What IS the current in a superconductor measured as?

mdeminico · Post by **mdeminico** » Wed Dec 29, 2010 3:48 pm

This has been bugging me for some time. I know it's not a Polywell related question, but this is the closest section I could find to ask the question, and Physicsforums.com is a piece of crap site (if you want the full story, PM me).

ANYHOW...

Since resistance is, I suppose, effectively zero, either current or voltage has to approach (or reach) zero, and the other goes to infinity, but I know this isn't the case.

Basically, how do I tell how much current goes into a superconductor? I presume it can't be infinite (because in that case, there would be infinite energy stored in the magnetic field of even a 1-inch long coil of wire.

So, how do you tell what that current is?

D Tibbets · Post by **D Tibbets** » Wed Dec 29, 2010 7:27 pm

Perhaps helpful, or possibly more confusing, but, using the formula I=V/R or Current = Volts/ Resistance.
Resistance is very close to zero (can it be absolute zero Ohms?)
I have heard that Voltage across the superconductor is also almost zero. So Current also will also be almost zero. Perhaps this basic formula does not apply to super conductors.

If there is no current, how can a magnetic field be generated? Confusing indeed!

Dan Tibbets

mdeminico · Post by **mdeminico** » Wed Dec 29, 2010 8:40 pm

That's what I'm wondering... this answer is something you'd think has a straightforward "google it" answer, but it's not out there.

rjaypeters · Post by **rjaypeters** » Wed Dec 29, 2010 8:42 pm

Tom Ligon wrote a post a few months back about how a superconducting current is created, he might know the answer to your question.

happyjack27 · Post by **happyjack27** » Wed Dec 29, 2010 8:49 pm

well resistance is very close to zero so presumably you only need a very small voltage to get large current. hence the appeal.

chrismb · Post by **chrismb** » Wed Dec 29, 2010 9:01 pm

0V, high current [current = charge carriers passing a given point in a given time]. V=IR; 0=I x 0. {LHS=RHS}

icarus · Post by **icarus** » Wed Dec 29, 2010 10:44 pm

Carver Mead has the answers on this one.

http://www.amazon.com/Collective-Electr ... 0262133784

Part 1, sect. 1.1;

Magnetic flux = inductance*current

total energy in loop, W = 0.5*inductance*current^2

Magnetic flux = n*quantum-fluxoid; n is an integer

KitemanSA · Post by **KitemanSA** » Thu Dec 30, 2010 1:22 am

Hmmm. My recollection is that V=IZ, not IR. Z=R+iX where X is the reactance. The resistance is perhaps 0 but is the reactance?

hanelyp · Post by **hanelyp** » Thu Dec 30, 2010 2:17 am

V = IZ is most useful dealing with AC at a known frequency where the reactance can be calculated. dI/dt = V/L may be more useful for a charging inductor, change in current over time = voltage/inductance.

mdeminico · Post by **mdeminico** » Thu Dec 30, 2010 3:47 am

icarus wrote:Carver Mead has the answers on this one.

http://www.amazon.com/Collective-Electr ... 0262133784

Part 1, sect. 1.1;

Magnetic flux = inductance*current

total energy in loop, W = 0.5*inductance*current^2

Magnetic flux = n*quantum-fluxoid; n is an integer

I know those formulas, just am wondering what the heck current could possibly be.

Will · Post by **Will** » Thu Dec 30, 2010 6:46 am

Current would simply be measured as Amps, like always.

V=IR, Ohm's Law doesn't apply in all cases, with superconductors being a notable one (solid state electronics is another).

Remember, amperage is coulombs per seconds, which is quite literally a measure of electron charges per second moving past a point. A superconductor simply passes a current, without needing a voltage to be present.

icarus · Post by **icarus** » Thu Dec 30, 2010 8:51 am

I know those formulas, just am wondering what the heck current could possibly be.

If you knew those formulae, you would not be asking the question in that way I think .... current (Amps) is charges (coulombs) per second flux ... literally.

KitemanSA · Post by **KitemanSA** » Thu Dec 30, 2010 12:35 pm

hanelyp wrote:V = IZ is most useful dealing with AC at a known frequency where the reactance can be calculated. dI/dt = V/L may be more useful for a charging inductor, change in current over time = voltage/inductance.

True, but while charging there is a dI/dt, so reactance works to determine I, no?

chrismb · Post by **chrismb** » Thu Dec 30, 2010 12:54 pm

Superconductivity is a DC effect.

Any real length of conductor has a non-zero Z, so one could argue that there are no such things as superconductors for AC.

mdeminico · Post by **mdeminico** » Thu Dec 30, 2010 1:40 pm

icarus wrote:
I know those formulas, just am wondering what the heck current could possibly be.
If you knew those formulae, you would not be asking the question in that way I think .... current (Amps) is charges (coulombs) per second flux ... literally.

I'm probably completely missing this (wouldn't be surprised, I need to brush up on my basic physics), but how do you go from, say, 100 amps at 120 volts input, to ???? amps at indeterminate voltage in the superconducting inductor? It's not 100 amps in the inductor, but is it 100,000,000 amps? Is it 10 amps? Somewhere it changes from 100A @ 120V into something different. I'm wondering what that something different is.