ELECTRICITY: THE FORCE THAT TRANSFORMED THE WORLD
1. In order to talk about electricity, it is necessary first to talk about the atom. The idea of the "atom" has a long history, one extending back to about 600 B.C. and the time of the ancient Greeks. They believed that all matter was made up of atoms. The word "atom," in fact, comes from the Greek word "atmos," which means "indivisible." It was not until 1897 that it was discovered that the atom is not indivisible but is composed of even smaller particles. Among these particles is one called the electron.

2. Electrons orbit around the center or nucleus of the atom, much as the planets in the solar system orbit around the sun. Electrons closer to the nu­cleus are held more tightly than those in the outer orbits. It is the electrons in the outermost orbit of certain kinds of atoms that can be made to flow as electric current.

3. Electrons flow easily through certain kinds of materials called "con­ductors." Many metals, such as silver, copper, gold, and aluminum, are good conductors. Good conductors are used in electric circuits to provide a path for the current.

4. Other substances provide strong resistance to the flow of current. These substances are called "insulators," which are used to confine a cur­rent to the desired path. Substances, such as hard rubber, glass, wax, and certain kinds of plastic, are good insulators. Thus, the cord on an electric appliance consists of a piece of wire, generally copper, surrounded by a type of plastic or vinyl, which is the insulator confining the current to its path.

5. The pressure that makes electrons flow along wires is called "voltage." Voltage may be created by a generator at a power plant or by an electric battery. When you turn on a light or an electric appliance, electrons are drawn from a generator at a power plant. When you turn the light or ap­pliance off, there will be electric pressure or voltage built up at the switch, but no current will flow. It is somewhat similar to the way a water system works.

6. When you turn on a water faucet, water flows through the pipes, which is like electric current flowing through wires. When you turn off a faucet, water pressure remains but no water flows through the faucet. Sim­ilarly, when you turn off an electric appliance, voltage remains, but no current flows. In a water system, the whole operation depends on water pressure generated by a water pump. In an electric system, the generator (or battery) creates the pressure called voltage.


EXERCISES
I. Multiple Choice: Circle the number of the item that best completes the statement.

1.Electrons will flow easily through copper and silver because they are good (1) particles (2) conductors (3) regulators (4) insulators.

2.Electrons are smaller particles of (1) circuits (2) currents (3) vol­tage (4) atoms.

3.One function of an insulator is to confine an electric current to its (1) voltage (2) orbit (3) path (4) generator.

4.Similar to the way that planets orbit around the sun, electrons in an atom orbit around the (1) nucleus (2) generator (3) flow (4) voltage.

5.A substance that offers strong resistance to the flow of electric current is called (1) a battery (2) a conductor (3) an appliance (4) an insulator.

6.Another term for electric pressure is (1) current (2) voltage (3) faucet (4) switch.

7.Rubber, glass, and wax are good (1) insulators (2) conductors (3) regulators (4) generators.

8.At a power plant a generator is used to create voltage, which is the pressure that makes (1) atoms (2) circuits (3) electrons (4) orbits flow along electric wires.