The Speed of Electricity
John Bell (Change John to Liberty for email)
It recently occured to me that this is not far from what you might
expect for the speed of light in a reasonably dense refractive medium,
which got me wondering:
What is the speed of electricity in a superconductor ?
With no resistance and, hence, no scattering (presumably), one would
expect it to be significantly faster.
Could it even be c (to within the limit of accuracy of observation)?
Presumably, the situation is a bit different in a vacuum tube or
electron microscope, since there, I would have thought the speed
depends both on the potential gradient and how far the electrons have
travelled between cathode and anode
For comparison purposes, does anyone know how fast electrons travel
(at the target) in modern electron microscopes, and the associated
wavelengths?
Gordon Stangler
the inductance and C is the capacitance.
Assuming the superconductor is a wire,
L = frac{mu_0}{2\pi} ln(r), where \mu_0 is the permeability of free
space, and r is the radius of the wire.
C = frac{1}{I}frac{dV}{dt}
Hope this helps.
Uncle Al
>
> IIRC, the speed of electricity in copper wire is ~2/3 c.
For DC, the signal is fast but the electrons are slow (mm/sec). One
electron/copper atom, FW = 63.456, d = 8.95 g/cm^3, 96,485.34
coulombs/mole electrons. Pick a wire diameter and an amperage.
For a single round copper wire, signal speed is lightspeed divided by
the square root of the surrounding dielectric constant near enough (re
permativity). Vacuum has permitivity and permeability. For ladder
line signal propagation can be 95% c. Coaxial cable, twisted pair,
circuit traces, etc.
> It recently occured to me that this is not far from what you might
> expect for the speed of light in a reasonably dense refractive medium,
> which got me wondering:
Permeability is generally inert in refractive indices and magnets tend
to be extremely opaque. Very high refractive index glasses using
magnetic lanthanide metal ions exist.
> What is the speed of electricity in a superconductor ?
> With no resistance and, hence, no scattering (presumably), one would
> expect it to be significantly faster.
Cooper pairs have a conjugate momenta. Superconductivity is strictly
a DC phenomenon. Are we talking the speed of the signal or the speed
of the electrons' centers of mass net drift?
> Could it even be c (to within the limit of accuracy of observation)?
The supercon is surrounded by insulation or vacuum. It is hard to see
how that doesn't enforce the usual speed limits,
Google
superconducting "delay line" 17,300 hits
superconductor "signal speed" 266 hits
> Presumably, the situation is a bit different in a vacuum tube or
> electron microscope, since there, I would have thought the speed
> depends both on the potential gradient and how far the electrons have
> travelled between cathode and anode
and Special Relativity at more than 60 kV potential difference - SLAC
when it was running.
> For comparison purposes, does anyone know how fast electrons travel
> (at the target) in modern electron microscopes, and the associated
> wavelengths?
Electron microscopy is ~40 to 400 keV. High Resolution transmission
EM (aberration corrections in software) will squeeze down just below
an angstrom, 50 million magnification.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2