Eternelle (eternele) wrote,

Я уже писала как-то про сверх-проводники. Повторяю на английском:

I would like to tell to you about an amazing property of some conductors - super-conductivity. I will try to be clear at the point that even your grandma would understand, as instructed us Einstein.

What is conductivity? As we all remember (or may be not) the Ohm's law from school, the current in a conductor is equal to the applied electric potential difference divided by the conductor resistance. It means that every conductor "resists" to the current. The greater the resistance is, the less the current for a fixed potential difference will be. For example, in your home you always have the potential difference of 220V, but the current in a light bulb or in a TVset won't be the same – it will depend on their resistance.

The Ohm’s law does not work all the time. Supra-conductors are so amazing, that they do not have any resistance, at all. Why it is so wonderful? The resistance provokes heating. Sometimes it can be useful – for instance, in electric oven the resistance of the metal disk is so high that produced heat allows to cook food and even to light up a cigarette. In a lightbulb, the tungsten filament heats up to 3000°C, it is the temperature of a most common star, like Proxima Centauri. At this temperature a heated body emits light. And in the tungsten filament the temperature goes up because of its resistance.

However, in general case, the heating means losing energy and danger. In case of a short circuit in the electric chain, the current, and therefore temperature, grows so high, it can burn everything out to hell.

Many years ago (more than hundred) it was noticed that the conductor resistance goes down with the temperature. "Hmmm, - ever curious scientists wondered, - what will happen if we go to the absolute zero - the lowest possible temperature?" As we all remember (once again, may be not), the absolute zero is about 273°Celcius below zero, much colder than anywhere on the Earth, including Antarctic, where the minimum registered temperature was 73°Celcius below zero.

But being scientist means to make the impossible possible. About the same time, thanks to laws of thermodynamics, they found a way to liquefy helium. The maximum temperature of the liquid helium is only four degrees above absolute zero. And the miracle happened.

Turns out, that at this temperature some metals – like mercury, plumb, aluminium – do not have the resistance at all, they turn into supra-conductors! Ironically, the resistance of the iron and the cuprum does not disappear.

In superconductors the electric current runs and runs in circles infinitely, without heat dissipation. “Yeahaa!” said scientists, “we can transmit the electricity from one end of the world to another”. But that means, we have to put all the wires in the liquid helium – very, very expensive.

So well for wires. If we can't do wires, than what? As we still (or don’t) remember from school, a current in a wire creates a rotating magnetic field around the wire. More the current is, more the magnetic field will be. So the most widespread application of the supra-conductors is supra-magnets – ones that you can see in IRM scanners or at CERN.

Moreover, this magnetic field is pushed away from supra-conductors (nobody knows exactly why). You can see it in this video:

Still, the extreme cost of this technology led to its decline. Scientists searched and searched supra-conductors working at higher temperatures but with no much success. Some very presumptuous persons even proved theoretically that supra-conductors can’t exist at “high temperatures”. Everybody started to lose interest in this subject, when, suddenly, in 1986 Bednorz and Müller found supra-conductors working at 35 degrees above absolute zero, above liquid hydrogen temperature.

They kept their discovery in secret for three months, repeating again and again experiments to verify their results. They chose a not very notorious journal Zeitschrift Physik to publish their results, because, unlike other scientific journals, it insist on articles being reviewed by other scientists. Bednorz and Müller were afraid they somebody can reproduce their experiments and publish results before them.

And history proved their right. When another scientist – Paul Chu send an article to Physical Review Letters, he made an intentional mistake in a supra-conductor’s formula in the draft submitted to the review. This faulty formula appeared in several articles right away. Ironically, it was a supra-conductor as well, but working at an inferior temperature. There was a big scandal – Paul Chu was blamed of scientific ethics violation !

Anyhow, Bednorz and Müller got a Nobel Prize next year after their discovery – incredibly rare incident. Rapidely, it gave rise to number of other supra-conductors, and finally their working temperature exceeded the temperature of the liquid azote, much more abundant than helium, hence, much cheaper.

What’s the catch? High temperature supra-conductors are not metals, that means, they are not easily deformable, they are very fragile and it is difficult to make wires with them. That’s why they search the same but more “metallic” and meantime some companies propose azote-containing cables:

Physicists like to dream, especially when they ask for a grant. The person, which presentation on supra-conductors I assisted, said with a passion in his eyes that someday we could fill the Sahara with solar panels and send the electricity everywhere in the world thanks to supra-conductors.

Still, less dreamy and more practical people work on magnetically levitating trains, capable to accelerate up to 500 kmh. Something likes this:

It’s all for the moment. See you.

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