Q: maximum current density in metal
sergius
I'm trying to find out if it's possible to have a current density in the order
of 10^8 amps / square cm in metal, disregarding the heat produced.
By rough estimate, to produce such current densities, electrons must move in
one direction with average speed comparable to that of their thermal motion.
That is, according to the classical model of electron gas.
My Physics Reference Guide, which is quite old, states that at high current
densities Ohm's Law is incorrect. My father tells me that he read about an
experiment on semiconductor breakdown that proved Ohm's Law for extremely high
current densities, but he cannot recall any details...
Could anyone shed some light on this or suggest a good reference?
Thanks in advance,
Sergei
PS this message has been posted to sci.physics, but so far I got no reply.
Perhaps, I'll have better luck here. Sorry if you've already seen this
message.
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Joe Fischer
: I'm trying to find out if it's possible to have a current density in the
Wire size is rated in amps, and 6 volt battery
cables are fatter than 12 volt, does that suggest
anything?
Current is a function of load, assuming capacity
is adequate.
Voltage is what enables current, so too high a
voltage will enable too high a current for any resistance
device, and burn it up.
And if the wire size is too small, the wire will
also burn.
Actually, even without superconducting, if you
can cool the wire fast enough, it will carry more
current.
That is why the wires on the poles can have
a smaller size for the same current.
But why are you asking? Current is only
related to power by the inverse of voltage, meant
to say, increase the voltage and get the same
power with less current.
There are places where current is needed,
such as in electroplating, you will have to
check Coulomb theory to see why current is needed
to separate atoms.
But for most uses, the devices are designed
to use voltages where wire size is kept nominal.
The bottom line may be, it isn't the metal,
it is the heat dissipation that sets the limits
on current, as all normal conductors have some
resistance, and are limited.
Maybe you are asking something less
complicated, like silver is best, copper
is next, at room temperature (for now).
Joe Fischer
Douglas Natelson
sergius wrote:
>
> Hi All!
>
> I'm trying to find out if it's possible to have a current density in the order
> of 10^8 amps / square cm in metal, disregarding the heat produced.
Yes, it is possible to have current densities that high, and even almost
an order of magnitude higher. For example, look at the following
website:
http://www.cismi.dk/main/RunProj/NANO97_98/task3_a.htm
These folks have been studying properties of very small wires, including
how they fail.
Turns out that macroscopic wires, like 20-gauge, tend to fail at high current
densities because they usually melt. Very small wires, however, are often
firmly attached to a substrate (single-crystal silicon, for example), and so
heat generated within the wires can be transported away easily. So, such wires
tend to fail by other means, such as electromigration, a process in which
the conduction electrons actually transfer enough momentum to defects in the
metal lattice to move atoms around.
> My Physics Reference Guide, which is quite old, states that at high current
> densities Ohm's Law is incorrect.
Well, Ohm's Law is a linear approximation. It can break down in a few
different limits. Probably the dominant reason for apparent breakdowns
at very high current densities is a combination of heating and the
temperature dependence of the resistivity.
Also, at very low temperatures in systems of appropriately small size,
quantum interference effects can seriously change the I-V characteristics
away from simple classical models. Check out Yoseph Imry's (pretty advanced)
book on mesoscopic physics for a pretty exhaustive set of references.
Doug Natelson
sergius
nate...@no-spam.erols.com wrote:
> Hello -
>
> sergius wrote:
> >
> > Hi All!
> >
> > I'm trying to find out if it's possible to have a current density in
the order
> > of 10^8 amps / square cm in metal, disregarding the heat produced.
>
> Yes, it is possible to have current densities that high, and even
almost
> an order of magnitude higher. For example, look at the following
> website:
> http://www.cismi.dk/main/RunProj/NANO97_98/task3_a.htm
> These folks have been studying properties of very small wires,
including
> how they fail.
>
Thank you very much, this is exactly the application that I'm interested
in -- high current in small wires. Very useful reference.
> Turns out that macroscopic wires, like 20-gauge, tend to fail at high
current
> densities because they usually melt. Very small wires, however, are
often
> firmly attached to a substrate (single-crystal silicon, for example),
and so
> heat generated within the wires can be transported away easily. So,
such wires
> tend to fail by other means, such as electromigration, a process in
which
> the conduction electrons actually transfer enough momentum to defects
in the
> metal lattice to move atoms around.
Interesting. I'll have to look into that.
>
> > My Physics Reference Guide, which is quite old, states that at high
current
> > densities Ohm's Law is incorrect.
>
> Well, Ohm's Law is a linear approximation. It can break down in a few
> different limits. Probably the dominant reason for apparent breakdowns
> at very high current densities is a combination of heating and the
> temperature dependence of the resistivity.
>
I think that the authors of the Reference Guide derived this idea from
whichever model of conductivity they were using. They were very vague
about it.
Later I found a more thorough treatment of the problem, plus
experimental data on current density up to about 10^9 amps / sq cm in
pure copper, for example. They used short pulses. The resistance was
more or less constant. All of this was in a book on Semiconductor
Physics by Karlheinz Seeger.
> Also, at very low temperatures in systems of appropriately small size,
> quantum interference effects can seriously change the I-V
characteristics
> away from simple classical models. Check out Yoseph Imry's (pretty
advanced)
> book on mesoscopic physics for a pretty exhaustive set of references.
>
> Doug Natelson
>
>
Thanks for this reference, too. I didn't have a chance to look at it
yet, but it may prove very helpful.
Sergei Shkarupo
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Phil_m...@yahoo.com
Physics Today. The electrons seem to travel ballistically so there is
little or no heating. I seem to remember that the largest measured
current was of the order of magnitude you are interested in.
Phil Morrison
> > sergius wrote:
> > >
> > > Hi All!
> > >
> > > I'm trying to find out if it's possible to have a current density
in
> the order
> > > of 10^8 amps / square cm in metal, disregarding the heat produced.
[Moderator's note: Much unnecessary quoted text deleted. -MM]
sergius
Phil_m...@yahoo.com wrote:
> Look at the current density in nanotubes - an article in this month's
> Physics Today. The electrons seem to travel ballistically so there is
> little or no heating. I seem to remember that the largest measured
> current was of the order of magnitude you are interested in.
> Phil Morrison
Thanks, that's very interesting. Libraries around here (SF Public
Library and UC Berkeley) don't have the May issue yet, I'll check it out
as soon as it arrives. And there's a book called "Physical Properties
of Carbon Nanotubes", but it's checked out.
> [Moderator's note: Much unnecessary quoted text deleted. -MM]
[Moderator's note: Ditto. -P.H.]