Anyone who has ever felt the zap of an electric shock after walking across a carpet and then touching a metal door knob has experienced that two objects rubbing together can create electrostatic charges. Whenever two different materials rub against each other it is likely that one will leave with more electrons than it started with...the other will leave with less. This is called Triboelectricity (tribo means friction). From the study of chemistry we learn that different materials have different desire for electrons. (This is called electronegativity.) Some materials are very greedy and will always steal electrons from things they come in contact with, others are more willing to give up electrons.
|As the neutrally charged person walks across the wool carpet, his leather
soled shoes have less desire for electrons than the wool carpet. As a result,
electrons get stolen from the shoe by the carpet. With every step the person becomes
more and more positively charged. That charge distributes itself over the body.
When the positively charged person gets near the metal door he will actually
attract charges from the door which jump in the form of a spark. Notice how only the
negative charges (electrons) are free to move.
It is important to point out that if he was wearing rubber soled shoes on a wool carpet, his shoes would steal electrons from the carpet. He would become more negatively charged with each step. When he gets near the door the electrons will jump from him to the door. From his point of view it would look and feel the same as it did in the first example. He can't tell whether charges jumped to or from him.
If we did a study of many materials and put them in order from those with the least desire for electrons to those with a very strong desire for electrons we would have created a Triboelectric series.
If two items from the list are rubbed together, then the item that is higher on the list will end up more positive and the lower one will end up more negatively charged. For example, if leather were rubbed with wool, the leather becomes positive and the wool negative. Yet if rubber is rubbed with wool, the rubber becomes negative and the wool positive.
It is important to note that this series is true only if the samples
are clean and dry. The presence of moisture, dirt, or oils may cause some of the
items to interact differently.
©1999 Science Joy Wagon