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Voltage Breakdown
All,
I've been reading about electrostatics and a very amazing revelation was
presented in the book Fundamentals of Applied Electrostatics by Crowley
ISBN 0-89464-535-8, 1991.
It seems that there are basically two major criteria which determine
breakdown in air.
1. voltage
2. field strength
As a rule, both criteria must be exceeded before breakdown occurs.
I was amazed to learn that below 300 volts electrical break down in air
is virtually impossible!! Even with sub-micron separations!!!
Next, it is impossible to achieve any breakdown in air at all until one
achieves an electrical field gradient of over 3 million volts per
meter!!!
This means if your coil is sparkin' the voltage is over 300 volts and
the electric field in the area of the arc breakout is over 3 million
volts per meter.
There is a critical size which separates the voltage controlled breakdown
from field controlled breakdown. This size is .1mm!
Naturally, the above is based on the typical dry air conditions. Also
the frequency of the signal is important (assumed DC level) and modifies
the above a bit.
Some other interesting tid bits are, that the span of charge in every day
life covers 25 orders of magnitude!!! Wow! Few variables in the real
world span so great a range. (earth charge 10^7 coulombs, electron/ion
charge 10^-18 coulomb)
Electrical breakdown can occur in air when 25 micro coulombs/square
meter of surface charge is present whether it is on an ink drop or a
person. This is the form of the electric displacement vector (D) and
relates to Gauss' law. D has the dimensions of charge per unit area.
A key statement was:
"A basic concern in electrostatics is the relation between the two
variables of voltage, or E-field and D, the electric displacement
vector." "THERE IS NO FUNDAMENTAL PHYSICAL LAW WHICH GIVES THIS
RELATION"!
This relation is determined by MATERIAL PROPERTIES and is rarely dealt
with, even in the better classic electrostatic instructional courses.
The whole deal is 100% emperical and is dealt with by applying voltage
to materials and measuring charge flow. (Relates back to the varying
insulating delectric constants covered on Chip's list)
There are two basic types of results for the above. Ions and the strange
material ELECTRETS can have charge present and be totally independent of
any voltage concerns.
In other systems, such as capacitors, the amount of charge is totally
dependent on the voltage applied. (it may also depend on the history of
the voltage applied) Remember, that capacitance is literally,
Coulombs/Volt. from C=q/v.
There are many real eye opening insights in this book. It seems the old
classic electrostatic mantle which had been cast to the ground as a dead
science has been snatched up of late due to researches in
Electrophotography (xerography) for using this abandoned voltage to
constructive ends, and in ESD protection research (semiconductor survival
techniques) in an effort to destroy on nullify this abandoned voltage.
This book is great because it involves itself only with practical
examples found in every day application engineering situations.
This is being sent to both the HV list and the Tesla list inorder to
uniformly share the data with possible interested parties.
Richard Hull, TCBOR