[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Longitudinal Forces in Tesla Coil Metallic Conductors
Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>
Peter,
I agree - at least for exploding coils.
However, things become quite a bit more interesting when you use
higher voltages, shorter pulses, and straight wires. In one
experiment, Peter Graneau suspended a freely hanging straight 1.2mm x
100 cm aluminum wire. He then discharged an 8 uF 100 kV capacitor
bank through small spark gaps at each end of the suspended wire. A
variable high voltage inductor (0-2000 uH, nominally 1000 uH) was
inserted into the current path to control peak current to 5-6 kA. The
capacitor bank was switched through the suspended wire and gaps using
a mechanical HV switch.
When a bank voltage above 60 kV was used, brittle fracture of the
wire was observed. This was NOT the stretching and necking ("sausage"
or "kink" instabilities) that we often see during coin shrinking.
These were brittle fractures - the wire seems to have simply
shattered. There is little doubt that substantial tensile forces are
at work, but their cause has been the subject of considerable debate.
A paper that describes Graneau's experiment and analysis can be found here:
Graneau, Peter, "AMPERE TENSION IN ELECTRIC CONDUCTORS", IEEE
Transactions on Magnetics, Volume 20, Issue 2, Mar 1984, Pages 444 - 455
Similar discussions appear in two of Graneau's books ("Newtonian
Electrodynamics" and "Ampere-Neumann Electrodynamics of Metals")
mentioned in my earlier post.
Similar behavior was seen in exploded copper wires by an earlier
researcher working on the behavior of electrical fuses in volume 3 of
the Exploding Wires series of books by Chace:
J. Nasilowski, "Unduloids and striated disintegration of wires,",
Exploding Wires. New York: Plenum, 1964, volume 3, pp. 295-313.
Although Graneau favors Ampere longitudinal forces as the
explanation, a number of later researches lean towards skin effect
and flexural vibrations as being responsible:
Wall, D.P.; Allen, J.E.; Molokov, S.; Lukyanov, A., "The
fragmentation of wires due to pulsed currents: influence of the skin
effect", Pulsed Power 2000 (Digest No. 2000/053), IEE Symposium, 2000
Lukyanov, A.; Molokov, S., "Why high pulsed currents shatter metal
wires?", Pulsed Power Plasma Science, 2001. PPPS-2001. Digest of
Technical Papers, Volume 2, 2001
A. Lukyanov, S. Molokov, "Do we need to recourse to Ampere-Neumann
electrodynamics to explain wire fragmentation in the solid state?", see:
http://arxiv.org/pdf/physics/0012029
As with overcoupling and racing sparks, the final chapter on wire
fragmentation may not have been written as yet... :^)
Bert
Tesla list wrote:
Original poster: "Peter Terren" <pterren@xxxxxxxxxxxx>
I have the pleasure of exploding a few wires and also have a wire
collection from my 16kJ cap bank. I did a test fire of a can crush
with fine wire (.024inch) in a plastic tube to see if the resulting
plasma channel was sufficient to give a crushing
current. Experimental setup is here.
http://tesladownunder.com/Pulse_Power.htm#Can%20crusher%204
The net result at 4 kJ was slight denting of the can (perhaps
equivalent to 500J worth) with no remaining charge on the cap. So
the plasma held until the cap emptied but did not allow a high
enough peak current compared to the wire. I did not control for the
reduced physical strength of the plastic tubing compared to the wire
however. With the heavy wire, the can is torn in three pieces in that setup.
I don't really see why new physics is suggested by this. The
radially expanding wire with a uniform force will bring out
inhomogeneities in the wire and magnify them into physical rupture
points. (in a way somewhat analagous to the Big Bang giving
microwave background irregulatities, but that would really imply
some new physics...).
Peter
Hi Bert,
I got a batch of your shrunken quarters years ago and I requested
to get the "wire fragments too" ;-)))
"Normal" people get a kick out of the coins, but "I" got the wires
too since that is what is "really cool" :o))
........ There are inertial and explosive forces that tend to bend
the wire up too along with impacts to the containment walls... The
wire fails in weak areas that become dominate fracture areas. I
suspect plasmas became conductors as the system ripped apart too.........
Cheers,
Terry