Unit 8. Electricity

Electricity has been known since the days of the ancient Greeks. The word “electricity” comes from the Greek word for amber. The Greeks discovered that, if a piece of amber was rubbed with fur, it would pick up bits of straw or other light-weight materials. Later scientists discovered that other materials would act like amber. They could be given charges of electricity. Charges of this kind are called charges of frictional, or static, electricity.

Perhaps one of the most useful accomplishments of the recent centuries is the development of electric circuits. The flow of charge through wires allows us to cook our food, light our houses, air-condition our work and living space, entertain us with movies and music and even allows us to drive to work or school safely.

How an electric field can influence charge within a circuit as it moves from one location to another. Electric force is described as a non-contact force. A charged balloon can have an attractive affect upon an oppositely charged balloon even when they are not in contact. The electric force acts over the distance separating the two objects.

A charged object creates an electric field. The electric field direction about a positive source charge is always directed away from the positive source. And the electric field direction about a negative source charge is always directed toward the negative source.

As a physical quantity, current is the rate at which charge flows past a point on a circuit. The standard metric unit for current is the ampere. Ampere is often shortened to Amp and is abbreviated by the unit symbol A. A current of 1 ampere means that there is 1 coulomb of charge passing through a cross section of a wire every 1 second. The push that forces a current through a circuit is measured in volts. Most household appliances are built for a voltage of either 127 or 220.

Electric circuits are designed to serve a useful function. The mere movement of charge from terminal to terminal is of little use if the electrical energy possessed by the charge is not transformed into another useful form. To equip a circuit with a battery and a wire leading from positive to negative terminal without an electrical device (light bulb, electric bell, motor, etc.) would lead to a high rate of charge flow. Such a circuit is referred to as a short circuit. With charge flowing rapidly between terminals, the rate at which energy would be consumed would be high. Such a circuit would heat the wires to a high temperature and drain the battery of its energy rather quickly. When a circuit is equipped with a light bulb, electric bell, or motor, the electrical energy supplied to the charge by the battery is transformed into other forms in the electrical device. In a light bulb, electrical energy is transformed into useful light energy (and some non-useful thermal energy). In an electric bell, electrical energy is transformed into sound energy. And in a motor, electrical energy is transformed into mechanical energy.