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HUMANS AND SPARKS
©1997 William J. Beaty
The Cause, Stopping the Pain, and "Electric People"
Part 3: CAR DOOR SPARKS
The cause of car-door sparking is well known: contact-electrification
between insulating surfaces, followed by separation of those surfaces. But
what does this mean? Well, *YOU* are one surface, and THE CAR SEAT is the
other. When you sit on a plastic car seat in dry weather, the contact
between your clothes and the seat's surface causes the electrical charges
within atoms of
the material to transfer between the surfaces. This is our old friend
"frictional" or "contact" charging. One surface ends up with more
negative charges than positive, and has a negative charge-imbalance. The
other surface has fewer negatives than positives, so it has a positive
imbalance. This is nearly same thing as rubbing a balloon upon your hair:
both surfaces become electrically charged. But rather than rubbing
just your hair, instead you're rubbing your entire back, but, and
legs upon the car seat surface.
However, nothing happens as long as you remain seated. Just keep yourself
in one place and you won't get zapped..
As long as the surfaces remain near each other, the positives and
negatives cancel out, and no overall "electricity"
appears and no sparks are possible.
But when you open the car door and step outside, you take
just one polarity of charge along with you, while the car seat has the
opposite polarity. At the same time, the charged-up
car seat causes the whole car to
become charged (by a process called "Faraday's Icepail
Effect.") As you step out of the car, the voltage between your
body and the car becomes huge, up to 10,000 or even 20,000 volts. Your
shoes are probably insulating, so the charge has no opportunity to leak
into the earth. You reach out to close the car door and ZAP!, the
opposite polarities rejoin by leaping through the air while giving you a
tiny, deep burn on your fingertip!
How to prevent this? One possibility: change the surface materials.
Identify and avoid the specific clothing which makes the problem worse.
These materials are usually wool sweaters and pants, certain manmade
fabrics, plastic raincoats, etc. Or, replace your cheap plastic car
seatcovers with cloth (stains easily!) or with leather (expensive dead
animals.) Another method: mix up some anti-static solution and spray your
car seats. This solution remains slightly damp for weeks, which halts the
contact-charging process. The formula: a teaspoon of fabric softener mixed
in one quart of water. This tends to work well at first, but after
days it wears off and needs a re-coating. Another sillier method: always
drive barefooted, so the charge will leak away when you step outside the
car. Not good in winter! You could cover your car seats with a conductor
aluminum foil, which screws up the contact-charging effect. Have a tailor
make some custom clothing out of black conductive carbon cloth? Or you
could eliminate the problem by eliminating your clothes. Skin is fairly
conductive, so it doesn't create charge-separation when held against
plastic. Driving while nude might cure the sparking problem (unless you
are a very hairy person!)
A less frivilous method: the car-keys trick I mentioned earlier.
Develop the habit of
holding your car keys as you leave the car, then grip the keys firmly and
touch the metal car door with the tip of the key. The spark will
but it will not be painful, since it blasts a little hole in the tip of
the key instead of in your finger. Or simply grasp the car door as
you climb out, and this will drain off the charge-imbalance faster than
it can build up on your body.
DANGER: GASOLINE FIRES. What happens when you climb out of your car at
the gas station? Usually you'll zap yourself on the car door, or on the
gas pump handle, or on the metal door that covers your gas cap, and
usually nothing bad happens. However, suppose your passengers climb out
of the car just as you're unscrewing the cap, or just as you jam the gas
pump nozzle into your tank? The whole car becomes momentarily charged.
ZAP! Or, suppose you turn on the gas pump and then climb back into your
car. When you climb out again, you body is highly charged from the car
seat. The very first thing you do is to reach into the cloud of gasoline
vapors to grab the metal handle of the gas pump. ZAP! FOOSH! This
obviously is a very rare event. However, it does occur sometimes,
especially in the winter. See the PEI site on static
A research paper: The
Control of Body Voltage Getting Out of a Car, from JCI
List of gasoline fires caused by static sparks.
Sparks and gasoline.
NEXT: Part 4: ELECTRIC PEOPLE