MOTION MOUNTAIN

electromagnetic effects and challenges 211
Challenge 204 d
Ref. 170
Ref. 171
Ref. 172
Ref. 169
below the Earth’s crust.The crust is typically 30 to40 km thick (under the continents),
though it is thicker under high mountains and thinner near volcanoes or under the
oceans. As already mentioned, the crust consists of large segments, theplates, that float
on magma and move with respect to one another. The Earth’s interior is divided into
the mantle – the first2900 km from the surface – and the core. The core is made up of
a liquid outer core,2210 km thick, and a solid inner core of1280 km radius. (The temperatureofthecoreisnotwellknown;itisbelieved
to be 6 to7 kK. Can you find a way
to determine it? The temperature might have decreased a few hundred kelvin during the
last 3000 million years.)
The Earth’s core consists mainly of iron that has been collected from the asteroids
that collided with the Earth during its youth. It seems that the liquid and electrically
conducting outer core acts as a dynamo that keeps the magnetic field going. The magneticenergycomesfromthekineticenergyoftheoutercore,whichrotateswithrespect
totheEarth’ssurface;thefluidcanactasadynamobecause,apartfromrotating,italso
convectsfrom deep inside theEarth to moreshallow depths,driven by the temperature
gradients between the hot inner core and the cooler mantle. Huge electric currents flow
in complex ways through these liquid layers, maintained by friction, and create themagneticfield.Whythisfieldswitchesorientationatirregularintervals
of between a few tens
of thousands and a few million years, is one of the central questions.The answers aredifficult;experimentsarenotyetpossible,150
yearsofmeasurementsisashorttimewhen
comparedwiththelasttransition–about730000yearsago–andcomputersimulations
areextremelyinvolved. Since the field measurements started, the dipole moment of the
magnetic field has steadily diminished, presently by 5 % a year, and the quadrupolemomenthassteadilyincreased.Maybeweareheadingtowardsasurprise.*(Bytheway,the
studyofgalacticmagneticfieldsiseven more complex, and still in its infancy.)
Levitation
We have seen that it is possible to move certain objects without touching them, using a
magnetic or electric field or, of course, using gravity. Is it also possible, without touching
an object, to keep it fixed, floating inmid-air?Doesthistypeofrestexist?
It turnsout thatthereare several methods of levitating objects.These are commonly
divided into two groups: those that consume energy and those who do not. Among the
methods that consume energy is the floating of objects on a jet of air or of water, the floatingofobjectsthroughsoundwaves,
e.g. on top of a siren, or through a laser beam coming
from below, and the floating of conducting material, even of liquids, in strong radiofrequency
fields. Levitation of liquids or solids by strong ultrasound waves is presently
becoming popular in laboratories. All these methods give stationary levitation. Another
group of energy consuming methods sense the way a body is falling and kick it up again
in the right way via a feedback loop; these methods are non-stationary and usually use
magnetic fields to keep the objects from falling.The magnetic train being built in ShanghaibyaGermanconsortiumislevitated
this way. The whole train, including the passengers,islevitated
and then moved forward using electromagnets. It is thus possible, using
magnets, to levitate many tens of tonnes of material.
*In2005,it hasbeenreported thattheinnercoreoftheEarthseemstorotate fasterthan theEarth’scrust
byupto halfadegreeperyear.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net