Superconductor question

Arrow · Post by **Arrow** » 2003-03-27 07:49am

Are there any limits to the amount of power that you can move across a superconductor? For instance, could you (concievibly) create a superconduct that allow megawatts of power to flow through it over an extended period of time? How about gigawatts? Terawatts?

Kuroneko · Post by **Kuroneko** » 2003-03-27 09:41am

Arrow Mk84 wrote:Are there any limits to the amount of power that you can move across a superconductor? For instance, could you (concievibly) create a superconduct that allow megawatts of power to flow through it over an extended period of time? How about gigawatts? Terawatts?

This question makes no sense.

By Ohm's Law, Voltage V, Current I, and Resistance R are related by V = IR. In a superconductor, R = 0, meaning there is no voltage. Consequently, VI = 0. If you think that irrelevant, please note that volt-amps are watts. To reiterate: there is no power.

A more meaningful question would be "Can one pass mega-amps of current through a superconductor?"

Superconductors stop being superconductive once a "critical current" or a "critical field" is reached. They vary vastly in their specs, but there is a Type I superconductor with a critical current density of around 20,000 A/mm^2 even at high magnetic field strengths. There are other superconductors that can handle mega-amp-range current if they're thick enough.

As for giga-amp-range current, I highly doubt it. Unless, of course, the superconductor is monstrously thick. Otherwise, it would be overpowered by its own magnetic field.

Arrow · Post by **Arrow** » 2003-03-27 10:30am

Ok, thanks.

Arrow · Post by **Arrow** » 2003-03-27 10:37am

How could one possibly move large amounts of energy then? Lasers, or some such?

Obviously, electrical engineering is something I know little about.

Kuroneko · Post by **Kuroneko** » 2003-03-27 09:04pm

I don't see what's so bad about good old copper wire.

Luke Starkiller · Post by **Luke Starkiller** » 2003-03-27 09:34pm

Kuroneko wrote:I don't see what's so bad about good old copper wire.

The resistance inherant in a copper wire causes losses in the form of heat from the energy flowing through the wire. Enough heat and the wire will melt, that is how fuses work. Superconductors have extremely low resistance causing them to be more effecient at transfering energy andd allowing higher amperages. They are of course prohibitvely expensive for any but the most highly funded research projects.

Kuroneko · Post by **Kuroneko** » 2003-03-27 09:43pm

Luke Starkiller wrote:The resistance inherant in a copper wire causes losses in the form of heat from the energy flowing through the wire. Enough heat and the wire will melt, that is how fuses work.

I know that, but having a very thick copper wire is a heck of a lot less expensive than a superconductor, even with the power loss due to heat.

Shaka[Zulu] · Post by **Shaka[Zulu]** » 2003-03-28 01:23am

yeah yeah... electrical superconductors are all well and good...

what I want is a Thermal superconductor -- cheap!

perfect spacecraft hull plating... heh

Kuroneko · Post by **Kuroneko** » 2003-03-28 02:33am

Shaka[Zulu] wrote:yeah yeah... electrical superconductors are all well and good...

what I want is a Thermal superconductor -- cheap!

perfect spacecraft hull plating... heh

I'm not familiar with that term.

A "normal" superconductor "holds" current in a loop almost indefinetely. An analogue for heat--a substance that "holds" thermal energy indefinetely is impossible by Stefan-Boltzmann law (in the end, all heat transfer, even conduction, is due to a transfer of EM radiation between the atoms of the materials).

Unless by "thermal superconductor" you meant something completely different.

Post by **Beowulf** » 2003-03-28 02:39am

I think what he means is something that has near infinite thermal conductivity, so that the surface would have an even temperature no matter where the heat is coming from.

Shaka[Zulu] · Post by **Shaka[Zulu]** » 2003-03-28 02:55am

exactomundo beowulf... that way, the material can radiate heat away just as fast (from the unheated side) if not faster than it takes it in (from the heated side)... no more TPS worries for reentry vehicles for one thing.

Kuroneko · Post by **Kuroneko** » 2003-03-28 03:40am

beowufl wrote:I think what he means is something that has near infinite thermal conductivity, so that the surface would have an even temperature no matter where the heat is coming from.

Shaka[Zulu] wrote:exactomundo beowulf... that way, the material can radiate heat away just as fast (from the unheated side) if not faster than it takes it in (from the heated side)... no more TPS worries for reentry vehicles for one thing.

Isotopically pure C12 diamond has the most insane thermal conductivity, something along the lines of 5000 W/(mK), about 12 times higher than silver (best of the metals). However, if you want temperature to be as homogeneous as possible, what you want is a high heat diffusivity, not necessarily high conductivity (diffusivity is κ = k/(rC), where k is conductivity, C specic heat, r density). The higher the thermal diffusivity is, the faster a thermal disturbance propagates throughout the material.

On the other hand, a high diffusivity would probably come at the cost of a lower specific heat, which is a property one would't want in re-entry hulls (the lower the specific heat, the faster the increase in temperature in the material).

Shaka[Zulu] · Post by **Shaka[Zulu]** » 2003-03-28 04:22am

dammit... doesnt the whole 'no free lunch' motto of the universe AFAWCT really bite?

Kuroneko · Post by **Kuroneko** » 2003-03-28 04:29am

Shaka[Zulu] wrote:dammit... doesnt the whole 'no free lunch' motto of the universe AFAWCT really bite?

In the words of Hawking, the Universe is the ultimate free lunch.

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