Greetings Squaddies! Welcome back.
Today's question comes from Kojubat, a fellow gamer and fine purveyor of Slayer poetry. That's Slayer as in Buffy, not as in speed metal, but perhaps he dabbles in both. You'll have to ask him. His question, and it's a great one, is: Why do some materials/environments generate more static electricity than others? Example: a car on a brisk day can shock you, but not so much in the middle of summer. There are actually two questions in there, but because I love you twice as much as the average site, I'll answer both of them. The first has to do with materials, the second with environments, specifically cold days vs warm days.
First, lets talk a little about static electricity. Static electricity is a result of a transfer of electrons from one surface to another. When a surface either gives up enough electrons or accepts enough electrons to have a net positive or negative charge, and then comes into contact with either a neutrally charged conductive object, or an object with a significant opposite charge, there's a spark, an owie and possibly some yelling. If said spark happens at a gas station, there may be a boom, an owie, and definitely some yelling.
The charge built up to generate the spark of static electricity comes about as a result of adhesion between the two materials. When two materials come in contact with each other, a chemical bond is formed linking the two materials together. As a result of this bond, electrons will transfer from one surface to another to try and equalize the charge of the bonded materials. When the two materials are then separated, the transferred electrons are left behind. Do this enough times, say when rubbing your feet across a carpet, and you have a significantly large enough collection of abandoned electrons to either open up a Home for Wayward Electrons, or zap the hell out of your hand when you grab a doorknob.
Now, there are certain materials that have a greater tendency to obtain a positive charge, some materials that have a greater tendency to achieve a negative charge and some that won't achieve a charge at all. This series of materials is called the Triboelectric Series. The rankings of the materials is basically a generalization as there are a hojillion characteristics that come into play when determining tendency to accrue charge. Basically, objects are the top of the list, such as human skin, rabbit fur and glass have a greater tendency to give up electrons. Things like teflon and polyester have a greater tendency to capture electrons. Why this is the case, I have no idea, so for the first part of Koju's question, "why do some materials generate more static electricity than others?" the core answer is "Dunno". A simple answer would be that some materials are more apt to give up electrons and some are more apt to accept them, resulting in a large charge difference when the two items are rubbed together and then separated. So, when you and your dry skin scuff across your carpet, which is probably made from polyester fibers, you accrue a much larger charge imbalance than if you had walked across a tile floor, or a floor covered in rabbit fur. You then touch a doorknob and BAM! Shock city.
As for the second part of the question pertaining to environments, we need to look back a couple of weeks when we talked about the relationship between warm air and humidity. Generally speaking, when it's warm out, there's more humidity as warmer air has a greater capacity to hold water vapor. Cold air, not so much which leads to dryer air in the winter and more humid air in the summer. Why this matters for the old shockola is because due to the hydrogen bonding going on in all that water vapor, the more humid the air, the better it conducts electricity which means that all of those electrons that, in dryer times, would hang out on your skin just waiting to blast you, instead bleed off resulting in a smaller charge imbalance. Now, just because it's warm, it doesn't mean that it's humid, nor does it mean that cold air is always dry, but typically that's the case. So, add the dry air, which doesn't conduct electricity very well, your skin, which is typically dryer in the winter, and warm winter clothing which is traditionally chock full of polyester and your ass is getting sparked every time you touch a car door handle in February.
Static electricity, while just mildly annoying most of the time, can cause some pretty serious damage under the right circumstances. Despite all the trappings of it being an urban legend, there are numerous accounts of static electricity igniting gas vapors at gas stations (just not from cell phones, so keep on yapping if you need to) so if ever you're filling up and you see someone rubbing a rabbit with a frying pan, get the hell out of there. Similarly, while you discharging at the door knob won't cause much more than a minor spark of pain, were you to do that while working on your super sensitive integrated circuit, say in a clean room, you'd fry the sucker and it's bye-bye circuit. This is why clean rooms used for circuit construction are usually kept at a relative humidity of 50%. Any rabbits reading this, hoping to grow up and build transistors for a living may want to reset their career expectations.
So there you are Koju. Thanks for the question and please keep these tips in mind the next time you head out for a night on the town in your polyester disco suit with genuine rabbit trim. I'd hate to hear that you exploded while doing the Hustle. For the rest of you, keep the questions coming.
School for Champions - The Triboelectric Series of Materials Causing Static Electricity - Ron Kurtus
Wikipedia - Triboelectric Effect
Howstuffworks - How Van de Graaff Generators Work - John M. Zavisa
MadSci Network - How is static electricity produced in connection to low humidity? - Barry Kamrass
ESD Journal - Static Fires