Another important aspect of electricity that arises from experimental observations
is that electric charge is always conserved in an isolated system. That is,
when one object is rubbed against another, charge is not created in the process. The
electrified state is due to a transfer of charge from one object to the other. One
object gains some amount of negative charge while the other gains an equal amount
of positive charge. For example, when a glass rod is rubbed with silk, as in Figure
23.2, the silk obtains a negative charge that is equal in magnitude to the positive
charge on the glass rod. We now know from our understanding of atomic structure
that electrons are transferred from the glass to the silk in the rubbing process.
Similarly, when rubber is rubbed with fur, electrons are transferred from the fur to
the rubber, giving the rubber a net negative charge and the fur a net positive
charge. This process is consistent with the fact that neutral, uncharged matter
contains as many positive charges (protons within atomic nuclei) as negative charges (electrons).