Originally posted by Andrew Hamilton
“…A material cannot superconduct when you have an ac-current. It is a dielectric and such a current will always vibrate dipoles and emit EM-radiation….”
Obviously I am no expert on this but I have never heard of superconductors not being able to have ac-current and don’t understand his explanation.
If a superconductor really could not handle ac ...[text shortened]... tal electronics.
So, why all the talk about superconductors making ultra-efficient computers!?
He is talking nonsense. He is referring to the fact that a diode only conducts in one direction and calls that 'dielectric'.
A dielectric is the insulating layer between capacitors, you have to have an insulator with as high a dielectric constant as possible to make both high voltage and high capacitance. If you have two plates X distance apart, you get Y amount of capacitance measured in Farads or Microfarads or Picofarads.
If you introduce a insulator with a dielectric constant of 2 and the spacing and the area of the capacitor plates are the same, you will double the resulting capacitance. The dielectric strength is the maximum voltage it will take per unit thickness. Here is a wiki about this issue:
You notice in this entire article there is no mention of diode action, one way flow of electricity.
You can for sure transmit AC down a superconductive line, it is already being done but on a small scale.
Vibrating dipoles. I think he is referring to the fact you can make AC turn into radio waves of the same frequency if you have a dipole antenna, where the length of the dipole is half the wavelength.
For a hypothetical signal of one hertz, which would be a wavelength by definition of 186,000 miles or 299,000 Kilometers so a dipole of half that length or 93,000 miles or 150,000 kilometers would be a fairly efficient radiator of one hertz AC.
So what we use in the US, 60 Hz, divide 90,000 by 60 and a dipole would have to be 1,500 miles long to radiate 60 Hz into space. Most long distance lines are way shorter than that deliberately, they can be tuned so it is antiradiative with proper filtering and such.
If it is 750 miles long, it would be 1/4 wave long and not a very efficient radiator, and 500 miles, even less radiative. So theoretically he is right, AC will radiate but only if you have extremely long wires.
Also, if you have two wires close together, not close enough to arc but a bit past that distance, in a long haul, you have one wire be the + and the other the - to use DC terminology, that further reduces the radiation of energy by turning the power lines into what are called feedlines.
You may know them as the TV twinlead wires going from an external antenna, two wires held a certain distance apart, they inhibit radiation and feed the power at whatever frequency they are interested in down that line THEN it goes to an antenna which efficiently radiates that energy into space, like in cell phone towers.
Every one of those antennas have feedlines, probably coaxial, which is like tv cable wiring, a conductive tube with an inner insulating layer and a wire in the center, does the same job as two wires side by side in suppressing radiation, you don't want transmission lines to radiate, you want them to conduct the energy to the antenna and IT radiates because it is designed to.
So his statement "A material cannot superconduct when you have an AC current' is false. It is maybe only valid in his world where his supposed diamond transistors superconduct and only in one direction because a transistor is basically a diode with a controlling element.
Superconductive wires conduct AC just fine and without radiation for the most part. You can't get away from SOME radiation even from an AC wire, a single wire, say 50 feet long but the percentage of the energy turning into radiation, say at 60 hertz is very small indeed. Enough to screw up a radio if you drive under a high tension line but it does die down as you travel away from them.
Us hams hate the dam things, we will not knowingly buy property within a few miles of them for that reason, they do radiate but a small amount but enough sometimes to add ac hum to a received signal which has to be filtered out.
One note on capacitors, two plates with a dielectric insulator in between, could even be air, but whatever it is, a capacitor conducts ac across it just fine, it's like a resistor reacting to DC, a bit more complex but capacitors are like shorts sometimes to RF but holds back DC.
They can be used therefore to transmit energy across the glass of a windshield to have an antenna that does not need to have a hole cut in the roof or a cable sticking out a window. One plate about 3 cm square on one side of the glass and a matching one on the other side and RF gets through quite efficiently.
At the point of making this an essay but here is a link to dielectric material: