Plasma, what really is it? well here's a plasma,
here's another and other one number
plus don't forget the Suns made of plasms too.
You may have been introduced to
plasma as the fourth state of
matter and in my video about
lightsabers I called it essentially an
ionized gas, but neither of those
descriptions really covers what we
actually mean by a plasma in physics and
delving into the proper definition might
make you question everything you
originally told. Is plasma
really the fourth state of matter? I
think it's fair to say there's no simple
generally accepted one-sentence
definition of what a plasma actually is, but
if you were to try and make it would
have to contain a few key things. Like a gas plasmas do
not have a fixed shape or volume but
unlike a gas they must contain
electrically charged particles or
species including electrons, which are negatively
charged, and at least one type of
positively charged ion. You can have negatively
charged ions as well.
You can also have neutral species or
neutral particles, they're allowed in
there but for it to be a plasma there
have to be enough of those unbound
electrically charged particles. How much
is enough? We'll get on to that later. The next
requirement is that if we take a certain
amount of plasma all of the charges of
all of those particles have to
cancel one another out, plasmas are
quasi-neutral as we call them. Finally
all those charged particles have to be
affected by, but also contribute to,
electric and magnetic fields and that
leads them to exhibit collective
behavior. How does that work?
Each charged particles' location and
motion will add to the overall electric and magnetic
field, and the overall electric and magnetic
field affects each charged particles'
motion. It's this collective behavior
which most physicists state make
plasmas so different. And that right
there was the real definition of a
plasma, though to be honest I've been
pretty damn vague about a lot of those things.
If you wanna get properly technical,
plasmas have to satisfy three
criteria. The first is called the plasma
approximation and it says the charged
particles have to be close enough
together so that they will interact with
lots of different particles rather than
just its nearest neighbour. How close
is close enough though? The Debye length is the
distance over which ions' electrostatic
effects are essentially screened out
by the much lighter
electrons. So for collective effects we
need to have lots of ions within a Debye
Sphere. The second criterion is that we
don't want to have to worry about
effects of the edges of the plasma, so
they have to be a lot lot bigger than a
Debye Length. The final criterion relates to
the speed at which electrons will
naturally wobble. They can do this in two
ways, through electrostatic fields called
plasma oscillations or through the
presence of a magnetic field called
gyromotion. So for plasma to act differently
from a gas, we need the frequency of
these motions
to be much larger than the frequency of
particle collisions. There you have it
basically the first sub chapter of my
PhD thesis, so what's the problem? Well my quick
definition of plasma as just an ionized
gas doesn't cover all of those points
that I just raised. For example
many people will claim that a naked flame
is an everyday example of a plasma, but it isn't.
Yes most flames are weakly ionized and
they will interact with a magnetic
fields but there really aren't enough
charged particles around for them to
exhibit collective behavior, so we
can't call them a plasma. OK what
about labeling plasmas as the fourth state?
Sound exotic doesn't it.
And whilst here on Earth solids, liquids
and gases are more prevalent, throughout
the universe plasmas are by far the most
abundant state of matter out there. 99% of
all known matter is in the plasma state,
that's not counting dark matter as we
don't know what that is. All the stars,
Solar and stellar winds, interstellar
medium, accretion disks, nebulae... they're
all examples of space and astrophysical
plasmas. So from the point of view of how
common the states of matter are, surely
plasma should be the first state of
matter? Though ordering the rest of them
might be a bit tricky. But you might be
saying well take a solid, heat it up and it
will melt forming a liquid, keep on heating
it up it will boil to a gas and go further again and it
will ionize and form a plasma. But
again it's not as simple as that. For instance
you can form plasmas at low temperatures
like this by using very strong electric
fields to rip the elections off the
atoms or even by using electromagnetic
waves like laser light to kick those
electrons off. But ignoring that, you have
to remember that depending on the
pressure you might not have that
simple solid to liquid to gas to plasma
route as you amp up the temperature.
Helium exists as a solid at temperatures
just a smidge over absolute zero only at
pressures of greater than 25 atmospheres,
whereas at low pressures most materials
will actually forgoe the liquid state
go straight from a solid to a gas called
sublimation, and even some metals
strictly obey the plasma criterion
because of their free charge carriers.
Going beyond just solids liquids and
gases, there are other states that we
have discovered. Glass is a separate
state otherwise known as non-crystalline
amorphous solid, then you've got the liquid
crystal state particularly important for
that other type of television technology,
magnetically organized states like
ferromagnets, there are low temperature
quantum states of matter like the Bose
Einstein condensate and superfluids, and
even high-temperature states of matter
like the quark-gluon plasma. So you can
see that simply numbering the states of
matter isn't really an option.
The problem is that defining states of
matter is really hard. Historically it
was done by just qualitative differences
in the properties, now we say a different
state of matter is a distinct form of that
substance. You'll often hear people
talk about phases rather than states, but
the two aren't necessarily the same
thing. The description of a phase is a
region of a material that is chemically
the same everywhere,
physically distinct, and separable from
anything else that might be around. What
we know about plasmas though they don't
really follow that definition, they're made
up of multiple different components
which are naturally tied together, any
neutral species which can be gases or
even solids are part of the plasma and will
affect it.
And the formation of a plasma is an
incredibly gradual process unlike the
very distinct phase transitions that
we go through when you transfer between
solids, liquids and gases. Basically the
definitions that we've built up over the
era of chemistry before we had
discovered or really understood plasmas
are either just way too vague or far too
rigid for plasma to really fits in. So
really, plasma is just plasma and it is everywhere.
