Plasma
What Is It?
•
A hot ionized gas
•
The ‘fourth state of matter’
–
Unlike gases, solids, or liquids, plasma does not
contain molecules
–
Instead, it is a gas that is composed of ions
•
Composes more than 99% of the known visible
universe
So What’s In It?
•
Some, or all, of the electrons in the outer
orbitals have been stripped away
•
The result is a collection of ions and
electrons, which are no longer bound
together
What Did That Mean?
•
Because the particles are not neutral:
–
Plasma behaves differently then regular
gases
–
For instance, in the presence of
electromagnetic fields
Who Found It?
•
First discovered by Sir William Crookes, in
1879
•
But it wasn’t called ‘plasma’ until 1928,
when Irving Langmuir coined the term
Characteristics:
Temperature
•
Defines two kinds of plasma:
Cold
and
Hot
•
Refers to the electron temperature
•
Ion temperature may be very different (lower)
•
Lightning
is an example of plasma present at
Earth's surface. Typically, lightning discharges
30,000 amperes, at up to 100 million volts, and
emits light, radio waves, X
-
rays and even
gamma rays.
Cold Plasma
•
Isn’t really cold
–
Typical electron temperatures for cold plasmas are in
the thousands of degrees
•
Only a small fraction of the gas molecules are
ionized (degree of ionization)
–
Usually on the order of 1%
•
Often created using strong electric fields
Cool
Where Can I Find Cold Plasma?
•
Fluorescent Lights
•
Strobe Lights
•
Experimental Fusion
Research Devices
Hot Plasma
•
Really hot
–
Like the Sun (15,000,000
°
C at the core)
•
The molecules are nearly if not fully
ionized
•
Created by heating the molecules to
extremely high temperatures
Scorching
Where Can I Find Hot Plasma?
•
In Nature, actually
–
The Sun and other stars
–
Lightning
–
The Aurora Borealis
Characteristics:
Potentials
•
Plasmas are excellent conductors
•
Simple view:
–
Due to the above, the electric fields in
plasmas tend to be very small
•
Quasineutrality:
–
On the one hand, we can assume that
densities of positive and negative charges are
equal
–
However, we can assume that electric fields
exist as needed for the physics at hand
An Application:
Let’s Get Hands On (sort of)
•
Plasma Globes
–
An electrode sitting inside a vessel containing
some kind of inert gas
–
The electrode is energized by a high
-
voltage,
high
-
frequency power supply
–
This globe uses voltages around 10 000 volts,
and frequencies ranging from a few kilohertz
to a few 10’s of kilohertz
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