> Ti ringrazio se puoi posta l'estratto di cui parli.
Come promesso...
Tratto da: Patrik Fazekas, "Lecture Notes on Electron Correlation
and Magnetism" - Series in Modern Condensed Matter Physics, Vol.5,
World Scientific.
E' il regalo di Natale di un carissimo amico; non ho avuto ancora
modo di leggerlo ma solamente di sfogliare distrattamente i primi
capitoli. Comunque, giusto all'inizio, ricordavo di poter trovare
alcune risposte alla tua domanda...
Dal paragrafo 1.2 "Sources of Magnetic Fields", estraggo solo le
parti piu' interessanti e direttamente legati al tuo quesito. I
riferimenti bibliografici dati in questo paragrafo sono:
[1] Boebinger, G., A. Passner and J. Benk: Sci. Am. 272, 34 (June 1995)
(dovrebbe trovarsi in italiano, aggiungo io)
[2] Verschuur, G. I.: "Hidden Attraction: The Mystery and History
on Magnetism", Oxford University Press, New York, and Oxford, 1993.
Ecco l'estratto, non credo che l'autore se ne avra' a male per poche
righe.
"[...] The Earth's magnetic field is about half a gauss (1T=1e4G).
It was extraordinarily important in the history of physics [2] to
have a source of much more intense fields to experiment with: the
naturally magnetic magnetite and magnetized iron. [...] The most
powerful permanent magnet (like samarium-cobalt or neodymium-iron-
boron magnets) have fields of 3000-4000G. The density of polarizable
electrons in solids puts theoretical limit of ~3T to fields we can
expect from permanet magnets. [...] In our laboratories, we now
routinely have fields of 5-30T. These are produced by electromagnets
[...] If you try to drive too much current, your electromagnet
breaks or melts. The current record for a DC resistive magnet is
~30T [1].
We need not worry about the heating effect if we use superconducting
coils. Here, however, the critical field of the superconductor limits
the the performance (to below ~20T). Apossibility to circumvent this
limitation is to build a hybrid magnet, putting a resistive electromagnet
insidea larger superconducting one. Hybrid magnets with fields in excess
of 20T are now commercially available, and the current record field
from a source is ~38.5T. [...] Higher fields can be produced in
pulses of duration from �seconds to ~100ms. This may look short
for a human observer but many solid state relaxation phenomena take
much less time, thus for research, a field pulse is often as good as
a DC field. With a non-destructive pulsed magnet you have a pulse, and
then wait for the magnet to cool off. Field exceding 70T can be
achieved. A quite radical approach is to build a self-destructing
magnet which disrupts upon producing the pulse. Apparently ~150T
can be produced with relative ease and regularity. In extreme
circumstances, fields approaching ~1000T have been detected.
Wheter it is DC or pulsed magnets, producing near-record fields
takes clever design, and a lot of money and electric power. There are
renowned high-field laboratories (Grenoble, Tallahassee, Tokyo,
Amsterdam, etc...) which are visited by researcher wishing to do
experiments in intense magnetic fields. However, these human
achievements are dwarfed by what Nature can do: the magnetic field
at the surface of a pulsar is estimated to be ~1e8T."
(nota: la notazione esponenziale e' mia, in originale e' 10 alla...
e anche la conversione gauss-tesla l'ho introdotta per convenienza:
nell'originale e' in un altro punto)
Hope it helps...
Andrea
--------------------------------
Inviato via
http://usenet.libero.it
Received on Wed Mar 12 2003 - 09:49:47 CET