Re: Science fiction or a wrong model again.

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

A conceptual model of electric current developed in an introductory
physics course is too naive to be useful in making predictions about
superconductors. Here is another illustration. Suppose that the emf=1
volt is suddenly inserted into a superconducting loop. Predict the
electric current. Ohm’s law predicts that I should keep increasing
when R --> 0. To overcome the "division by zero" dilemma one can
reason in a different way. The current is normally calculated as:

I=n*q*A*v,

where n is the density of free electrons (such as 10^28 per cubic
meter), q is the charge of one electron (1.6*10^-19 C), A is the
wire cross section (for example, 1 mm^2=10^-6 m^2) and v is
the velocity of electrons. To estimate v let me assume that the
length of the wire, L is one meter. After covering this distance
(along the wire) the kinetic energy of each electron will be 1 eV
and the corresponding v will be nearly 6*10^5 m/s. With this v
the above formula yields I~10^9 A, which is enormous and
ridiculous. Why ridiculous?  Because 1 eV per electron would
mean a total kinetic energy increase of 10^28 eV =1.600,000 J
(enough to elevate 10,000 kg to a height of one mile)!

A "Modern Physics" textbook I consulted (Serway, Moses and
Moyer, 1989) states that "supercurrents have been observed to
persist in a superconducting loop for several years with no
measurable decay." Does anybody know how such currents are
started and how large they are, typically? The textbook table
shows that in several Type 2 superconductoprs Bc2 exceeds
30 Teslas. What kind of power supply is used in commercial
superconducting electromagnets? Such devices are often used
in research to produce B~2 T, and above.

Larry Woolf wrote:

> Also, a superconductor has a critical temperature, a critical current
> Density, and a critical magnetic field, which is generally considered
> to be Hc2 since most commercially used superconducting wires are
> made of Type 2 superconductors.  For magnetic fields greater than
> Hc2,  there are no more superconducting regions in the material.
>
> By the way, some of you may be interested in knowing that Richard
> Hake did some of the major early work on high field superconductors
> before he became an physics education researcher.

I hope those familiar with practical supercoductivity will answer my
questions and will criticize my naive speculations in this thread.
Ludwik Kowalski