There’s been a long hiatus for Everyday Physics because I was occupied writing my PhD thesis. Now that that’s complete, we can jump right back in!
If you’re living somewhere with a relatively cold winter, you’ve probably experienced a lot of small shocks as you touch metallic objects like doorknobs or coat hangers. Now that the weather is turning warmer you’ll experience it less and less. Many people know that this is built up electric charge on your body leaving, but have you ever wondered why it seems to happen more often in the winter?
Static electricity is a charge imbalance that builds up on an object
You may already know that static electricity can build up on an object if it is rubbed against another one. We like to demonstrate this today by rubbing balloons in someone’s hair, but it’s actually a very old idea: the Greeks originally discovered this behavior when they rubbed fur against amber rods. In fact, the Greek word for amber is elektros, which was used as the origin for the modern word electricity.
What’s happening here is electrons are moving from the fur to the amber because of the rubbing process. It turns out the atomic structure of the amber holds electrons more strongly than the structure in the fur, and so some electrons will be pulled from the fur when the two materials touch. This happens to some degree upon initial contact, but is a really prominent when the two materials are rubbed together.
Which way the electrons move depends on the atomic details of the materials. Amber will pull electrons from fur when they are rubbed together, but (for example) glass will give up electrons to silk if they come into contact, such that the glass rod will become positively charged from now having fewer electrons. Rubber can also accept electrons from carpet, which is one common way that you will become electrically charged in everyday life.
The excess charge will move to another object in order to restore balance if it can
In general, the extra electrons will try move out of an object as soon as it comes into contact with another material. In the case of a positively charged rod, electrons from another material will move into the rod when it comes into contact. Certain materials are better than others at moving electrons back and forth in this manner, and are called electrical conductors. You probably already know that metals are generally excellent conductors, which is why they are used in circuits.
Electrons travel through the air when your charged finger gets close to something metal
It’s easy for electrons to move between two materials in direct contact, but not otherwise. This is because the air acts as an insulator, preventing electrical flow. As the charge on an object builds up, however, it can eventually “jump” through the air in order to reach another material and restore balance. This occurs because the air is of course comprised of atoms with electrons just like a solid, only more dispersed: in the same way that electrons can move between two solids in contact, they can also move from a solid to the atoms and molecules that comprise the air.
Air is a relatively good insulator, though, so it takes a large charge buildup on a surface and a nearby accepting surface before the electrons can travel through the air. Static discharge in the air occurs at around 10 kilovolts per centimeter. In other words, it’s not uncommon to build up electrical charge on your body such that the electrical potential between you and another object is a few thousand volts, so that electrical discharge occurs when you come within several millimeters of an object that is more electrically neutral.
The more electrical charge buildup there is, though, the larger the distance over which electrons can jump from surface to surface. For example, lightning is the same sort of electrical buildup and then discharge as you experience by walking on carpets, only many orders of magnitude more charge, which lets the discharge jump from clouds all the way to the ground.
Airborne water vapor impedes the flow of static discharge, and there is typically less humidity in the winter months
The 10 kV/cm number for electrical discharge will change, however, depending on the humidity present in the air. This is because higher water vapor content makes air a better insulator, preventing the flow of electricity.
As previously discussed, lower temperatures of air contain less humidity. As a result, winter air is typically drier than summer air, which means it is easier for electrical charge to jump through the air between materials.
This sort of static discharge can be slightly painful, and produces a small flash and tiny shock wave when it occurs. That’s not a big deal for humans, but it can be devastating to sensitive electronics.