What happens in a glass rubbed with silk glass becomes positively charged and the silk negatively charged?

By John Papiewski

When silk is rubbed on a glass rod, an electrostatic charge is generated in a phenomenon known as the triboelectric effect. It is this electric charge that gives you a shock in winter when your rubber-soled shoes rub against a carpeted floor. The triboelectric series is a list of materials sorted according to their electrostatic behavior; using the list, you can predict the electrostatic effects when silk is rubbed on a glass rod.

The triboelectric series starts at the top with materials that strongly acquire a positive electric charge; the list continues down with materials becoming more negative. This occurs because some materials will lose electrons to other materials when they come in contact. When you rub two items together, the materials closer to the top of the series lose electrons and take on a positive charge; the materials lower on the series gain electrons and become negatively charged.

Because glass is closer to the top of the triboelectric series than silk, it develops a positive charge; silk becomes just as strongly charged, but the sign of the charge is negative. If you rub the two materials together, then touch the glass rod to two small bits of styrofoam, both pieces of styrofoam will lose electrons to the glass rod, become positively charged and will repel each other, because like charges repel. If you touch the rod to one piece of styrofoam and the silk to the other piece, the former piece will lose electrons take on a positive charge and the latter piece will gain electrons and take on a negative charge. The opposite charges will cause the pieces to attract.

You might know that all matter is made up out of atoms. Now, atoms themselves have a core, or nucleus, and electrons orbiting around the nucleus. The core has positive charge, the electrons have negative charge.

When you are rubbing the glass rod with the silk cloth, electrons are stripped away from the atoms in the glass and transferred to the silk cloth. This leaves the glass rod with more positive than negative charge, so you get a net positive charge.

Why do the electrons go from glass to silk and not from silk to glass? That depends a lot on the minute details of the material. Ultimately, for every two materials there is one of them where electrons are happier. It just turns out that for glass and silk, electrons are happier at the silk cloth.

Now to your second question. Here, the important thing to note is that in your typical solid material, the positive charges, which are the cores of the atoms, cannot move around much. They are locked into a rigid structure. The tiny electrons, however, can move around much better. That's why the glass rod can induce a net negative charge at one end of the paper clips.

EDIT: Let me add that there should also be some attraction between the silk and a bunch of paper: The electrons in the paper will be pushed away by the electrons in the silk, leaving the end of the paper that is closer to the silk with a net positive charge that then gets attracted. However, it might very well be that in your silk cloth the electrons are overall too spread out to have a strong enough attractive effect.