Hacker News Reader

Comment by steve19 | Hacker News Reader

Can someone explain what it means to carry current at room temperature without any heat loss.

As a layman I interpret this as the material not having any resistance. If it does have resistance, what is that resistance generating if not heat? What happens if I put 200 amps through a thin stand of it?

View on HN

I interpreted the comment as quantum ballistic conduction, similar to what has been observed in carbon nanotubes and some other systems. There's resistance, but it's effectively quantised (ie, there's an integer number of quantum 'channels'). It's independent of the length of the material, so it's non-Ohmic. Which in this case effectively means other than an initial resistance of basically getting current into the quantum channel, there's no further conductive loss.

FYI - Ballistic conduction is _not_ super conductivity, as the electrons haven't paired up to form Cooper pairs via a phonon lattice distortion, the current carriers are not bosons, and doesn't display most of the other characteristics of superconductivity (eg Meissner effect).

https://en.m.wikipedia.org/wiki/Ballistic_conduction https://en.m.wikipedia.org/wiki/Conductance_quantum

"Loss" is the term we use when we want heat, but it's going away against our wishes. The correct term for generating and eliminating unwanted heat is "dissipation".

That this is the intended meaning is clear from the later wording "... being able to conduct electricity without generating any waste heat".

> What happens if I put 200 amps through a thin stand of it?

That's a good question. Even if a material can carry some small current with apparently zero R, that doesn't mean the same R value applies at a high current. It could be non-ohmic, that is.

If you want to read the details on how it works, I suggest this paper [1] "Large-gap quantum spin Hall insulators in tin films". The real hope is that this will be a replacement for the conductors in traditional silicon. You still have the heat bottleneck when you hit a traditional conductor. The dissipation free property breaks down at about 100C for this material, so you are going to have an issue with what ever is interacting with that 200A. [1] http://arxiv.org/pdf/1306.3008

Don't know about the implementation specifics, but Landauer's principle says that heat dissipation corresponds directly to information erasure. So, however they do it, it would have to be some kind of reversible circuit.

https://en.m.wikipedia.org/wiki/Landauer%27s_principle https://en.m.wikipedia.org/wiki/Reversible_computing

I read it as meaning that it should be a superconductor. Why they do not mention it, I don't understand...

Couldn't there be electromagnetic resistance too ?

From the sound of it, you'll probably be able to transmit much lower amounts of electricity over a much longer distance in a conductor, and expend less effort sending the weak transmission.

200 amps proportional to an amount of work, measured in watts. If the wire demands less work, but you expend a constant amount of work, you'll just be sending that many more electrons to the other side of the wire. Like hitting a baseball with a baseball bat in fresh air, as opposed to under water.

I guess that means it's a super conductor, or maybe just a really good conductor? Did they say zero resistance, or only very low resistance?

First off in the scenario you're describing it makes no sense to assume a constant voltage, so you shouldn't be conflating amps and watts.

Second, the amount of watts going through a wire is not at all the same as the amount of watts being dissipated/expended in a wire. You're assuming that it's being used as a space heater, and it's not. The baseball is going through a very thin or empty environment and 99+% of the energy is still there at the other end.

Reply on news.ycombinator.com