It IS the humidity August 9, 2010Posted by mareserinitatis in electromagnetics, physics.
Tags: electricity, electrostatics, humidity, triboelectric
It’s the time of year that I love: beautiful sunrises, not having to wear a coat outside, yards full of gorgeous flowers, and lack of electrical shocks.
The last one may sound funny, but one of the drawbacks to winter is the ever present shock that one gets when encountering the wrong combination of synthetic and natural fabrics. In fact, I have a lovely polyester lined winter jacket that turns into a van de graff generator every time I wear my favorite wool sweater. (For the record, I hate van de graff generators. Every stinking physical science class I took involved a demo of how they worked, and having long hair, I always was asked to hold onto it while they charged it up.)
Usually most people don’t think about the shocks until the cold weather sets in, at which point they wonder why they keep getting shocked. The explanation usually involves explaining rubbing balloons on hair or fur and causing it to stick to a wall.
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The reason this happens, as most people know, is that rubbing causes a transfer of electrons from one object to another. Fur or hair will tend to let go of electrons while other objects, like packing peanuts will tend to gain them. One way of phrasing this is saying that some materials have an affinity for electrons while others prefer to let them go. This process is called triboelectric charging.
A chart was developed to give people a rough idea of a material’s affinity for electrons. It’s called the triboelectric series. This does not tell you how much charge will be transferred from one material to another. There are a lot of factors that affect this, and even under the most controlled laboratory conditions, static charge due to the triboelectric effect is difficult to quantify. The rough list is
Back to the original question: why don’t we get shocked in the summer? The answer is many people’s summer nemesis: humidity. As the air temperature warms up, more water vapor is held in the air. Hopefully you’ll remember from high school chemistry that water is a polar molecule; that is, it has a polarity, being more negative on one side and more positive on the other.
The interesting thing, in my mind, is that I’ve seen three different explanations in literature as to how humidity decreases discharge (shock) events. (The reason, of course, depends on which author you read.) The first is that water molecules in the air, will attract electrons from the surface, plucking them off and thus dissipating the charge. The second, slightly related, is that the water molecules nearby will change orientation so that the a direction of polarity is facing any build up of charge. This behaves somewhat like Debye shielding so that the charge looks neutral from the outside. It reduces induction on another object nearby and therefore dielectric breakdown (which is necessary for a spark to flow) doesn’t happen. The final explanation is that water will collect on an object creating a slightly conductive path for charge to flow off an insulator.
I’m not sure which explanation is correct, and I wouldn’t be surprised if the actual effect was caused by a combination of two or all three of the explanations. Regardless, we now have at least one reason to appreciate humidity.