Superconductivity


INTRODUCTION
We\'ve all heard about superconductivity. But, do we all know what it is?
How it works and what are its uses? To start talking about superconductivity, we
must try to understand the how "normal" conductivity works. This will make it
much easier to understand how the "super" part functions. In the following
paragraphs, I will explain how superconductivity works, some of the current
problems and some examples of its uses.

CONDUCTIVITY
Conductivity is the ability of a substance to carry electricity. Some
substances like copper, aluminium, silver and gold do it very well. They are
called conductors. Others conduct electricity partially and they are called
semi-conductors. The concept of electric transmission is very simple to
understand. The wire that conducts the electric current is made of atoms which
have equal numbers of protons and electrons making the atoms electrically
neutral. If this balance is disturbed by gain or loss of electrons, the atoms
will become electrically charged and are called ions. Electrons occupy energy
states. Each level requires a certain amount of energy. For an electron to move
to a higher level, it will require the right amount of energy. Electrons can
move between different levels and between different materials but to do that,
they require the right amount of energy and an "empty" slot in the band they
enter. The metallic conductors have a lot of these slots and this is where the
free electrons will head when voltage (energy) is applied. A simpler way to look
at this is to think of atoms aligned in a straight line (wire). if we add an
electron to the first atom of the line, that atom would have an excess of
electrons so it releases an other electron which will go to the second atom and
the process repeats again and again until an electron pops out from the end of
the wire. We can then say that conduction of an electrical current is simply
electrons moving from one empty slot to another in the atoms\' outer shells.
The problem with these conductors is the fact that they do not let all the
current get through. Whenever an electric current flows, it encounters some
resistance, which changes the electrical energy into heat. This is what causes
the wires to heat. The conductors become themselves like a resistance but an
unwanted one. This explains why only 95% of the power generated by an AC
generator reaches consumers. The rest is converted into useless heat along the
way. The conducting wire is made of vibrating atoms called lattice. The higher
the temperature, the more the lattice shakes making it harder for the electrons
to travel through that wire. It becomes like a jungle full of obstacles. Some
of the electrons will bump with the vibrating atoms and impurities and fly off
in all directions and lose energy in form of heat. This is known as friction.
This is where superconductivity comes into work. Inside a superconductor, the
lattice and the impurities are still there, but their state is much different
from that of an ordinary conductor.

SUPERCONDUCTIVITY (Theory / history)
Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes, a Dutch
physicist. It is the ability to conduct electricity without resistance and
without loss. At that time, it took liquid helium to get extremely low
temperatures to make a substance superconduct, around 4 kelvins. That wasn\'t
very far from absolute Zero (The theoretical temperature at which the atoms and
molecules of a substance lose all of their frantic heat-dependent energy and at
which all resistance stops short.) Kelvin believed that electrons travelling in
a conductor would come to a complete stop as the temperature got close to
absolute zero. But others were not so sure. Kelvin was wrong. The colder it gets,
the less the lattice shakes, making it easier for electrons to get through.
There\'s one theory that explains best what happens in a superconducting wire:
When a conductor is cooled to super low temperatures, the electrons travelling
inside it would join up in some way and move as a team. The problem with this
notion was that electrons carry negative charges and like charges repel. This
repulsion would prevent the electrons from forming their team. The answer to
that was phonons. It is believed that packets of sound waves (phonons) that are
emitted by the vibrating lattice overcome the electrons natural repulsion making
it possible for them to travel in team. It\'s as if they were all holding hands
together. If one of them falls in a hole or bumps into something, the preceding
electron would pull him and the