Why does rubbing plastic and wool together create electricity?
In the more technical writings, this phenomenon is called "triboelectricity" where the first part comes from the Greek word "tribein" which means "to rub." In fact the rubbing is not really the cause, although it can be a big help in creating the effect. Solid matter is a collection of atoms held together by electrical forces. This electrical force is generated by the electrons associated with the atoms. At the surface of a material, this force presents a boundary that prevents another piece of matter from simply passing through it. (This is a good thing as otherwise, we would be falling through the floor!) However, the exact structure of the electrical force at the surface for a particular material allows some materials to be able to exchange some electrons from one material to the other if the materials can be brought sufficiently close together. You only need to have very close physical contact to do this. However, the number of electrons exchanged can be increased by increasing the area that is brought into close contact. The rubbing together of the two materials allows one to increase this area of contact.
Once you have achieved this exchange of electrons, you now have one material with an excess number of electrons, and the other has an equal number of missing electrons. This imbalance of electron number is what one means when something becomes electrically charged. Although this phenomenon seems simple enough, its control is a vital part of modern technology. As examples of this, the production of modern electronic microchips requires the absolute elimination of static charge buildup. Otherwise, those tiny electrical parts would be fried by the spark from a nearby charged object. Also, those nice printouts from a modern laser printer are only possible because many people used their talents and knowledge to precisely control how electrical charge is distributed on the surface of the paper during the printing process. For more reading, an "oldie but goodie" is an article called "Static Electricity" by A.D. Moore and was published in the March, 1972 edition of Scientific American.
Carl Zorn, Detector Scientist (Other answers by Carl Zorn)