20/04/2015
Levitation (from Latin levitas "lightness")[1] is the process by which an object is
held aloft, without mechanical
support, in a stable position. Levitation is accomplished by
providing an upward force that counteracts the
downward force of gravity, plus a smaller stabilizing force
that pushes the object
toward a home position
whenever it is a small
distance away from that
home position. The force can be a fundamental force such
as magnetic or electrostatic,
or it can be a reactive force
such as optical, buoyant,
aerodynamic, or hydrodynamic.[citation needed] Levitation excludes floating at the surface of a liquid
because the liquid provides
direct mechanical support.
Levitation excludes hovering
flight by insects,
hummingbirds, helicopters, rockets, and balloons because
the object provides its own
counter-gravity force.[citation needed] Physics Levitation (on Earth or any planetoid) requires an upward
force that cancels out the weight of the object, so that the object does not fall
(accelerate downward) or rise
(accelerate upward). For
positional stability, any small
displacement of the levitating
object must result in a small change in force in the
opposite direction.[citation needed] The small changes in force can be
accomplished by gradient field
(s) or by active regulation. If
the object is disturbed, it
might oscillate around its final
position, but it motions eventually decrease to zero
due to damping effects. (In a turbulent flow, the object
might oscillate indefinitely.)[citation needed] Levitation techniques are
useful tools in physics
research. For example,
levitation methods are useful
for high-temperature melt
property studies because they eliminate the problem of
reaction with containers and
allow deep undercooling of
melts. The containerless
conditions may be obtained by
opposing gravity with a levitation force, instead of
allowing an entire experiment to freefall.[2] Magnetic levitation Main article: Magnetic levitation a high-temperature superconductor levitating above magnet Magnetic levitation is the
most commonly seen and used
form of levitation. Diamagnetic materials are commonly used for
demonstration purposes. In
this case the returning force
appears from the interaction
with the screening currents. For example, a superconducting sample, which can be considered either as a
perfect diamagnet or an ideally hard superconductor, easily levitates in an ambient
external magnetic field. The
superconductor is first
heated strongly, then cooled
with liquid nitrogen to levitate
on top of a diamagnet. In very strong magnetic field, by
means of diamagnetic levitation even small live animals have been levitated. It is possible to levitate
pyrolytic graphite by placing
thin squares of it above four
cube magnets with the north
poles forming one diagonal
and south poles forming the other diagonal.[3] A magnetically levitated
(maglev) train departing
Pudong International Airport
on the first commercial high-
speed maglev line in the
world. Magnetic levitation is in
development for use for
transportation systems. For
example the Maglev includes trains that are levitated by a
large number of magnets. Due
to the lack of friction on the
guide rails, they are faster,
quieter, and smoother than
wheeled mass transit systems. Electrodynamic suspension uses AC magnetic fields. Electrostatic levitation Main article: Electrostatic levitation In electrostatic levitation an electric field is used to counteract gravitational
force. Aerodynamic levitation Main article: Aerodynamic levitation In aerodynamic levitation, the
levitation is achieved by
floating the object on a
stream of gas, either
produced by the object or
acting on the object. For example, a ping pong ball can be levitated with the stream
of air from a vacuum cleaner
set on 'blow'. With enough
thrust, very large objects can
be levitated using this
method. This technique enables the
levitation of an object against gravitational force by floating it on a thin gas film formed by gas flow through a porous membrane. Using this
technique, high temperature
melts can be kept clean from
contamination and be supercooled.[2] A common example in general usage
includes air hockey, where the puck is lifted by a thin
layer of air. Hovercraft also use this technique, producing
a large region of high-
pressure air underneath
them. Acoustic levitation Main article: Acoustic levitation Acoustic levitation uses sound
waves to provide a levitating
force. Optical levitation Main article: Optical levitation Optical levitation is a
technique in which a material
is levitated against the
downward force of gravity by
an upward force stemming
from photon momentum transfer (radiation pressure). Buoyant levitation Gases at high pressure can
have a density exceeding
that of some solids. Thus they
can be used to levitate solid objects through buoyancy.[4] Noble gases are preferred for their non-reactivity. Xenon is the densest non-radioactive
noble gas, at 5.894g/L. Xenon
has been used to levitate polyethylene, at a pressure of 154atm. Casimir force Scientists have discovered a
way of levitating ultra small
objects by manipulating the
so-called Casimir force, which normally causes objects to
stick together due to forces
predicted by quantum field theory. This is, however, only possible for micro-objects.[5] [6] Uses Main article: Maglev Magnetic levitation is used to
suspend trains without
touching the track. This
permits very high speeds, and
greatly reduces the
maintenance requirements for tracks and vehicles, as little
wear occurs. This also means
there is no friction, so the
only force acting against it is
air resistance. Diamagnetic levitation of a live frog. Scientists have levitated frogs,[7] grasshoppers, and mice by means of powerful
electromagnets utilizing
superconductors, producing
diamagnetic repulsion of body
water. The mice acted
confused at first, but adapted to the levitation
after approximately four
hours, suffering no immediate ill effects.[8][9
