(Molecular Magnetism and Levitation)

The image of a high-temperature superconductor levitating above a
magnet in fog of liquid nitrogen can hardly surprise anyone these days
- it has become common knowledge that superconductors are ideal
diamagnetics and magnetic field must expel them. On the other hand,
the enclosed photographs of water and a frog hovering inside a magnet
(not on board a spacecraft) are somewhat counterintuitive and will
probably take many people (even physicists) by surprise. This is the
first observation of magnetic levitation of living organisms as well
as the first images of diamagnetics levitated in a normal, room-
temperature environment (if we disregard the tale about Flying Coffin
of Mohammed as such evidence, of course). In fact, it is possible to
levitate magnetically every material and every living creature on the
earth due to the always present molecular magnetism. The molecular
magnetism is very weak (millions times weaker than ferromagnetism) and
usually remains unnoticed in everyday life, thereby producing the
wrong impression that materials around us are mainly nonmagnetic. But
they are all magnetic. It is just that magnetic fields required to
levitate all these "nonmagnetic" materials have to be approximately
100 times larger than for the case of, say, superconductors.



Whether an object will or will not levitate in a magnetic field B is
defined by the balance between the magnetic force F = MB and gravity
mg = V g where is the material density, V is the volume and g = 9.8m/
s2. The magnetic moment M = (/ µ0)VB so that F = (/µ0)BVB = (/2µ0)VB2.
Therefore, the vertical field gradient B2 required for levitation has
to be larger than 2µ0 g/. Molecular susceptibilities are typically
10-5 for diamagnetics and 10-3 for paramagnetic materials and, since
is most often a few g/cm3, their magnetic levitation requires field
gradients ~1000 and 10 T2/m, respectively. Taking l = 10cm as a
typical size of high-field magnets and B2 ~ B2/l as an estimate, we
find that fields of the order of 1 and 10T are sufficient to cause
levitation of para- and diamagnetics. This result should not come as a
surprise because, as we know, magnetic fields of less than 0.1T can
levitate a superconductor (= -1) and, from the formulas above, the
magnetic force increases as B2.

.

Relevant Pages

  • =?ISO-8859-1?Q?A_little_frog_(alive_!)_and_a_water_ball_levitate_in?= =?ISO-8859-1?Q
    ... magnet in fog of liquid nitrogen can hardly surprise anyone these days ... Whether an object will or will not levitate in a magnetic field B is ... the vertical field gradient B2 required for levitation has ... levitate a superconductor and, from the formulas above, the ...
    (sci.physics)
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