MadSci Network: Physics

Re: If plasma rarely forms on Earth, then how do we know that it exists?

Date: Tue Jun 19 00:55:44 2001
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Physics
ID: 991017741.Ph

Dear Melissa:
You ask many questions.  To avoid confusion I will answer them one at a 
time.   1. We know the temperature of the sun by analyzing the wavelength 
of the light which is emitted from it.  The wavelength of the light is a 
measure of the temperature so the temperature of the sun can be measured 
without having to go to the sun. 2. A plasma per se is a gas containing 
may ions and free electrons.  A simple bolt of lightning in a thunderstorm 
is a giant plasma, which we see all the time.   The gas which glows 
brightly which you see in the lightning is very hot-thousands of degrees-
but the hot gas is confined to a very small volume and as you move away 
from the plasma the temperature drops very quickly.

Without a constant voltage to drive electric current which is the heart of 
the plasma, then the plasma disappears.  In lightning the voltage comes 
from the electrostatic charging of the clouds above the earth to a very 
high voltage, which then causes the electric spark to ground which is the 
plasma.  But the bolt of lightning is the discharge of this high voltage, 
changing into the very high current of ions and electrons.   The voltage 
then disappears and the plasma is gone!   It happens so fast that the 
surrounding area does not have time to heat up before the voltage is all 
gone!  Scientists create a plamsa by generating very high voltages which 
cause the gases to break down just like lightning, making the plasma.

A simple neon sign is a plasma!  That is a cold plasma!  The temperature 
of the plasma depends on the pressure of the gas and the current flowing 
through it.  When the pressure is low, as in a neon sign, the temperatures 
are low because the currents are low!  Usually thay are less than one 
ampere.  But a lightning bolt carries currents of thousands of amperes-big 
difference!  The pressure is high (about 1,000 times the pressure in the 
neon sign).  And it is not true that plasma forms rarely on earth.  Every 
time there is a lightning bolt there is a plasma!

Without the presence of a voltage to separate the positive ions and the 
free electrons,  the positive and negative charges recombine to form 
neutral atoms.  The opposite charges attract each 
other very strongly, so when the voltage is gone the plasma vanishes 
immediately by the recombination of the positive ions with the negative 

I hope this answers your questions.  If you have more questions after this 
please ask them and I will explain in further detail for you.

R. Bersin......

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