screening of the
magnetic field might be useful for shielding highly sensitive superconducting
quantum interference devices, SQUIDs. Finally, the motion of the magnet through
a superconducting pipe is compared and contrasted to the flow of ions through a
trans-membrane channel
Size-dependence of Strong-Coupling Between Nanomagnets and Photonic Cavities
The coherent dynamics of a coupled photonic cavity and a nanomagnet is
explored as a function of nanomagnet size. For sufficiently strong coupling
eigenstates involving highly entangled photon and spin states are found, which
can be combined to create coherent states. As the size of the nanomagnet
increases its coupling to the photonic mode also monotonically increases, as
well as the number of photon and spin states involved in the system's
eigenstates. For small nanomagnets the crystalline anisotropy of the magnet
strongly localized the eigenstates in photon and spin number, quenching the
potential for coherent states. For a sufficiently large nanomagnet the
macrospin approximation breaks down and different domains of the nanomagnet may
couple separately to the photonic mode. Thus the optimal nanomagnet size is
just below the threshold for failure of the macrospin approximation.Comment: 10 pages, 7 figure
Comment on `About the magnetic field of a finite wire'
A flaw is pointed out in the justification given by Charitat and Graner [2003
Eur. J. Phys. vol. 24, 267] for the use of the Biot--Savart law in the
calculation of the magnetic field due to a straight current-carrying wire of
finite length.Comment: REVTeX, 3 pages. A slightly expanded version that has been accepted
for publication by Eur. J. Phy
Variational principle for the Wheeler-Feynman electrodynamics
We adapt the formally-defined Fokker action into a variational principle for
the electromagnetic two-body problem. We introduce properly defined boundary
conditions to construct a Poincare-invariant-action-functional of a finite
orbital segment into the reals. The boundary conditions for the variational
principle are an endpoint along each trajectory plus the respective segment of
trajectory for the other particle inside the lightcone of each endpoint. We
show that the conditions for an extremum of our functional are the
mixed-type-neutral-equations with implicit state-dependent-delay of the
electromagnetic-two-body problem. We put the functional on a natural Banach
space and show that the functional is Frechet-differentiable. We develop a
method to calculate the second variation for C2 orbital perturbations in
general and in particular about circular orbits of large enough radii. We prove
that our functional has a local minimum at circular orbits of large enough
radii, at variance with the limiting Kepler action that has a minimum at
circular orbits of arbitrary radii. Our results suggest a bifurcation at some
radius below which the circular orbits become saddle-point extrema. We give a
precise definition for the distributional-like integrals of the Fokker action
and discuss a generalization to a Sobolev space of trajectories where the
equations of motion are satisfied almost everywhere. Last, we discuss the
existence of solutions for the state-dependent delay equations with slightly
perturbated arcs of circle as the boundary conditions and the possibility of
nontrivial solenoidal orbits
A closed form for the electrostatic interaction between two rod-like charged objects
We have calculated the electrostatic interaction between two rod-like charged
objects with arbitrary orientations in three dimensions. we obtained a closed
form formula expressing the interaction energy in terms of the separation
distance between the centers of the two rod-like objects, , their lengths
(denoted by and ), and their relative orientations (indicated by
and ). When the objects have the same length (),
for particular values of separations, i.e for , two types of
minimum are appeared in the interaction energy with respect to . By
employing the closed form formula and introducing a scaled temperature , we
have also studied the thermodynamic properties of a one dimensional system of
rod-like charged objects. For different separation distances, the dependence of
the specific heat of the system to the scaled temperature has been studied. It
is found that for , the specific heat has a maximum.Comment: 10 pages, 9 figures, 1 table, Accepted by J. Phys.: Condens. Matte
On the universality of the scaling of fluctuations in traffic on complex networks
We study the scaling of fluctuations with the mean of traffic in complex
networks using a model where the arrival and departure of "packets" follow
exponential distributions, and the processing capability of nodes is either
unlimited or finite. The model presents a wide variety of exponents between 1/2
