Electricity and Magnetism 99
charge would be enough to completely destroy all the molecular bonds,
which hold matter together and scatter it to the four corners of the
universe.
Fortunately for our existence the equality of the magnitude of the
charges for the proton and electron, two particles, which differ in so
many other ways, is apparently identical. Macroscopic matter becomes
electrically charged, however, whenever there is a slight excess or
shortage of electrons. This occurs, for example, when a rubber rod is
stroked by a piece of cat fur in which case electrons are transferred by
friction from the fur to the rubber rod. The rubber rod has an excess of
electrons and hence, has a net negative charge whereas the fur contains
more protons than electrons and hence, has a net positive charge. If, after
rubbing the rod with the fur, one were to put these two objects in contact,
electrons attracted by the positive charge of the fur would flow from the
rod to the fur until electric neutrality was once again established. Electric
neutrality can also be established by placing a copper wire between the
rod and the fur, which would permit the passage of an electric current of
electrons to flow between them.
Not all materials, however, permit the passage of an electric current.
Some materials such as wood, asbestos and rubber, referred to as
insulators, do not permit the flow of an electric current because all of
their electrons are tightly bound by the chemical bonds holding these
materials together. In certain materials, however, such as metals, not all
of the electrons are so tightly bound in their atoms. These materials,
referred to as conductors, permit the flow of an electric current. When an
electric current flows, electrons do not flow from one end of the wire to
the other. The electrons in a conductor behave more or less like the
molecules in a gas. When there is no current flowing in the conductor,
the free electrons move back and forth within the wire in a random
fashion colliding with each other and the atoms making up the wire.
When a current is flowing there is a general drift of the electron in a
particular direction. The net effect is that a current flows from one end of
the wire to the other although no actual electron makes this trip. The wire
is actually electrically neutral with as many electrons flowing into any
one segment as flow out of that segment. If more electrons flow in one
direction than the other then there is a flow of current in the opposite
direction because the charge on an electron is negative and the flow of
current is defined as the direction of the flow of positive charge as is
illustrated in Fig. 11.2. The heat generated by an electric current is due to