Image 1 Caption: A diagram of
a spectral tube (AKA discharge tube)
(a special case of a gas-filled tube).
A direct current (DC) is
used for simplicity.
The electrical potential difference
(AKA voltage)
causes an electric field which
causes the free cations (positive ions)
to move toward the cathode (negative electrode)
and the anions (negative ions)
and free electrons
(negatively charged
particles) to move toward
the anode (positive electrode).
The ammeter (circle-I)
and voltmeter (circle-V) are shown by
standard electric symbols.
The electric field
in the spectral tube
causes an electrical breakdown
of the neutral
gas in the
spectral tube and turns it into a
plasma (i.e., an ionized gas).
Explication of plasmas (i.e., ionized gases):
- An atom
contains a small nucleus
made of protons
and neutrons:
size scale 1 fm = 10**(-5) Å = 10**(-15) m
which is 10**5 times smaller than the atomic size scale.
The protons and
neutrons make up nearly all the
mass and they
are bound by the strong nuclear force.
- Electrons surround the
nucleus in
quantum mechanical
atomic orbitals
(the quantum mechanical analogs
of orbits though they are very different
from orbits in many ways) and are bound to the
nucleus by the
electric force.
They make up only a tiny bit of the mass,
but give the
atom its size scale of 1 Å = 10**(-10) m.
- Atoms and
molecules
(which are bound collections of atoms)
with equal numbers of protons
and electrons are
uncharged (i.e., electrically neutral).
This is the usual state for
atoms and
molecules
under relatively low temperatures
and sufficiently high
densities (but NOT
extremely high densities).
- If an atom or
molecule
becomes charged
by losing or gaining
electrons, it is called
an ion.
Losing/gaining electrons creates
positively/negatively charged
ion.
The losing of electrons is
called ionization.
In fact, negatively charged
ions can only exist if there
free electrons, and so
there must be positively charged
ions for there to be
negatively charged
ions, but the reverse is NOT true.
Highly ionized gases
usually have NO
negatively charged
ions at all.
Negatively charged
ions though sometimes important
in chemical reactions
and opacity
are usually a small minority species.
- Electrical breakdowns,
sufficient heat energy,
Townsend discharge, and
other ionization process
can cause ionization.
- A gas with a significant
fraction of atoms and/or
molecules
ionized
is a state of matter called a
plasma.
- Note yours truly like many other
astronomers just considers
a plasma
to be a kind of gas (i.e.,
an ionized gas)
and seldom uses
the word
plasma.
- Most plasmas
we encounter in experiments are low-ionization
plasmas
where the atoms
have lost (or gained) only a few electrons.
- Deep in the interior of stars,
one gets an extreme plasmas
where all an atom's
electrons are stripped off
(i.e., the atom is completely
ionized)
and one just has a gas
consisting of
unbound electrons and
bare atomic nuclei.
The stellar photospheres and
adjacent regions are usually only partially ionized.
Image 2 Caption: "Townsend discharge
is a gas
ionization process
(AKA plasma creation process)
where free electrons, accelerated
by a sufficiently strong electric field,
give cause
electrical conduction
through a gas
by electron avalanche
(a kind of avalanche breakdown
caused by the ionization
of atoms and/or
molecules by
a cascade of
particle collisions
of free electrons."
(Somewhat edited.)
Townsend discharge probably acts in ordinary
spectral tubes, but
yours truly does NOT know how important it is.
Images:
- Credit/Permission: ©
User:Rudolfensis,
2010 /
CC BY-SA 3.0.
Image link: Wikimedia Commons:
File:Simple representation of a discharge tube - plasma.png.
- Credit/Permission: ©
User:Denis Fadeev,
2014/
CC BY-SA 3.0.
Image link: Wikimedia Commons:
File:Townsend Discharge.svg.
Local file: local link: plasma.html.
File: Thermodynamics file:
plasma.html.