Name: John M. S.
The plasma cutting process works by passing a gas between
a cathode and anode which is conducting a very high electrical current.
As this gas pass through the electrical current it becomes ionized. The
temperature of the gas now increases to approximately 50,000 degrees F.
What causes the gas to become so hot?
A normal electric current is a flow of electrons. Moving electrons have
energy and will heat whatever atoms or molecules they strike. As you have
noticed, normal air does not conduct electricity very well. If a voltage is
imposed across two metal electrodes, no current flows because the electrons
cannot jump the gap from the negative to the positive electrode, nor do the
electrons travel on any air molecules. If ionized gas (free electrons and
positively charged atoms) is present between the electrodes then the
situation is different. The electrons move toward the positive electrode
and the ions toward the negative electrode.
A voltage difference of only a few volts will accelerate an electron to
pretty high energies, and the electron will transfer energy to anything it
strikes, including atoms in the gas. So the gas gets hot because moving
electrons, which get energy from the electric field, strike atoms and heat
them up. (Neutral atoms in the gas will then ionize because the impact of
the electrons, which then can participate in more ionization, so the plasma
keeps on going. )
A plasma can be started by either using high temperature, or a really large
voltage, or by touching the electrodes together.
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Update: June 2012