Be careful saying "never".

Post #71,922 by Another Scott 1/1/03 8:48:57 PM Reply	Be careful saying "never". Ashton writes: In the physical word, there is never 0 resistance or infinite resistance; nor can a delta-function exist (a pulse with 0-rise-time). Emphasis added. As I'm sure you're aware, [link\|http://www.suptech.com/super101.html\|superconductors] have 0 (zero), not "really, really small" resistance. :-) They're used in several real systems, e.g., [link\|http://www.brown.edu/Administration/News_Bureau/2001-02/01-083.html\|MRI machines] (as discussed in the link). Cheers, Scott. (Pedants R Us. :-)
Post #71,924 by Ashton 1/1/03 9:17:06 PM Reply	Methinks that "0 resistance" does not transmit EMF across those er "frictionless outer valence electrons?" at precisely C - a velocity indistinguishable from that of the [also impossible to attain] "perfect vacuum". So, I'll take 'never' as a good approximation ;-) You'll note that 'DC' is the preferred mode of operating superconducting magnets.. and as the electrons dribble FIFO, this question of degraded velocity only appears not-to matter. And it doesn't (!) in these primitive applications. (And maybe hedge the bet. Given that the trend towards proliferation of a few of those kilotons of fissionable material which we have accumulated for our illusion of security: this fact + homo-sap suicidal tendency pretty much places a Species Survival Limit upon this species' imaginations of the word forever, and by implication never.) My mere guess is that TANSTAAFL Rulez. Ashton [cackle]
Post #71,941 by Another Scott 1/1/03 10:41:32 PM Reply	Electrical resistance and c are different issues. Electrical resistance of zero means that there's no power loss - i.e. P = IV = I^2R = 0. Thus, no heating of the material due to the current (no energy is lost to the material by phonons (lattice vibrations)). As discussed [link\|http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html#c1\|here], the "drift velocity" or the net electron speed through a conductor is very slow - a few cm/hr to a few m/s - far less than c. They're [link\|http://www.lns.cornell.edu/spr/2002-01/msg0038305.html\|faster] in superconductors but still far less than c. Superconductors aren't otherworldly. They're simply materials that have the proper combinations of electronic and lattice energy states such that conduction electrons can "pair", act as bosons, and have a forbidden "energy gap" such that they [link\|http://hyperphysics.phy-astr.gsu.edu/hbase/solids/bcs.html#c1\|cannot lose energy to the lattice] when the material is kept below a certain characteristic temperature. Cheers, Scott. (Who agrees that perfect vacuum, like "empty space", is ideal and doesn't exist. And who agrees that superconductors are lossy using AC.)
Post #71,948 by Ashton 1/1/03 11:42:01 PM Reply	Well... DC resistivity does seem bulletproof and like Newton's notes re fluxions, (IIRC he had an aside ~ "assuming mass remains constant" - which some take to be a prescience of Relativity ;-) I must eat the non-0 resistivity claim prima facie, since E=IR presumes 'DC'! :( Now as to impedance and the analogues of capacitive or inductive reactance: looks as if we have here not much likelihood of superconducting transmission, rectification? for much beyond milliHertz frequencies. Also kinda amazing data point distributions in the experimental evidence: can one say "straight line"? (The EE ideal is of course, "the straight wire with gain", as some early transistorized amplifier mfgs. loved to spin.) I have some experience of cryo-vacuum systems, incl. He reefers - but not with superconducting magnets (hands-on). Maybe.. had the SCSC gotten past the pork-barrel brigade, though moving to Texas would have cut it for me. Now I shall have to imagine Bosons! in practical day-day application. Nicely complete links. Thanks, Ashton
Post #71,950 by dmarker 1/2/03 12:15:07 AM Reply	A/C for runing a super conductor The only reason I can see for any inefficiency is that the electrons have to slow down & reverse direction & the logistics required to do that are inefficient plus the beneficial effect of the magnetic flux provided by the electron flow through the superconductor material, gets reversed every cycle. The 1st problem is losses due to latency in the collapse of the magnetic field then there is the effect of the revers polarity... I mean can you imagine what the ride would be like in Shanghai's maglev supertrain if the field holding it up reversed 50 or so times per second. That would be one brain scrambling ride <grin> Cheers Doug

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