Q&A
with TJ Byers
Electronics Q&A
In this column, I answer
questions about all aspects
of electronics, including
computer hardware,
software, circuits, electronic
theory, troubleshooting, and
anything else of interest to
the hobbyist.
Feel free to participate
with your questions, as
well as comments and
suggestions.
You can reach me at:
TJBYERS@aol.com
Lightning Physics
Q. During a lightning strike, is the
earth considered positive or
negative?
Gene Bozek
via Internet
What's Up:
Have questions about
lightning? I have answers.
A. Positive. In an electrical storm,
the storm cloud is charged like a
giant capacitor. The upper portion of
the cloud is positive and the lower
portion is negative.
Like all capacitors, an electric
field gradient exists between the
upper positive and lower negative
regions. The strength or intensity of
the electric field is directly related to
the amount of charge build-up in the
cloud. The charge is created by
colliding water droplets.
As the collisions continue and the
charges at the top and bottom of the
cloud increase, the electric field
becomes more intense — so intense,
in fact, that the electrons at the
earth’s surface are repelled deeper
into the earth by the strong negative
charge at the lower portion of the
cloud. This repulsion of electrons
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causes the earth’s surface to acquire
a strong, positive charge.
The strong electric field also
causes the air around the cloud to
break down and become ionized (a
plasma). A point is reached (usually
when the gradient exceeds tens of
thousands of volts per inch) where
the ionized air begins to act like a
conductor. At this point, the ground
sends out feelers to the cloud,
searching for a path of least
resistance. Once that path is
established, the cloud-to-earth
capacitor discharges in a bright flash
of lightning (Figure 1).
Because there’s an enormous
amount of current in a lightning
strike, there’s also an enormous
amount of heat. (In fact, a bolt of
lightning is hotter than the surface of
the sun.) The air around the strike
becomes super heated — so hot that
the air immediate to the strike actually explodes. The explosion creates a
sound wave that we call thunder.
Some say that lightning strikes like
this in the early days of the Earth led
to the creation of life.
Cloud to ground strikes are not
the only form of lightning, though. There are
also ground to cloud
(usually originating from
a tall structure) and
cloud to cloud strikes.
These strikes are further
defined into normal lightning (discussed above),
sheet lightning, heat
lightning, ball lightning,
red sprite, blue jet, and
others that are lesser
defined. For more information on lightning,
check out http://scie
nce.howstuffworks.
com/ lightning.htm
SEPTEMBER 2004
Figure 1