and 1 for this scaling, revealing their dependence on the few parameters
considered, and questioning the existence of universality classes. We also
report the experimental scaling of the fluctuations in the Internet for the
Abilene backbone network. We found scaling exponents between 0.71 and 0.86 that
do not fit with the exponent 1/2 reported in the literature.Comment: 4 pages, 4 figure
Quadrupole collective modes in trapped finite-temperature Bose-Einstein condensates
Finite temperature simulations are used to study quadrupole excitations of a
trapped Bose-Einstein condensate. We focus specifically on the m=0 mode, where
a long-standing theoretical problem has been to account for an anomalous
variation of the mode frequency with temperature. We explain this behavior in
terms of the excitation of two separate modes, corresponding to coupled motion
of the condensate and thermal cloud. The relative amplitudes of the modes
depends sensitively on the temperature and on the frequency of the harmonic
drive used to excite them. Good agreement with experiment is found for
appropriate drive frequencies.Comment: 4 pages, 3 figure
Theory of elastic interaction between colloidal particles in the nematic cell in the presence of the external electric or magnetic field
The Green function method developed in Ref.[S. B. Chernyshuk and B. I. Lev,
Phys. Rev. E \textbf{81}, 041707 (2010)] is used to describe elastic
interactions between axially symmetric colloidal particles in the nematic cell
in the presence of the external electric or magnetic field. General formulas
for dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions in
the homeotropic and planar nematic cells with parallel and perpendicular field
orientations are obtained. A set of new results has been predicted: 1)
\textit{Deconfinement effect} for dipole particles in the homeotropic nematic
cell with negative dielectric anisotropy and perpendicular
to the cell electric field, when electric field is approaching it's Frederiks
threshold value . This means cancellation of the
confinement effect found in Ref. [M.Vilfan et al. Phys.Rev.Lett. {\bf 101},
237801, (2008)] for dipole particles near the Frederiks transition while it
remains for quadrupole particles. 2) New effect of \textit{attraction and
stabilization} of the particles along the electric field parallel to the cell
planes in the homeotropic nematic cell with . The minimun
distance between two particles depends on the strength of the field and can be
ordinary for . 3) Attraction and repulsion zones for all elastic interactions
are changed dramatically under the action of the external field.Comment: 15 pages, 17 figure
Poynting Vector Flow in a Circular Circuit
A circuit is considered in the shape of a ring, with a battery of negligible
size and a wire of uniform resistance. A linear charge distribution along the
wire maintains an electrostatic field and a steady current, which produces a
constant magnetic field. Earlier studies of the Poynting vector and the rate of
flow of energy considered only idealized geometries in which the Poynting
vector was confined to the space within the circuit. But in more realistic
cases the Poynting vector is nonzero outside as well as inside the circuit. An
expression is obtained for the Poynting vector in terms of products of
integrals, which are evaluated numerically to show the energy flow. Limiting
expressions are obtained analytically. It is shown that the total power
generated by the battery equals the energy flowing into the wire per unit time.Comment: 19 pages, 8 figure
Strongly coupled plasma with electric and magnetic charges
A number of theoretical and lattice results lead us to believe that
Quark-Gluon Plasma not too far from contains not only electrically
charged quasiparticles -- quarks and gluons -- but magnetically charged ones --
monopoles and dyons -- as well. Although binary systems like charge-monopole
and charge-dyon were considered in details before in both classical and quantum
settings, it is the first study of coexisting electric and magnetic particles
in many-body context. We perform Molecular Dynamics study of strongly coupled
plasmas with particles and different fraction of magnetic charges.
Correlation functions and Kubo formulae lead to such transport properties as
diffusion constant, shear viscosity and electric conductivity: we compare the
first two with empirical data from RHIC experiments as well as results from
AdS/CFT correspondence. We also study a number of collective excitations in
these systems.Comment: 2nd version, 22 pages, 32 figures: two important new figures have
been included to compare our results with RHIC experiments and AdS/CFT
results; a few new references and comments are added as wel
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