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The Cause, Stopping the Pain, and "Electric People"

©1997 William J. Beaty


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 such as 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 still jump, 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 hazards.

A research paper: The Control of Body Voltage Getting Out of a Car, from JCI

Safety issues:
List of gasoline fires caused by static sparks.
Sparks and gasoline.

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