From: bds@ipp-garching.mpg.de (Bruce Scott TOK )
Newsgroups: sci.physics.plasma
Subject: Re: Plasma questions
Date: 14 Jan 1997 19:10:45 GMT
Organization: Rechenzentrum der Max-Planck-Gesellschaft in Garching


Mike Greenberg (fodder@ix.netcom.com) wrote:
: Hello; my name is David Greenberg. I am fourteen years old, and am
: very interested in science and mathematics. I have been doing a
: plasma-related project, and require some clarification of the
: properties of matter. I have not yet taken calculus, but can get
: someone to explain anything you tell me.
: I appreciate any assitance you can give me, and please do not feel
: like I am asking you to answer all your questions. Thanks in advance.

: 1)What part of a flame is plasma? Does it have a magnetic field?

Flames are mostly chemical -- they are at very low temperatures (1000 K)
by comparison to most plasmas. What you see in a candle flame are
small, macroscopic (containing lots of atoms) carbon bits which haven't
been completely burned, glowing by their own heat.

A plasma is a gas containing charged particles, in which there are
enough charged particles in a given volume such that they react as a
group to electric and magnetic fields. There is a length scale, called
the Debye length, in a plasma which gives the range within which a
particle's individual electric field will affect its neighbors. Beyond
this range the slight motions all the near neighbors make in response to
an individual particle screen that particle from other neighbors farther
away. This distance depends on the density and temperature of a
particle distribution, and the individual charge on each particle. It
is called the Debye length. You have a plasma if there are lots of
neighboring particles within one Debye length of any given particle
(this is primarily a condition on the temperature of a cloud of charged
particles, given their density). This means that lots of particles will
react to anything any one of them does, so that the response to anything
that happens will be collective.

: 2)Does the earth's magnetic field affect fire or plasma?

The latter, yes. Near-Earth space is a very rarefied plasma, and the
properties of this plasma are determined by the interaction between the
solar wind and the Earth's magnetic field.

On terrestrial fires, the action of the planetary magnetic field is
really very small compared to anything the local volume of atmospheric
gas does.

: 3)Since electric current is random in a plasm, there should be no
: field. But can you make an diode plasma? Or what about lighting a
: diode on fire? If you ignite an LED, will it light?

The electric field is not random in a plasma. Individual atoms and
other particles are randomly arranged, but the electric field depends on
the densities of the charged particles. In an otherwise neutral plasma,
the amount of local charge imbalance needed to provide a substantial
electric field is very small. If the scale is large compared to a Debye
length (see above), you can consider the plasma to be neutral (if it
started that way) although there can be an electric field. This is
called "quasineutrality". An obvious exception is something like a pure
electron plasma, which is very exotic. People at U Cal San Diego have
been studying such nonneutral plasmas for many years.

: 4)What is the resistance of plasma when conducting electricity, and
: what factors influence it?

This depends on the temperature. A rough approximation useful in
nondynamical situations is that the electric field is balanced by the
current density times the resistivity. In an electron-ion plasma, the
current density is the electron number density times the charge on an
electron times the drift of electrons relative to the ions, with a minus
sign to reflect the electrons' negative charge. The resistivity is a
constant depending on the electron mass and the average charge on the
ions, times the temperature to the minus 3/2 power.

--
Mach's gut!
Bruce Scott The deadliest bullshit is
Max-Planck-Institut fuer Plasmaphysik odorless and transparent
bds@ipp-garching.mpg.de -- W Gibson