http://www.ck12.org Chapter 15. Electrostatics
For example, when the cat inFigure15.1 is petted, it is now known that the negative charge from the cat’s fur is
transferred to your hand (a brush even works better at accumulating negative charge). Because of a “loss” of negative
charge, the cat has become positively charged. No new charge has been created. The work done in petting the cat
has redistributed the negative charges.
It will be explained below that it is the negative charge that moves when an electric shock is experienced. If enough
negative charge accumulates on your hand (the best chance of this is on a dry day), the electrostatic attraction
between the negative charge on your hand and positive charge on the cat will result in a discharge of the negative
charges. The cat (and your hand) experience an electrical shock.
A Simple Model of the Atom
During the 20th century, it was discovered that atoms are composed of three types of particles: theelectron, the
proton, and theneutron. The electron and proton are oppositely charged particles without which atoms could not
be held together; the electrons and the protons attract each other. The neutron has no electric charge (it is neutral).
Protons and neutrons reside at the center of an atom (in the nucleus) and the electrons, in the simplest atomic model,
travel around the nucleus like planets around a star. We know today that this model is vastly oversimplified.
The charges of the electron and the proton are of exactly equal magnitude but of opposite polarity (sign).
The smallest unit of electric charge (an elementary charge), found on an electron and a proton, has a magnitude of
- 6 × 10 −^19 C.
The symbolCrepresents the fundamental unit of electric charge, coulomb, which we will discuss later.
By convention, we say that electrons carry negative charge and protons carry positive charge. An atom is always
neutral, having the same number of protons in the nucleus as electrons in orbit about the nucleus. Though the
magnitudes of their charges are equal, their masses are quite different. The proton has about eighteen hundred times
more mass then the electron.
The atoms of solids remain in relatively fixed positions due to the electrostatic bonds between their atoms. Electrons,
as we have stated, are in motion about the nucleus of an atom. Some of the farther (outer) electrons in an atom are
held very weakly by the electrical attraction from the nucleus. Friction (or merely contact if the other substance
strongly attracts electrons) provides enough energy to completely free some of the electrons from their atoms. These
electrons are then able to transfer from one object to another.
When an atom loses or gains electrons we call the atom anion. Ions do not have an equal number of positive and
negative charges.
Conductors
The fact that electrons can easily move gives rise to what is known as an electric current, or a flow of electrons. We
will further discuss electric current later.
Some materials allow electrons to move more easily than others. In metals, for example,Figure15.2, electrons can
move very easily. The metals are considered to be the bestconductorsof electric current. Any material that permits
electrons to readily move is called a conductor. You may be familiar with the fact that current is “conducted” by
wires. The wires of electrical devices such as lamps, computers, and TV sets conduct electrons. We will discuss the
flow (current) of electricity later.
Check Your Understanding
How do free electrons distribute themselves on a conducting sphere?